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The disturbance caused to school teachers by noise
Journal of Sound and Vibration (1980) 70(4), 557-572
THE DISTURBANCE
CAUSED
TO SCHOOL
TEACHERS
BY NOISE J. W. SARGENT, M. I. GIDMAN, M. A. HUMPHREYS AND W. A. UTLEY Building Research Station, Garston, Watford WD2 IJR, England (Received 25 September 1979, and in revised form
14 January 1980)
A survey to investigate the disturbance caused to secondary school teachers by noise is described. Although the survey sample was selected on the basis of road traffic noise exposure it has also been possible to draw conclusions about the disturbance by aircraft noise. Quantitative relationships have been established between the proportions of teachers bothered by noise and the noise level to which they are exposed. The results of this school survey are compared with dwelling noise surveys.
1.
INTRODUCTION
During the last decade a considerable amount of research has been carried out to determine the effects of transportation noise on the occupants of dwellings [l]. In the United Kingdom the information obtained from this type of research has been used as the basis of planning procedures and schemes to provide increased sound insulation for dwellings exposed to high levels of external noise. Considerably less effort has been devoted to determining the effects of external noise on the occupants of other types of building. For example, the only published study of the effects of traffic noise on schools appears to be that undertaken by Bruckmayer and Lang in Vienna [2], based on use of a questionnaire. Although the study was limited in size, covering only 46 classrooms, it showed that traffic noise could produce serious disturbance in schools facing main roads. Crook and Langdon [3] investigated the effects of aircraft noise on school teaching. By direct observation in the classroom they were able to determine a relationship between the peak noise level and the probability of a teacher pausing during an aircraft fly-over. The observers were not able to detect other effects despite the fact that teachers claimed that on noisy days pupils were less well behaved and had greater difficulty in maintaining their concentration. Ko [4] in a survey of teachers’ response to aircraft noise in Hong Kong has shown a high correlation between mean annoyance and NNI. Attempts at determining the effect of noise on pupil performance have produced conflicting results. Wyon [5] obtained significant differences both in performance and behaviour between classes working in quiet conditions and classes exposed to bursts of white noise (55-78 dB(A)). However, Slater [6] found no significant difference in performance between children exposed to quiet (45-55 dB(A)), average (55-70 dB(A)) and noisy (75-90 dB(A)) conditions. A general lack of consistency in studies of the effects of noise on non-auditory tasks has been noted by Kryter [7]. When it was suggested that schools subjected to high levels of road traffic noise from new roads should be treated to increase their sound insulation it was clear that there was insufficient information available to enable a “qualifying level” to be chosen on a rational basis. In order to provide this information the Building Research Establishment has carried out an investigation to determine the disturbance to school teachers by noise. A questionnaire survey was used, restricted to teachers in secondary schools. It is expected 557
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J. W. SARGENT
ET AL.
that the results will assist in the planning and design of new schools as well as being a basis for any future scheme of remedial treatment to existing schools exposed to high levels of noise.
2. SURVEY
SAMPLE
Initially, a sample of secondary schools was selected to be representative of the population density distribution given in the 1971 Census for England and Wales. Areas of Hertfordshire, Buckinghamshire, Berkshire, Bedfordshire and London were chosen to give approximately the following population distribution: 32% in conurbations (e.g., London); 13% in towns of more than lo0 000 people; 10% in towns of 50 000-100 000 people; 22.5% in towns of less than 50 000 people; 22.5% in rural areas. A total of 300 secondary schools in the selected areas formed the primary sample. For each of the 300 schools the road traffic noise level (L10 dB(A)) at the noisiest facade was predicted by using the method described in “Calculation of Road Traffic Noise” [8]. These predicted levels can be used to give some indication of the national incidence of traffic noise on schools. The most significant result to emerge at this stage was that 50% of the schools had no classroom which was exposed to road traffic noise above a level of 50 dB(A) Lo.
Available resources permitted a sample of no more than 100 schools to be covered by a questionnaire survey. Clearly, if the final sample had been chosen at random from the primary sample of 300 schools it would have been poorly balanced for the analysis of the effects of traffic noise. Therefore the final sample was chosen to include approximately equal numbers of classrooms in noise bands 5 dB(A) wide from under 50 dB(A) to over 70 dB(A). A control sample of classrooms on the quiet side of each school was also included. It was hoped to cover about 10 classrooms (5 noisy and 5 quiet) in each of 100 schools giving a total of about 1000 completed questionnaires. After classrooms had been selected, a questionnaire was given to the teacher who worked most frequently in each classroom and subsequently a measurement of the noise exposure of the classroom was made. In the event, only 78 schools were used because a large proportion (50%) of the schools which were approached refused to take part in the survey. Such a high refusal rate is unusual for school surveys. It appears from the reasons which were given for refusal that the high rate of refusal was not linked to the teachers’ attitudes to noise. Because of this and the fact that refusals came from head teachers rather than individual teachers it is considered that the high refusal rate is unlikely to have biased the results. One result of the poor response rate from schools was that it was not possible to have as many classrooms with the highest noise levels as had been hoped, there being insufficient schools with classrooms in the high noise level bands to replace those which refused to take part. At the 78 schools, 1148 teachers completed questionnaires. In some cases it was not possible to identify the relevant classroom for noise measurements while in others it was not possible to obtain a traffic noise measurement because of problems of access or extraneous noise. Therefore the sample for road traffic noise had to be reduced to 999 questionnaires from 73 schools. The locations of these schools are shown in Figure 1. Of these 999 questionnaires, 33 1 were completed by teachers who taught in classrooms with a direct view of the road. The distribution of this sample with noise level is shown in Figure 2. A further 336 questionnaires related to classrooms in the same schools on the side away from the road. Most of the remainder were completed by teachers in classrooms with an oblique view of the road while for a few schools none of the classrooms had a view of the road. Of the 73 schools used in the road traffic noise sample all but two were also found to
DISTURBANCE
TO TEACHERS
BY NOISE
Figure 1. Locations of the 73 schools used in the noise survey. 0, Greater/Central London; 0, towns> 100 000; A towns 50 000-100 000; A, small towns (
have a measurable aircraft noise exposure. The number of questionnaires completed for the aircraft noise sample was 971. Approximately equal numbers of male and female teachers completed questionnaires. They taught a wide variety of subjects. 3. THE QUESTIONNAIRE
The questionnaire was self-administered and was handed to and collected from each teacher individually. Information and instructions which were given at the beginning of the questionnaire were as follows: “The Building Research Establishment carries out research on the design, structure and function of buildings. Part of its programme is to study the needs of those who use the buildings.
Under45 45-50
External
Figure
2. Distribution
50-55
55-60
60-65
65-70
70-75
75-60
rood trofflc nose level, Lb0 (dB(A))
of traffic noise levels outside
331 classrooms
with a direct view of a road.
n = 331
560
J.W. SARGENT
ET AL.
At present we are making a study of school buildings in order to find out what design characteristics result in the most satisfactory working conditions. We would like you to help by answering the questions on this reply sheet. Anything you say will be treated in strict confidence. Remember that any information you can give will be valuable, so do not hesitate to say just what you think. You will find most questions can be answered quickly by putting a tick in the appropriate box. There is no need to consider your replies at length; simply give the answer that first comes to mind. Please do not discuss with others what you should say, as we want your own personal opinion. Thank you for your co-operation.” Before it was used in the main survey the questionnaire was tested in a pilot study at four schools, two primary and two secondary. Each school was chosen because it had a noisy and a quiet facade and therefore could be used to test the resolution of the questions related to noise. The pilot study showed that the questionnaire, in general, was satisfactory for the secondary schools but not for the primary schools, since for the latter the resolution of the questions about noise was inadequate. Teachers were asked to give their impressions of a particular classroom and the number of the room was indicated at the head of the questionnaire. Questions about the general environment in the classroom were included in addition to specific questions on noise. It was hoped that these questions would enable noise disturbance to be assessed in relation to other aspects of the classroom environment. There were also questions relating to the teacher who completed the questionnaire (e.g., age, subject taught) and these included a self-rating question on sensitivity to noise. The principal question about noise disturbance (question 7) asked the teachers how much they were bothered, by noise from a number of listed sources, during the summer term (i.e., the period when they completed the questionnaire). For each source the teachers could choose one of five boxes labelled “not at all”, “a little”, “quite a lot”, “very much” or “does not apply”. The listed sources were as follows: “Children playing outside “, “aircraft”, “lessons in adjacent rooms”, “doors banging”, “road traffic”, “noise from factories”, “children in passages or on stairs ”, “the classroom above” and “lawnmowers and tractors”. Other questions specifically about noise asked teachers whether they were more disturbed by noise from inside or outside the building, whether there was more disturbance from outside noise in winter or in summer (or no difference) and how often in the summer term outside noise caused them to keep their windows closed. In addition there was an open question which asked those with experience of teaching in a noisy room to indicate effects the noise had on themselves, the children or in general. Finally, at the end of the questionnaire a space was left for any further comments which the teacher wished to make. 4. NOISE MEASUREMENTS
After the questionnaires had been returned noise measurements were made at each school in the sample. These measurements were made during the school holidays and therefore, in general, were not affected by noise generated within the school. A road tratlic noise level was required for each classroom on the road side of each school and also a level representative of the external noise at classrooms to the rear of each school. Where there were many classrooms on the same facade measurements were obtained for a representative sample of rooms which permitted noise levels for other rooms on the facade to be obtained by interpolation.
DISTURBANCE
TO
TEACHERS
BY
NOISE
561
Calibrated tape recordings were made at each school on a weekday between 9 a.m. and 4 p.m. One microphone was set 1 m from the external facade of the classroom to record the noise outside and a second microphone was set 1.2 m high in the centre of the room to record the internal noise. A ten minute recording was made with the windows closed followed by a two minute recording with half the openable window area open. Internal and external recordings were made simultaneously. The recordings were made by using one of the following systems: (a) : inch capacitor microphone (Briiel and Kjaer (B and K) type 4149) connected to preamplifier (B and K 2619) outside the classroom, 1 inch capacitor microphone (B and K type 4145) connected to preamplifier (B and K 2619) inside classroom, 2 channel tape recorder (Nagra IVSJ); (b) 2 sound level meters (B and K type 2203) and 2 tape recorders (Nagra III). At the beginning of each tape, equal amplitude pure tone signals of frequency 125,250, 500, 1000,200O and 4000 Hz were recorded for 10 seconds each from a portable signal generator. These were used during analysis to check the frequency linearity of the recording/replay system. If necessary the replay system was adjusted to improve this linearity. Before each noise recording a calibration tone with a level of 94 dB at 1000 Hz from a sound level calibrator (B and K type 4230) was recorded. To obtain levels of road traffic noise the analysis of the tapes was carried out by using a measuring amplifier (B and K type 2606) set to A-weighting, a level recorder (B and K type 2305), and a statistical analyzer (B and K type 4420). During recording no attempt had been made to eliminate noise from sources other than road traffic for example, aircraft. Therefore, during the analysis an attempt was made to reduce the effect of extraneous noise by switching off the statistical analyzer whenever such noise was heard. Noise levels in terms of Li,,, LSO, LgO and L,, were calculated from the statistical data. In some cases where there were no roads near to the school the variability of the noise level was small (less than 3 dB(A)) and therefore a single noise level was read from the level recorder trace. Although the external traffic noise levels quoted in this paper are based on measurements over a period of about 12 minutes it is considered that they are reasonably representative of the levels over the whole of the school day. To obtain levels of aircraft noise the tapes were analyzed in two ways. First, the maximum level in dB(A) for each fly-over was measured from a level recorder trace (writing speed 100 dB/s) and the average maximum level and the number of fly-overs per hour were computed for each school. These were used to calculate Noise and Number Index (NM) type measures. In the second method of analysis a General Radio real time analyzer (type 1926) and a PDP 11 computer were used to determine the single event noise exposure level LAX [9] for each fly-over. The values of Lax were combined to give the value of L,, for the sampling period (i.e., the total recording time) for each school. It should be noted that all fly-overs which contributed significantly to the overall L,, were included. When calculating NM it is normal to ignore fly-overs with peak levels below a given fixed level.
5. RESULTS
5.1. ROAD TRAFFIC NOISE In the analyses to be described the sample used, unless stated otherwise, comprised the 331 teachers who taught in classrooms with a clear view of the road. The data pertaining to road traffic noise from question 7 (“When you are teaching in the room during the summer term, how much are you bothered by noise from the sources listed below?“) was analyzed in two ways.
562
J. W. SARGENT
ET AL.
For the first method of analysis the degree of bother for each teacher was scored by assigning a number to each level of response. Thus the response “not at all” was scored as 1, “a little” as 2, “quite a lot” as 3 and “very much” as 4. Those responding “does not apply” were scored as 1, since it is reasonable to assume that they were not bothered. Those failing to make any response to the part of the question concerned with road traffic were also scored as 1. Only 29 of the 33 1 teachers in classrooms on the road side of schools came into these last two categories. A plot of the individual bother scores against the Llo road traffic noise level outside the classroom gives the scatter diagram shown in Figure 3. The linear regression line is also shown and the correlation coefficient is 0.66 with 330 degrees of freedom. The corresponding correlation coefficients when the noise exposure is expressed in terms of L,, and Lso are 0.64 and 0.63 respectively.
e 0
::
121
3-
2
1
1
I 40
95ll22231
I 60
road traffic
1
2265765324
3623455
I 50
External
3
1
70
22
80
noise level. Llo (dB(A))
Figure 3. Scatter diagram for individual responses to traffic noise (figures indicate the number of responses at eachlevel). y=-4.72+0.114X;r=0.66; n=331.
If the mean bother score is plotted for each 1 dB(A) noise level interval a clearer picture of the relationship between response and noise level is obtained as shown in Figure 4. Only mean values obtained when there were 5 or more individual scores in a noise band were used thus excluding 18 of the sample of 331 in 10 noise bands. All except one of the noise bands excluded were outside the range of noise levels covered in Figure 4 (i.e., 5 1 dB(A) 73 dB(A)). Data outside this range of noise levels would need to be excluded in any case if a linear relationship is postulated since the mean bother score cannot be less than 1 or exceed 4. The correlation coefficient for the linear regression to the data in Figure 4 is 0.97 with 20 degrees of freedom. 4
I
I
40
I 50
External
Figure 4. Relationship I = 0.97; p =
I 60
road traffic
m
Bo
nose level, Llo (dB(A))
between mean bother score and road traffic noise level LIo. y =-5.43+0*125X;
DISTURBANCE
TO TEACHERS
563
BY NOISE
Another way of analyzing the data is to relate the percentage of teachers bothered at a given degree of bother to the noise level. This gives a sigmoid shaped curve and a statistical fit has been obtained by probit analysis with use of the principle of maximum likelihood [lo]. Figure 5 shows a plot of the percentage of teachers bothered “quite a lot” or “very much” (scores 3 and 4) by road traffic noise against external Lo road traffic noise level and also gives the probit regression of bother on L 1otransformed back into percentages. An advantage of using a probit fit to the data is that it gives a realistic weight to points at the ends of the scale. Figure 5 also shows the 95% fiducial limits. These limits indicate the confidence which can be placed in the fitted curve. When the total sample of 999 teachers is used the fitted curve lies within the fiducial limits for the sample of 331 teachers with a clear view of the road. The goodness of fit of the probit line to the data can be tested by using a x2 test. The probit line from which Figure 5 has been produced is a good fit since ,y* = 16 with 30 degrees of freedom.
External
rood
traffbc
Norse level,
L,o
(dB(A))
Figure 5. Percentage of teachers bothered “quite a lot” or “very much” at different external road traffic noise levels Li,,. x2 = 16; degrees of freedom (df) = 30; n = 331; 5% fiducial band.
Sigmoid curves were also fitted to the data for other degrees of bother and the complete set of curves is shown in Figure 6. This figure shows, for example, that at an external road traffic noise level of 60 dB(A) Llo, 9% of the teachers were bothered “very much”, 20% were bothered “quite a lot” and 32% were bothered “a little”. The three probit curves from which these sigmoid curves have been obtained are parallel and equally spaced. I
I
40
50 External
60 rood
trofflc
nose
70 level,
80
Llo (dB(A))
Figure 6. Response curves showing the relationship between the percentage of teachers bothered to different degrees and the external road traffic noise level L ia. Bother scale: 1, not at all; 2, a little; 3, quite a lot; 4, very much. n = 331.
564
J. W.
SARGENT
ET AL.
Curves for other noise indices have been produced and Figure 7 shows the differences between Lo, Lso and L,, for a response of “quite a lot” or “very much”. L,, is about 3 dB(A) lower than LIO for the same response and L50 about 5 dB(A) lower than Lo dB(A).
External
traffic
nose
level (dB(A))
Figure 7. Response curves showing the relationship between the percentage of teachers bothered “quite a lot” or “very much” and the external traffic noise level expressed in terms of LIo, LSOand L,,. n = 331; Llo,$ = 16, df = 30; L,x2 = 33, df = 30; Lsox2 = 28 df = 27.
Teachers were also asked to rate their general sensitivity to noise as “sensitive to noise”, “noise doesn’t bother me” or “neither”. The 19.3% who indicated that they were not sensitive to noise were found to be significantly less bothered by traffic noise than the rest of the sample. There was no significant difference in the response curves of those teachers indicating that they were “sensitive to noise” and those indicating “neither”. No significant difference was found between the responses of male and female teachers to road traffic noise. However there was a tendency for a slightly higher proportion of female teachers to be bothered by the lower noise levels. The maximum difference between the two response curves occurred at 58-60 dB(A) Llo. Such a result might be expected since speech interference levels for women’s voices are lower than those for men’s voices [ 111. While question 7 was the only question specifically related to road traffic noise there were a number of other questions which either asked teachers about noise in general or asked teachers about the classroom environment. Although it was not possible to establish significant relationships between the response to those questions and the level of road traffic noise it is clear that at the higher noise levels the teachers’ response is affected by the traffic noise. For example, when asked to select words to describe the classroom, only 28% of teachers chose the descriptor “noisy” at a traffic noise level of 50-54 dB(A) Llo. However, almost 80% of teachers in rooms exposed to traffic noise at 70-74 dB(A) Llo chose the descriptor “noisy”. 5.2. AIRCRAFT NOISE It has already been pointed out that the survey sample was chosen on the basis of road traffic noise exposure. The distribution of aircraft noise exposure was not similarly controlled and as a result is not an ideal one for examining relationships between teacher disturbance and noise exposure. Also, the method of noise measurement used, which involved relatively short period samples for each classroom, was inadequate for estimating the aircraft noise exposure of each individual classroom. Because of the inherent variability in exposure to aircraft noise it has been normal to predict rather than measure exposure from this source. However since many of the schools in the survey were outside the area of the published noise contours it was not possible to do that in this case. Despite
DISTURBANCE
TO TEACHERS
BY NOISE
565
these difficulties it was felt that useful information could be obtained about the disturbance caused by aircraft noise. In order to obtain the best estimate of the aircraft noise exposure the measured values for individual classrooms were combined to give an overall exposure for the school. It was then assumed that all teachers in the school were exposed to that level of noise. Where changes in mode of operation at the airport were likely to influence the measured noise level significantly (e.g., at schools close to airports) an attempt was made to measure the highest noise level experienced (i.e., the worst mode). As with the road traffic noise the data from question 7 was analyzed in two ways. First, by allotting a score to each level of response it was possible to establish a relationship between the average bother score for each school and the aircraft noise exposure of the school expressed in L, (Figure 8). Despite the high value of correlation coefficient (r = 0.81) it can be seen that the best fit linear regression curve tends to underpredict the bother score at the higher noise levels. This is because the slope of the curve is determined mainly by the large amount of data at the lower values of noise exposure.
25
1
I
I
30
35
40 External
,,,,,,I
45
50
aircraft
55
60
noise level,
65 I&
70
75
(dB(A))
Figure 8. Relationship between the mean bother score for each school and the external aircraft noise level L,,. y=1.28+0.06X; r=0.81; n =971.
The second method of analysis in which the percentage of teachers bothered to a given degree is related to the noise level gives a set of sigmoid type curves. Figure 9 shows the curve for a response of “quite a lot” or “very much”. The data have been grouped in 5 dB(A) bands and the curve shown has been fitted by using probit analysis. This method of analyzing the data has an advantage over the first method in that the form of the best fit curve is not influenced to the same degree by the large amount of data at low noise exposure. It was found that, in general, the goodness of fit of the probit curves to the data was not as good as that found for responses to road traffic noise. This is probably due to the difficulties mentioned above concerning the distribution of the sample and the determination of aircraft noise exposure. The curve in Figure 9 for aircraft noise has a value of x2 = 26 with 8 degrees of freedom. It appears that several schools with noise exposures in the range 69-74 dB(A) are not responding in the same way as the majority of schools for some reason not identifiable in this survey. It should be noted that for a response of “very much” bothered there is no
566
J. W. SARGENT
ET
AL.
55
60
80 -
60 -
25
30
35
40
External
45
50
65
70
75
nalse level, L,, (dB(A)l
Figure 9. Percentage of teachers bothered “quite a lot” or “very much” at different external noise levels L,, ) and road traffic noise (- . - .). For aircraft noise x2 = 26, df = 8; IZL, fiducial band. for aircraft noise (-
evidence that these schools behave differently (typically x2 = 4.4 with 9 degrees of freedom). Since the aircraft noise exposure has been expressed in terms of L,, it is possible to compare the disturbance caused by aircraft noise with that caused by road traffic noise. As can be seen in Figure 9 there is little difference between the percentage of teachers bothered by the two sources at a given noise level. In order to assist those who wish to make comparisons with noise levels expressed in NM the measured value of L,, for each school has been related to the NM calculated on the basis of the measured noise exposure. It should be remembered that this is essentially a worst mode NM The equation of the linear regression line for schoo!s with an NNI of over 30 is L,, = O-83 NNI + 24.7. This is in broad agreement with the equation linking L,, with NNI quoted in the recently published L,, guide [9]. 6. DISCUSSION 6.1. COMPARISON WITH DWELLING TRAFFIC NOISE SURVEYS A comparison of the results of this survey for road traffic with the results of other noise surveys shows some differences between the response of teachers and the response of occupants of dwellings to external noise. One usual feature of all noise surveys is the rather low correlation between individual response and noise level. Typical correlation coefficients quoted are in the range O-2 to O-4. The value of O-66 found in the present survey for the correlation coefficient of individual responses against the road traffic noise level L10 is much higher. There are a number of factors which could have influenced the degree of correlation of individual responses. First, the teachers were performing a single task. Occupants of dwellings, however, are involved in a number of different activities which will be affected to different degrees by noise. Thus an individual’s response to noise in a dwelling is likely to be influenced by the particular range of activities in which he is involved. A second factor is that teachers replied to the questionnaire in relation to a particular room. Some respondents in dwellings may spend much of their time in rooms away from the facade for which measurements of noise exposure are made. Thus the effective noise exposure of the teachers was more closely defined. A third factor which could influence the degree of correlation between noise level and response is the particular
DISTURBANCE
TO TEACHERS
BY
567
NOISE
scale used to measure the response. In the study described here a four point scale has been used with all points named. In recent surveys of traffic noise and dwellings a seven point semantic differential scale was used. In the pilot study which was carried out prior to the schools survey some teachers indicated that they would prefer to have all points on the scale named rather than just the end points. Another difference between the results of this survey and surveys concerned with dwellings is the actual relationship between response and noise level. It is not possible to make a direct comparison between the surveys because of the different scales which have been used. However Schultz [l] has made an attempt to compare data from a number of surveys concerned with transportation noise and dwellings. Figure 10 shows the relationship between the percentage of people highly annoyed and the noise exposure in terms of
Schwls (road (vs. Lw durmg
traffic) the
--
I/
school day)
I
,, 1’ ,’ ,’
,‘Schools /’ (aIrcroft) (vs Lw dwng the / school day) ’ / // /_Dwelhngs (after
(vs. L,) Schultz
[I])
External wxe level(dB(A))
Figure 10. Comparison of the percentage of teachers very bothered by noise from road traffic (-) and aircraft (- - --) with the percentage of dwelling occupants highly annoyed by transportation noise (- - -) (after Schultz [l]).
the day night exposure level Ldn which was derived by Schultz together with the curve from the present school survey for the percentage of teachers who were bothered “very much” by road traffic noise. No attempt has been made to adjust the schools curve to a response to noise expressed in terms of Ldn since the correction would vary from situation to situation. However it should be noted that the curve for teachers’ response is related to the noise level in L, during the school day (09.00-16.00) and that this will in general be slightly higher than the corresponding value of Ldn. It can be seen from Figure 10 that for schoolteachers the proportions who are highly annoyed increases much more rapidly with increasing noise level. Thus, while the curves are in broad agreement below a noise level of 60 dB(A), 50% of teachers are very much bothered at an L,, value (working day) of 66 dB(A) compared to 50% of residents being highly annoyed at an Ldn value of 79 dB(A). Some of the reasons which were put forward earlier to explain the higher correlation between response and noise level could also explain the differences in response between teachers and occupants of dwellings. In particular, the fact that teachers are involved in an auditory task is clearly significant. Figure 10 also shows the curve which indicates the proportion of teachers very much bothered by aircraft noise. The differences between the road traffic noise and aircraft noise curves is quite small (less than 3 dB(A) at a 50% response level). Thus the results of this
568
J. W. SARGENT ET AL
school survey tend to support the conclusion of Schultz, which was based on dwelling surveys, that a single curve would adequately predict the proportion of respondents highly annoyed or bothered by either aircraft noise or road traffic noise. However the results also indicate that the curve derived by Schultz does not adequately predict the proportion of school teachers highly bothered by transportation noise. 6.2. MEANING OF THE NOISE SCALE Figure 6 indicates clear and significant relationships between the response of teachers and the noise levels to which they are exposed. It has the particular advantage that it covers a wide range of noise levels from a level where no teachers are bothered to a level where all teachers are bothered. However it does not give an absolute indication of how an “acceptable” level can be determined. Much of the analysis reported here has been concerned with the proportion of teachers who are bothered either “quite a lot” or “very much”, i.e., the top half of the bother scale. This division between the top and bottom halves of a noise scale could be argued to be quite logical provided that the scale is linear. Nevertheless some would argue that one should instead be concerned mainly with the proportion of people highly annoyed: i.e., those responding “very much” in this survey. Others might even suggest that the third response curve (i.e., the proportion bothered “a little”, “quite a lot” or “very much”) should be used. Even when a particular degree of response has been selected it is necessary to decide what proportion of teachers who respond is acceptable. Table 1 shows a range of noise levels covering different degrees of bother and different percentages of teachers who would be bothered to each degree. TABLE 1 Traffic noise levels (LIO dB(A)) at which given percentages of teachers are bothered to different degrees
r At least ,‘a little” Percentage response 10 20 50 70 90
Degree of bother h At least “quite a lot” “very much”
Traffic noise level L1,, dB(A)
49
55
52
58 64 67 72
2; 67
61 64 69 73 78
One point which should be made arises from the way in which the data have been analyzed. Confidence limits on the probit curves are at their smallest at the point where 50% of the teachers are bothered. The limits increase as one moves away from this central point on the curve. Therefore the choice of an acceptable level based on the highest response level (very much bothered) may be subject to a larger error than one based on a moderate response level (quite a lot or very much bothered). Data obtained from responses to other questions not specifically concerned with road traffic noise may help those seeking to determine acceptable levels for design purposes. Details of responses to the four questions in Table 2 are plotted against traffic noise level in Figure 11. The first point to note is that unlike the response to question 7 the response to these questions does not reach zero at low levels of road traffic noise. This is because the
DISTURBANCE
TO TEACHERS TABLE
569
BY NOISE
2
Certain questions not specifically concerned with road traffic noise Question number
2(c)
Possible
Question In general, work in?
how would you rate this room as a place to
Satisfactory
responses
or unsatisfactory
3
Please tick any of the following items which you think generally describes the room at this time of year
20 descriptors of aspects environment
5
During the summer term how often do you find your room to be too noisy?
Rarely,
6
In general during the summer term are you more disturbed by noise originating within the building, or by noise entering the building from outside?
Within, from difference
covering most the classroom
sometimes
or often
outside
or no
100
60
20
35-39
40-44
45-49
External road
50-54 traffic
55-59 noise
60-64 level,
65-69
70-74
L,a (dB(A))
Figure 11. Responses to four questions plotted against external traffic noise level. 0 - - 0, Response “unsatisfactory” to question 2(c); X - - - - X , response “noisy” to question 3; 0 - - - - 0, response “often” to question 5; . - -. , response “from outside” to question 6. n = 999.
questions do not refer specifically to road traffic noise. At low levels of road traffic noise the level of response is determined by other noise sources, or in the case of question 2(c) by other aspects of the classroom environment. Figure 11 shows that above an external traffic noise level LIO in the region of 60 dB(A) the level of road traffic noise begins to affect the response to questions about noise in general. Perhaps more importantly, above this level of road traffic noise the proportion of teachers who consider their classroom to be an unsatisfactory place in which to work increases. Conversely one might conclude that below an external traffic noise level of about 60 dB(A) LIo the overall teachers’ response to general noise questions or questions about the classroom environment are not significantly influenced by the level of traffic noise. Another set of data which helps to put the response curves for road traffic noise into perspective is the response in question 7 to noise sources within the school. Thus Figure 12 indicates that above a level of 59 dB(A) LIO a higher proportion of teachers is bothered by road traffic noise than by noise from any source within the school.
570
ET AL.
J. W. SARGENT loo -
60 -
60
-
40
20 -
/
40 External
Figure 12. Comparison
//,I
50 rood traffic
60 “me
70
60
level, e& (dB(A))
of bother from road traffic noise with bother from internal noises. n = 999.
It may be of interest to note here the effects attributed to noise by the teachers. Twenty-seven per cent of all teachers considered that noise caused them to become irritated with many mentioning specific reasons for the irritation such as having to raise their voices or disruption to the lesson. The greatest effect which noise had on children was considered to be in disturbing concentration. Thirty-two per cent of all teachers attributed this effect to noise while 21% of teachers considered that noise distracted the children. 6.3.
INTERNAL
OR EXTERNAL
NOISE
LEVELS
In all of the analyses carried out in the work reported here the external traffic noise levels were used as a measure of the noise exposure. It might be argued that the use of internal noise levels from road traffic would give a better measure of the teachers’ noise exposure. There are problems however in determining internal noise levels, particularly for a sample such as the one used here which covers a wide range of noise exposures down to quite low levels. As described in an earlier section, when traffic noise levels were obtained from the tape recordings parts of the recordings where noise from sources other than road traffic was significant were excluded. It is much more difficult to use this technique with recordings made inside the school since it is less easy to identify noise sources on internal recordings. Furthermore, since the internal noise level from road traffic will depend for each classroom on whether and to what degree windows are opened it would be necessary to make some assumption about window opening patterns if teacher disturbance was to be related to internal noise levels. There was insufficient information from the survey on which to base such an assumption. In order to provide information on typical values of facade insulation, the insulation against traffic noise was measured for classrooms in 15 of the schools exposed to the highest noise levels. The average level difference for 15 rooms with open windows was 8.0 dB(A) (standard deviation 2.5 dB(A)) and for 16 rooms with closed windows was 18.7 dB(A) (standard deviation 2.0 dB(A)). 6.4. THE SCALE OF THE PROBLEM It is clear from this survey that for some classrooms the level of traffic noise is high enough to bother a high proportion of teachers. For classrooms with an external level of 65 dB(A) LiO almost 60% of teachers are bothered “quite a lot” or “very much”. However it appears that the proportion of secondary school classrooms exposed to such high levels is rather small. Only 18% of schools in the primary sample of 300 secondary schools had any classroom exposed to an external traffic noise level over 65 dB(A) LlO. Therefore if the primary sample is representative of England and Wales as a whole, and it was designed to meet this condition, approximately 900 of the total of 5000 secondary schools [12] will
DISTURBANCE
TO
TEACHERS
BY
571
NOISE
have some classrooms exposed to a traffic noise level over 65 dB(A) LlO. It is not known what proportion of classrooms in these schools is exposed to noise over that level but it is certainly no more than 50% and may be much less. Thus it seems that no more than 9% of secondary school classrooms are exposed to traffic noise levels over 65 dB(A) Llo. Similarly, less than 15% of classrooms are exposed to levels over 60 dB(A) LlO. 7. CONCLUSIONS The following conclusions may be drawn from this survey of teachers disturbance caused by noise. (a) Quantitative relationships have been established between the proportion of teachers bothered by road traffic noise and the level of road traffic noise outside the classroom expressed in L10 dB(A) and L,, dB(A). (b) There is no significant difference between the response of male and female teachers to road traffic noise. (c) The proportion of teachers who claim to be not sensitive to noise and who are bothered at a given level of road traffic noise is significantly less than the proportion of other teachers. There is no significant difference between the proportion of sensitive and neutral teachers bothered at a given noise level. (d) High levels of road traffic noise appear to disturb higher proportions of school teachers than occupants of dwellings. (e) Above an external road traffic noise level of 60 dB(A) L10 there is an increase in the response to questions about noise in general and a higher percentage of teachers consider the classroom to be an unsatisfactory working environment. Also, for the survey sample as a whole, a higher proportion of teachers were bothered by road traffic noise than by any internal noise source above an external road traffic noise level of about 60 dB(A) LlO. (f) Quantitative relationships have been established between the proportion of teachers bothered by aircraft noise and the level of aircraft noise in L,,dB(A) outside the classroom. (g) There appears to be little difference between the proportion of teachers bothered by road traffic noise and the proportion bothered by aircraft noise at a given level of L,, dB(A). (h) The percentages of classrooms in existing secondary schools which are exposed to very high levels of road traffic noise is not high. It appears that only 18% of secondary schools in England and Wales have any classrooms which are exposed to noise levels of more than 65 dB(A) Llo. About half of the schools have no classroom exposed to a level above 50 dB(A) L10 from road traffic.
ACKNOWLEDGMENT The authors wish to thank the staff of the schools which took part in the survey. The work described has been carried out as part of the research programme of the Building Research Establishment of the Department of the Environment and this paper is published by permission of the Director.
REFERENCES 1. T. J. SCHULTZ 1978 Journal of the Acoustical Society of America 64, 377-405. social surveys on noise annoyance.
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of
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2. F.BRUCKMAYER and J. LANG 1968 Osterreichische Ingenieur-Zeitschrift 11,73-77. Storung durch Verkehrslarm in Unterrichtersraumen. The 3. M. A. CROOK and F. J. LANGDON 1974 Journal of Sound and Vibration 34,221-232. effects of aircraft noise in schools around London airport. 4. N. W. M. KO 1979 Journal of Sound and Vibration 62, 277-292. Responses of teachers to aircraft noise. 5. D. P. WYON 1970 Ljudklimat, Byggforskingsradets forskningskonferens, Report R 36170, Stockholm. Performance and behaviour of school children during low-level but intermittent noise (in Swedish). 6. BARBARA R. SLATER 1968 Journal of Educational Psychology 59,239-243. Effects of noise on pupil performance. 7. K. D. KRYTER 1970 The Effects of Noise on Man. London and New York: Academic Press. 8. DEPARTMENT OFTHE ENVIRONMENT 1975 Calculation of Road Trafic Noise. London: Her Majesty’s Stationery Office. 9. NOISE ADVISORY COUNCIL 1978A Guide to Measurement and Prediction of the Equivalent Continuous Sound Level L,,. London: Her Majesty’s Stationery Office. 10. D. J. FINNEY 1952 Probit Analysis. Cambridge University Press, second edition. 11. L. L. BERANEK (Editor) 1971 Noise and Vibration Control. New York: McGraw-Hill. 12. DEPARTMENT OF EDUCATION AND SCIENCE 1977 A Study of School Building. London: Her Majesty’s Stationery Office.
THE DISTURBANCE
CAUSED
TO SCHOOL
TEACHERS
BY NOISE J. W. SARGENT, M. I. GIDMAN, M. A. HUMPHREYS AND W. A. UTLEY Building Research Station, Garston, Watford WD2 IJR, England (Received 25 September 1979, and in revised form
14 January 1980)
A survey to investigate the disturbance caused to secondary school teachers by noise is described. Although the survey sample was selected on the basis of road traffic noise exposure it has also been possible to draw conclusions about the disturbance by aircraft noise. Quantitative relationships have been established between the proportions of teachers bothered by noise and the noise level to which they are exposed. The results of this school survey are compared with dwelling noise surveys.
1.
INTRODUCTION
During the last decade a considerable amount of research has been carried out to determine the effects of transportation noise on the occupants of dwellings [l]. In the United Kingdom the information obtained from this type of research has been used as the basis of planning procedures and schemes to provide increased sound insulation for dwellings exposed to high levels of external noise. Considerably less effort has been devoted to determining the effects of external noise on the occupants of other types of building. For example, the only published study of the effects of traffic noise on schools appears to be that undertaken by Bruckmayer and Lang in Vienna [2], based on use of a questionnaire. Although the study was limited in size, covering only 46 classrooms, it showed that traffic noise could produce serious disturbance in schools facing main roads. Crook and Langdon [3] investigated the effects of aircraft noise on school teaching. By direct observation in the classroom they were able to determine a relationship between the peak noise level and the probability of a teacher pausing during an aircraft fly-over. The observers were not able to detect other effects despite the fact that teachers claimed that on noisy days pupils were less well behaved and had greater difficulty in maintaining their concentration. Ko [4] in a survey of teachers’ response to aircraft noise in Hong Kong has shown a high correlation between mean annoyance and NNI. Attempts at determining the effect of noise on pupil performance have produced conflicting results. Wyon [5] obtained significant differences both in performance and behaviour between classes working in quiet conditions and classes exposed to bursts of white noise (55-78 dB(A)). However, Slater [6] found no significant difference in performance between children exposed to quiet (45-55 dB(A)), average (55-70 dB(A)) and noisy (75-90 dB(A)) conditions. A general lack of consistency in studies of the effects of noise on non-auditory tasks has been noted by Kryter [7]. When it was suggested that schools subjected to high levels of road traffic noise from new roads should be treated to increase their sound insulation it was clear that there was insufficient information available to enable a “qualifying level” to be chosen on a rational basis. In order to provide this information the Building Research Establishment has carried out an investigation to determine the disturbance to school teachers by noise. A questionnaire survey was used, restricted to teachers in secondary schools. It is expected 557
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ET AL.
that the results will assist in the planning and design of new schools as well as being a basis for any future scheme of remedial treatment to existing schools exposed to high levels of noise.
2. SURVEY
SAMPLE
Initially, a sample of secondary schools was selected to be representative of the population density distribution given in the 1971 Census for England and Wales. Areas of Hertfordshire, Buckinghamshire, Berkshire, Bedfordshire and London were chosen to give approximately the following population distribution: 32% in conurbations (e.g., London); 13% in towns of more than lo0 000 people; 10% in towns of 50 000-100 000 people; 22.5% in towns of less than 50 000 people; 22.5% in rural areas. A total of 300 secondary schools in the selected areas formed the primary sample. For each of the 300 schools the road traffic noise level (L10 dB(A)) at the noisiest facade was predicted by using the method described in “Calculation of Road Traffic Noise” [8]. These predicted levels can be used to give some indication of the national incidence of traffic noise on schools. The most significant result to emerge at this stage was that 50% of the schools had no classroom which was exposed to road traffic noise above a level of 50 dB(A) Lo.
Available resources permitted a sample of no more than 100 schools to be covered by a questionnaire survey. Clearly, if the final sample had been chosen at random from the primary sample of 300 schools it would have been poorly balanced for the analysis of the effects of traffic noise. Therefore the final sample was chosen to include approximately equal numbers of classrooms in noise bands 5 dB(A) wide from under 50 dB(A) to over 70 dB(A). A control sample of classrooms on the quiet side of each school was also included. It was hoped to cover about 10 classrooms (5 noisy and 5 quiet) in each of 100 schools giving a total of about 1000 completed questionnaires. After classrooms had been selected, a questionnaire was given to the teacher who worked most frequently in each classroom and subsequently a measurement of the noise exposure of the classroom was made. In the event, only 78 schools were used because a large proportion (50%) of the schools which were approached refused to take part in the survey. Such a high refusal rate is unusual for school surveys. It appears from the reasons which were given for refusal that the high rate of refusal was not linked to the teachers’ attitudes to noise. Because of this and the fact that refusals came from head teachers rather than individual teachers it is considered that the high refusal rate is unlikely to have biased the results. One result of the poor response rate from schools was that it was not possible to have as many classrooms with the highest noise levels as had been hoped, there being insufficient schools with classrooms in the high noise level bands to replace those which refused to take part. At the 78 schools, 1148 teachers completed questionnaires. In some cases it was not possible to identify the relevant classroom for noise measurements while in others it was not possible to obtain a traffic noise measurement because of problems of access or extraneous noise. Therefore the sample for road traffic noise had to be reduced to 999 questionnaires from 73 schools. The locations of these schools are shown in Figure 1. Of these 999 questionnaires, 33 1 were completed by teachers who taught in classrooms with a direct view of the road. The distribution of this sample with noise level is shown in Figure 2. A further 336 questionnaires related to classrooms in the same schools on the side away from the road. Most of the remainder were completed by teachers in classrooms with an oblique view of the road while for a few schools none of the classrooms had a view of the road. Of the 73 schools used in the road traffic noise sample all but two were also found to
DISTURBANCE
TO TEACHERS
BY NOISE
Figure 1. Locations of the 73 schools used in the noise survey. 0, Greater/Central London; 0, towns> 100 000; A towns 50 000-100 000; A, small towns (
have a measurable aircraft noise exposure. The number of questionnaires completed for the aircraft noise sample was 971. Approximately equal numbers of male and female teachers completed questionnaires. They taught a wide variety of subjects. 3. THE QUESTIONNAIRE
The questionnaire was self-administered and was handed to and collected from each teacher individually. Information and instructions which were given at the beginning of the questionnaire were as follows: “The Building Research Establishment carries out research on the design, structure and function of buildings. Part of its programme is to study the needs of those who use the buildings.
Under45 45-50
External
Figure
2. Distribution
50-55
55-60
60-65
65-70
70-75
75-60
rood trofflc nose level, Lb0 (dB(A))
of traffic noise levels outside
331 classrooms
with a direct view of a road.
n = 331
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ET AL.
At present we are making a study of school buildings in order to find out what design characteristics result in the most satisfactory working conditions. We would like you to help by answering the questions on this reply sheet. Anything you say will be treated in strict confidence. Remember that any information you can give will be valuable, so do not hesitate to say just what you think. You will find most questions can be answered quickly by putting a tick in the appropriate box. There is no need to consider your replies at length; simply give the answer that first comes to mind. Please do not discuss with others what you should say, as we want your own personal opinion. Thank you for your co-operation.” Before it was used in the main survey the questionnaire was tested in a pilot study at four schools, two primary and two secondary. Each school was chosen because it had a noisy and a quiet facade and therefore could be used to test the resolution of the questions related to noise. The pilot study showed that the questionnaire, in general, was satisfactory for the secondary schools but not for the primary schools, since for the latter the resolution of the questions about noise was inadequate. Teachers were asked to give their impressions of a particular classroom and the number of the room was indicated at the head of the questionnaire. Questions about the general environment in the classroom were included in addition to specific questions on noise. It was hoped that these questions would enable noise disturbance to be assessed in relation to other aspects of the classroom environment. There were also questions relating to the teacher who completed the questionnaire (e.g., age, subject taught) and these included a self-rating question on sensitivity to noise. The principal question about noise disturbance (question 7) asked the teachers how much they were bothered, by noise from a number of listed sources, during the summer term (i.e., the period when they completed the questionnaire). For each source the teachers could choose one of five boxes labelled “not at all”, “a little”, “quite a lot”, “very much” or “does not apply”. The listed sources were as follows: “Children playing outside “, “aircraft”, “lessons in adjacent rooms”, “doors banging”, “road traffic”, “noise from factories”, “children in passages or on stairs ”, “the classroom above” and “lawnmowers and tractors”. Other questions specifically about noise asked teachers whether they were more disturbed by noise from inside or outside the building, whether there was more disturbance from outside noise in winter or in summer (or no difference) and how often in the summer term outside noise caused them to keep their windows closed. In addition there was an open question which asked those with experience of teaching in a noisy room to indicate effects the noise had on themselves, the children or in general. Finally, at the end of the questionnaire a space was left for any further comments which the teacher wished to make. 4. NOISE MEASUREMENTS
After the questionnaires had been returned noise measurements were made at each school in the sample. These measurements were made during the school holidays and therefore, in general, were not affected by noise generated within the school. A road tratlic noise level was required for each classroom on the road side of each school and also a level representative of the external noise at classrooms to the rear of each school. Where there were many classrooms on the same facade measurements were obtained for a representative sample of rooms which permitted noise levels for other rooms on the facade to be obtained by interpolation.
DISTURBANCE
TO
TEACHERS
BY
NOISE
561
Calibrated tape recordings were made at each school on a weekday between 9 a.m. and 4 p.m. One microphone was set 1 m from the external facade of the classroom to record the noise outside and a second microphone was set 1.2 m high in the centre of the room to record the internal noise. A ten minute recording was made with the windows closed followed by a two minute recording with half the openable window area open. Internal and external recordings were made simultaneously. The recordings were made by using one of the following systems: (a) : inch capacitor microphone (Briiel and Kjaer (B and K) type 4149) connected to preamplifier (B and K 2619) outside the classroom, 1 inch capacitor microphone (B and K type 4145) connected to preamplifier (B and K 2619) inside classroom, 2 channel tape recorder (Nagra IVSJ); (b) 2 sound level meters (B and K type 2203) and 2 tape recorders (Nagra III). At the beginning of each tape, equal amplitude pure tone signals of frequency 125,250, 500, 1000,200O and 4000 Hz were recorded for 10 seconds each from a portable signal generator. These were used during analysis to check the frequency linearity of the recording/replay system. If necessary the replay system was adjusted to improve this linearity. Before each noise recording a calibration tone with a level of 94 dB at 1000 Hz from a sound level calibrator (B and K type 4230) was recorded. To obtain levels of road traffic noise the analysis of the tapes was carried out by using a measuring amplifier (B and K type 2606) set to A-weighting, a level recorder (B and K type 2305), and a statistical analyzer (B and K type 4420). During recording no attempt had been made to eliminate noise from sources other than road traffic for example, aircraft. Therefore, during the analysis an attempt was made to reduce the effect of extraneous noise by switching off the statistical analyzer whenever such noise was heard. Noise levels in terms of Li,,, LSO, LgO and L,, were calculated from the statistical data. In some cases where there were no roads near to the school the variability of the noise level was small (less than 3 dB(A)) and therefore a single noise level was read from the level recorder trace. Although the external traffic noise levels quoted in this paper are based on measurements over a period of about 12 minutes it is considered that they are reasonably representative of the levels over the whole of the school day. To obtain levels of aircraft noise the tapes were analyzed in two ways. First, the maximum level in dB(A) for each fly-over was measured from a level recorder trace (writing speed 100 dB/s) and the average maximum level and the number of fly-overs per hour were computed for each school. These were used to calculate Noise and Number Index (NM) type measures. In the second method of analysis a General Radio real time analyzer (type 1926) and a PDP 11 computer were used to determine the single event noise exposure level LAX [9] for each fly-over. The values of Lax were combined to give the value of L,, for the sampling period (i.e., the total recording time) for each school. It should be noted that all fly-overs which contributed significantly to the overall L,, were included. When calculating NM it is normal to ignore fly-overs with peak levels below a given fixed level.
5. RESULTS
5.1. ROAD TRAFFIC NOISE In the analyses to be described the sample used, unless stated otherwise, comprised the 331 teachers who taught in classrooms with a clear view of the road. The data pertaining to road traffic noise from question 7 (“When you are teaching in the room during the summer term, how much are you bothered by noise from the sources listed below?“) was analyzed in two ways.
562
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ET AL.
For the first method of analysis the degree of bother for each teacher was scored by assigning a number to each level of response. Thus the response “not at all” was scored as 1, “a little” as 2, “quite a lot” as 3 and “very much” as 4. Those responding “does not apply” were scored as 1, since it is reasonable to assume that they were not bothered. Those failing to make any response to the part of the question concerned with road traffic were also scored as 1. Only 29 of the 33 1 teachers in classrooms on the road side of schools came into these last two categories. A plot of the individual bother scores against the Llo road traffic noise level outside the classroom gives the scatter diagram shown in Figure 3. The linear regression line is also shown and the correlation coefficient is 0.66 with 330 degrees of freedom. The corresponding correlation coefficients when the noise exposure is expressed in terms of L,, and Lso are 0.64 and 0.63 respectively.
e 0
::
121
3-
2
1
1
I 40
95ll22231
I 60
road traffic
1
2265765324
3623455
I 50
External
3
1
70
22
80
noise level. Llo (dB(A))
Figure 3. Scatter diagram for individual responses to traffic noise (figures indicate the number of responses at eachlevel). y=-4.72+0.114X;r=0.66; n=331.
If the mean bother score is plotted for each 1 dB(A) noise level interval a clearer picture of the relationship between response and noise level is obtained as shown in Figure 4. Only mean values obtained when there were 5 or more individual scores in a noise band were used thus excluding 18 of the sample of 331 in 10 noise bands. All except one of the noise bands excluded were outside the range of noise levels covered in Figure 4 (i.e., 5 1 dB(A) 73 dB(A)). Data outside this range of noise levels would need to be excluded in any case if a linear relationship is postulated since the mean bother score cannot be less than 1 or exceed 4. The correlation coefficient for the linear regression to the data in Figure 4 is 0.97 with 20 degrees of freedom. 4
I
I
40
I 50
External
Figure 4. Relationship I = 0.97; p =
I 60
road traffic
m
Bo
nose level, Llo (dB(A))
between mean bother score and road traffic noise level LIo. y =-5.43+0*125X;
DISTURBANCE
TO TEACHERS
563
BY NOISE
Another way of analyzing the data is to relate the percentage of teachers bothered at a given degree of bother to the noise level. This gives a sigmoid shaped curve and a statistical fit has been obtained by probit analysis with use of the principle of maximum likelihood [lo]. Figure 5 shows a plot of the percentage of teachers bothered “quite a lot” or “very much” (scores 3 and 4) by road traffic noise against external Lo road traffic noise level and also gives the probit regression of bother on L 1otransformed back into percentages. An advantage of using a probit fit to the data is that it gives a realistic weight to points at the ends of the scale. Figure 5 also shows the 95% fiducial limits. These limits indicate the confidence which can be placed in the fitted curve. When the total sample of 999 teachers is used the fitted curve lies within the fiducial limits for the sample of 331 teachers with a clear view of the road. The goodness of fit of the probit line to the data can be tested by using a x2 test. The probit line from which Figure 5 has been produced is a good fit since ,y* = 16 with 30 degrees of freedom.
External
rood
traffbc
Norse level,
L,o
(dB(A))
Figure 5. Percentage of teachers bothered “quite a lot” or “very much” at different external road traffic noise levels Li,,. x2 = 16; degrees of freedom (df) = 30; n = 331; 5% fiducial band.
Sigmoid curves were also fitted to the data for other degrees of bother and the complete set of curves is shown in Figure 6. This figure shows, for example, that at an external road traffic noise level of 60 dB(A) Llo, 9% of the teachers were bothered “very much”, 20% were bothered “quite a lot” and 32% were bothered “a little”. The three probit curves from which these sigmoid curves have been obtained are parallel and equally spaced. I
I
40
50 External
60 rood
trofflc
nose
70 level,
80
Llo (dB(A))
Figure 6. Response curves showing the relationship between the percentage of teachers bothered to different degrees and the external road traffic noise level L ia. Bother scale: 1, not at all; 2, a little; 3, quite a lot; 4, very much. n = 331.
564
J. W.
SARGENT
ET AL.
Curves for other noise indices have been produced and Figure 7 shows the differences between Lo, Lso and L,, for a response of “quite a lot” or “very much”. L,, is about 3 dB(A) lower than LIO for the same response and L50 about 5 dB(A) lower than Lo dB(A).
External
traffic
nose
level (dB(A))
Figure 7. Response curves showing the relationship between the percentage of teachers bothered “quite a lot” or “very much” and the external traffic noise level expressed in terms of LIo, LSOand L,,. n = 331; Llo,$ = 16, df = 30; L,x2 = 33, df = 30; Lsox2 = 28 df = 27.
Teachers were also asked to rate their general sensitivity to noise as “sensitive to noise”, “noise doesn’t bother me” or “neither”. The 19.3% who indicated that they were not sensitive to noise were found to be significantly less bothered by traffic noise than the rest of the sample. There was no significant difference in the response curves of those teachers indicating that they were “sensitive to noise” and those indicating “neither”. No significant difference was found between the responses of male and female teachers to road traffic noise. However there was a tendency for a slightly higher proportion of female teachers to be bothered by the lower noise levels. The maximum difference between the two response curves occurred at 58-60 dB(A) Llo. Such a result might be expected since speech interference levels for women’s voices are lower than those for men’s voices [ 111. While question 7 was the only question specifically related to road traffic noise there were a number of other questions which either asked teachers about noise in general or asked teachers about the classroom environment. Although it was not possible to establish significant relationships between the response to those questions and the level of road traffic noise it is clear that at the higher noise levels the teachers’ response is affected by the traffic noise. For example, when asked to select words to describe the classroom, only 28% of teachers chose the descriptor “noisy” at a traffic noise level of 50-54 dB(A) Llo. However, almost 80% of teachers in rooms exposed to traffic noise at 70-74 dB(A) Llo chose the descriptor “noisy”. 5.2. AIRCRAFT NOISE It has already been pointed out that the survey sample was chosen on the basis of road traffic noise exposure. The distribution of aircraft noise exposure was not similarly controlled and as a result is not an ideal one for examining relationships between teacher disturbance and noise exposure. Also, the method of noise measurement used, which involved relatively short period samples for each classroom, was inadequate for estimating the aircraft noise exposure of each individual classroom. Because of the inherent variability in exposure to aircraft noise it has been normal to predict rather than measure exposure from this source. However since many of the schools in the survey were outside the area of the published noise contours it was not possible to do that in this case. Despite
DISTURBANCE
TO TEACHERS
BY NOISE
565
these difficulties it was felt that useful information could be obtained about the disturbance caused by aircraft noise. In order to obtain the best estimate of the aircraft noise exposure the measured values for individual classrooms were combined to give an overall exposure for the school. It was then assumed that all teachers in the school were exposed to that level of noise. Where changes in mode of operation at the airport were likely to influence the measured noise level significantly (e.g., at schools close to airports) an attempt was made to measure the highest noise level experienced (i.e., the worst mode). As with the road traffic noise the data from question 7 was analyzed in two ways. First, by allotting a score to each level of response it was possible to establish a relationship between the average bother score for each school and the aircraft noise exposure of the school expressed in L, (Figure 8). Despite the high value of correlation coefficient (r = 0.81) it can be seen that the best fit linear regression curve tends to underpredict the bother score at the higher noise levels. This is because the slope of the curve is determined mainly by the large amount of data at the lower values of noise exposure.
25
1
I
I
30
35
40 External
,,,,,,I
45
50
aircraft
55
60
noise level,
65 I&
70
75
(dB(A))
Figure 8. Relationship between the mean bother score for each school and the external aircraft noise level L,,. y=1.28+0.06X; r=0.81; n =971.
The second method of analysis in which the percentage of teachers bothered to a given degree is related to the noise level gives a set of sigmoid type curves. Figure 9 shows the curve for a response of “quite a lot” or “very much”. The data have been grouped in 5 dB(A) bands and the curve shown has been fitted by using probit analysis. This method of analyzing the data has an advantage over the first method in that the form of the best fit curve is not influenced to the same degree by the large amount of data at low noise exposure. It was found that, in general, the goodness of fit of the probit curves to the data was not as good as that found for responses to road traffic noise. This is probably due to the difficulties mentioned above concerning the distribution of the sample and the determination of aircraft noise exposure. The curve in Figure 9 for aircraft noise has a value of x2 = 26 with 8 degrees of freedom. It appears that several schools with noise exposures in the range 69-74 dB(A) are not responding in the same way as the majority of schools for some reason not identifiable in this survey. It should be noted that for a response of “very much” bothered there is no
566
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ET
AL.
55
60
80 -
60 -
25
30
35
40
External
45
50
65
70
75
nalse level, L,, (dB(A)l
Figure 9. Percentage of teachers bothered “quite a lot” or “very much” at different external noise levels L,, ) and road traffic noise (- . - .). For aircraft noise x2 = 26, df = 8; IZL, fiducial band. for aircraft noise (-
evidence that these schools behave differently (typically x2 = 4.4 with 9 degrees of freedom). Since the aircraft noise exposure has been expressed in terms of L,, it is possible to compare the disturbance caused by aircraft noise with that caused by road traffic noise. As can be seen in Figure 9 there is little difference between the percentage of teachers bothered by the two sources at a given noise level. In order to assist those who wish to make comparisons with noise levels expressed in NM the measured value of L,, for each school has been related to the NM calculated on the basis of the measured noise exposure. It should be remembered that this is essentially a worst mode NM The equation of the linear regression line for schoo!s with an NNI of over 30 is L,, = O-83 NNI + 24.7. This is in broad agreement with the equation linking L,, with NNI quoted in the recently published L,, guide [9]. 6. DISCUSSION 6.1. COMPARISON WITH DWELLING TRAFFIC NOISE SURVEYS A comparison of the results of this survey for road traffic with the results of other noise surveys shows some differences between the response of teachers and the response of occupants of dwellings to external noise. One usual feature of all noise surveys is the rather low correlation between individual response and noise level. Typical correlation coefficients quoted are in the range O-2 to O-4. The value of O-66 found in the present survey for the correlation coefficient of individual responses against the road traffic noise level L10 is much higher. There are a number of factors which could have influenced the degree of correlation of individual responses. First, the teachers were performing a single task. Occupants of dwellings, however, are involved in a number of different activities which will be affected to different degrees by noise. Thus an individual’s response to noise in a dwelling is likely to be influenced by the particular range of activities in which he is involved. A second factor is that teachers replied to the questionnaire in relation to a particular room. Some respondents in dwellings may spend much of their time in rooms away from the facade for which measurements of noise exposure are made. Thus the effective noise exposure of the teachers was more closely defined. A third factor which could influence the degree of correlation between noise level and response is the particular
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scale used to measure the response. In the study described here a four point scale has been used with all points named. In recent surveys of traffic noise and dwellings a seven point semantic differential scale was used. In the pilot study which was carried out prior to the schools survey some teachers indicated that they would prefer to have all points on the scale named rather than just the end points. Another difference between the results of this survey and surveys concerned with dwellings is the actual relationship between response and noise level. It is not possible to make a direct comparison between the surveys because of the different scales which have been used. However Schultz [l] has made an attempt to compare data from a number of surveys concerned with transportation noise and dwellings. Figure 10 shows the relationship between the percentage of people highly annoyed and the noise exposure in terms of
Schwls (road (vs. Lw durmg
traffic) the
--
I/
school day)
I
,, 1’ ,’ ,’
,‘Schools /’ (aIrcroft) (vs Lw dwng the / school day) ’ / // /_Dwelhngs (after
(vs. L,) Schultz
[I])
External wxe level(dB(A))
Figure 10. Comparison of the percentage of teachers very bothered by noise from road traffic (-) and aircraft (- - --) with the percentage of dwelling occupants highly annoyed by transportation noise (- - -) (after Schultz [l]).
the day night exposure level Ldn which was derived by Schultz together with the curve from the present school survey for the percentage of teachers who were bothered “very much” by road traffic noise. No attempt has been made to adjust the schools curve to a response to noise expressed in terms of Ldn since the correction would vary from situation to situation. However it should be noted that the curve for teachers’ response is related to the noise level in L, during the school day (09.00-16.00) and that this will in general be slightly higher than the corresponding value of Ldn. It can be seen from Figure 10 that for schoolteachers the proportions who are highly annoyed increases much more rapidly with increasing noise level. Thus, while the curves are in broad agreement below a noise level of 60 dB(A), 50% of teachers are very much bothered at an L,, value (working day) of 66 dB(A) compared to 50% of residents being highly annoyed at an Ldn value of 79 dB(A). Some of the reasons which were put forward earlier to explain the higher correlation between response and noise level could also explain the differences in response between teachers and occupants of dwellings. In particular, the fact that teachers are involved in an auditory task is clearly significant. Figure 10 also shows the curve which indicates the proportion of teachers very much bothered by aircraft noise. The differences between the road traffic noise and aircraft noise curves is quite small (less than 3 dB(A) at a 50% response level). Thus the results of this
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school survey tend to support the conclusion of Schultz, which was based on dwelling surveys, that a single curve would adequately predict the proportion of respondents highly annoyed or bothered by either aircraft noise or road traffic noise. However the results also indicate that the curve derived by Schultz does not adequately predict the proportion of school teachers highly bothered by transportation noise. 6.2. MEANING OF THE NOISE SCALE Figure 6 indicates clear and significant relationships between the response of teachers and the noise levels to which they are exposed. It has the particular advantage that it covers a wide range of noise levels from a level where no teachers are bothered to a level where all teachers are bothered. However it does not give an absolute indication of how an “acceptable” level can be determined. Much of the analysis reported here has been concerned with the proportion of teachers who are bothered either “quite a lot” or “very much”, i.e., the top half of the bother scale. This division between the top and bottom halves of a noise scale could be argued to be quite logical provided that the scale is linear. Nevertheless some would argue that one should instead be concerned mainly with the proportion of people highly annoyed: i.e., those responding “very much” in this survey. Others might even suggest that the third response curve (i.e., the proportion bothered “a little”, “quite a lot” or “very much”) should be used. Even when a particular degree of response has been selected it is necessary to decide what proportion of teachers who respond is acceptable. Table 1 shows a range of noise levels covering different degrees of bother and different percentages of teachers who would be bothered to each degree. TABLE 1 Traffic noise levels (LIO dB(A)) at which given percentages of teachers are bothered to different degrees
r At least ,‘a little” Percentage response 10 20 50 70 90
Degree of bother h At least “quite a lot” “very much”
Traffic noise level L1,, dB(A)
49
55
52
58 64 67 72
2; 67
61 64 69 73 78
One point which should be made arises from the way in which the data have been analyzed. Confidence limits on the probit curves are at their smallest at the point where 50% of the teachers are bothered. The limits increase as one moves away from this central point on the curve. Therefore the choice of an acceptable level based on the highest response level (very much bothered) may be subject to a larger error than one based on a moderate response level (quite a lot or very much bothered). Data obtained from responses to other questions not specifically concerned with road traffic noise may help those seeking to determine acceptable levels for design purposes. Details of responses to the four questions in Table 2 are plotted against traffic noise level in Figure 11. The first point to note is that unlike the response to question 7 the response to these questions does not reach zero at low levels of road traffic noise. This is because the
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TO TEACHERS TABLE
569
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2
Certain questions not specifically concerned with road traffic noise Question number
2(c)
Possible
Question In general, work in?
how would you rate this room as a place to
Satisfactory
responses
or unsatisfactory
3
Please tick any of the following items which you think generally describes the room at this time of year
20 descriptors of aspects environment
5
During the summer term how often do you find your room to be too noisy?
Rarely,
6
In general during the summer term are you more disturbed by noise originating within the building, or by noise entering the building from outside?
Within, from difference
covering most the classroom
sometimes
or often
outside
or no
100
60
20
35-39
40-44
45-49
External road
50-54 traffic
55-59 noise
60-64 level,
65-69
70-74
L,a (dB(A))
Figure 11. Responses to four questions plotted against external traffic noise level. 0 - - 0, Response “unsatisfactory” to question 2(c); X - - - - X , response “noisy” to question 3; 0 - - - - 0, response “often” to question 5; . - -. , response “from outside” to question 6. n = 999.
questions do not refer specifically to road traffic noise. At low levels of road traffic noise the level of response is determined by other noise sources, or in the case of question 2(c) by other aspects of the classroom environment. Figure 11 shows that above an external traffic noise level LIO in the region of 60 dB(A) the level of road traffic noise begins to affect the response to questions about noise in general. Perhaps more importantly, above this level of road traffic noise the proportion of teachers who consider their classroom to be an unsatisfactory place in which to work increases. Conversely one might conclude that below an external traffic noise level of about 60 dB(A) LIo the overall teachers’ response to general noise questions or questions about the classroom environment are not significantly influenced by the level of traffic noise. Another set of data which helps to put the response curves for road traffic noise into perspective is the response in question 7 to noise sources within the school. Thus Figure 12 indicates that above a level of 59 dB(A) LIO a higher proportion of teachers is bothered by road traffic noise than by noise from any source within the school.
570
ET AL.
J. W. SARGENT loo -
60 -
60
-
40
20 -
/
40 External
Figure 12. Comparison
//,I
50 rood traffic
60 “me
70
60
level, e& (dB(A))
of bother from road traffic noise with bother from internal noises. n = 999.
It may be of interest to note here the effects attributed to noise by the teachers. Twenty-seven per cent of all teachers considered that noise caused them to become irritated with many mentioning specific reasons for the irritation such as having to raise their voices or disruption to the lesson. The greatest effect which noise had on children was considered to be in disturbing concentration. Thirty-two per cent of all teachers attributed this effect to noise while 21% of teachers considered that noise distracted the children. 6.3.
INTERNAL
OR EXTERNAL
NOISE
LEVELS
In all of the analyses carried out in the work reported here the external traffic noise levels were used as a measure of the noise exposure. It might be argued that the use of internal noise levels from road traffic would give a better measure of the teachers’ noise exposure. There are problems however in determining internal noise levels, particularly for a sample such as the one used here which covers a wide range of noise exposures down to quite low levels. As described in an earlier section, when traffic noise levels were obtained from the tape recordings parts of the recordings where noise from sources other than road traffic was significant were excluded. It is much more difficult to use this technique with recordings made inside the school since it is less easy to identify noise sources on internal recordings. Furthermore, since the internal noise level from road traffic will depend for each classroom on whether and to what degree windows are opened it would be necessary to make some assumption about window opening patterns if teacher disturbance was to be related to internal noise levels. There was insufficient information from the survey on which to base such an assumption. In order to provide information on typical values of facade insulation, the insulation against traffic noise was measured for classrooms in 15 of the schools exposed to the highest noise levels. The average level difference for 15 rooms with open windows was 8.0 dB(A) (standard deviation 2.5 dB(A)) and for 16 rooms with closed windows was 18.7 dB(A) (standard deviation 2.0 dB(A)). 6.4. THE SCALE OF THE PROBLEM It is clear from this survey that for some classrooms the level of traffic noise is high enough to bother a high proportion of teachers. For classrooms with an external level of 65 dB(A) LiO almost 60% of teachers are bothered “quite a lot” or “very much”. However it appears that the proportion of secondary school classrooms exposed to such high levels is rather small. Only 18% of schools in the primary sample of 300 secondary schools had any classroom exposed to an external traffic noise level over 65 dB(A) LlO. Therefore if the primary sample is representative of England and Wales as a whole, and it was designed to meet this condition, approximately 900 of the total of 5000 secondary schools [12] will
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have some classrooms exposed to a traffic noise level over 65 dB(A) LlO. It is not known what proportion of classrooms in these schools is exposed to noise over that level but it is certainly no more than 50% and may be much less. Thus it seems that no more than 9% of secondary school classrooms are exposed to traffic noise levels over 65 dB(A) Llo. Similarly, less than 15% of classrooms are exposed to levels over 60 dB(A) LlO. 7. CONCLUSIONS The following conclusions may be drawn from this survey of teachers disturbance caused by noise. (a) Quantitative relationships have been established between the proportion of teachers bothered by road traffic noise and the level of road traffic noise outside the classroom expressed in L10 dB(A) and L,, dB(A). (b) There is no significant difference between the response of male and female teachers to road traffic noise. (c) The proportion of teachers who claim to be not sensitive to noise and who are bothered at a given level of road traffic noise is significantly less than the proportion of other teachers. There is no significant difference between the proportion of sensitive and neutral teachers bothered at a given noise level. (d) High levels of road traffic noise appear to disturb higher proportions of school teachers than occupants of dwellings. (e) Above an external road traffic noise level of 60 dB(A) L10 there is an increase in the response to questions about noise in general and a higher percentage of teachers consider the classroom to be an unsatisfactory working environment. Also, for the survey sample as a whole, a higher proportion of teachers were bothered by road traffic noise than by any internal noise source above an external road traffic noise level of about 60 dB(A) LlO. (f) Quantitative relationships have been established between the proportion of teachers bothered by aircraft noise and the level of aircraft noise in L,,dB(A) outside the classroom. (g) There appears to be little difference between the proportion of teachers bothered by road traffic noise and the proportion bothered by aircraft noise at a given level of L,, dB(A). (h) The percentages of classrooms in existing secondary schools which are exposed to very high levels of road traffic noise is not high. It appears that only 18% of secondary schools in England and Wales have any classrooms which are exposed to noise levels of more than 65 dB(A) Llo. About half of the schools have no classroom exposed to a level above 50 dB(A) L10 from road traffic.
ACKNOWLEDGMENT The authors wish to thank the staff of the schools which took part in the survey. The work described has been carried out as part of the research programme of the Building Research Establishment of the Department of the Environment and this paper is published by permission of the Director.
REFERENCES 1. T. J. SCHULTZ 1978 Journal of the Acoustical Society of America 64, 377-405. social surveys on noise annoyance.
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of
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2. F.BRUCKMAYER and J. LANG 1968 Osterreichische Ingenieur-Zeitschrift 11,73-77. Storung durch Verkehrslarm in Unterrichtersraumen. The 3. M. A. CROOK and F. J. LANGDON 1974 Journal of Sound and Vibration 34,221-232. effects of aircraft noise in schools around London airport. 4. N. W. M. KO 1979 Journal of Sound and Vibration 62, 277-292. Responses of teachers to aircraft noise. 5. D. P. WYON 1970 Ljudklimat, Byggforskingsradets forskningskonferens, Report R 36170, Stockholm. Performance and behaviour of school children during low-level but intermittent noise (in Swedish). 6. BARBARA R. SLATER 1968 Journal of Educational Psychology 59,239-243. Effects of noise on pupil performance. 7. K. D. KRYTER 1970 The Effects of Noise on Man. London and New York: Academic Press. 8. DEPARTMENT OFTHE ENVIRONMENT 1975 Calculation of Road Trafic Noise. London: Her Majesty’s Stationery Office. 9. NOISE ADVISORY COUNCIL 1978A Guide to Measurement and Prediction of the Equivalent Continuous Sound Level L,,. London: Her Majesty’s Stationery Office. 10. D. J. FINNEY 1952 Probit Analysis. Cambridge University Press, second edition. 11. L. L. BERANEK (Editor) 1971 Noise and Vibration Control. New York: McGraw-Hill. 12. DEPARTMENT OF EDUCATION AND SCIENCE 1977 A Study of School Building. London: Her Majesty’s Stationery Office.