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The assessment of noise from industrial plants by direct measurement and by calculation
THE ASSESSMENT OF NOISE FROM INDUSTRIAL PLANTS BY DIRECT MEASUREMENT AND BY CALCULATION
M. GRASHOF
Bayer AG Leverkusen, IN Anlagenplanung Erd61chemie, c/o Erdiilchemie GmbH, 5000 K61n 71, Postfach 75 2002 (West Germany)
SUMMARY
The noise levels measured at a distance o f 1000 m from an industrial plant can vary within a range of 20 dB(A) due to the effects o f weather and extraneous sources. This paper examines the use o f various statistical parameters for assessing the noise due to the plant and describes a method o f minimising the effect o f extraneous noise due to traffic. The choice of parameter influences the attenuation data to be used in calculating the noise from new plants. Some typical measurements from a large industrial plant are presented. The paper emphasises the need to agree on a single parameter which can be used for calculations, for assessing measured noise levels, and for comparison with legal or noise criteria, and recommends the general adoption of
Leq, INTRODUCTION
This paper examines the effect of using various parameters for assessing the noise in residential areas due to an industrial plant. The noise emitted from a plant is frequently limited by law, and it is customary to define the limit in terms of an allowable noise level in a residential area; where there is no legal limit, the criterion for an acceptable noise level may be obtained from a standard such as ISO 1996 or BS 4142. In designing a new plant it is therefore important to be able to calculate the expected noise levels in order to meet the legal limit or criterion; this involves estimating the attenuation between the plant and the residential area. Where prior planning permission has to be obtained it may be necessary to use attenuation data acceptable to the local government. Finally, when the plant is operating, it will be necessary to monitor the noise levels to ensure that they comply with the legal limits. 177 Applied Acoustics (9) (1976)--© Applied Science Publishers Ltd, England, 1976 Pnnted in Great Britain
178
M. GRASHOF
It is not always appreciated that these are all aspects of the same problem and that the same concepts must be used at all stages; in particular, consistent parameters must be used for defining noise levels. Using noise data from a typical industrial plant, this paper will indicate the problems which arise when inconsistent parameters are used for defining noise levels and when different criteria are used for fixing noise limits.
COMMUNITY NOISE LIMITS IN GERMANY
In Germany, the authorities have been setting noise limits for industrial plants since 1968, using as a guide the criteria defined in the code of practice, TA-L/irm, or VDI 2058 (Part 1). Frequently, the limit in a community area is set at 40 dB(A) for the allowable noise from a particular plant. It is not always clear, however, whether this is the maximum acceptable noise in the area or whether it merely applies to a single plant; clearly, the levels would be much higher if a number of plants were each allowed to contribute 40dB(A). Another anomaly is that these levels are set for residential areas regardless of their distance from the plant. This can lead to severe requirements for the noise emitted from the plant ifa housing area is built close to its perimeter, and could make the operation of the plant completely uneconomic. It is important, therefore, that authorities should establish transition areas around large industrial plants where higher noise levels would be allowed. It may be noted that the corresponding criterion in BS 4142 for an urban residential area is 50 dB(A) for night-time, and it is suggested that this is a more realistic level.
MEASURING COMMUNITY NOISE LEVELS FROM A PLANT
When the noise level due to a plant is measured in a community area at a distance from the plant, it is found to vary considerably, even though the noise emission from the plant is steady. This is due to the effect of weather, and the measured noise level may vary from day to day within a range of about 20 dB(A) at a distance of 1000 m. There will also be local additions to the noise level from the plant, due, for example, to traffic, trains or aircraft. These may cause sudden peak noise levels on top of the plant noise, which usually will only vary slowly due to weather changes. Figure 1 shows a typical record of noise levels measured close to the plant (the plant noise emission) and of the measured noise level in a nearby residential area (the immission). The plant noise emission is seen to be fairly steady, whereas the measured noise level has a slow variation with superimposed peaks due to local sources of noise. This illustrates the basic problem of continuously monitoring noise from a plant; how to eliminate the effect of extraneous noise sources and how to define the slowly varying plant noise in terms of a single parameter.
ASSESSMENT OF NOISE FROM INDUSTRIAL PLANTS
179
Plant Noise Emission
dB (A) 90
80
50 40
3~o
4'00
5'oo
6'o0
Time.Interval: 3 seconds
Immission
dB (A) 90 _ _ 80 70
5O 4O 46
I--4 ,215 2'41 Time Interval: 3 minutes
Paper 1.46 2.15 2.41 3.45
Speed 0,03 mm/s h 50 dB {A) ,, 54 ,, ,, 44 ,, ,, 42 ,,
21.9.74
! Class Interval: 5 d B (A) 2,5 dB(A) or 1 dB(A)
Fig. 1. Variation of background noise caused by meteorologicalconditions.
DESIGNING PLANTS FOR NOISE CONTROL Usually, the noise reduction which can be achieved in an existing plant is limited to about 5-7 dB(A) and can often only be effected at considerable cost.l This places greater emphasis on including noise control measures at the design stage of new plants. I f the plant is being designed to meet a particular criterion for community noise, some agreed attenuation data must be used. In Germany, this is defined in VDI 2714, s which uses the following equation: H A = LNA + 10log 2xr 2 + K L + K B + K s + K M where LNa is the required noise limit for the plant in a particular community area. Table 1 illustrates how the required sound power level of the plant m a y be derived for design purposes when a noise limit has been set at 40 dB(A) at a distance ofS00 m from the plant; it also defines the terms in the above equation. In the case of open-air petrochemical plants and refineries, where there are continuously operating noise sources with predominant frequencies between 500 Hz and 2000 Hz, the following approximate formulas may be used for the calculation of overall noise levels in dB(A).
180
M. GRASHOF TABLE 1 ACOUSTIC PLANNING
Allowable sound power level H A
#~ = L N a + 101og2nr 2 + K LNA = r = 10 log 2nr 2 = KL = K, = Ks = KM =
40dB(A) 66 3.5 0 to 2 0 to 10 4.5
(in dB(A)) t + K B + Ks + Ku
Community noise limit for a single plant 800 m radius, distance Hemispherical sound propagation Air transmission loss Ground effects loss Screening, scattering by barriers Meteorological effects loss
H A = 114 to 126dB(A)
K L = 10log(1 + 1 . 5 . r . 10 -3 ) dB(A) K M = (12"500r -2 + 0"2) -1 dB(A) KL + Ku + Ks = 10log(3 + r. 1 6 0 - ' ) dB(A) K s = 10log(3 + 20z) dB(A) z = (r 2 + h2) j'2 - r 1 + (r22 + h2) 1'2 - r 2 where: z = path difference; h = height o f a barrier a b o u t twice as long as the noise source; rj = distance between source and barrier and r 2 = distance between barrier and receiver. Where z tends to zero and r > 4 0 0 m : 6 r K s + K L + K B + K ~ ~ lOlog~6
dB(A)
CALCULATING NOISE LEVELS DUE TO OPEN-AIR PLANTS
Most petroleum and petrochemical plants are built in the open, for safety reasons. As a result there is usually little attenuation o f the noise within the plant itself, most o f the attenuation occurring between the plant and the community areas. A frequently used method o f estimating noise levels due to a plant is based on measuring the sound power level o f the whole plant. This is obtained from measurements o f sound pressure level m a d e near the plant and from the areas o f an appropriate measuring surface, 2 using the following equation: H A = (L)A + 101og2S where: HA = sound power level o f complete plant in dB(A); (L)A = mean sound pressure level measured a r o u n d plant in dB(A) and S = surface area o f plant within the measuring line in m 2. As an example from a typical plant, where 70 dB(A) was measured at the plant boundary: H A = 70 + 101og[2(160 x 120)] = l l 8 d B ( A ) It m a y be noted that values o f 3 to 10 m a y be used for the screening factor, K s.
ASSESSMENT OF NOISE FROM INDUSTRIAL PLANTS
] 81
T h e three principal types of propagation in the atmosphere to a receiver located near the ground (taken from Beranek)
Free Atmosphere
.~ q4~ve
h -zooo (500-600 rn)
S
Transition region Air to ground
100- 200 ft ( 3 0 - 6 0 m ) Surface boundary layer : S ¢.---
I=
i
Ground wave
S = Noise Source
R R = Receiver
Fig. 2. Sound propagation.
Normally, only the minimum value should be used. According to Beranek (Noise and Vibration Control, p. 165) there are three main types of sound propagation in the atmosphere and these are illustrated in Fig. 2. For industrial plants only two types of propagation are of concern; the ground wave and the sky wave and, in practice, for distances up to 1000m only the ground wave is important, although, in Germany, VDI 2714 requires the sky wave to be used as the basis for calculations. When the wind is blowing directly from the noise source to the community area there is a decrease in the effect of screening from cooling towers or buildings. This causes an increase in the measured noise level. The curves given in VDI 2714 for the parameters K L and K B + K M are given in Fig. 3.
MONITORING NOISE FROM AN INDUSTRIAL PLANT There is increasing pressure from environmentalists for the reduction of industrial noise and this places greater emphasis on the need to measure the noise from plants.
182
M. GRASHOF
Where there is a particular concern about plant noise--or where there are legal limits on plant noise--it may be necessary to monitor the noise continuously. Various equipment is now available commercially for continuous monitoring--mainly developed for traffic noise--but the problem in using this is to separate the plant noise from the local noise. However, as shown in Fig. 1, the plant noise usually varies slowly, whereas the local noise causes sudden peaks and, by discriminating against the latter, it is possible to separate the background noise, which may then be attributed to the plant. Suitable 12.5 I
0
25 I
50 I
100 I
200 I
400 I
800 I
i[ d B (A KL=Airtransmission loss 1
1600 I
3200 m I
...... ......................Traffic noise
..........
(250Hz)
""°%,.. Plant "',..,noise
O-
1
(1000 Hz)
12.5
25
50
100
200
400
800
1600
3200
I
I
I
I
I
I
I
I
I
10d B (A)
KBeteKrMc~Go;°:alndefloss fndcts
"O%o.%,,,,..,,.,,,,.,iirange ,i,io]n
20-
(VDI- draft 2714) Fig. 3. Community noise levels. Variation range due to meteorological effects loss.
equipment for doing this has been developed at Erd61chemie and will be described briefly; a fuller description of the equipment has been given elsewhere. 3'4. The equipment consists of the following items: microphone, preamplifier, measuring amplifier, level recorder and a statistical distribution analyser with a maximum selector. Sampling times can be chosen between a few minutes and several hours; for example, sampling times can be selected to cover 8 h at night and 16 h during the day. The maximum selector can be set to operate within intervals of 1 to 10 sec and the class interval for counting events can be set at 1, 2.5 or 5 dB(A). To
183
ASSESSMENT OF NOISE FROM INDUSTRIAL PLANTS
measure background noise, a further device is required which selects the minimum noise level within each interval of 1-10 sec. Various time and class intervals were examined to find the most suitable arrangement. Ideally, the time interval should be two to three times longer than the duration of the peak noises due to traffic and aircraft, so that the minimum noise is a reasonable measure of the background. In the original equipment the maximum time interval available was 10 sec, although this was considered to be rather short. Later, new equipment was built in which it was possible to use a time interval of 3 min for determining the maximum and minimum noise levels. The smaller class intervals of 1 or 2.5 dB(A) are to be preferred for assessing background noise. Occasionally, of course, the plant itself emits peaks of noise, due, for example, to the operation of valves or safety vents, and it is important to be able to distinguish these effects from the other peaks. This is done by making a recording of peak noise events on a tape recorder so that they can be distinguished by ear. To avoid continuous operation o f the recorder, it is only switched on by an automatic circuit when the peak noise is about 10dB(A) above the highest background noise level.
Relative Frequency % 807060-
r
50-
oooooo,
! ! !
40302010"
vs. Overall Noise Level and Background Noise Level
Z,,, !
i
t .
|
t
~ * o l o a . l o l * * e l geeaoo
0 30 Fig. 4.
!
!
40
50
60 '
7b
80 '
9o d B (A)
Histograms. Time intervals: 5 and 10sec. Class interval: 5dB(A).
184
M. GRASHOF EXAMPLES OF NOISE MONITORING FROM A PLANT
Continuous noise measurements were made during 1973 and 1974 at monitoring stations situated between 400 m and 1200 m from an open-air petrochemical plant. The peak noise levels (overall noise) and the minimum noise levels (background) within each interval were analysed separately. Figures 4 and 5 show the histograms and cumulative frequency distributions for night-time measurements in July, 1973; Figs. 6 and 7 show the same information obtained in May, 1974. Further details of these two monitoring programmes are given in Table 2. Some typical data from Fig. 7 are presented in Table 3 together with the energyaveraged level, L=q. From Table 3 it can be seen that in May, 1974 the background noise level exceeded 40 dB(A) on 95 per cent of the occasions, while the corresponding level for the overall (peak) noise was 44 dB(A). The background noise exceeded 50 dB(A) on only 10 per cent of the occasions whereas the overall noise exceeded 58 dB(A) on 10 per cent of the occasions. For comparison, the values of L=q were 48.5 and 58 dB(A) for the background and overall noise. Cumulative frequency % 99.98-
ael,oeoe B
99"
90-
eve,
503010-
1"
0.130
20
i
"'%,. ..
6'o
7'0
do
9b ae (AI
Fig. 5. Distribution curves. Time intervals: 5 and 10sec. Class interval: 5dB(A).
ASSESSMENT OF NOISE FROM INDUSTRIAL PLANTS
185
In Table 4, the value o f Leq has been calculated for the individual months from March to August and for the whole period in 1973 and 1974. Table 4 shows that the average measured noise level had decreased from 52.4 to 49.7 dB(A) for the corresponding periods in 1973 and 1974. To some extent this reflects a greater accuracy in the assessment of the measurements--due to increasing the time interval from l0 sec to 3 min and to decreasing the class interval from 5 to 2.5 dB(A)--but it also indicates a real reduction in the noise from the plant, due to noise control measures.
Relative Frequency % 8070firS. Overall Noise Level and Background Noise Level
60504030I I I
I I
20-
I I I
10-
I . . . .
I
0
I
30
40
I
50
1
60
L
7()
dB
810
(A)
910
Fig. 6. Histograms. Time intervals: 3 sec and 3 min. Class interval: 2.5 dB(A).
THE ASSESSMENT OF ADDITIONAL ATTENUATION The equation of VDI 2714 contains two groups of attenuation factors; the first is 10 log 2nr 2, which represents the effect of the inverse square law; the second is the group of K factors which represent additional attenuation due to ground and weather effects. The additional attenuation, represented by ~ K , can be calculated if the values of sound power level, H A, and the measured noise level, LNA, are known.
186
M. GRASHOF
However, a decision has to be made about what value to use for the measured noise level; whether to use the lowest, average, or highest individual measurements. The choice will affect the apparent additional attenuation. Take, for example, the case of a plant where the lowest background level was 40 dB(A), the highest was 55 dB(A) and the L=q was found to be 50 dB(A) at a point 1000 m from the centre of the plant. If the sound power level of the plant is known to be 133dB(A), three corresponding values can be calculated for the additional attenuation. The calculation is given in Table 5.
Cumulative frequency %
99.98 ! 99.95 I 99.9 99.8 l
99.5" 98 97 95" 90" 80-
vs. Overall Noise Level and Background Noise Level
50403020" 105 0,5 0,2 0,1
0,05 0,02 30
40
50
60
70
~30
90 OB (A)
Fig. 7. Distribution curves. Time intervals: 3 sec and 3 min. Class interval: 2.5 dB(A). Table 5 shows that the following values are obtained for the additional attenuation over 1000m: Maximum attenuation 25 dB(A)--corresponding to 40 dB(A) Minimum attenuation 10 dB(A)--corresponding to 55 dB(A) Mean attenuation 15 dB(A)---corresponding to Leq = 50 dB(A) If it is required to calculate the value of L=q, the additional attenuation of 15 dB(A) should be used, corresponding to 1.5 dB(A) per 100 m; beyond 1000 m, this should
] 87
A S S E S S M E N T O F NOISE F R O M I N D U S T R I A L P L A N T S
TABLE 2 NOISE MONITORING STATION: 400 TO 1200M FROM OPEN PLANTS
Overall noise levels Maximum selector July, 1973 May, 1974
Sampling time: Each night In Figure Time interval Class interval M a x i m u m noise level M i n i m u m noise level Variation range Energetic mean Noise level Leq
Background noise levels Minimum selector July, 1973 May, 1974
2"3
4.5
2.3
4.5
5 sec 5 dB
3 sec 5 dB
10 sec 5 dB
3 min 2.5 dB
dB(A) 80 40 40
dB(A) 85 (35) (50)
dB(A) 60 40 20
dB(A) 55 38 17
58
58
52
48.5
TABLE 3 PER CENT NOISE LEVELS, MAY, 1974
Per cent time
Background noise level dB(A)
Overall noise level dB(A )
99.98 95 90 68 50 32 IL(~ q 5 0-02
37.5 40 41 43 46 48 48.5 50 50.5 55
40 44 45 47 51 53 58 58 60 85
TABLE 4 MEAN NOISE LEVELS FROM MARCHTO AUGUST, 1973 AND 1974. CLASS INTERVALS: 5 riB(A) AND 2"5 dB(A)
Sampling time
March April May June July August Mean noise level for a period of 6 m o n t h s
Mean noise level for each month
Lea in dB(A)
(1973)
(1974)
53 54 52 51.5 52 51.5 52.4
48.5 50 48-5 50 51.5 49 49.7
188
M. GRASHOF
be reduced to 1 dB(A) per 100 m, decreasing to 0.75 dB(A) per 100m at a distance of 2000 m. The average noise level, Leq , a t a distance from the plant should then be calculated from: 1.5 Zeq --- H a - 101og27tr 2 - 1--0-6r dB(A) where: H A = L A + 101og2S dB(A) L,, = mean sound pressure level measured on a line at 0.15(a) 1/2 from the plant centre, a = plant area in m 2. S -- area within the measuring line.
TABLE 5 CALCULATIONOF ADDITIONALATTENUATION Sound power level of all plants Distance monitoring station from acoustic r = 1000m L= -101og2nr 2 Sound pressure level without losses Backgroundlnoise levels measured: LA m~. = 55, Lml. = 40, Lcq = 50 dB(A)
centre:
133 dB(A) 68 dB(A) 65 dB(A)
Additional attenuation ~ K
Yr~,o Per 1000m Per 100m •
10 1-0
~Kma x 25 2.5
~K .... 15 dB(A) I'5dB(A)
DOWNWIND MEASUREMENTS
It can be seen that the amount of additional attenuation to be used in a calculation depends on the parameter used for rating the measured noise level and, clearly, the same parameter should be used for defining the noise limit. In Germany, some authorities require the noise measurements to be made downwind of the planti normally they require the mean value of at least three measurements made at night, under downwind and temperature-inversion conditions. This is equivalent to using the maximtim values of the distribution of noise levels for assessing the plant noise; the mean value calculated under these conditions would only be about 3 dB(A) below the 5 per cent point in the distribution. The additional attenuation to be used in a calculation should therefore be the lowest of the three values quoted above, that is, 10dB(A) or l dB(A) per 100m. To obtain a reliable assessment of the measured noise level under downwind conditions it is necessary to record the meteorological data at the same time and to assess the noise level for different wind directions. This limits the number of measurements which can be used and hence the reliability of the result. The,effect of limiting the number of measurements is shown in Table 6 which gives values of Leq f o r 2 h and 8 h sampling periods. It shows that differences of 1 to
189
A S S E S S M E N T O F N O I S E F R O M I N D U S T R I A L PLANTS
TABLE 6 ERRORS
DUE TO LIMITING
THE NUMBER
OF MEASUREMENTS
Noise levels in dB(A) Time interval: 3 rain = 20 counts per hour Class interval 1 dB( A ) Class interval 2-5 dB( A ) 2h each night 8h each night
Sampling time 1974 June July August
52 52-6 49.9
50 51.5 49
Devmtion 2 1.1 0.9
2 dB(A) can occur and it is recommended that sampling periods should be not less than 2 h per night for 3 min intervals. Figures 8 and 9 show some typical distributions of noise levels from a plant when the measurements are made under various wind conditions. They indicate the large variations in the mean values which occur when they are made under restricted weather conditions.
Relative Frequency % 45.-"=i ............ 1, 5. to 2. 5. 74 22"" to 6""DowrNgrnd L~1=54 dB (Aj := :'" . . . . 31.8. to 1.9. 74 22°~ to 6"" U ~ , ~ L~q=45 ~ (A)
40-
~
4.6. to 1.9. 74 22 °o ~ 2 °° E v ~ ' Wind ~ Leq~ 52(:18 IA)
3530
z |
25-
! =-
j il
..=-
ii Ii
20-
j"
''
# I
:=
m t
:I
15iP
#,
:
Ir
l
II
•
I
,I
10-
I
, I
5-
&.:
, i
I
I
II
i I
v
I 1 w i
I
s
# s
l
11 I1
.='- I
!
.-"
| J
i
II
13
I" I ~-
I"
|
40 Fig. 8.
=
45
r
i
50
55
dB (A) 60
Relative frequency of background noise. Time interval: 3 min. Class interval: I d B(A).
190
M. GRASHOF
Cumulative Frequency % .q~.9.
99.899.5¸ 99
__ t .
96 95 9O
•............ t. 5. to ?. 5. 74 2 2 ° 0 to 6 °o Downwin0 Leq 54 clBtA) .... 3 t . 8 . t o l . 9 . 7 4 2 2 o~ Io6OOUpwind Leq 45dB~A') , 4.6. tO 1 . 9 . 7 4 22 °o to 2 °0 Every Wind Direction ' Leq 5 2 t i B I A ) .
\\
8O 70
,t
6O 5O
\
|
40
20
\
10
t I
5
I It
|
0.5
!
0,2
,
|
0,1 0,05 0.02
| I~ t
4b Fig, 9.
!i
1
,:5
5b
,
~= ' ::,
s5
~o
65
dB (A)
Cumulative frequency of background noise, Time interval: 3 min. Class interval: I dB(A).
THE EFFECT OF USING INCONSISTENT PARAMETERS FOR COMPARING MEASURED NOISE LEVELS WITH NOISE CRITERIA
Unfortunately there are as yet no agreed parameters for assessing the measured noise levels from a plant. Figures 5 and 7 show the distribution of noise levels which can typically occur, and various points on the cumulative frequency distributions can be chosen to typify the distribution; Lj o, L9o and Lso values have been used on various occasions--and more recently the Leq has become more widely used. To these may be added the downwind values required by the German authorities. The chosen parameter has to be compared with a criterion selected from BS 4142 or ISO 1996 or with a legal noise limit, but the parameters in these criteria are not clearly defined in statistical terms. There are also differences in the criteria themselves which can give rise to varying national standards.
ASSESSMENT
OF NOISE FROM
INDUSTRIAL
191
PLANTS
To illustrate the variations which can occur it is interesting to calculate the distance between a plant and a residential area which would satisfy these different requirements. If it is defined as an urban area with light industry, the night-time criterion according to BS 4142 is 55 dB(A) and according to TA-L/irm 45 dB(A). Assuming that the sound power level of the plant is 133 dB(A), the required distance, r, is obtained from the following equation by inserting appropriate values for the additional attenuation, ~ K . 10 log 2nr 2 = H A - L A - ~ K (where L A is the noise criterion which the plant has to satisfy). The choice of value for ~ K will depend upon the parameter which is to be compared with the criterion and Table 7 gives approximate values of r where it corresponds to the maximum, average (Lcq), minimum, and downwind measured noise levels.
TABLE 7 REQUIRED DISTANCES BETWEEN PLANT AND RESIDENTIAL AREA TO SATISFY VARIOUS CRITERIA AND ATTENUATION FACTORS
Noise ~vel ~K
L=a x
L,~
-10
-15
Lmi.
L~. . . . i.n
-25
- 13
178 562
708 2239
r(m) C r i t e r i o n L a = 55 L a = 45
1000 3162
562 1778
Table 7 illustrates the confusion which can arise when the parameters for assessing noise levels are not clearly defined and when they are compared with different national criteria. Until new criteria are defined in terms of some generally accepted parameter it is therefore recommended as a temporary compromise that the criteria of BS 4142 should be used for comparison with L e q . Eventually an agreed parameter should be used for assessing measured noise levels and for noise criteria and this should be incorporated in a revised version of I SO 1996; preferably, this parameter should be Z e q .
ACKNOWLEDGEMENTS
This paper was presented at the London Chemical Engineering Congress in May, 1975, organised by the London and Southeastern Branch of the Institution of Chemical Engineers. I should like to thank K. J. Marsh for preparing the English text and the directors of Erd61chemie for permission to publish it.
192
M. GRASHOF REFERENCES
I. 2. 3. 4. 5. 6.
H.W. DUNKER,M. GRASHOF,H. SMOLENund O. ZIEGERT,Schallquellen und Schallschutzmassnahmen in Verfahrenstechnischen Anlagen, insbesondere im Himblick auf offene lndustrieanlagen. Erd61 und Kohle Erdgas, 25 (1972) pp. 659-70. B. STCIBER,Messmethode zur Ermittlung der Schalleistung ausgedehnter Schallquellen, Akustik und Schwingungstechnik, VDE-Verlag, Berlin, 1972, pp. 241-4. O. Z1EGERT,Schallmesseinrichtungen der Erd61chemie GmbH und einige Messergebnisse. Erddl und Kohle Erdgas, 28(8) (1975). M. GRASHOF,'Das Grundger/iuschspeicherverfahren.' Ein automatisches Schallmessverfahren zur getrennten Erfassung zeitlich untersehiedlicher Ger~uschimmissionen, Erddl und Kohle Erdgas, 28(7) (1975). VDI, Schallausbreitung im Freien (Sound propagation outside), Guideline No. 2714 (draft), April 1974. M. GRASHOF, Sicherheitsventile im Apparatebau--Bauarten, Auslegung fiir den Brandfall, Vorausberechnung der Ger/iuschemissionen und -immissionen, Chemie lng. Technik, 45(11) (1973) pp. 784-9.
M. GRASHOF
Bayer AG Leverkusen, IN Anlagenplanung Erd61chemie, c/o Erdiilchemie GmbH, 5000 K61n 71, Postfach 75 2002 (West Germany)
SUMMARY
The noise levels measured at a distance o f 1000 m from an industrial plant can vary within a range of 20 dB(A) due to the effects o f weather and extraneous sources. This paper examines the use o f various statistical parameters for assessing the noise due to the plant and describes a method o f minimising the effect o f extraneous noise due to traffic. The choice of parameter influences the attenuation data to be used in calculating the noise from new plants. Some typical measurements from a large industrial plant are presented. The paper emphasises the need to agree on a single parameter which can be used for calculations, for assessing measured noise levels, and for comparison with legal or noise criteria, and recommends the general adoption of
Leq, INTRODUCTION
This paper examines the effect of using various parameters for assessing the noise in residential areas due to an industrial plant. The noise emitted from a plant is frequently limited by law, and it is customary to define the limit in terms of an allowable noise level in a residential area; where there is no legal limit, the criterion for an acceptable noise level may be obtained from a standard such as ISO 1996 or BS 4142. In designing a new plant it is therefore important to be able to calculate the expected noise levels in order to meet the legal limit or criterion; this involves estimating the attenuation between the plant and the residential area. Where prior planning permission has to be obtained it may be necessary to use attenuation data acceptable to the local government. Finally, when the plant is operating, it will be necessary to monitor the noise levels to ensure that they comply with the legal limits. 177 Applied Acoustics (9) (1976)--© Applied Science Publishers Ltd, England, 1976 Pnnted in Great Britain
178
M. GRASHOF
It is not always appreciated that these are all aspects of the same problem and that the same concepts must be used at all stages; in particular, consistent parameters must be used for defining noise levels. Using noise data from a typical industrial plant, this paper will indicate the problems which arise when inconsistent parameters are used for defining noise levels and when different criteria are used for fixing noise limits.
COMMUNITY NOISE LIMITS IN GERMANY
In Germany, the authorities have been setting noise limits for industrial plants since 1968, using as a guide the criteria defined in the code of practice, TA-L/irm, or VDI 2058 (Part 1). Frequently, the limit in a community area is set at 40 dB(A) for the allowable noise from a particular plant. It is not always clear, however, whether this is the maximum acceptable noise in the area or whether it merely applies to a single plant; clearly, the levels would be much higher if a number of plants were each allowed to contribute 40dB(A). Another anomaly is that these levels are set for residential areas regardless of their distance from the plant. This can lead to severe requirements for the noise emitted from the plant ifa housing area is built close to its perimeter, and could make the operation of the plant completely uneconomic. It is important, therefore, that authorities should establish transition areas around large industrial plants where higher noise levels would be allowed. It may be noted that the corresponding criterion in BS 4142 for an urban residential area is 50 dB(A) for night-time, and it is suggested that this is a more realistic level.
MEASURING COMMUNITY NOISE LEVELS FROM A PLANT
When the noise level due to a plant is measured in a community area at a distance from the plant, it is found to vary considerably, even though the noise emission from the plant is steady. This is due to the effect of weather, and the measured noise level may vary from day to day within a range of about 20 dB(A) at a distance of 1000 m. There will also be local additions to the noise level from the plant, due, for example, to traffic, trains or aircraft. These may cause sudden peak noise levels on top of the plant noise, which usually will only vary slowly due to weather changes. Figure 1 shows a typical record of noise levels measured close to the plant (the plant noise emission) and of the measured noise level in a nearby residential area (the immission). The plant noise emission is seen to be fairly steady, whereas the measured noise level has a slow variation with superimposed peaks due to local sources of noise. This illustrates the basic problem of continuously monitoring noise from a plant; how to eliminate the effect of extraneous noise sources and how to define the slowly varying plant noise in terms of a single parameter.
ASSESSMENT OF NOISE FROM INDUSTRIAL PLANTS
179
Plant Noise Emission
dB (A) 90
80
50 40
3~o
4'00
5'oo
6'o0
Time.Interval: 3 seconds
Immission
dB (A) 90 _ _ 80 70
5O 4O 46
I--4 ,215 2'41 Time Interval: 3 minutes
Paper 1.46 2.15 2.41 3.45
Speed 0,03 mm/s h 50 dB {A) ,, 54 ,, ,, 44 ,, ,, 42 ,,
21.9.74
! Class Interval: 5 d B (A) 2,5 dB(A) or 1 dB(A)
Fig. 1. Variation of background noise caused by meteorologicalconditions.
DESIGNING PLANTS FOR NOISE CONTROL Usually, the noise reduction which can be achieved in an existing plant is limited to about 5-7 dB(A) and can often only be effected at considerable cost.l This places greater emphasis on including noise control measures at the design stage of new plants. I f the plant is being designed to meet a particular criterion for community noise, some agreed attenuation data must be used. In Germany, this is defined in VDI 2714, s which uses the following equation: H A = LNA + 10log 2xr 2 + K L + K B + K s + K M where LNa is the required noise limit for the plant in a particular community area. Table 1 illustrates how the required sound power level of the plant m a y be derived for design purposes when a noise limit has been set at 40 dB(A) at a distance ofS00 m from the plant; it also defines the terms in the above equation. In the case of open-air petrochemical plants and refineries, where there are continuously operating noise sources with predominant frequencies between 500 Hz and 2000 Hz, the following approximate formulas may be used for the calculation of overall noise levels in dB(A).
180
M. GRASHOF TABLE 1 ACOUSTIC PLANNING
Allowable sound power level H A
#~ = L N a + 101og2nr 2 + K LNA = r = 10 log 2nr 2 = KL = K, = Ks = KM =
40dB(A) 66 3.5 0 to 2 0 to 10 4.5
(in dB(A)) t + K B + Ks + Ku
Community noise limit for a single plant 800 m radius, distance Hemispherical sound propagation Air transmission loss Ground effects loss Screening, scattering by barriers Meteorological effects loss
H A = 114 to 126dB(A)
K L = 10log(1 + 1 . 5 . r . 10 -3 ) dB(A) K M = (12"500r -2 + 0"2) -1 dB(A) KL + Ku + Ks = 10log(3 + r. 1 6 0 - ' ) dB(A) K s = 10log(3 + 20z) dB(A) z = (r 2 + h2) j'2 - r 1 + (r22 + h2) 1'2 - r 2 where: z = path difference; h = height o f a barrier a b o u t twice as long as the noise source; rj = distance between source and barrier and r 2 = distance between barrier and receiver. Where z tends to zero and r > 4 0 0 m : 6 r K s + K L + K B + K ~ ~ lOlog~6
dB(A)
CALCULATING NOISE LEVELS DUE TO OPEN-AIR PLANTS
Most petroleum and petrochemical plants are built in the open, for safety reasons. As a result there is usually little attenuation o f the noise within the plant itself, most o f the attenuation occurring between the plant and the community areas. A frequently used method o f estimating noise levels due to a plant is based on measuring the sound power level o f the whole plant. This is obtained from measurements o f sound pressure level m a d e near the plant and from the areas o f an appropriate measuring surface, 2 using the following equation: H A = (L)A + 101og2S where: HA = sound power level o f complete plant in dB(A); (L)A = mean sound pressure level measured a r o u n d plant in dB(A) and S = surface area o f plant within the measuring line in m 2. As an example from a typical plant, where 70 dB(A) was measured at the plant boundary: H A = 70 + 101og[2(160 x 120)] = l l 8 d B ( A ) It m a y be noted that values o f 3 to 10 m a y be used for the screening factor, K s.
ASSESSMENT OF NOISE FROM INDUSTRIAL PLANTS
] 81
T h e three principal types of propagation in the atmosphere to a receiver located near the ground (taken from Beranek)
Free Atmosphere
.~ q4~ve
h -zooo (500-600 rn)
S
Transition region Air to ground
100- 200 ft ( 3 0 - 6 0 m ) Surface boundary layer : S ¢.---
I=
i
Ground wave
S = Noise Source
R R = Receiver
Fig. 2. Sound propagation.
Normally, only the minimum value should be used. According to Beranek (Noise and Vibration Control, p. 165) there are three main types of sound propagation in the atmosphere and these are illustrated in Fig. 2. For industrial plants only two types of propagation are of concern; the ground wave and the sky wave and, in practice, for distances up to 1000m only the ground wave is important, although, in Germany, VDI 2714 requires the sky wave to be used as the basis for calculations. When the wind is blowing directly from the noise source to the community area there is a decrease in the effect of screening from cooling towers or buildings. This causes an increase in the measured noise level. The curves given in VDI 2714 for the parameters K L and K B + K M are given in Fig. 3.
MONITORING NOISE FROM AN INDUSTRIAL PLANT There is increasing pressure from environmentalists for the reduction of industrial noise and this places greater emphasis on the need to measure the noise from plants.
182
M. GRASHOF
Where there is a particular concern about plant noise--or where there are legal limits on plant noise--it may be necessary to monitor the noise continuously. Various equipment is now available commercially for continuous monitoring--mainly developed for traffic noise--but the problem in using this is to separate the plant noise from the local noise. However, as shown in Fig. 1, the plant noise usually varies slowly, whereas the local noise causes sudden peaks and, by discriminating against the latter, it is possible to separate the background noise, which may then be attributed to the plant. Suitable 12.5 I
0
25 I
50 I
100 I
200 I
400 I
800 I
i[ d B (A KL=Airtransmission loss 1
1600 I
3200 m I
...... ......................Traffic noise
..........
(250Hz)
""°%,.. Plant "',..,noise
O-
1
(1000 Hz)
12.5
25
50
100
200
400
800
1600
3200
I
I
I
I
I
I
I
I
I
10d B (A)
KBeteKrMc~Go;°:alndefloss fndcts
"O%o.%,,,,..,,.,,,,.,iirange ,i,io]n
20-
(VDI- draft 2714) Fig. 3. Community noise levels. Variation range due to meteorological effects loss.
equipment for doing this has been developed at Erd61chemie and will be described briefly; a fuller description of the equipment has been given elsewhere. 3'4. The equipment consists of the following items: microphone, preamplifier, measuring amplifier, level recorder and a statistical distribution analyser with a maximum selector. Sampling times can be chosen between a few minutes and several hours; for example, sampling times can be selected to cover 8 h at night and 16 h during the day. The maximum selector can be set to operate within intervals of 1 to 10 sec and the class interval for counting events can be set at 1, 2.5 or 5 dB(A). To
183
ASSESSMENT OF NOISE FROM INDUSTRIAL PLANTS
measure background noise, a further device is required which selects the minimum noise level within each interval of 1-10 sec. Various time and class intervals were examined to find the most suitable arrangement. Ideally, the time interval should be two to three times longer than the duration of the peak noises due to traffic and aircraft, so that the minimum noise is a reasonable measure of the background. In the original equipment the maximum time interval available was 10 sec, although this was considered to be rather short. Later, new equipment was built in which it was possible to use a time interval of 3 min for determining the maximum and minimum noise levels. The smaller class intervals of 1 or 2.5 dB(A) are to be preferred for assessing background noise. Occasionally, of course, the plant itself emits peaks of noise, due, for example, to the operation of valves or safety vents, and it is important to be able to distinguish these effects from the other peaks. This is done by making a recording of peak noise events on a tape recorder so that they can be distinguished by ear. To avoid continuous operation o f the recorder, it is only switched on by an automatic circuit when the peak noise is about 10dB(A) above the highest background noise level.
Relative Frequency % 807060-
r
50-
oooooo,
! ! !
40302010"
vs. Overall Noise Level and Background Noise Level
Z,,, !
i
t .
|
t
~ * o l o a . l o l * * e l geeaoo
0 30 Fig. 4.
!
!
40
50
60 '
7b
80 '
9o d B (A)
Histograms. Time intervals: 5 and 10sec. Class interval: 5dB(A).
184
M. GRASHOF EXAMPLES OF NOISE MONITORING FROM A PLANT
Continuous noise measurements were made during 1973 and 1974 at monitoring stations situated between 400 m and 1200 m from an open-air petrochemical plant. The peak noise levels (overall noise) and the minimum noise levels (background) within each interval were analysed separately. Figures 4 and 5 show the histograms and cumulative frequency distributions for night-time measurements in July, 1973; Figs. 6 and 7 show the same information obtained in May, 1974. Further details of these two monitoring programmes are given in Table 2. Some typical data from Fig. 7 are presented in Table 3 together with the energyaveraged level, L=q. From Table 3 it can be seen that in May, 1974 the background noise level exceeded 40 dB(A) on 95 per cent of the occasions, while the corresponding level for the overall (peak) noise was 44 dB(A). The background noise exceeded 50 dB(A) on only 10 per cent of the occasions whereas the overall noise exceeded 58 dB(A) on 10 per cent of the occasions. For comparison, the values of L=q were 48.5 and 58 dB(A) for the background and overall noise. Cumulative frequency % 99.98-
ael,oeoe B
99"
90-
eve,
503010-
1"
0.130
20
i
"'%,. ..
6'o
7'0
do
9b ae (AI
Fig. 5. Distribution curves. Time intervals: 5 and 10sec. Class interval: 5dB(A).
ASSESSMENT OF NOISE FROM INDUSTRIAL PLANTS
185
In Table 4, the value o f Leq has been calculated for the individual months from March to August and for the whole period in 1973 and 1974. Table 4 shows that the average measured noise level had decreased from 52.4 to 49.7 dB(A) for the corresponding periods in 1973 and 1974. To some extent this reflects a greater accuracy in the assessment of the measurements--due to increasing the time interval from l0 sec to 3 min and to decreasing the class interval from 5 to 2.5 dB(A)--but it also indicates a real reduction in the noise from the plant, due to noise control measures.
Relative Frequency % 8070firS. Overall Noise Level and Background Noise Level
60504030I I I
I I
20-
I I I
10-
I . . . .
I
0
I
30
40
I
50
1
60
L
7()
dB
810
(A)
910
Fig. 6. Histograms. Time intervals: 3 sec and 3 min. Class interval: 2.5 dB(A).
THE ASSESSMENT OF ADDITIONAL ATTENUATION The equation of VDI 2714 contains two groups of attenuation factors; the first is 10 log 2nr 2, which represents the effect of the inverse square law; the second is the group of K factors which represent additional attenuation due to ground and weather effects. The additional attenuation, represented by ~ K , can be calculated if the values of sound power level, H A, and the measured noise level, LNA, are known.
186
M. GRASHOF
However, a decision has to be made about what value to use for the measured noise level; whether to use the lowest, average, or highest individual measurements. The choice will affect the apparent additional attenuation. Take, for example, the case of a plant where the lowest background level was 40 dB(A), the highest was 55 dB(A) and the L=q was found to be 50 dB(A) at a point 1000 m from the centre of the plant. If the sound power level of the plant is known to be 133dB(A), three corresponding values can be calculated for the additional attenuation. The calculation is given in Table 5.
Cumulative frequency %
99.98 ! 99.95 I 99.9 99.8 l
99.5" 98 97 95" 90" 80-
vs. Overall Noise Level and Background Noise Level
50403020" 105 0,5 0,2 0,1
0,05 0,02 30
40
50
60
70
~30
90 OB (A)
Fig. 7. Distribution curves. Time intervals: 3 sec and 3 min. Class interval: 2.5 dB(A). Table 5 shows that the following values are obtained for the additional attenuation over 1000m: Maximum attenuation 25 dB(A)--corresponding to 40 dB(A) Minimum attenuation 10 dB(A)--corresponding to 55 dB(A) Mean attenuation 15 dB(A)---corresponding to Leq = 50 dB(A) If it is required to calculate the value of L=q, the additional attenuation of 15 dB(A) should be used, corresponding to 1.5 dB(A) per 100 m; beyond 1000 m, this should
] 87
A S S E S S M E N T O F NOISE F R O M I N D U S T R I A L P L A N T S
TABLE 2 NOISE MONITORING STATION: 400 TO 1200M FROM OPEN PLANTS
Overall noise levels Maximum selector July, 1973 May, 1974
Sampling time: Each night In Figure Time interval Class interval M a x i m u m noise level M i n i m u m noise level Variation range Energetic mean Noise level Leq
Background noise levels Minimum selector July, 1973 May, 1974
2"3
4.5
2.3
4.5
5 sec 5 dB
3 sec 5 dB
10 sec 5 dB
3 min 2.5 dB
dB(A) 80 40 40
dB(A) 85 (35) (50)
dB(A) 60 40 20
dB(A) 55 38 17
58
58
52
48.5
TABLE 3 PER CENT NOISE LEVELS, MAY, 1974
Per cent time
Background noise level dB(A)
Overall noise level dB(A )
99.98 95 90 68 50 32 IL(~ q 5 0-02
37.5 40 41 43 46 48 48.5 50 50.5 55
40 44 45 47 51 53 58 58 60 85
TABLE 4 MEAN NOISE LEVELS FROM MARCHTO AUGUST, 1973 AND 1974. CLASS INTERVALS: 5 riB(A) AND 2"5 dB(A)
Sampling time
March April May June July August Mean noise level for a period of 6 m o n t h s
Mean noise level for each month
Lea in dB(A)
(1973)
(1974)
53 54 52 51.5 52 51.5 52.4
48.5 50 48-5 50 51.5 49 49.7
188
M. GRASHOF
be reduced to 1 dB(A) per 100 m, decreasing to 0.75 dB(A) per 100m at a distance of 2000 m. The average noise level, Leq , a t a distance from the plant should then be calculated from: 1.5 Zeq --- H a - 101og27tr 2 - 1--0-6r dB(A) where: H A = L A + 101og2S dB(A) L,, = mean sound pressure level measured on a line at 0.15(a) 1/2 from the plant centre, a = plant area in m 2. S -- area within the measuring line.
TABLE 5 CALCULATIONOF ADDITIONALATTENUATION Sound power level of all plants Distance monitoring station from acoustic r = 1000m L= -101og2nr 2 Sound pressure level without losses Backgroundlnoise levels measured: LA m~. = 55, Lml. = 40, Lcq = 50 dB(A)
centre:
133 dB(A) 68 dB(A) 65 dB(A)
Additional attenuation ~ K
Yr~,o Per 1000m Per 100m •
10 1-0
~Kma x 25 2.5
~K .... 15 dB(A) I'5dB(A)
DOWNWIND MEASUREMENTS
It can be seen that the amount of additional attenuation to be used in a calculation depends on the parameter used for rating the measured noise level and, clearly, the same parameter should be used for defining the noise limit. In Germany, some authorities require the noise measurements to be made downwind of the planti normally they require the mean value of at least three measurements made at night, under downwind and temperature-inversion conditions. This is equivalent to using the maximtim values of the distribution of noise levels for assessing the plant noise; the mean value calculated under these conditions would only be about 3 dB(A) below the 5 per cent point in the distribution. The additional attenuation to be used in a calculation should therefore be the lowest of the three values quoted above, that is, 10dB(A) or l dB(A) per 100m. To obtain a reliable assessment of the measured noise level under downwind conditions it is necessary to record the meteorological data at the same time and to assess the noise level for different wind directions. This limits the number of measurements which can be used and hence the reliability of the result. The,effect of limiting the number of measurements is shown in Table 6 which gives values of Leq f o r 2 h and 8 h sampling periods. It shows that differences of 1 to
189
A S S E S S M E N T O F N O I S E F R O M I N D U S T R I A L PLANTS
TABLE 6 ERRORS
DUE TO LIMITING
THE NUMBER
OF MEASUREMENTS
Noise levels in dB(A) Time interval: 3 rain = 20 counts per hour Class interval 1 dB( A ) Class interval 2-5 dB( A ) 2h each night 8h each night
Sampling time 1974 June July August
52 52-6 49.9
50 51.5 49
Devmtion 2 1.1 0.9
2 dB(A) can occur and it is recommended that sampling periods should be not less than 2 h per night for 3 min intervals. Figures 8 and 9 show some typical distributions of noise levels from a plant when the measurements are made under various wind conditions. They indicate the large variations in the mean values which occur when they are made under restricted weather conditions.
Relative Frequency % 45.-"=i ............ 1, 5. to 2. 5. 74 22"" to 6""DowrNgrnd L~1=54 dB (Aj := :'" . . . . 31.8. to 1.9. 74 22°~ to 6"" U ~ , ~ L~q=45 ~ (A)
40-
~
4.6. to 1.9. 74 22 °o ~ 2 °° E v ~ ' Wind ~ Leq~ 52(:18 IA)
3530
z |
25-
! =-
j il
..=-
ii Ii
20-
j"
''
# I
:=
m t
:I
15iP
#,
:
Ir
l
II
•
I
,I
10-
I
, I
5-
&.:
, i
I
I
II
i I
v
I 1 w i
I
s
# s
l
11 I1
.='- I
!
.-"
| J
i
II
13
I" I ~-
I"
|
40 Fig. 8.
=
45
r
i
50
55
dB (A) 60
Relative frequency of background noise. Time interval: 3 min. Class interval: I d B(A).
190
M. GRASHOF
Cumulative Frequency % .q~.9.
99.899.5¸ 99
__ t .
96 95 9O
•............ t. 5. to ?. 5. 74 2 2 ° 0 to 6 °o Downwin0 Leq 54 clBtA) .... 3 t . 8 . t o l . 9 . 7 4 2 2 o~ Io6OOUpwind Leq 45dB~A') , 4.6. tO 1 . 9 . 7 4 22 °o to 2 °0 Every Wind Direction ' Leq 5 2 t i B I A ) .
\\
8O 70
,t
6O 5O
\
|
40
20
\
10
t I
5
I It
|
0.5
!
0,2
,
|
0,1 0,05 0.02
| I~ t
4b Fig, 9.
!i
1
,:5
5b
,
~= ' ::,
s5
~o
65
dB (A)
Cumulative frequency of background noise, Time interval: 3 min. Class interval: I dB(A).
THE EFFECT OF USING INCONSISTENT PARAMETERS FOR COMPARING MEASURED NOISE LEVELS WITH NOISE CRITERIA
Unfortunately there are as yet no agreed parameters for assessing the measured noise levels from a plant. Figures 5 and 7 show the distribution of noise levels which can typically occur, and various points on the cumulative frequency distributions can be chosen to typify the distribution; Lj o, L9o and Lso values have been used on various occasions--and more recently the Leq has become more widely used. To these may be added the downwind values required by the German authorities. The chosen parameter has to be compared with a criterion selected from BS 4142 or ISO 1996 or with a legal noise limit, but the parameters in these criteria are not clearly defined in statistical terms. There are also differences in the criteria themselves which can give rise to varying national standards.
ASSESSMENT
OF NOISE FROM
INDUSTRIAL
191
PLANTS
To illustrate the variations which can occur it is interesting to calculate the distance between a plant and a residential area which would satisfy these different requirements. If it is defined as an urban area with light industry, the night-time criterion according to BS 4142 is 55 dB(A) and according to TA-L/irm 45 dB(A). Assuming that the sound power level of the plant is 133 dB(A), the required distance, r, is obtained from the following equation by inserting appropriate values for the additional attenuation, ~ K . 10 log 2nr 2 = H A - L A - ~ K (where L A is the noise criterion which the plant has to satisfy). The choice of value for ~ K will depend upon the parameter which is to be compared with the criterion and Table 7 gives approximate values of r where it corresponds to the maximum, average (Lcq), minimum, and downwind measured noise levels.
TABLE 7 REQUIRED DISTANCES BETWEEN PLANT AND RESIDENTIAL AREA TO SATISFY VARIOUS CRITERIA AND ATTENUATION FACTORS
Noise ~vel ~K
L=a x
L,~
-10
-15
Lmi.
L~. . . . i.n
-25
- 13
178 562
708 2239
r(m) C r i t e r i o n L a = 55 L a = 45
1000 3162
562 1778
Table 7 illustrates the confusion which can arise when the parameters for assessing noise levels are not clearly defined and when they are compared with different national criteria. Until new criteria are defined in terms of some generally accepted parameter it is therefore recommended as a temporary compromise that the criteria of BS 4142 should be used for comparison with L e q . Eventually an agreed parameter should be used for assessing measured noise levels and for noise criteria and this should be incorporated in a revised version of I SO 1996; preferably, this parameter should be Z e q .
ACKNOWLEDGEMENTS
This paper was presented at the London Chemical Engineering Congress in May, 1975, organised by the London and Southeastern Branch of the Institution of Chemical Engineers. I should like to thank K. J. Marsh for preparing the English text and the directors of Erd61chemie for permission to publish it.
192
M. GRASHOF REFERENCES
I. 2. 3. 4. 5. 6.
H.W. DUNKER,M. GRASHOF,H. SMOLENund O. ZIEGERT,Schallquellen und Schallschutzmassnahmen in Verfahrenstechnischen Anlagen, insbesondere im Himblick auf offene lndustrieanlagen. Erd61 und Kohle Erdgas, 25 (1972) pp. 659-70. B. STCIBER,Messmethode zur Ermittlung der Schalleistung ausgedehnter Schallquellen, Akustik und Schwingungstechnik, VDE-Verlag, Berlin, 1972, pp. 241-4. O. Z1EGERT,Schallmesseinrichtungen der Erd61chemie GmbH und einige Messergebnisse. Erddl und Kohle Erdgas, 28(8) (1975). M. GRASHOF,'Das Grundger/iuschspeicherverfahren.' Ein automatisches Schallmessverfahren zur getrennten Erfassung zeitlich untersehiedlicher Ger~uschimmissionen, Erddl und Kohle Erdgas, 28(7) (1975). VDI, Schallausbreitung im Freien (Sound propagation outside), Guideline No. 2714 (draft), April 1974. M. GRASHOF, Sicherheitsventile im Apparatebau--Bauarten, Auslegung fiir den Brandfall, Vorausberechnung der Ger/iuschemissionen und -immissionen, Chemie lng. Technik, 45(11) (1973) pp. 784-9.