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Performance of a motorway noise barrier at Heston
PERFORMANCE OF A MOTORWAY NOISE BARRIER AT HESTON W. E. SCHOLES*, A. m. MACKIE'I', G. H. VULKAn** and D. G. HARLAND't"
* Building Research Establishment, Garston, Warford, Herts. (Great Britain) t Transport and Road Research Laboratory, Dept. of the Enrironment, Crowthorne, Berks. (Great Britain) ** Scientific Branch, Greater London Council (Great BritahO
(Received: 2 May, 1973)
SUMMA R Y This paper reports on the performance and costs of a 300 m long experimental noise barrier erected alongside an elerated section of the M4 motorway to protect existing two-storey housing which is as close as 20 m to the motorway. Measured reductions in L t o values, at the facades of these dwellings, were around 8 d B(A) at first floor windows and around 4 d B(A) at ground floor windows. These measured reductions were based on the leeels with no barrier but with some screening due to the eleration of the road. A wooden fence in the barrier position at the start of the e.vperiment gave reductions of about 3 dB(A) at the facades of the nearby houses. The subjective impressions of the occupants showed a markedly fat'ourable overall reaction to the change from the wooden fence to the experimental noise barrier. A reduction in the variability of the received noise levels from the partially screened situation without a barrier to the screened situation wasfound. There was no significant increase in noise levels for positions on the opposite side of the motorway.
INTRODUCTION
As part of current studies into the means of controlling nuisance due to the noise from road traffic, an experimental noise barrier has been constructed along a section of the M4 motorway at Heston. Measurements have been made of its effects on the noise levels at nearby housing and of the subjective reactions of the people in these houses. The work was carried out under the auspices of the Barrier Sub-Gr0up ofthe Working G r o u p on Traffic Noise by teams from the Building Research Station and the Transport and Road Research Laboratory of the Department of the 1
Applied Acoustics (7) (1974)--© Applied Science Publishers Ltd, England, 1974 Printed in Great Britain
W. E. SCHOLES, A. M. MACKIE, G. H. VULKAN, D. G. HARLAND
Environment and from the Scientific Branch of the Greater London Council, working jointly. The G L C initiated the experiment, suggested the form o f t h e noise barrier--taking into account its appearance--and suitable sites. The detailed design, construction and safety aspects were the responsibility of the T R R L who also carried out the subjective studies. BRS carried out the analysis of the combined results of the three measuring teams and measured the transmission loss of the experimental barrier. At the selected site, the M4 passes within about 20 m of a row of two-storey semi-detached houses in Winchester Avenue, Heston. Before the experiment started a close-boarded fence had been built along the edge of the motorway. Noise measurements were made with this wooden fence in position, without the fence and with the experimental barrier. Subjective reactions to the noise levels were determined with the wooden fence in position and with the experimental noise barrier. This paper summarises the results both of the noise measurements and of the subjective studies.
SITE DETAILS AND BARRIER CONSTRUCTION
The M4 at this site is elevated and carries about 73,000 vehicles in an 18-hour day (6 am to midnight) along its six lanes, making it one of the busiest roads in Europe. Typically the proportion of heavy vehicles is about 30 per cent and the mean traffic speed is about 85 km/h. Winchester Avenue used to have houses on both sides but the construction of the M4 took in the houses and gardens of one side, leaving the remaining semi-detached houses about 20 m from, and running roughly parallel to the edge of the motorway. The general situation is illustrated in Fig. I and Fig. 2 shows a plan of the area. Winchester Avenue rises slightly along its length relative to the motorway so that, at the western end of the avenue (House No. 74) the motorway is about first floor level and. at the eastern end, it is about level with the roofs of the houses. Figure 3 shows a generalised section with the values of the relevant Ibarameters along the avenue. The wooden fence and, later, the noise barrier, ran the full length of Winchester Avenue, about 300 m, and, at the western end of its run, the noise barrier was returned towards some outbuildings in an attempt to extend its effective length. Due to the elevation of the motorway the road surface itself shielded many of the measuring positions from the traffic noise, even with no fence or barrier present. The wooden fence, on site at the start of the experiment, had been erected to keep children and animals offthe motorway, but it also provided some protection against noise. It was 1.5 m high and was built on a line approximately 5 m from the edge of the nearest motorway traffic lane. It was close-boarded with feather-edged boards, generally 5 to 10 mm thick. The experimental noise barrier was built on a line approximately 4.5 m from the edge of the nearest motorway traffic lane. It was
PERFORMANCE OF A MOTORWAY NOISE BARRIER AT HESTON
constructed of two leaves of double skin hollow vinyl panels supported in a metal framework. Each panel was nominally 20 mm thick and 2.4 m long by 0-5 m wide. The panels were fitted into the metal framework, long edges horizontal, and the joints between the vertically adjacent panels were sealed with plastic strip. The joint to the ground was sealed with no fines concrete which provided an acoustic seal and allowed some drainage. The air space between the two leaves of the barrier was 45 mm wide and the cavity was closed at the top by means of a plastic capping
Fig. 1.
View of M4 and Winchester Avenue houses immediately behind noise barrier.
piece. The overall surface mass of the barrier was 7.7 kg/m 2 and the overall height 2.7 m above the ground near to the motorway. Small variations in this ground surface relative to the motorway gave a range of barrier height from 2-7 to 3.3 m. The cost of this barrier was approximately £60 per metre run. Of this cost £38 covered the manufacture and erection of the framework.
BARRIER TRANSMISSION LOSS
The transmission loss of the barrier construction was measured between two reverberant rooms at the Building Research Establishment. Measurements were
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PERFORMANCE OF A MOTORWAY NOISE BARRIER AT HESTON
made of the barrier transmission loss using the plastic sealing strip between the individual panels--as constructed at Winchester Avenue and also with all possible sound leakage paths sealed with plasticine. The results are shown in Fig. 4. The full
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line in this figure is for the barrier sealed with plastic strip--as constructed at Winchester Avenue. The line with circles refers to the transmission loss with all possible leakage paths sealed with. plasticine. Also shown in Fig. 4 is an indication
W. E. SCHOLES, A. M. MACKIE, G. H. VULKAN, D. G. HARLAND
of the transmission loss to be expected from a single leaf homogeneous partition of the same surface mass as the barrier construction 7.7 kg/m 2, calculated by a method due to Sewell.l It will be seen that at low to mid frequencies the better sealing by means of plasticine has little effect on the transmission loss but that from mid to high frequencies the curve for the plasticine sealed barrier exceeds the curve for the plastic strip sealed barrier by up to 7 dB. More important, in terms of the properties of the barrier in insulating against motorway noise, is the dip in the transmission loss in the region 200 to 630 Hz compared with what could be expected from a single leaf barrier of the same surface mass. The effects of this dip i~ the low to mid frequency transmission loss and also the imperfect sealing of the barrier as used at Winchester Avenue have been calculated in dB(A) using a typical spectrum of L io for motorway noise. 2 The calculations show that the high frequency improvements in the transmission los; due to sealing the barrier with plasticine have no significant effects on the potential barrier performance in dB(A) but that the dip in the transmission loss curve in the region of 200-630 Hz could reduce the potential performance compared with that of a single homogeneous barrier of the same surface mass by up to about 5 dB(A).
SITE NOISE MEASUREMENTS
Site noise measurements were made in three stages, first with the wooden fence in position, second with the wooden fence removed and with only the metal framework of the experimental barrier in position--giving the without barrier condition--and third, with the completed barrier in position. The main noise measurements were made at twelve measuring positions all I m from the exposed faqades ofthe houses in Winchester Avenue. Some of the measurements were made at the level of ground floor windows and the others at the level of the bedroom windows. Subsidiary measurements were made in the gardens behind Winchester Avenue, in Eton Avenue and in Durham Avenue. The measuring positions are indicated in plan in Fig. 2 and in section in Fig. 3. There were differences between the detailed techniques used by the three measuring teams. At some of the measuring positions measurements were made over 24 hours, at others only short-term measurements were made. At all measuring positions the noise levels were recorded and subsequently analysed statistically or directly analysed on site. It is not thought that these differences in technique had any significant effect on the measured results. At most of the measuring sites in Winchester Avenue the noise levels were measured for each of the three screening conditions, but at three positions in that avenue and at all the positions behind it, measurements were not made for the wooden fence condition. Measurements on the north side of the motorway were not made until the barrier was constructed.
PERFORMANCE OF A MOTORWAY NOISE BARRIER AT HESTON
In view of the recommendations of the Noise Advisory Council 3 that L to (18 h) should be adopted as the index for measuring disturbance by traffic noise, the effects of the barrier in terms of this unit are of prime interest. However, because the measurements at some of the houses were only short-term measurements, the main presentation of results will refer to these short-term (13.00 to 16.00 h) values of L t0 to include results from all the measuring positions in Winchester Avenue. In terms of noise level reduction by the fence and barrier, the results from the shortterm measurements may be taken for practical purposes as applying equally to 18 h values of L,o. The value of L,o averaged over the short period (13.00 to 16.00 h) was from I to 3 dB(A) higher than the value of Lto averaged over the normal period (06.00 t o 24.00 h). As has been mentioned, the chronological order of the noise measurements was first with the wooden fence, second with no barrier (metal framework anly) and third with the experimental noise barrier in position. For the sake of clarity, in what follows, the levels with no barrier will be given first, since this is the reference condition for the noise measurements. Table I shows the measured levels of L~o averaged over the period 13.00 to 16.00 h. Without any barrier there was considerable variation between the measuring points in Winchester Avenue because of the varying amount of shielding provided by the elevation of the road above the microphone positions. The I-5 m wooden fence provided Llo reductions of from I to 5 dB(A) while the 3 m experimental barrier gave reductions of from 3 to 9 dB(A). The experimental barrier was most effective at bedroom level, giving reductions of 5-9 dB(A). At ground floor level the motorway embankment provided considerable shielding without any barrier and the additional effect arising from the extra height of the barrier may have been cancelled, in part, by sound leaking through the barrier. The last column of Table I shows the improvement of the experimental barrier over the wooden fence. This is the improvement that influenced the subjective responses discussed in the following section. The principal benefit of the experimental barrier over the wooden fence is a reduction of about 4 dB(A) in the first floor noise levels. For the area between Winchester Avenue and Eton Avenue the Winchester Avenue houses provide considerable screening and the erection of the barrier gives reductions of L to, of about 2 dB(A). The change between the wooden fence and the experimental barrier would probably be less. In Table 2 the measured L lo values at the positions in Winchester Avenue are compared with the values predicted using the method of Scholes and Sargent.'* In general the predictions for the no barrier condition agree well with the measured values. At position 1 the microphone could just see the traffic across the edge of the embankment. For such marginally-screened positions the reductions vary very rapidly as the receiver moves across the edge of the shadow. Consequently. quite a slight error in determining the elevation of the road with respect to the microphone could lead to a prediction error as great as 3 dB(A). At position 12 the microphone
W. E. SCHOLES, A. M. MACKIE, G. H. VULKAN, D. G. HARLAND
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was screened not only by the embankment but also by the Heston Grange garages. No account was taken of this additional screening in calculating the predicted level which, consequently, is higher than the measured level. The comparison between predicted and measured levels for the experimental barrier condition in general shows poor agreement. However, for predicted attenuations of 13 dB(A.) or less (positions 4, 6 and 7), corresponding to predicted noise levels of 69 dB(A.) or more, the differences do not exceed 2 dB(A.). For the remaining positions the predicted attenuations were 15 dB(A.) or more, corresponding to predicted noise levels of 67 dB(A.) or less and for these positions the Lie 80 dBIA)
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may indicate some significant sound transmission through the barrier--because of the dip in the transmission loss curve discussed in the section on barrier transmission loss above--which could be expected to become important for positions well within the shadow of the screening. Alternatively, the prediction method could be in error for predicted attenuations in excess of about 15 dB(A). This uncertainty could be resolved by replacing one leaf of the experimental barrier with a panel having a sound transmission characteristic free from the dip shown in Fig. 4. The effects of barriers on L~o through a 24 h period and also on the variability (L t o-Lgo) are illustrated in Fig. 5. This refers to the noise levels at 3 m high and I m from the faqade of house No. 36, roughly midway along the length of Winchester
10
W. E. SCHOLES, A. M. MACKIE, G. H. VULKAN, D. G. HAKLAND
Avenue for the two conditions, without barrier and with the experimental barrier. It should be noted that, because of the elevation of the motorway, the measuring position was marginally screened even without the fence or barrier and so there is a possibility that the variability could already have been reduced by this screening. Even so, there is a significant reduction in the variability from the no-barrier (marginally screened) condition to the with-barrier condition. The values of L t o and Ego for these two conditions averaged over 18 h (6 am to midnight) and also over 24 h are shown in Table 3, together with the corresponding values of L Io minus Lgo, to represent the variability. TABLE 3 EFFECT OF BARRIER ON NOISE LEVELS AND VARIABILITY, NO. 36, FIRST FLOOR
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It will be seen from Table 3 that at House No. 36, 3 m high, the barrier gives improvements of 7 dB(A) for L~0 over 18 h and of 6 dB(A) for Llo over 24 h. This compares with a 6 dB(A) improvement over the period of 13.00 to 16.00 h used in Talkie 1 for the purposes of comparison. In addition to these reductions in LIo, the change from the no-barrier condition (marginally screened) to the with-barrier condition reduces the variability (Lt0-Lg0), by 3 dB(A) for the 18 h period and by 2 dB(A) over 24 h. Thus the expectation that barriers would reduce the variability of the noise 5 seems to have been confirmed and, because there was marginal screening even without a barrier, the change in variability from completely unscreened conditions to screened conditions may well be greater than the 2 to 3 dB(A) measurement discussed here. Other measurements were made on the opposite side of the M4 to see if the noise barrier, which was on the south side of the motorway, caused any increase in noise, by reflection, at housing to the north of the motorway. These measurements were made at three positions, about 50 m north of the edge of the motorway, two positions opposite the barrier and the other further east Where there was no barrier. As expected, there was no significant difference in the measured values of L lo between the sites and so it can be concluded that the experimental barrier did not cause any significant increase in noise levels to the hou'sing on the opposite side of the motorway. SUBJECTIVE NOISE EFFECTS
Studies of subjective response to traffic noise have been made by McKennel and Hunt and by Griffiths and Langdon. 6"~ The Heston Surveys differed from previous
PERFORMANCE OF A MOTORWAYNOISE BARRIER AT HESTON
I1
studies in that they attempted to assess not only the subjects' attitudes to noise but also any change in attitude brought about by the construction of the barrier. Surveys of the subjective reactions of the residents on the shielded side of the motorway and of the reactions of control groups on the unshielded side of the motorway were made first before the erection of the barrier and with the wooden fence in position and secondly after the erection of the experimental barrier. The first survey was made in September, 1970 and the second a year later, so that recent experience of the use of gardens and the opening of windows would be similar for both surveys. The considered need for this similarity of recent experience and for an appreciable settling down period precluded a survey during the short time that there was no barrier. Normally, in a study ofthis kind, to avoid bias the after survey would not sample subjects interviewed in the before study. However, because of the small number of houses protected by the barrier, this was not practical. Instead, a compromise scheme was adopted. In this half the population was interviewed in the before study and the total population in the after study. The before study sampled clusters of adjoining houses, rather than alternate houses, to reduce inter-respondent contact. The data was collected by means of a questionnaire administered by interviewers. The questionnaire dealt with three aspects of noise; noise as a disadvantage in this environment; the detailed effects of motorway noise, including a nine point scale of noise dissatisfaction; and opinions about the barrier and its effects. Some 142 people were interviewed before the erection of the barrier and 316 people after. This represented a response rate of 85 per cent. The motorway and its noise was spontaneously given as the main disadvantage of the area by more than 50 per cent of the respondents, both before and after the barrier was constructed. A direct question indicated that, after the barrier construction, the proportion of respondents who thought the neighbourhood noisy decreased on both sides of the motorway but the decrease was statistically significant only on the side protected by the barrier. The response to rating the noise on a nine point scale followed the same trend. Table 4 shows the dissatisfaction scores before and after the erection of the barrier. The change is greatest for Winchester Avenue and suggests that after the TABLE 4 Changes in noise levels (Lto) and median dissatisfaction scores Median dissatisfaction scores Site Wooden fence
Experimental barrier
Winchester Avenue
6.8
5.5
Eton Avenue Durham Avenue
5.0 6"5
4.3 6.2
Reduction in noise level from wooden fence to experimental barrier
4 or 5 dB(A) 1st floor level About 1 dB(A) ground floor l to 3 dBtA) Not significant
12
W. E. SCHOLES, A. M. MACKIE, G. H. VULKAN, D. G. HARLAND
barrier was constructed dissatisfaction with traffic noise in Winchester Avenue decreased to approximately the level of dissatisfaction experienced in Eton Avenue, the second road back from the motorway, before the barrier was constructed. Questions into the effects of noise revealed that a considerable proportion of the total population claimed that some activities were disturbed. In the before stage more than a third of those interviewed reported disturbed rest and relaxation and about half the people in the two roads nearest the motorway reported interference with conversation and that their houses were made to vibrate. In Winchester Avenue, after the barrier construction, the fraction whose rest and relaxation was disturbed dropped to a fifth, a quarter still experienced some interference with conversation and the proportion experiencing vibration was not significantly affected. Two-thirds of all the people questioned thought the barrier was a good idea. mainly because it reduced noise. Only about 10 per cent of the respondents disapproved of the barrier and their main criticisms were that it cut off the view of the traffic and that it was unsightly. In Winchester Avenue 69 per cent of the respondents thought the appearance o f the barrier would be improved if it were screened by a hedge. When asked what more the authorities could do to reduce noise, even in the after survey of Winchester Avenue, the most frequent suggestion was 'build more of a barrier' with 'give grants for house insulation' second. During the interviews of the respondents in Winchester Avenue it was noted that several of the houses had been fitted with sound insulating windows and ventilators under the H e a t h r o w Noise Insulation Grants Scheme. s It was also noted that, although the backs of these houses were exposed to the noise of aircraft using the Heath.row flight paths, the windows at the fronts of these houses, the M4 side, were the ones which had been insulated. This will be reported on in greater detail elsewhere. 9
CONCLUSIONS
In this practical situation, in which the houses were already partially screened by the elevation of the motorway, the experimental barrier gave a small but worthwhile reduction in the noise levels. From the no-barrier to the experimental-barrier condition typical improvements in the short-term L io values were from 79 dB(A) to 71 dB(A) at first floor windows and from 74 dB(A) to 70 dB(A) at ground floor windows. The wooden fence, in position at the start of the experiment, was already giving significant reductions from the no-barrier noise levels at first floor level of typically 3 dB(A). From the laboratory measurements of sound transmission by the experimental barrier construction it appears possible that its efficiency in screening traffic noise was reduced by the dip in the insulation curve in the region 200-630 Hz and that a homogeneous barrier of the same surface mass could have given noise reductions on site of up to 5 dB(A) higher than those measured. Nevertheless, the
PERFORMANCEOF A MOTORWAYNOISE BARRIERAT HESTON
13
measured noise reductions between the wooden fence and the experimental barrier, of about 4 to 5 dB(A) at first floor level, were matched with subjective improvements for which most of the people in Winchester Avenue were willing to accept the barrier, 20 m from their bedroom windows. Had the barrier been designed solely on acoustic requirements it would have been of solid construction and it seems reasonable to assume that any resulting increase in performance of the barrier would have been matched by an even more favourable subjective reaction. Subsidiary findings confirmed the expectation that noise barriers reduce the variability of traffic noise and the measurements also provided some confirmation of a current prediction method in the cases of unscreened and screened motorway noise for predicted attenuations of up to 13 dB(A). The subjective survey indicated that construction of the barrier had reduced dissatisfaction with noise among the population shielded by the barrier.
ACKNOWLEDGEMENTS We are grateful to Mr. S. V. Gardner who prepared the initial design of the experimental barrier and to Dr. W. A. Utley who provided the data on its sound transmission characteristics. We also acknowledge the assistance given by our colleagues in finalising the design and in carrying out the measurements and the analysis of the results. The work reported here was carried out as part of the respective research programmes of the Building Research Establishment, the Transport and Road Research Laboratory and the Greater London Council, and is reported with the permission of the Director of the BRE, of the Director of the T R R L and of the Scientific Adviser to the GLC.
REFERENCES 1. E. C. SEWELL,Transmission of reverberant sound through a single leaf partition surrounded by an infinite rigid baffle, d. Sound Vib. 12 (1971). 2. The Working Group on Research into Road Traffic Noise. A review of road traffic noise, RR.L Report LR357. Crowthorne, 1970. 3. Hansard Parliamentary Debates. Vol. 819, No. 165, 1971. 4. W. E. SCHOLESand J. W. SARGENT,Designingagainst noise from road traffic, App/iedAcoustics, 4 (1971). Also BRS CP 20/71. Barrier performance aspects, also in Design Bulletin 26, New Housing and Traffic Noise, HMSO, 1972. 5. F. J. LANGDONand W. E. SC.OLES,The traffic noise index--a method of controlling noise nuisance, BRS Current Paper CP 38/68. 6. A. C. McKENNELand E. A. HUNT, Noise annoyance in Central Lond.on, The Government Social Survey, SS 332, March, 1966. 7. I. L. GRIFFITHSand F. J. LANGDON,Subjective response to road traffic noise, J. Sound Vib., 8 (1968). 8. Statutory Instrument No. 424, HMSO, 1966. 9. G. M. B. WEBaEaand A. G. LONE)ON,The Heathrow Airport noise insulation grants scheme. (To be published.)
* Building Research Establishment, Garston, Warford, Herts. (Great Britain) t Transport and Road Research Laboratory, Dept. of the Enrironment, Crowthorne, Berks. (Great Britain) ** Scientific Branch, Greater London Council (Great BritahO
(Received: 2 May, 1973)
SUMMA R Y This paper reports on the performance and costs of a 300 m long experimental noise barrier erected alongside an elerated section of the M4 motorway to protect existing two-storey housing which is as close as 20 m to the motorway. Measured reductions in L t o values, at the facades of these dwellings, were around 8 d B(A) at first floor windows and around 4 d B(A) at ground floor windows. These measured reductions were based on the leeels with no barrier but with some screening due to the eleration of the road. A wooden fence in the barrier position at the start of the e.vperiment gave reductions of about 3 dB(A) at the facades of the nearby houses. The subjective impressions of the occupants showed a markedly fat'ourable overall reaction to the change from the wooden fence to the experimental noise barrier. A reduction in the variability of the received noise levels from the partially screened situation without a barrier to the screened situation wasfound. There was no significant increase in noise levels for positions on the opposite side of the motorway.
INTRODUCTION
As part of current studies into the means of controlling nuisance due to the noise from road traffic, an experimental noise barrier has been constructed along a section of the M4 motorway at Heston. Measurements have been made of its effects on the noise levels at nearby housing and of the subjective reactions of the people in these houses. The work was carried out under the auspices of the Barrier Sub-Gr0up ofthe Working G r o u p on Traffic Noise by teams from the Building Research Station and the Transport and Road Research Laboratory of the Department of the 1
Applied Acoustics (7) (1974)--© Applied Science Publishers Ltd, England, 1974 Printed in Great Britain
W. E. SCHOLES, A. M. MACKIE, G. H. VULKAN, D. G. HARLAND
Environment and from the Scientific Branch of the Greater London Council, working jointly. The G L C initiated the experiment, suggested the form o f t h e noise barrier--taking into account its appearance--and suitable sites. The detailed design, construction and safety aspects were the responsibility of the T R R L who also carried out the subjective studies. BRS carried out the analysis of the combined results of the three measuring teams and measured the transmission loss of the experimental barrier. At the selected site, the M4 passes within about 20 m of a row of two-storey semi-detached houses in Winchester Avenue, Heston. Before the experiment started a close-boarded fence had been built along the edge of the motorway. Noise measurements were made with this wooden fence in position, without the fence and with the experimental barrier. Subjective reactions to the noise levels were determined with the wooden fence in position and with the experimental noise barrier. This paper summarises the results both of the noise measurements and of the subjective studies.
SITE DETAILS AND BARRIER CONSTRUCTION
The M4 at this site is elevated and carries about 73,000 vehicles in an 18-hour day (6 am to midnight) along its six lanes, making it one of the busiest roads in Europe. Typically the proportion of heavy vehicles is about 30 per cent and the mean traffic speed is about 85 km/h. Winchester Avenue used to have houses on both sides but the construction of the M4 took in the houses and gardens of one side, leaving the remaining semi-detached houses about 20 m from, and running roughly parallel to the edge of the motorway. The general situation is illustrated in Fig. I and Fig. 2 shows a plan of the area. Winchester Avenue rises slightly along its length relative to the motorway so that, at the western end of the avenue (House No. 74) the motorway is about first floor level and. at the eastern end, it is about level with the roofs of the houses. Figure 3 shows a generalised section with the values of the relevant Ibarameters along the avenue. The wooden fence and, later, the noise barrier, ran the full length of Winchester Avenue, about 300 m, and, at the western end of its run, the noise barrier was returned towards some outbuildings in an attempt to extend its effective length. Due to the elevation of the motorway the road surface itself shielded many of the measuring positions from the traffic noise, even with no fence or barrier present. The wooden fence, on site at the start of the experiment, had been erected to keep children and animals offthe motorway, but it also provided some protection against noise. It was 1.5 m high and was built on a line approximately 5 m from the edge of the nearest motorway traffic lane. It was close-boarded with feather-edged boards, generally 5 to 10 mm thick. The experimental noise barrier was built on a line approximately 4.5 m from the edge of the nearest motorway traffic lane. It was
PERFORMANCE OF A MOTORWAY NOISE BARRIER AT HESTON
constructed of two leaves of double skin hollow vinyl panels supported in a metal framework. Each panel was nominally 20 mm thick and 2.4 m long by 0-5 m wide. The panels were fitted into the metal framework, long edges horizontal, and the joints between the vertically adjacent panels were sealed with plastic strip. The joint to the ground was sealed with no fines concrete which provided an acoustic seal and allowed some drainage. The air space between the two leaves of the barrier was 45 mm wide and the cavity was closed at the top by means of a plastic capping
Fig. 1.
View of M4 and Winchester Avenue houses immediately behind noise barrier.
piece. The overall surface mass of the barrier was 7.7 kg/m 2 and the overall height 2.7 m above the ground near to the motorway. Small variations in this ground surface relative to the motorway gave a range of barrier height from 2-7 to 3.3 m. The cost of this barrier was approximately £60 per metre run. Of this cost £38 covered the manufacture and erection of the framework.
BARRIER TRANSMISSION LOSS
The transmission loss of the barrier construction was measured between two reverberant rooms at the Building Research Establishment. Measurements were
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made of the barrier transmission loss using the plastic sealing strip between the individual panels--as constructed at Winchester Avenue and also with all possible sound leakage paths sealed with plasticine. The results are shown in Fig. 4. The full
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W. E. SCHOLES, A. M. MACKIE, G. H. VULKAN, D. G. HARLAND
of the transmission loss to be expected from a single leaf homogeneous partition of the same surface mass as the barrier construction 7.7 kg/m 2, calculated by a method due to Sewell.l It will be seen that at low to mid frequencies the better sealing by means of plasticine has little effect on the transmission loss but that from mid to high frequencies the curve for the plasticine sealed barrier exceeds the curve for the plastic strip sealed barrier by up to 7 dB. More important, in terms of the properties of the barrier in insulating against motorway noise, is the dip in the transmission loss in the region 200 to 630 Hz compared with what could be expected from a single leaf barrier of the same surface mass. The effects of this dip i~ the low to mid frequency transmission loss and also the imperfect sealing of the barrier as used at Winchester Avenue have been calculated in dB(A) using a typical spectrum of L io for motorway noise. 2 The calculations show that the high frequency improvements in the transmission los; due to sealing the barrier with plasticine have no significant effects on the potential barrier performance in dB(A) but that the dip in the transmission loss curve in the region of 200-630 Hz could reduce the potential performance compared with that of a single homogeneous barrier of the same surface mass by up to about 5 dB(A).
SITE NOISE MEASUREMENTS
Site noise measurements were made in three stages, first with the wooden fence in position, second with the wooden fence removed and with only the metal framework of the experimental barrier in position--giving the without barrier condition--and third, with the completed barrier in position. The main noise measurements were made at twelve measuring positions all I m from the exposed faqades ofthe houses in Winchester Avenue. Some of the measurements were made at the level of ground floor windows and the others at the level of the bedroom windows. Subsidiary measurements were made in the gardens behind Winchester Avenue, in Eton Avenue and in Durham Avenue. The measuring positions are indicated in plan in Fig. 2 and in section in Fig. 3. There were differences between the detailed techniques used by the three measuring teams. At some of the measuring positions measurements were made over 24 hours, at others only short-term measurements were made. At all measuring positions the noise levels were recorded and subsequently analysed statistically or directly analysed on site. It is not thought that these differences in technique had any significant effect on the measured results. At most of the measuring sites in Winchester Avenue the noise levels were measured for each of the three screening conditions, but at three positions in that avenue and at all the positions behind it, measurements were not made for the wooden fence condition. Measurements on the north side of the motorway were not made until the barrier was constructed.
PERFORMANCE OF A MOTORWAY NOISE BARRIER AT HESTON
In view of the recommendations of the Noise Advisory Council 3 that L to (18 h) should be adopted as the index for measuring disturbance by traffic noise, the effects of the barrier in terms of this unit are of prime interest. However, because the measurements at some of the houses were only short-term measurements, the main presentation of results will refer to these short-term (13.00 to 16.00 h) values of L t0 to include results from all the measuring positions in Winchester Avenue. In terms of noise level reduction by the fence and barrier, the results from the shortterm measurements may be taken for practical purposes as applying equally to 18 h values of L,o. The value of L,o averaged over the short period (13.00 to 16.00 h) was from I to 3 dB(A) higher than the value of Lto averaged over the normal period (06.00 t o 24.00 h). As has been mentioned, the chronological order of the noise measurements was first with the wooden fence, second with no barrier (metal framework anly) and third with the experimental noise barrier in position. For the sake of clarity, in what follows, the levels with no barrier will be given first, since this is the reference condition for the noise measurements. Table I shows the measured levels of L~o averaged over the period 13.00 to 16.00 h. Without any barrier there was considerable variation between the measuring points in Winchester Avenue because of the varying amount of shielding provided by the elevation of the road above the microphone positions. The I-5 m wooden fence provided Llo reductions of from I to 5 dB(A) while the 3 m experimental barrier gave reductions of from 3 to 9 dB(A). The experimental barrier was most effective at bedroom level, giving reductions of 5-9 dB(A). At ground floor level the motorway embankment provided considerable shielding without any barrier and the additional effect arising from the extra height of the barrier may have been cancelled, in part, by sound leaking through the barrier. The last column of Table I shows the improvement of the experimental barrier over the wooden fence. This is the improvement that influenced the subjective responses discussed in the following section. The principal benefit of the experimental barrier over the wooden fence is a reduction of about 4 dB(A) in the first floor noise levels. For the area between Winchester Avenue and Eton Avenue the Winchester Avenue houses provide considerable screening and the erection of the barrier gives reductions of L to, of about 2 dB(A). The change between the wooden fence and the experimental barrier would probably be less. In Table 2 the measured L lo values at the positions in Winchester Avenue are compared with the values predicted using the method of Scholes and Sargent.'* In general the predictions for the no barrier condition agree well with the measured values. At position 1 the microphone could just see the traffic across the edge of the embankment. For such marginally-screened positions the reductions vary very rapidly as the receiver moves across the edge of the shadow. Consequently. quite a slight error in determining the elevation of the road with respect to the microphone could lead to a prediction error as great as 3 dB(A). At position 12 the microphone
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was screened not only by the embankment but also by the Heston Grange garages. No account was taken of this additional screening in calculating the predicted level which, consequently, is higher than the measured level. The comparison between predicted and measured levels for the experimental barrier condition in general shows poor agreement. However, for predicted attenuations of 13 dB(A.) or less (positions 4, 6 and 7), corresponding to predicted noise levels of 69 dB(A.) or more, the differences do not exceed 2 dB(A.). For the remaining positions the predicted attenuations were 15 dB(A.) or more, corresponding to predicted noise levels of 67 dB(A.) or less and for these positions the Lie 80 dBIA)
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may indicate some significant sound transmission through the barrier--because of the dip in the transmission loss curve discussed in the section on barrier transmission loss above--which could be expected to become important for positions well within the shadow of the screening. Alternatively, the prediction method could be in error for predicted attenuations in excess of about 15 dB(A). This uncertainty could be resolved by replacing one leaf of the experimental barrier with a panel having a sound transmission characteristic free from the dip shown in Fig. 4. The effects of barriers on L~o through a 24 h period and also on the variability (L t o-Lgo) are illustrated in Fig. 5. This refers to the noise levels at 3 m high and I m from the faqade of house No. 36, roughly midway along the length of Winchester
10
W. E. SCHOLES, A. M. MACKIE, G. H. VULKAN, D. G. HAKLAND
Avenue for the two conditions, without barrier and with the experimental barrier. It should be noted that, because of the elevation of the motorway, the measuring position was marginally screened even without the fence or barrier and so there is a possibility that the variability could already have been reduced by this screening. Even so, there is a significant reduction in the variability from the no-barrier (marginally screened) condition to the with-barrier condition. The values of L t o and Ego for these two conditions averaged over 18 h (6 am to midnight) and also over 24 h are shown in Table 3, together with the corresponding values of L Io minus Lgo, to represent the variability. TABLE 3 EFFECT OF BARRIER ON NOISE LEVELS AND VARIABILITY, NO. 36, FIRST FLOOR
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It will be seen from Table 3 that at House No. 36, 3 m high, the barrier gives improvements of 7 dB(A) for L~0 over 18 h and of 6 dB(A) for Llo over 24 h. This compares with a 6 dB(A) improvement over the period of 13.00 to 16.00 h used in Talkie 1 for the purposes of comparison. In addition to these reductions in LIo, the change from the no-barrier condition (marginally screened) to the with-barrier condition reduces the variability (Lt0-Lg0), by 3 dB(A) for the 18 h period and by 2 dB(A) over 24 h. Thus the expectation that barriers would reduce the variability of the noise 5 seems to have been confirmed and, because there was marginal screening even without a barrier, the change in variability from completely unscreened conditions to screened conditions may well be greater than the 2 to 3 dB(A) measurement discussed here. Other measurements were made on the opposite side of the M4 to see if the noise barrier, which was on the south side of the motorway, caused any increase in noise, by reflection, at housing to the north of the motorway. These measurements were made at three positions, about 50 m north of the edge of the motorway, two positions opposite the barrier and the other further east Where there was no barrier. As expected, there was no significant difference in the measured values of L lo between the sites and so it can be concluded that the experimental barrier did not cause any significant increase in noise levels to the hou'sing on the opposite side of the motorway. SUBJECTIVE NOISE EFFECTS
Studies of subjective response to traffic noise have been made by McKennel and Hunt and by Griffiths and Langdon. 6"~ The Heston Surveys differed from previous
PERFORMANCE OF A MOTORWAYNOISE BARRIER AT HESTON
I1
studies in that they attempted to assess not only the subjects' attitudes to noise but also any change in attitude brought about by the construction of the barrier. Surveys of the subjective reactions of the residents on the shielded side of the motorway and of the reactions of control groups on the unshielded side of the motorway were made first before the erection of the barrier and with the wooden fence in position and secondly after the erection of the experimental barrier. The first survey was made in September, 1970 and the second a year later, so that recent experience of the use of gardens and the opening of windows would be similar for both surveys. The considered need for this similarity of recent experience and for an appreciable settling down period precluded a survey during the short time that there was no barrier. Normally, in a study ofthis kind, to avoid bias the after survey would not sample subjects interviewed in the before study. However, because of the small number of houses protected by the barrier, this was not practical. Instead, a compromise scheme was adopted. In this half the population was interviewed in the before study and the total population in the after study. The before study sampled clusters of adjoining houses, rather than alternate houses, to reduce inter-respondent contact. The data was collected by means of a questionnaire administered by interviewers. The questionnaire dealt with three aspects of noise; noise as a disadvantage in this environment; the detailed effects of motorway noise, including a nine point scale of noise dissatisfaction; and opinions about the barrier and its effects. Some 142 people were interviewed before the erection of the barrier and 316 people after. This represented a response rate of 85 per cent. The motorway and its noise was spontaneously given as the main disadvantage of the area by more than 50 per cent of the respondents, both before and after the barrier was constructed. A direct question indicated that, after the barrier construction, the proportion of respondents who thought the neighbourhood noisy decreased on both sides of the motorway but the decrease was statistically significant only on the side protected by the barrier. The response to rating the noise on a nine point scale followed the same trend. Table 4 shows the dissatisfaction scores before and after the erection of the barrier. The change is greatest for Winchester Avenue and suggests that after the TABLE 4 Changes in noise levels (Lto) and median dissatisfaction scores Median dissatisfaction scores Site Wooden fence
Experimental barrier
Winchester Avenue
6.8
5.5
Eton Avenue Durham Avenue
5.0 6"5
4.3 6.2
Reduction in noise level from wooden fence to experimental barrier
4 or 5 dB(A) 1st floor level About 1 dB(A) ground floor l to 3 dBtA) Not significant
12
W. E. SCHOLES, A. M. MACKIE, G. H. VULKAN, D. G. HARLAND
barrier was constructed dissatisfaction with traffic noise in Winchester Avenue decreased to approximately the level of dissatisfaction experienced in Eton Avenue, the second road back from the motorway, before the barrier was constructed. Questions into the effects of noise revealed that a considerable proportion of the total population claimed that some activities were disturbed. In the before stage more than a third of those interviewed reported disturbed rest and relaxation and about half the people in the two roads nearest the motorway reported interference with conversation and that their houses were made to vibrate. In Winchester Avenue, after the barrier construction, the fraction whose rest and relaxation was disturbed dropped to a fifth, a quarter still experienced some interference with conversation and the proportion experiencing vibration was not significantly affected. Two-thirds of all the people questioned thought the barrier was a good idea. mainly because it reduced noise. Only about 10 per cent of the respondents disapproved of the barrier and their main criticisms were that it cut off the view of the traffic and that it was unsightly. In Winchester Avenue 69 per cent of the respondents thought the appearance o f the barrier would be improved if it were screened by a hedge. When asked what more the authorities could do to reduce noise, even in the after survey of Winchester Avenue, the most frequent suggestion was 'build more of a barrier' with 'give grants for house insulation' second. During the interviews of the respondents in Winchester Avenue it was noted that several of the houses had been fitted with sound insulating windows and ventilators under the H e a t h r o w Noise Insulation Grants Scheme. s It was also noted that, although the backs of these houses were exposed to the noise of aircraft using the Heath.row flight paths, the windows at the fronts of these houses, the M4 side, were the ones which had been insulated. This will be reported on in greater detail elsewhere. 9
CONCLUSIONS
In this practical situation, in which the houses were already partially screened by the elevation of the motorway, the experimental barrier gave a small but worthwhile reduction in the noise levels. From the no-barrier to the experimental-barrier condition typical improvements in the short-term L io values were from 79 dB(A) to 71 dB(A) at first floor windows and from 74 dB(A) to 70 dB(A) at ground floor windows. The wooden fence, in position at the start of the experiment, was already giving significant reductions from the no-barrier noise levels at first floor level of typically 3 dB(A). From the laboratory measurements of sound transmission by the experimental barrier construction it appears possible that its efficiency in screening traffic noise was reduced by the dip in the insulation curve in the region 200-630 Hz and that a homogeneous barrier of the same surface mass could have given noise reductions on site of up to 5 dB(A) higher than those measured. Nevertheless, the
PERFORMANCEOF A MOTORWAYNOISE BARRIERAT HESTON
13
measured noise reductions between the wooden fence and the experimental barrier, of about 4 to 5 dB(A) at first floor level, were matched with subjective improvements for which most of the people in Winchester Avenue were willing to accept the barrier, 20 m from their bedroom windows. Had the barrier been designed solely on acoustic requirements it would have been of solid construction and it seems reasonable to assume that any resulting increase in performance of the barrier would have been matched by an even more favourable subjective reaction. Subsidiary findings confirmed the expectation that noise barriers reduce the variability of traffic noise and the measurements also provided some confirmation of a current prediction method in the cases of unscreened and screened motorway noise for predicted attenuations of up to 13 dB(A). The subjective survey indicated that construction of the barrier had reduced dissatisfaction with noise among the population shielded by the barrier.
ACKNOWLEDGEMENTS We are grateful to Mr. S. V. Gardner who prepared the initial design of the experimental barrier and to Dr. W. A. Utley who provided the data on its sound transmission characteristics. We also acknowledge the assistance given by our colleagues in finalising the design and in carrying out the measurements and the analysis of the results. The work reported here was carried out as part of the respective research programmes of the Building Research Establishment, the Transport and Road Research Laboratory and the Greater London Council, and is reported with the permission of the Director of the BRE, of the Director of the T R R L and of the Scientific Adviser to the GLC.
REFERENCES 1. E. C. SEWELL,Transmission of reverberant sound through a single leaf partition surrounded by an infinite rigid baffle, d. Sound Vib. 12 (1971). 2. The Working Group on Research into Road Traffic Noise. A review of road traffic noise, RR.L Report LR357. Crowthorne, 1970. 3. Hansard Parliamentary Debates. Vol. 819, No. 165, 1971. 4. W. E. SCHOLESand J. W. SARGENT,Designingagainst noise from road traffic, App/iedAcoustics, 4 (1971). Also BRS CP 20/71. Barrier performance aspects, also in Design Bulletin 26, New Housing and Traffic Noise, HMSO, 1972. 5. F. J. LANGDONand W. E. SC.OLES,The traffic noise index--a method of controlling noise nuisance, BRS Current Paper CP 38/68. 6. A. C. McKENNELand E. A. HUNT, Noise annoyance in Central Lond.on, The Government Social Survey, SS 332, March, 1966. 7. I. L. GRIFFITHSand F. J. LANGDON,Subjective response to road traffic noise, J. Sound Vib., 8 (1968). 8. Statutory Instrument No. 424, HMSO, 1966. 9. G. M. B. WEBaEaand A. G. LONE)ON,The Heathrow Airport noise insulation grants scheme. (To be published.)