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Noise and the docklands light railway
Applied Acoustics 26 (1989) 305-315
Technical Note Noise and the Docklands Light Railway
ABSTRACT Railway noise has long been regarded as one of the least offensive forms of transportation noise, with relatively few complaints arising from normal operations. However, certain components of the noise from new inner-city light railway systems are generating a substantial hostile community reaction. This note reports on noise measurements along the section of the London Docklands Light Railway (DLR) running from Tower Bridge to the Isle of Dogs. An explanation of the extent and depth of community reaction to the railway is sought in terms of both the character of the noise radiated and its loudness.
INTRODUCTION Previous surveys 1'2 of community response to noise in the U.K. have shown that noise from railways has generally been regarded as less annoying than that from other forms of transport. Such studies were limited to investigating the response to noise from conventional railways. The criteria derived (in terms of dB(A) and 24-hour LAeq),however, have since been used to specify noise limits for urban light railway systems, and in such cases as the instances reported here significant differences have been found between expected and actual community response. The D L R commenced operations in August 1987. At present the railway runs due east from Tower Hill to Limehouse, where it divides into two sections, one going north to Stratford and the other south through the 305 Applied Acoustics 0003-682X/89/$03.50 © 1989 Elsevier SciencePublishers Ltd, England. Printed in Great Britain
306
B. Shield, J. Roberts & M. Vuillermoz
Stratford
MillLane/
.//
Road
//J -~j
J_~
ConwneeclRo~ aa
=/=
Street
t~iv¢
Fig. 1.
Map of the Docklands Light Railway. e, Measurement sites.
middle of the Isle of Dogs, as shown in Fig. 1. For much of its length the track passes close to dwellings of various types including blocks of flats, terraced housing and new office builings. The railway consists of several different types of track construction. In some places the rails run on ballasted track at ground level; some sections consist of embankments or Victorian brick viaducts; and there are new elevated sections of relatively lightweight steel and concrete construction. During the period when the trains and track were being tested a significantly large number of local residents complained about the noise level from the trains. After the railway started to operate a full service the number of complaints increased along the entire length of the track. As a result of the strong local reaction the authors were commissioned to carry out a noise survey and investigate likely reasons for the adverse public reaction. They were also asked to determine whether or not the complaints made by members of the local community could be justified; and to
Noise and the Docklands Light Railway
307
investigate whether the noise levels measured complied with the specifications laid down by the Docklands Light Railway Company prior to the construction of the railway.
THE SURVEY The main survey was carried out over a period of three months from September to November 1987, with measurements made at a number of sites along the track, as shown in Fig. 1. Octave and one-third octave spectra were obtained from recordings made with Bruel & Kjaer type 2203 and CEL type 175/1 sound level meters and a Nagra T tape recorder, and Lax values were measured on site using a CEL meter (type 193/1). An exploratory survey had shown that there was no discernible difference in the noise of individual trains with time of day or day of the week, so after the initial period measurements were largely made in the evening or at weekends to reduce the effects of ambient noise due to the considerable building work taking place in the area. Unless otherwise specified, external noise measurements were made 1 m from the faqade facing the railway of the dwelling where a complaint had originated. Simultaneous readings were made inside the dwellings with the measuring microphone placed in the centre of that room where the noise from the railway caused most annoyance to the occupants. Each measurement visit included a discussion with the complainant. In these circumstances it was easy to obtain comments on the noise from the railway. A common complaint was interruption of television viewing but others included: 'Visitors in the front bedroom couldn't sleep because of the railway.' (site 1, Cable St) 'It keeps us awake at night.' (site 3, Gill St) 'It's like a monster coming into our lives.' (site 11, Manchester Rd) 'I'm woken up by the trains at 5"30 every morning.' (site 11, Manchester Rd) 'It's like thunder on wheels.' (site 9, East Ferry Rd) In broad terms there were two major types of noise which gave rise to complaints from those living near the railway: noise from stations and noise from trains running on the elevated sections. Annoyance caused by station announcements, the hiss of air brakes and the pneumatic control of doors are well known problems, the solutions of which are understood, and so measurements made adjacent to stations are not reported in this paper. From the interviews it was soon obvious that the residents who are most
308
B. Shield, J. Roberts & M. Vuillermoz
upset by the noise from the railway are those living near to the new elevated sections of track. The measurement survey showed that the rumbling thunder-like noise of which residents complained occurred near every section of the track built on new viaducts and coincided with very high levels of noise in the 63 Hz octave band, with individual trains generating peaks of up to 100dB r e 20#Pa at some sites. The extent of subjective reaction from those local residents experiencing the highest noise levels can be gauged from various incidents that occurred during the measurement survey. While anecdotal, they do give some idea of the depth and extent of community reaction to the noise from sections of the DLR. For example, in East Ferry Rd (site 9), one measurement position was in the street alongside a long section of elevated track. Here residents from nearly every dwelling came out to lobby, and an i m p r o m p t u street meeting took place so that those attempting to take the noise measurements were forced instead to listen to complaints. Individual residents and passing motorists often took the opportunity to voice their concern at the noise from the railway, and on one memorable occasion a refuse van digressed from its normal route to offer us a swift lift to the d u m p until assured we were carrying out an independent assessment!
RESULTS It became clear during the measurement period that noise levels varied both between trains and for any given train over a period of time. (A major contribution to this, possibly up to 10 dB(A) or more, was the development of wheel flats.) All results quoted are thus the average of at least three D L R trains in each direction. Measurements were made at seven sites where trains travelled over the new elevated sections of track and three sites where the train travelled over the old brick viaduct or at ground level. Results are given in Table 1. The dwellings where complaints originated were at different distances from the track. It would not be a sensible exercise to attempt to normalize the readings taken by making some allowance for distance, as the parameters and relationships involved are not yet fully understood. Thus it is necessary to describe each measurement site relative to the railway (see Table 1 and Fig. 1). It can be seen from Table 1 that the different types of-track have an important effect on the noise produced as a train passes over it. The older brick viaduct structures, as at NeweU St and Gill St (sites 2 and 3), and ballasted ground level track as at Undine Rd (site 7), give a sound spectrum
30 34 14 30 Underneath 20 35 20 20 40 10 20
Approx. distance from edge of track (m) LWV BV BV BV LWV LWV LWV BGLT LWV LWV Emb LWV
Type of track
63 58 ----54 -54 51 57 47
81 70 75 71 -78 62 70 77 75 75 76
Inside Outside
Average max. level (dB(A))
64
54 61 57 58 57 56
55 61 58 58 58 57
Eve.
65
Day
53
53 54
57 54
50
60
N~ht
Calculated external LA~q
79 76 66 68 84 69 78 72 79 70 75 77
31.5 86 74 75 76 100 95 81 70 97 93 85 90
63
250 75 68 71 67 81 80 59 66 73 72 73 74
125 83 69 71 67 90 82 69 66 88 87 77 76
75 66 72 70 77 77 59 69 74 65 73 73
500
79 66 74 69 73 72 55 66 69 63 71 69
lk
68 65 62 60 61 65 49 57 62 59 63 68
2k
58 59 52 50 51 56 46 56 59 59 59 66
4k
Mean maximum external SPL (dB re 20 #Pa) Mid-frequency octave band (Hz)
Average max. level (dB(A)) is the average of the m a x i m u m dB(A) readings of at least three D L R trains in each direction. The max. octave band SPLs are the average of max. octave band levels measured for at least three D L R trains in each direction. The figures quoted for BR trains are the average of three sets of readings. Type of track: LWV, lightweight viaduct; BV, brick viaduct; BGLT, ballasted ground level track; Emb, embankment.
1 Cable St (DLR) (BR) 2 Newell St 3 Gill St 4 BT Corner 5 Marsh Wall 6 Glengall Grove 7 Undine Rd 8 Thermopylae Gate 9 East Ferry Rd 10 Manchester Grove 11 Manchester Rd
Measurement site and location (see Fig. 1)
TABLE I L o c a t i o n s o f M e a s u r e m e n t Sites a n d P o s i t i o n s , a n d C o r r e s p o n d i n g N o i s e L e v e l s
48 48 44 44 -50 41 49 53 55 52 60
8k
E"
310
B. Shield, J. Roberts & M. Vuillermoz
in which the energy content of the lower frequency noise is much less than that from the new elevated sections of track. The differences in levels may be, typically, 20dB in the 63 Hz octave band and 15dB in the 125 Hz octave band. It should be noted that where the trains pass over new elevated structures all the spectra show the same general characteristics, with a pronounced peak in the 63Hz octave band, which for one particularly noisy train reached over 100dB r e 20/~Pa at 1 m from the faqade of a complainant's dwelling, and 70 dB in the lounge. For comparison, the noise produced by three British Rail trains running parallel to the Cable St section of the D L R track at site 1 was measured. Here the BR track runs on a brick viaduct and the D L R on a new lightweight viaduct. Table 1 shows that from 63 Hz to 4 kHz the noise from the D L R trains exceeds that of the BR trains, and the average level of the D L R trains at this site exceeds that of BR trains by about 10dB(A). Comparison of the noise level of BR trains on the brick viaduct in Cable St with that of D L R trains running on similar brick viaduct (sites 2 and 3) shows D L R trains to be 4 dB(A) quieter on average. This emphasizes that it is the construction of the track, rather than the trains themselves, which determines the character of the noise radiated.
T H E USE OF dB(A) It has been known for some time that dB(A) may not be the most suitable measure for the prediction of community reaction to intrusive noise where that noise contains a significant low-frequency component, a- 5 This is an important factor here as the prescribed maximum permissible level for individual trains at an unspecified distance from the track was 75 dB(A). Dawson a was concerned with local community response to continuous noise from static sources containing large amounts of low-frequency energy. He proposed the C o m m u n i t y Noise Criterion (CNC) shown in Fig. 2 as the sound spectrum at the nearest dwelling which, if exceeded, was likely to cause complaints. In contrast to dB(A) weighting, Dawson does not progressively increase the attentuation as the frequency falls. When the onethird octave band spectra of noise from individual trains, measured at the facade of the dwellings of two of the complainants, are compared with the C N C as shown in Fig. 2 it can be seen that the criterion is generally exceeded at all frequencies, sometimes by as much as 30 dB. The C N C was developed for use in other circumstances and may not be applied directly to the evaluation of noise from light transit systems. However, it would be expected to give some indication of likely reaction.
Noise and the Docklands Light Railway
311
0 (M
6o
_
,o
e-
~ 20 0
I
I
I
16
31"5
63
Octave
I
I
I
I
I
125 250 500 1K 2K band centre f r e q u e n c y (Hz)
I
I
4K
BK
Fig. 2. Comparison of train noise with Dawson's Community Noise Criterion curve, a - - , Community Noise Criterion curve; - - - • - - - , one-third octave band spectrum of a train at site 11; - - o - - , one-third octave band spectrum of a train at site 1.
Broner and Leventhall 4 have suggested that where the linear S P L is greater than the dB(A) reading by 20 dB or more, the dB(A) value is not the best basis for assessing the likelihood of complaint. As a result of their investigations (again of continuous sources) they proposed a Low Frequency Noise Rating, as shown in Fig. 3, where noise levels of two trains are plotted on L F N R curves. The similarity with Dawson and the difference compared to the dB(A) weighting curve is obvious. Broner and Leventhall also showed that the frequency range 30-50 Hz was particularly annoying, and that discernible intrusive noise occurring in this range was likely to cause complaints. Allan et aL s showed that for the noise from heavy goods vehicles (HGVs) measured according to relevant international and national standards, 6- 8 dB(C) correlated better with subjective assessment than dB(A). Figure 4 shows the mean maximum octave band noise spectrum obtained at 20 m from the drive-by line of the HGVs used in the experiment. Also shown is the mean maximum octave band spectrum for D L R trains passing over the new elevated structures. It can be seen that there is significant similarity in the spectral shape and levels of the two types of noise source. The major difference is that the train noise spectrum has its maximum energy at an even lower frequency than that of the HGVs. Given that the time history of the noise spectrum of HGVs in drive-by tests is similar to that of trains passing over elevated sections of track, this is a further strong indication that dB(A) is highly likely
B. Shield, J. Roberts & M. Vuillermoz
312
~20 110
115 110
100
105 100
9O
95 90
8O
85 80
70
75 70
60 55
r~
50
~ 5o
55
0
50 45
t
40
~ 3o
35 30
20
25 31.5
63
125
250
500 1000 2000 4 0 0 0 8000
Frequency (Hz)
Fig. 3. Trainnoise plotted on proposed Low Frequency Noise Rating curves of Broner & Leventhall.4 - - o - - , one-third octave band spectrum of a train at site 1; ---#---, one third octave band spectrum of a train at site 11. to underestimate the degree o f subjective response to such noise and thus underestimate likely community reaction. Thus for noise from the new elevated track structure, there is a case to be made that setting the limit o f maximum permitted noise levels in terms o f a unit appropriate for conventional railways (dB(A)) was a significant factor in underestimating community response.
T H E U S E O F LAeq LAe~ is increasingly being used as a measure o f environmental noise, and for the assessment o f the impact o f new noise sources on the surrounding
Noise and the Docklands Light Railway
313
100
~90 o ¢4
~- 8 0 m
~ 7o ig
L.
~ 6o Ill L
•~
C
50
o
ffl
I
31,5
Fig.
4.
I
I
I
I
l
63 125 250 500 lk Octave band c e n t r e f r e q u e n c y
I
2k (Hz)
, I
I
4k
8k
Comparison o f m a x i m u m noise levels from HGVs and D L R trains. - - - e - - - ,
Average spectrum of maximum levelsof HGV noise at 20.m;s --o--, averagespectrum of maximum levels of train noise near new viaducts. community. The LAe q noise limits set by the D L R prior to operation were 60dB(A) for daytime (07.00-19.00), 55dB(A) for evening (19.00-23.00) and 50 dB(A) for night-time (23.0(0-07.00). Because Laeq essentially averages over the times in question, the 5 dB(A) decreases going from day to evening to night are obtained solely by reduction of the frequency of operation, and not the noise of each individual event. Noise appears to have the greatest adverse effect on the community during the period 19.00-07.00, when it is more discernible owing to the lower level of background noise. It is during this time that the disturbance due to the trains takes such forms as interfering with television viewing or waking people. The use of Laeq for the periods 19.00-23.00 and 23.00-07.00 can be misleading because, while a reduction in the number of trains alone during these periods gives significant reductions in La~q the 'startle' effect of each train remains the same (the last trains at night still prevent sleep and the first train of the day still wakes up individuals), so the disruption of social life can remain largely unchanged. The measured Lax values and known frequency of service enabled calculation of daytime, evening and night-time La~q levels as shown in Table 1. With present services the daytime LA~q is calculated to be within the 60dB(A) limit originally proposed, the evening LA~a typically about
314
B. Shield, J. Roberts & M. Vuillerrnoz
1-2 dB(A) higher, and the night-time La,q typically about 3 dB(A) higher than the specified limits. When the frequency of the trains is increased, for example, to service the new Canary Wharf development or the proposed eastern extension, the Laeq levels will, of course, rise. CONCLUSIONS The most vociferous and active community response to noise from the DLR is to the high levels of very intrusive low-frequency noise radiated from the new elevated structures. The levels of noise measured in this survey would appear to justify the response of the community and the levels of complaint. To avoid such widespread and hostile community reaction to any future extension of the railway, the design of the new viaducts should be significantly changed. The techniques for such constructions are known and may be applied in future developments. The noise which causes the most severe annoyance is a low-frequency rumble, but the dB(A) weighting used for its assessment is designed to follow the 40 phon equal loudness contour of Fletcher and Munson 6 and discriminates strongly against low frequencies. Transmission through a facade into a dwelling will tend to preferentially attenuate the higher frequencies, a process which will relatively accentuate the low-frequency content and make the dB(A) scale even less appropriate for assessing likely community reaction to this particular noise. This suggests that noise criteria for the D L R based solely on the dB(A) will be misleading. The nature of the noise generating the hostile community response makes the noise criteria originally adopted by the D L R now appear inappropriate. A criterion is required which will adequately represent community response to a noise which contains a significant level of low-frequency rumble. Whether this will require a new scale such as the Community Noise Criterion or something as simple as dB(C) will need further investigation. The use of L,~eq, which averages individual events over a time period, does not appear to represent adequately the full disturbing impact of sudden loud noises on individual dwellings, particularly in the early hours of the morning or late at night. A criterion similar to L~o, which represents a mean maximum level, or even individual peak levels set realistically would appear preferable, and its possible use for inner-city light railway systems needs to be investigated. REFERENCES Fields, J. M., Railway noise and vibration annoyance in residential areas. J. Sound Vibr., 66 (1979) 445-58.
Noise and the Docklands Light Railway
315
2. Fields, J. M. & Walker, J. G., The response to railway noise in residential areas in Great Britain. J. Sound Vibr., 88 (1982) 177-255. 3. Dawson, H., Practical aspects of the low frequency noise problem. J. Low Freq. Noise Vibr., 1 (1982) 28-44. 4. Broner, N. & Leventhall, H., Low frequency noise annoyance assessment by low frequency noise rating (LFNR) curves. J. Low Freq. Noise Vibr., 2 (1983) 20-26. 5. Allan, S., Roberts, J. & Vuillermoz, M., Subjective assessment and objective measurement of noise from heavy goods vehicles. FASE, Sopron, Hungary, August 1986. 6. Fletcher, H. & Munson, W., Loudness, its definition, measurement and calculation. J. Acoust. Soc. Am., 5 (1933) 82-108.
B. Shield, J. Roberts & M. Vuillermoz Acoustics Group, Institute of Environmental Engineering, South Bank Polytechnic, London S E 1 0 A A , UK (Received 15 October 1988; accepted 9 November 1988)
Technical Note Noise and the Docklands Light Railway
ABSTRACT Railway noise has long been regarded as one of the least offensive forms of transportation noise, with relatively few complaints arising from normal operations. However, certain components of the noise from new inner-city light railway systems are generating a substantial hostile community reaction. This note reports on noise measurements along the section of the London Docklands Light Railway (DLR) running from Tower Bridge to the Isle of Dogs. An explanation of the extent and depth of community reaction to the railway is sought in terms of both the character of the noise radiated and its loudness.
INTRODUCTION Previous surveys 1'2 of community response to noise in the U.K. have shown that noise from railways has generally been regarded as less annoying than that from other forms of transport. Such studies were limited to investigating the response to noise from conventional railways. The criteria derived (in terms of dB(A) and 24-hour LAeq),however, have since been used to specify noise limits for urban light railway systems, and in such cases as the instances reported here significant differences have been found between expected and actual community response. The D L R commenced operations in August 1987. At present the railway runs due east from Tower Hill to Limehouse, where it divides into two sections, one going north to Stratford and the other south through the 305 Applied Acoustics 0003-682X/89/$03.50 © 1989 Elsevier SciencePublishers Ltd, England. Printed in Great Britain
306
B. Shield, J. Roberts & M. Vuillermoz
Stratford
MillLane/
.//
Road
//J -~j
J_~
ConwneeclRo~ aa
=/=
Street
t~iv¢
Fig. 1.
Map of the Docklands Light Railway. e, Measurement sites.
middle of the Isle of Dogs, as shown in Fig. 1. For much of its length the track passes close to dwellings of various types including blocks of flats, terraced housing and new office builings. The railway consists of several different types of track construction. In some places the rails run on ballasted track at ground level; some sections consist of embankments or Victorian brick viaducts; and there are new elevated sections of relatively lightweight steel and concrete construction. During the period when the trains and track were being tested a significantly large number of local residents complained about the noise level from the trains. After the railway started to operate a full service the number of complaints increased along the entire length of the track. As a result of the strong local reaction the authors were commissioned to carry out a noise survey and investigate likely reasons for the adverse public reaction. They were also asked to determine whether or not the complaints made by members of the local community could be justified; and to
Noise and the Docklands Light Railway
307
investigate whether the noise levels measured complied with the specifications laid down by the Docklands Light Railway Company prior to the construction of the railway.
THE SURVEY The main survey was carried out over a period of three months from September to November 1987, with measurements made at a number of sites along the track, as shown in Fig. 1. Octave and one-third octave spectra were obtained from recordings made with Bruel & Kjaer type 2203 and CEL type 175/1 sound level meters and a Nagra T tape recorder, and Lax values were measured on site using a CEL meter (type 193/1). An exploratory survey had shown that there was no discernible difference in the noise of individual trains with time of day or day of the week, so after the initial period measurements were largely made in the evening or at weekends to reduce the effects of ambient noise due to the considerable building work taking place in the area. Unless otherwise specified, external noise measurements were made 1 m from the faqade facing the railway of the dwelling where a complaint had originated. Simultaneous readings were made inside the dwellings with the measuring microphone placed in the centre of that room where the noise from the railway caused most annoyance to the occupants. Each measurement visit included a discussion with the complainant. In these circumstances it was easy to obtain comments on the noise from the railway. A common complaint was interruption of television viewing but others included: 'Visitors in the front bedroom couldn't sleep because of the railway.' (site 1, Cable St) 'It keeps us awake at night.' (site 3, Gill St) 'It's like a monster coming into our lives.' (site 11, Manchester Rd) 'I'm woken up by the trains at 5"30 every morning.' (site 11, Manchester Rd) 'It's like thunder on wheels.' (site 9, East Ferry Rd) In broad terms there were two major types of noise which gave rise to complaints from those living near the railway: noise from stations and noise from trains running on the elevated sections. Annoyance caused by station announcements, the hiss of air brakes and the pneumatic control of doors are well known problems, the solutions of which are understood, and so measurements made adjacent to stations are not reported in this paper. From the interviews it was soon obvious that the residents who are most
308
B. Shield, J. Roberts & M. Vuillermoz
upset by the noise from the railway are those living near to the new elevated sections of track. The measurement survey showed that the rumbling thunder-like noise of which residents complained occurred near every section of the track built on new viaducts and coincided with very high levels of noise in the 63 Hz octave band, with individual trains generating peaks of up to 100dB r e 20#Pa at some sites. The extent of subjective reaction from those local residents experiencing the highest noise levels can be gauged from various incidents that occurred during the measurement survey. While anecdotal, they do give some idea of the depth and extent of community reaction to the noise from sections of the DLR. For example, in East Ferry Rd (site 9), one measurement position was in the street alongside a long section of elevated track. Here residents from nearly every dwelling came out to lobby, and an i m p r o m p t u street meeting took place so that those attempting to take the noise measurements were forced instead to listen to complaints. Individual residents and passing motorists often took the opportunity to voice their concern at the noise from the railway, and on one memorable occasion a refuse van digressed from its normal route to offer us a swift lift to the d u m p until assured we were carrying out an independent assessment!
RESULTS It became clear during the measurement period that noise levels varied both between trains and for any given train over a period of time. (A major contribution to this, possibly up to 10 dB(A) or more, was the development of wheel flats.) All results quoted are thus the average of at least three D L R trains in each direction. Measurements were made at seven sites where trains travelled over the new elevated sections of track and three sites where the train travelled over the old brick viaduct or at ground level. Results are given in Table 1. The dwellings where complaints originated were at different distances from the track. It would not be a sensible exercise to attempt to normalize the readings taken by making some allowance for distance, as the parameters and relationships involved are not yet fully understood. Thus it is necessary to describe each measurement site relative to the railway (see Table 1 and Fig. 1). It can be seen from Table 1 that the different types of-track have an important effect on the noise produced as a train passes over it. The older brick viaduct structures, as at NeweU St and Gill St (sites 2 and 3), and ballasted ground level track as at Undine Rd (site 7), give a sound spectrum
30 34 14 30 Underneath 20 35 20 20 40 10 20
Approx. distance from edge of track (m) LWV BV BV BV LWV LWV LWV BGLT LWV LWV Emb LWV
Type of track
63 58 ----54 -54 51 57 47
81 70 75 71 -78 62 70 77 75 75 76
Inside Outside
Average max. level (dB(A))
64
54 61 57 58 57 56
55 61 58 58 58 57
Eve.
65
Day
53
53 54
57 54
50
60
N~ht
Calculated external LA~q
79 76 66 68 84 69 78 72 79 70 75 77
31.5 86 74 75 76 100 95 81 70 97 93 85 90
63
250 75 68 71 67 81 80 59 66 73 72 73 74
125 83 69 71 67 90 82 69 66 88 87 77 76
75 66 72 70 77 77 59 69 74 65 73 73
500
79 66 74 69 73 72 55 66 69 63 71 69
lk
68 65 62 60 61 65 49 57 62 59 63 68
2k
58 59 52 50 51 56 46 56 59 59 59 66
4k
Mean maximum external SPL (dB re 20 #Pa) Mid-frequency octave band (Hz)
Average max. level (dB(A)) is the average of the m a x i m u m dB(A) readings of at least three D L R trains in each direction. The max. octave band SPLs are the average of max. octave band levels measured for at least three D L R trains in each direction. The figures quoted for BR trains are the average of three sets of readings. Type of track: LWV, lightweight viaduct; BV, brick viaduct; BGLT, ballasted ground level track; Emb, embankment.
1 Cable St (DLR) (BR) 2 Newell St 3 Gill St 4 BT Corner 5 Marsh Wall 6 Glengall Grove 7 Undine Rd 8 Thermopylae Gate 9 East Ferry Rd 10 Manchester Grove 11 Manchester Rd
Measurement site and location (see Fig. 1)
TABLE I L o c a t i o n s o f M e a s u r e m e n t Sites a n d P o s i t i o n s , a n d C o r r e s p o n d i n g N o i s e L e v e l s
48 48 44 44 -50 41 49 53 55 52 60
8k
E"
310
B. Shield, J. Roberts & M. Vuillermoz
in which the energy content of the lower frequency noise is much less than that from the new elevated sections of track. The differences in levels may be, typically, 20dB in the 63 Hz octave band and 15dB in the 125 Hz octave band. It should be noted that where the trains pass over new elevated structures all the spectra show the same general characteristics, with a pronounced peak in the 63Hz octave band, which for one particularly noisy train reached over 100dB r e 20/~Pa at 1 m from the faqade of a complainant's dwelling, and 70 dB in the lounge. For comparison, the noise produced by three British Rail trains running parallel to the Cable St section of the D L R track at site 1 was measured. Here the BR track runs on a brick viaduct and the D L R on a new lightweight viaduct. Table 1 shows that from 63 Hz to 4 kHz the noise from the D L R trains exceeds that of the BR trains, and the average level of the D L R trains at this site exceeds that of BR trains by about 10dB(A). Comparison of the noise level of BR trains on the brick viaduct in Cable St with that of D L R trains running on similar brick viaduct (sites 2 and 3) shows D L R trains to be 4 dB(A) quieter on average. This emphasizes that it is the construction of the track, rather than the trains themselves, which determines the character of the noise radiated.
T H E USE OF dB(A) It has been known for some time that dB(A) may not be the most suitable measure for the prediction of community reaction to intrusive noise where that noise contains a significant low-frequency component, a- 5 This is an important factor here as the prescribed maximum permissible level for individual trains at an unspecified distance from the track was 75 dB(A). Dawson a was concerned with local community response to continuous noise from static sources containing large amounts of low-frequency energy. He proposed the C o m m u n i t y Noise Criterion (CNC) shown in Fig. 2 as the sound spectrum at the nearest dwelling which, if exceeded, was likely to cause complaints. In contrast to dB(A) weighting, Dawson does not progressively increase the attentuation as the frequency falls. When the onethird octave band spectra of noise from individual trains, measured at the facade of the dwellings of two of the complainants, are compared with the C N C as shown in Fig. 2 it can be seen that the criterion is generally exceeded at all frequencies, sometimes by as much as 30 dB. The C N C was developed for use in other circumstances and may not be applied directly to the evaluation of noise from light transit systems. However, it would be expected to give some indication of likely reaction.
Noise and the Docklands Light Railway
311
0 (M
6o
_
,o
e-
~ 20 0
I
I
I
16
31"5
63
Octave
I
I
I
I
I
125 250 500 1K 2K band centre f r e q u e n c y (Hz)
I
I
4K
BK
Fig. 2. Comparison of train noise with Dawson's Community Noise Criterion curve, a - - , Community Noise Criterion curve; - - - • - - - , one-third octave band spectrum of a train at site 11; - - o - - , one-third octave band spectrum of a train at site 1.
Broner and Leventhall 4 have suggested that where the linear S P L is greater than the dB(A) reading by 20 dB or more, the dB(A) value is not the best basis for assessing the likelihood of complaint. As a result of their investigations (again of continuous sources) they proposed a Low Frequency Noise Rating, as shown in Fig. 3, where noise levels of two trains are plotted on L F N R curves. The similarity with Dawson and the difference compared to the dB(A) weighting curve is obvious. Broner and Leventhall also showed that the frequency range 30-50 Hz was particularly annoying, and that discernible intrusive noise occurring in this range was likely to cause complaints. Allan et aL s showed that for the noise from heavy goods vehicles (HGVs) measured according to relevant international and national standards, 6- 8 dB(C) correlated better with subjective assessment than dB(A). Figure 4 shows the mean maximum octave band noise spectrum obtained at 20 m from the drive-by line of the HGVs used in the experiment. Also shown is the mean maximum octave band spectrum for D L R trains passing over the new elevated structures. It can be seen that there is significant similarity in the spectral shape and levels of the two types of noise source. The major difference is that the train noise spectrum has its maximum energy at an even lower frequency than that of the HGVs. Given that the time history of the noise spectrum of HGVs in drive-by tests is similar to that of trains passing over elevated sections of track, this is a further strong indication that dB(A) is highly likely
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Fig. 3. Trainnoise plotted on proposed Low Frequency Noise Rating curves of Broner & Leventhall.4 - - o - - , one-third octave band spectrum of a train at site 1; ---#---, one third octave band spectrum of a train at site 11. to underestimate the degree o f subjective response to such noise and thus underestimate likely community reaction. Thus for noise from the new elevated track structure, there is a case to be made that setting the limit o f maximum permitted noise levels in terms o f a unit appropriate for conventional railways (dB(A)) was a significant factor in underestimating community response.
T H E U S E O F LAeq LAe~ is increasingly being used as a measure o f environmental noise, and for the assessment o f the impact o f new noise sources on the surrounding
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Comparison o f m a x i m u m noise levels from HGVs and D L R trains. - - - e - - - ,
Average spectrum of maximum levelsof HGV noise at 20.m;s --o--, averagespectrum of maximum levels of train noise near new viaducts. community. The LAe q noise limits set by the D L R prior to operation were 60dB(A) for daytime (07.00-19.00), 55dB(A) for evening (19.00-23.00) and 50 dB(A) for night-time (23.0(0-07.00). Because Laeq essentially averages over the times in question, the 5 dB(A) decreases going from day to evening to night are obtained solely by reduction of the frequency of operation, and not the noise of each individual event. Noise appears to have the greatest adverse effect on the community during the period 19.00-07.00, when it is more discernible owing to the lower level of background noise. It is during this time that the disturbance due to the trains takes such forms as interfering with television viewing or waking people. The use of Laeq for the periods 19.00-23.00 and 23.00-07.00 can be misleading because, while a reduction in the number of trains alone during these periods gives significant reductions in La~q the 'startle' effect of each train remains the same (the last trains at night still prevent sleep and the first train of the day still wakes up individuals), so the disruption of social life can remain largely unchanged. The measured Lax values and known frequency of service enabled calculation of daytime, evening and night-time La~q levels as shown in Table 1. With present services the daytime LA~q is calculated to be within the 60dB(A) limit originally proposed, the evening LA~a typically about
314
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1-2 dB(A) higher, and the night-time La,q typically about 3 dB(A) higher than the specified limits. When the frequency of the trains is increased, for example, to service the new Canary Wharf development or the proposed eastern extension, the Laeq levels will, of course, rise. CONCLUSIONS The most vociferous and active community response to noise from the DLR is to the high levels of very intrusive low-frequency noise radiated from the new elevated structures. The levels of noise measured in this survey would appear to justify the response of the community and the levels of complaint. To avoid such widespread and hostile community reaction to any future extension of the railway, the design of the new viaducts should be significantly changed. The techniques for such constructions are known and may be applied in future developments. The noise which causes the most severe annoyance is a low-frequency rumble, but the dB(A) weighting used for its assessment is designed to follow the 40 phon equal loudness contour of Fletcher and Munson 6 and discriminates strongly against low frequencies. Transmission through a facade into a dwelling will tend to preferentially attenuate the higher frequencies, a process which will relatively accentuate the low-frequency content and make the dB(A) scale even less appropriate for assessing likely community reaction to this particular noise. This suggests that noise criteria for the D L R based solely on the dB(A) will be misleading. The nature of the noise generating the hostile community response makes the noise criteria originally adopted by the D L R now appear inappropriate. A criterion is required which will adequately represent community response to a noise which contains a significant level of low-frequency rumble. Whether this will require a new scale such as the Community Noise Criterion or something as simple as dB(C) will need further investigation. The use of L,~eq, which averages individual events over a time period, does not appear to represent adequately the full disturbing impact of sudden loud noises on individual dwellings, particularly in the early hours of the morning or late at night. A criterion similar to L~o, which represents a mean maximum level, or even individual peak levels set realistically would appear preferable, and its possible use for inner-city light railway systems needs to be investigated. REFERENCES Fields, J. M., Railway noise and vibration annoyance in residential areas. J. Sound Vibr., 66 (1979) 445-58.
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2. Fields, J. M. & Walker, J. G., The response to railway noise in residential areas in Great Britain. J. Sound Vibr., 88 (1982) 177-255. 3. Dawson, H., Practical aspects of the low frequency noise problem. J. Low Freq. Noise Vibr., 1 (1982) 28-44. 4. Broner, N. & Leventhall, H., Low frequency noise annoyance assessment by low frequency noise rating (LFNR) curves. J. Low Freq. Noise Vibr., 2 (1983) 20-26. 5. Allan, S., Roberts, J. & Vuillermoz, M., Subjective assessment and objective measurement of noise from heavy goods vehicles. FASE, Sopron, Hungary, August 1986. 6. Fletcher, H. & Munson, W., Loudness, its definition, measurement and calculation. J. Acoust. Soc. Am., 5 (1933) 82-108.
B. Shield, J. Roberts & M. Vuillermoz Acoustics Group, Institute of Environmental Engineering, South Bank Polytechnic, London S E 1 0 A A , UK (Received 15 October 1988; accepted 9 November 1988)