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Development of audible locating signals for use at pedestrian crossings
Applied Acoustics 62 (2001) 15±27
www.elsevier.com/locate/apacoust
Development of audible locating signals for use at pedestrian crossings G.R. Watts *, N.S. Godfrey, T.A. Savill Transport Research Laboratory, Crowthorne, Berkshire, UK RG45 6AU Received 10 September 1999; received in revised form 8 March 2000
Abstract At light controlled crossings in several countries audible ticking sounds are used to assist visually impaired pedestrians locate the control box so that they can readily request the crossing phase. A literature review has shown that a wide range of sound signals are used for this purpose and it was concluded that further research was required to select a suitable locating sound for possible use in the UK. Laboratory studies were conducted to assist in the selection of suitable locating sounds which will not only be easy to locate but also will not be readily confused with the ``walk'' signal or reversing alarms used by trucks and buses etc. There was also a need to assess the possible annoying eects of such signals especially for residents or workers located close to the crossing. For this purpose a wide range of sounds which have potential as locating signals have been evaluated by panels of sighted listeners so that the more annoying sounds can be screened out at an early stage. # 2000 Transport Research Laboratory. Published by Elsevier Science Ltd. All rights reserved. Keywords: Sound location; Visual impairment; Trac signals
1. Introduction In recent years facilities have been added to light-controlled crossings in the UK to assist their use by visually-impaired pedestrians. These include tactile paving and audible signals to augment the visual green pedestrian signal (or ``walk'' phase). However, although these advances have greatly improved controlled crossings visually impaired people have reported that they sometimes ®nd it dicult to locate the push button control box. A recent study [1] involving 55 visually impaired subjects, * Corresponding author. Tel.: +44-1344-770414; fax: +44-1344-770918. E-mail address: [email protected] (G.R. Watts). 0003-682X/01/$ - see front matter # 2000 Transport Research Laboratory. Published by Elsevier Science Ltd. All rights reserved. PII: S0003-682X(00)00027-X
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who were asked to evaluate locating sounds at a simulated road crossing, concluded that an additional sound signal would be helpful to assist in the location of this control box. As a result of this recommendation a worldwide literature review was commissioned to gather information about the ecacy of such signals. It was clear that they were in widespread use in Northern Europe and Australia and varied in signal characteristics from simple ticks to relatively long tones. However, the survey highlighted the need for further studies in order to evaluate locating sounds under well controlled conditions so that an appropriate sound could be identi®ed which can be readily perceived by pedestrians and yet not cause confusion or annoyance in the environment [2]. This paper reports on a series of tests that have been carried out by TRL in order to identify a suitable sound signal. The ®rst test examined the perceived nuisance of 18 test sounds using sighted members of the public. Six sounds were selected as being least annoying when their loudness level was taken into account and these were used in a further study. This second test employed visually impaired people to assess the ease with which the sounds could be located and additionally to rate the risks of confusing each of the sounds with the standard UK crossing signal and a typical lorry reversing alarm. 2. Description of locating signals Locating signals are normally operated when the ``wait'' signal is being shown to the pedestrian. A common form of locating signal is a ticking sound or bleep repeated about every 2 s. The signal is typically automatically adjusted by an automatic volume control circuit (AVC) so that it is just audible above trac noise. If correctly adjusted this results in a very low level under quiet conditions so that nuisance to residents is minimised. When the ``wait'' phase changes to ``invitation to cross'' the crossing signal is heard. The crossing signal is typically much louder than the locating signal and tone variations are more rapid. Fig. 1 shows an example of locating and cross signals and gives values of frequencies, pulse duration and repeat interval. The sound signals were based on previous experience of signals used at pedestrian crossings in other countries [2] and sounds which were based on a consideration of the literature on sound localisation [3±7]. Table 1 gives a description of the sounds employed. Included are the sounds used at crossings in Australia and Sweden and sounds containing a wide range of frequencies e.g. pink noise bursts and square wave signals. The chirp sound was based on the sound produced by the ®eld cricket (Gryllus campestris) [8] but its fundamental frequency (carrier) was lowered from 5 to 1 kHz so that it would be more readily perceived by the general population. A fuller description of this sound is given in Section 5 below. Some sounds were composites containing two dierent sounds e.g. sound 12 a square wave signal followed by a chirp which was a combination of sounds similar to the Swedish tone (sound 3) and the chirp (sound 9). Table 1 indicates the lengths and separation between these components together with the frequencies involved and waveform type. The waveforms were either sinusoidal, square wave, chirp or broadband pink noise. The pink
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Fig. 1. Example of locating and ``cross'' signals.
Table 1 Speci®cation of test sounds for annoyance study Sound
Description
Pulse duration (s)
Pulse separation (s)
Repeat interval (s)
Frequency (kHz)
Wave type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Short high tone Australian Swedish TRL1 double tick TRL2 double tick Short pink pulse Long pink pulse Short pink pulse Chirp ISVR test signal Short square wave/chirp Long square wave/Chirp Short square wave/Chirp TRL2/chirp Short square wave/Chirp Short square wave/Short pink pulse Short square wave/Long pink pulse Short pink pulse/Chirp
0.025 0.025 0.400 0.025 0.025 0.025 0.100 0.025 0.120 0.025 0.012/0.120 0.185/0.120 0.012/0.120 0.025/0.120 0.012/0.120 0.012/0.025
Ð Ð Ð 0.225 0.225 Ð Ð Ð Ð Ð 0.240 0.100 0.014 0.22/0.25 0.360 0.250
1.8 1.8 2.0 2.0 2.0 1.8 1.8 1.8 1.8 1.5 1.8 1.8 1.8 1.8 1.8 1.8
3.1 1.0 0.9 1.0 0.5±15 0.5±15 0.5±15 0.5±15 1.0 1.0 2.1/1.0 1.8/1.0 2.1/1.0 0.5±15/1.0 2.1/1.0 2.1/0.5±15
Sine Square Square Sine Pink (high) Pink (high) Pink (high) Pink (low) Chirp Sine Square/chirp Square/chirp Square/chirp Pink (high)/Chirp Square/chirp Square/pink (high)
0.012/0.100
0.250
1.8
2.1/0.5±15
Square/pink (high)
0.025/0.120
0.250
1.8
0.5±15/1.0
Pink (high)/chirp
17 18
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noise signals were divided into high and low frequency ranges. The high range covered one-third octave bands from 0.5 to 15 kHz and the low range from 0.1 to 5 kHz. 3. Annoyance tests The TRL listening room shown in Fig. 2 was used to obtain evaluations of annoyance. It is furnished as a lounge and has a single glazed window on one wall that opens onto another room that contains the loudspeaker source. The loudspeaker was positioned 2 m from the middle of the window. The various ticking signals were reproduced by this loudspeaker and the participants (2 at a time) evaluated these sounds while seated in the listening room. To obtain a range of conditions, sessions were run with the window open and closed and also with and without simulated masking trac noise. This masking noise was produced by shaping the one-third octave spectrum of pink noise so that the input to the speaker conformed to a typical urban trac noise spectrum [9]. The A-weighted spectrum when plotted in one-third octave bands is approximately triangular in shape with a peak at 1 kHz and is 10 dB down at 160 Hz and 5 kHz. Subjectively this produces a reasonable simulation of constant trac noise. The masking trac noise was reproduced from an additional speaker placed adjacent to the ®rst speaker. The A-weighted level LAeq of the masking noise was adjusted to a constant SPL of 69 dB outside the listening room at a position 1 m from the centre of the closed window. This is approximately the average level of trac noise measured at a wide range of residential sites [10]. The sound signals fed to the loudspeaker were of identical voltage amplitude and they were adjusted so that the 0.4 s tone of the Swedish sound (sound 2) produced an Aweighted level at the same level as the masking trac noise i.e. 69 dB. Because of the dierent duration and frequency content of the signals the maximum output levels at the loudspeaker produced by the test sounds varied. Inside at the listener's ear position the masking noise level LAeq was 33.5 dB with the windows closed and 50.7 dB with the windows opened. Without the masking noise present the maximum Aweighted levels of the sounds varied from 26.3 to 36.5 dB with the window closed
Fig. 2. Experimental arrangement for annoyance and loudness assessments.
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and from 40.3 to 59.2 dB with the window open. Thus the level of the masking noise under each condition was within the range of the maximum levels of the test sounds. Adult listeners were recruited through advertisements placed in local newspapers. An audiometer calibrated to ISO 389-1991 [11] was used to check hearing ability and it was found that only 2 had hearing diculties likely to interfere with the required assessment task. The criterion used was the ``referral level'' given by age in the Health and Safety Executive's guideline [12]. This indicates a level of hearing loss (without hearing aid) based on age at or below which further medical investigation is recommended. The results from these participants were not used in subsequent analysis and further participants were recruited to reach the target number. Thirty-six listeners took part; 18 listening to the sounds with the window open and 18 with the window closed. During each session the 18 sound signals were ®rst presented without masking trac noise and then with masking noise present. The description of these test sounds is given in Table 1. The presentation was balanced to avoid order eects. Their task was to rate the noise nuisance (assuming they have been exposed for several hours to the sound) under both conditions and then the procedure was repeated but on this occasion the loudness of the sounds were rated. The following 10 point scales were used: Noise annoyance scale: ``Not annoying at all''
0123456789
Loudness scale: ``Very quiet'' 0123456789
``Very annoying''
``Very loud''
For rating loudness an 11±12 s signal duration was used, however, for the more dicult task of assessing annoyance the sound duration was lengthened to 20 s. An 8 s period without the signal present was used for the assessment period. It was considered important to include ratings of loudness since the sounds will almost certainly be perceived to have dierent levels of loudness and this will in¯uence judgements of annoyance. Clearly quiet sounds will tend to be less intrusive and, therefore, less annoying than loud sounds. In a practical installation volume can be controlled so the aim was to identify sounds which are particularly annoying for their level of perceived loudness. 3.1. Results Eighteen men and 18 women took part in the study. Ages ranged from 18 to 73 the average age being 43 years. None of the listeners were wearing a hearing aid. The average annoyance and loudness ratings for the test signals were calculated for each condition i.e. with and without masking noise and with the window open and closed. The overall annoyance and loudness ratings were then calculated by averaging over all listening conditions. Fig. 3 shows the average annoyance score plotted against the average loudness score together with the regression line drawn through
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Fig. 3. Average annoyance by average loudness.
the origin. Sounds falling below this line are to be preferred since the average annoyance they cause is relatively small when their loudness level is taken into account. As expected loudness was an important determinant of annoyance as can be seen from the excellent relationship between average annoyance and average loudness. The Pearson correlation coecient is 0.94 which is highly signi®cant (0.1% level). Reducing the noise level should reduce the loudness ratings of the sounds and hence limit annoyance. However, the signal needs to be detectable against trac noise so there is a limit to how quiet the signal can be made and yet remain viable as a locating signal. It is important that annoyance is as low as possible, whatever the chosen level of loudness. The six sounds with the lowest and highest overall annoyance to loudness ratio are tabulated in Table 2 together with the range in the ratio calculated for each of the four listening conditions. In general the ratios for the preferred sounds were consistently low across listening conditions although there was some tendency for the ratios to be higher with masking noise present. In general the Swedish and Australian signals, composite sounds ending with a chirp, and the chirp sound alone have the lowest ratios whereas the TRL double tick sound and those containing a pink noise pulse appear to be the most annoying. For example,
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Table 2 Sounds with lowest and highest overall annoyance to loudness ratio Sound number
Description
Average annoyance/average loudness (range of annoyance/loudness ratio)
Lowest 12 3 13 2 9 15
Long square wave/chirp Swedish Short square wave/chirp (short gap) Australian Chirp Short square wave/chirp (long gap)
0.93 0.96 0.98 0.99 1.03 1.04
(0.86±1.04) (0.82±1.03) (0.64±1.25) (0.89±1.14) (0.75±1.12) (0.90±1.12)
Highest 14 6 5 16 17 7
TRL2/chirp Short pink pulse TRL2 double tick Short square wave/short pink pulse Short square wave/long pink pulse Long pink pulse
1.41 1.33 1.27 1.26 1.21 1.21
(1.19±1.95) (1.08±1.79) (1.07±1.90) (1.13±1.55) (1.06±1.56) (1.04±1.42)
the result for the TRL2 chirp indicates a 50% higher degree of average annoyance relative to the loudness than the Swedish chirp. Based on these tests the six sounds with the lowest ratios were taken through to the next series of tests. 4. Sound location and confusion tests These further tests employed visually impaired people to assess the ease with which the sounds could be located and additionally to rate the risk of confusing the sounds with the standard UK crossing signal and a typical lorry reversing alarm. Thirty-six listeners were recruited from local groups for visually impaired people. Listeners were required to undergo an audiometric test on each ear identical to that used for the nuisance study. None were found to be wearing a hearing aid at the time of testing. Details such as age and gender were recorded as well as the extent of visual impairment. The TRL anechoic chamber was used for these studies. The ¯oor and one wall were arranged to be sound re¯ective to simulate a footway and building facade as shown in Fig. 4. All other surfaces were sound absorptive. A swivel chair was placed in the middle of a circle of loudspeakers of radius 2 m. There were six equi-spaced speakers along the perimeter of the circle with the loudspeaker cone axes at a height of 1.0 m directed towards the centre. Above the six speakers were placed a further six speakers with axes at a height of 1.18 m through which the simulated urban trac noise (referred to in the description of the nuisance study) was played at a constant level. This was intended to partially mask the test signals in order to reproduce to some extent the conditions existing near a junction with trac passing in all directions.
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Fig. 4. Experimental arrangement for sound location and confusion tests.
4.1. Sound location Each listener was seated in the swivel chair and was required to point towards the speaker from which the test sounds were played. The test sounds were played out of the speakers in a balanced order. This was carried out by storing the test sounds in a digital sampler, which was programmed to reproduce the sounds in a predetermined order. This order was balanced so that each sound was reproduced at each speaker the same number of times but the order was suciently complex that the listener would not have been aware of any pattern which would allow prediction of the position of the sound source. The speci®cation of the six selected test sounds are described in Table 3. Prior to the commencement of testing the LAeq level of the masking noise was set at 68 dB at the listening position. The levels of the test sounds were adjusted so that the subjectively judged loudness of the sounds on replay were similar as judged by a panel of listeners and were clearly audible against the masking noise. The maximum A-weighted levels of the test sounds at the listening position ranged from 65.6 to 80.8 dB. The results from 36 visually impaired people were used. Some listeners were totally without sight and others had partial sight. The range of impairment was
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Table 3 Speci®cation of test sounds for sound location study Description
Pulse duration (s)
Pulse separation (s)
Repeat interval (s)
Frequency (kHz)
Wave type
2. Australian 3. Swedish 9. Chirp 12. Long square wave/chirp 13. Short square wave/chirp 15. Short square wave/chirp
0.025 0.400 0.120 0.185/0.120 0.012/0.120 0.012/0.120
± ± ± 0.100 0.014 0.360
1.8 2.0 1.8 1.8 1.8 1.8
1.0 0.9 1.0 1.8/1.0 2.1/1.0 2.1/1.0
Square Square Chirp Square/chirp Square/chirp Square/chirp
considered to cover broadly that found in the population of the visually impaired. During each session the six test sounds were presented six times (once at each speaker position) in the balanced order described above. On hearing the test sound the listener was required to turn the swivel chair so that it faced towards the sound and to point towards the perceived source of sound. Each test sound lasted for about 10 s and there was a further 8 s before the next sound started. An experimenter scored an error if the direction of pointing was greater than 30 from the correct line i.e. to the centre of the speaker from which the sound was reproduced. In this case the direction of pointing would be closer to a speaker other than the correct one. Radial lines placed on the ¯oor were used to aid error assessment. If the listener could not decide on the correct direction he or she was asked to raise their arm. This was also scored as an error. Following this test, listeners were then asked to rate each of the test sounds in turn. For this purpose the sounds were replayed from a single speaker only. As before the test sound lasted for 10 s and following that there was a period of 8 s in which to make the assessment. The rating scale ranged from 1 (``not at all dicult to locate the sound'') to 5 (``very dicult to locate the sound''). 4.2. Sound confusion Tests were also conducted to assess the risk of confusing the test sounds with the standard ``cross'' signal used in the UK (shown in Fig. 1) and a typical lorry reversing alarm which had been recorded for this purpose (slow ``beep-beep'' sound). For evaluating the confusion with the cross signal, each test signal (replayed at the same level as before) was paired with a 10 s sample of the crossing signal leaving a 1 s gap. The maximum A-weighted level of the crossing signal was adjusted to a subjectively assessed realistic level of 76 dB. The pairs were presented in a balanced order through a single loudspeaker. The masking trac noise used in the direction tests was played throughout the test. There was an 8 s gap following the pairs in which the listener was required to rate the risk of confusion. The rating scale ranged from 1 (``not at all likely to confuse the two sounds'') to 5 (``very likely to confuse the two sounds'').
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An identical procedure was used for assessing the risk of confusion with the reversing alarm sound although a dierent balanced order was used. The maximum A-weighted level was adjusted to 77 dB which was considered realistic. For both direction and rating tasks listeners were given a period of practice before the main tests began. 4.3. Results The results from 36 listeners were analysed. Ages ranged from 16 to 82 with a mean age of 44 years, 19 women and 17 men took part. Eight listeners had hearing impairment levels below the referral level [12] but their results were included as both hearing and vision impairments are present in the pedestrian population and especially in the older age groups. For each test sound the number of direction errors made, N, were accumulated over the 36 subjects. Each subject was presented with each test sound six times and so the total number of presentations of a sound was 216. The error rate (expressed as a percentage) was calculated as: Error-rate 100
N 216
Table 4 gives the error rate by test sound. It can be seen that sound 13 (the short square wave signal followed by the chirp) had the lowest error rate of approximately 1.4%. This can be compared with an error rate of nearly ten times that for the chirp alone (i.e. sound 9). This dierence was found to be statistically dierent at the 1% level using the non-parametric sign test [13]. In terms of perceived diculty of locating the sounds, the highest average rating of 2.8 (greatest perceived diculty) was given for the chirp (sound 9) which agrees with the results of the error analysis. The Swedish sound (sound 3) had the lowest rating of 1.4. Table 4 also lists the overall confusion rating for the test sounds based on the average rating over all listeners. In terms of the risk of confusion with the crossing
Table 4 Summary of results Average rating (risk of confusion) Description
Error rate (%)
Average rating (location)
Crossing signal
Reversing alarm
2. Australian 3. Swedish 9. Chirp 12. Long square wave/chirp 13. Short square wave/chirp 15. Short square wave/chirp
7.41 6.94 12.5 5.56 1.39 6.02
2.17 1.42 2.83 1.75 2.61 2.78
1.39 1.58 1.39 1.86 1.50 1.64
1.39 1.61 1.31 1.56 1.28 1.36
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Fig. 5. Time history of sound 13 containing pulse of square waves and chirp.
signal the sounds were rated in a similar way. The average ratings were low ranging from 1.39 to 1.86. The Australian sound (sound 2) and the chirp (sound 9) were judged on average to be least likely to be confused with the crossing sound and the long square wave and chirp (sound 12) was judged most likely to be confused. These dierences were statistically signi®cant at the 5% level or better. Average ratings for confusing the sounds with the reversing alarm were also low and similar ranging from 1.28 to 1.61. In this case the Swedish sound (3) had the highest rating and the short square wave and chirp (sound 13) the lowest rating. However, these dierences were not statistically signi®cant. Testing with further groups of listeners would be required to con®rm this ®nding. 5. Discussion and conclusions Taking all the results into account it can be seen that the short square wave combined with the chirp (sound 13) performed best in the tests. For the subjects tested it had a very low direction error rate though in terms of perceived diculty of location it was not rated the most favourable. However, it can be argued that because this assessment is subjective and was based on listeners' memories of diculties encountered in the previous test, it was not such a reliable indicator as the objective test. Previous studies have indicated that sounds with a relatively wide frequency
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content are more easily localised than narrow band sounds or pure tones [4,7]. For this reason square waves which generate odd harmonics are likely to be more easily localised than pure tones of the same fundamental frequency. The chirp (sound 9) that contained a single sinusoidal frequency of 1 kHz was on average more dicult to locate than the other sounds containing square waves. In addition composite sounds containing pulses with dierent fundamental frequencies should also provide advantages. Fig. 5 gives the time history of this preferred sound signal (sound 13). It can be seen that before the four tone bursts which produces the chirp there is a pulse of length 12 ms which contains square waves at a frequency of 2.1 kHz. There will be a signi®cant harmonic at three times this frequency (6.3 kHz) which will be less than 10 dB down on the fundamental. However, because the loss in hearing sensitivity with age tends to increases with frequency it is likely that for the elderly the higher harmonics would not contribute signi®cantly to localisation. The chirp following the square wave will add a further frequency (1 kHz ) to aid localisation. Sound 13 was least likely to be confused with a typical vehicle reversing alarm and was fourth most likely to be confused with the crossing signal of the six sounds tested. Although these ratings of confusion are low the results need to be con®rmed. Also there may be other sounds in the real environment apart from those tested that may cause confusion. It is planned to mount a street trial in a further phase of the study where visually impaired users of a crossing equipped with the signal can be questioned about the usefulness of the locating sound and about the potential for confusion. It should be noted that in the test of the annoyance of 18 sounds, sound 13 was the third least annoying sound when the judged loudness of the sounds was taken into account. It is possible that sounds containing a chirp are more acceptable as a consequence of producing a more ``natural'' sound than that produced by pulses of constant amplitude tones or pink noise. It is dicult to obtain reliable information about annoyance in an experimental situation and because the eects on annoyance of dierent levels of masking trac were not fully investigated in the present experiment, the results of this part of the study must be viewed as tentative and needing con®rmation. In a further phase of the project it is planned to test sounds in real street environments and to interview both users of the crossing and residents and shop workers in the vicinity. It is expected that tests will take place at a range of sites, both town centre and residential, with dierent levels of trac noise and with a range of window types and conditions (e.g. single and double glazed, closed, partly and fully open). Acknowledgements The study was commissioned by the Mobility Unit of the Department of Environment, Transport and the Regions. The co-operation of the Natural History Museum, London in providing a time history of a typical ®eld cricket chirp is gratefully acknowledged.
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References [1] Edwards MC, Savill TA. Experimental road crossing features for visually-impaired pedestrians. TRL Report 141, Transport Research Laboratory, Crowthorne, 1995. [2] Clark P, Watts GR, Savill TA. Review of audible locating signals used at pedestrian crossings. Trac Engineering and Control, October, 1997. [3] Wightman FL, Kistler DJ. The dominant role of low-frequency interaural time dierences in sound localization. J Acoust Soc Am 1992;91:1648±61. [4] Wightman FL, Kistler DJ. Sound localisation. In: Yost WA, Popper AN, Fay RR, editors. Human psychophysics. New York: Springer-Verlag, 1993. p. 155±92. [5] Blauert J. Spatial hearing: the psychophysics of human sound localization. Cambridge, MA: MIT, 1983. [6] Abel SM, Banerjee PJ. Accuracy versus choice response time in sound localization. Applied Acoustics 1996;49:405±17. [7] Withington D. The quest for better ambulance sirens. Ambulance 1996;11:20±1. [8] Reynolds WJ. Private communication, 1977. [9] British Standards Institution. BS EN 1793-3:1998, Road trac noise reducing devices Ð test method for determining the acoustic performance Ð Part 3: normalized trac noise spectrum. British Standards Institution, Chiswick, 1998. [10] Watts G. Vibration nuisance from road trac Ð results of a 50 site survey. Transport Research Laboratory Report LR 1119. Crowthorne, Berkshire, UK, 1990. [11] International Organization for Standardization. ISO 389-1991: acoustics: standard reference zero for the calibration of pure tone air conduction audiometers, 1991. [12] Health and Safety Executive. Audiometry in industry: a discussion document. London, HMSO, 1978. [13] Siegel S. Nonparametric statistics for the behavioural sciences. London: McGraw Hill, 1956. (p. 68± 75)
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Development of audible locating signals for use at pedestrian crossings G.R. Watts *, N.S. Godfrey, T.A. Savill Transport Research Laboratory, Crowthorne, Berkshire, UK RG45 6AU Received 10 September 1999; received in revised form 8 March 2000
Abstract At light controlled crossings in several countries audible ticking sounds are used to assist visually impaired pedestrians locate the control box so that they can readily request the crossing phase. A literature review has shown that a wide range of sound signals are used for this purpose and it was concluded that further research was required to select a suitable locating sound for possible use in the UK. Laboratory studies were conducted to assist in the selection of suitable locating sounds which will not only be easy to locate but also will not be readily confused with the ``walk'' signal or reversing alarms used by trucks and buses etc. There was also a need to assess the possible annoying eects of such signals especially for residents or workers located close to the crossing. For this purpose a wide range of sounds which have potential as locating signals have been evaluated by panels of sighted listeners so that the more annoying sounds can be screened out at an early stage. # 2000 Transport Research Laboratory. Published by Elsevier Science Ltd. All rights reserved. Keywords: Sound location; Visual impairment; Trac signals
1. Introduction In recent years facilities have been added to light-controlled crossings in the UK to assist their use by visually-impaired pedestrians. These include tactile paving and audible signals to augment the visual green pedestrian signal (or ``walk'' phase). However, although these advances have greatly improved controlled crossings visually impaired people have reported that they sometimes ®nd it dicult to locate the push button control box. A recent study [1] involving 55 visually impaired subjects, * Corresponding author. Tel.: +44-1344-770414; fax: +44-1344-770918. E-mail address: [email protected] (G.R. Watts). 0003-682X/01/$ - see front matter # 2000 Transport Research Laboratory. Published by Elsevier Science Ltd. All rights reserved. PII: S0003-682X(00)00027-X
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G.R. Watts et al. / Applied Acoustics 62 (2001) 15±27
who were asked to evaluate locating sounds at a simulated road crossing, concluded that an additional sound signal would be helpful to assist in the location of this control box. As a result of this recommendation a worldwide literature review was commissioned to gather information about the ecacy of such signals. It was clear that they were in widespread use in Northern Europe and Australia and varied in signal characteristics from simple ticks to relatively long tones. However, the survey highlighted the need for further studies in order to evaluate locating sounds under well controlled conditions so that an appropriate sound could be identi®ed which can be readily perceived by pedestrians and yet not cause confusion or annoyance in the environment [2]. This paper reports on a series of tests that have been carried out by TRL in order to identify a suitable sound signal. The ®rst test examined the perceived nuisance of 18 test sounds using sighted members of the public. Six sounds were selected as being least annoying when their loudness level was taken into account and these were used in a further study. This second test employed visually impaired people to assess the ease with which the sounds could be located and additionally to rate the risks of confusing each of the sounds with the standard UK crossing signal and a typical lorry reversing alarm. 2. Description of locating signals Locating signals are normally operated when the ``wait'' signal is being shown to the pedestrian. A common form of locating signal is a ticking sound or bleep repeated about every 2 s. The signal is typically automatically adjusted by an automatic volume control circuit (AVC) so that it is just audible above trac noise. If correctly adjusted this results in a very low level under quiet conditions so that nuisance to residents is minimised. When the ``wait'' phase changes to ``invitation to cross'' the crossing signal is heard. The crossing signal is typically much louder than the locating signal and tone variations are more rapid. Fig. 1 shows an example of locating and cross signals and gives values of frequencies, pulse duration and repeat interval. The sound signals were based on previous experience of signals used at pedestrian crossings in other countries [2] and sounds which were based on a consideration of the literature on sound localisation [3±7]. Table 1 gives a description of the sounds employed. Included are the sounds used at crossings in Australia and Sweden and sounds containing a wide range of frequencies e.g. pink noise bursts and square wave signals. The chirp sound was based on the sound produced by the ®eld cricket (Gryllus campestris) [8] but its fundamental frequency (carrier) was lowered from 5 to 1 kHz so that it would be more readily perceived by the general population. A fuller description of this sound is given in Section 5 below. Some sounds were composites containing two dierent sounds e.g. sound 12 a square wave signal followed by a chirp which was a combination of sounds similar to the Swedish tone (sound 3) and the chirp (sound 9). Table 1 indicates the lengths and separation between these components together with the frequencies involved and waveform type. The waveforms were either sinusoidal, square wave, chirp or broadband pink noise. The pink
G.R. Watts et al. / Applied Acoustics 62 (2001) 15±27
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Fig. 1. Example of locating and ``cross'' signals.
Table 1 Speci®cation of test sounds for annoyance study Sound
Description
Pulse duration (s)
Pulse separation (s)
Repeat interval (s)
Frequency (kHz)
Wave type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Short high tone Australian Swedish TRL1 double tick TRL2 double tick Short pink pulse Long pink pulse Short pink pulse Chirp ISVR test signal Short square wave/chirp Long square wave/Chirp Short square wave/Chirp TRL2/chirp Short square wave/Chirp Short square wave/Short pink pulse Short square wave/Long pink pulse Short pink pulse/Chirp
0.025 0.025 0.400 0.025 0.025 0.025 0.100 0.025 0.120 0.025 0.012/0.120 0.185/0.120 0.012/0.120 0.025/0.120 0.012/0.120 0.012/0.025
Ð Ð Ð 0.225 0.225 Ð Ð Ð Ð Ð 0.240 0.100 0.014 0.22/0.25 0.360 0.250
1.8 1.8 2.0 2.0 2.0 1.8 1.8 1.8 1.8 1.5 1.8 1.8 1.8 1.8 1.8 1.8
3.1 1.0 0.9 1.0 0.5±15 0.5±15 0.5±15 0.5±15 1.0 1.0 2.1/1.0 1.8/1.0 2.1/1.0 0.5±15/1.0 2.1/1.0 2.1/0.5±15
Sine Square Square Sine Pink (high) Pink (high) Pink (high) Pink (low) Chirp Sine Square/chirp Square/chirp Square/chirp Pink (high)/Chirp Square/chirp Square/pink (high)
0.012/0.100
0.250
1.8
2.1/0.5±15
Square/pink (high)
0.025/0.120
0.250
1.8
0.5±15/1.0
Pink (high)/chirp
17 18
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noise signals were divided into high and low frequency ranges. The high range covered one-third octave bands from 0.5 to 15 kHz and the low range from 0.1 to 5 kHz. 3. Annoyance tests The TRL listening room shown in Fig. 2 was used to obtain evaluations of annoyance. It is furnished as a lounge and has a single glazed window on one wall that opens onto another room that contains the loudspeaker source. The loudspeaker was positioned 2 m from the middle of the window. The various ticking signals were reproduced by this loudspeaker and the participants (2 at a time) evaluated these sounds while seated in the listening room. To obtain a range of conditions, sessions were run with the window open and closed and also with and without simulated masking trac noise. This masking noise was produced by shaping the one-third octave spectrum of pink noise so that the input to the speaker conformed to a typical urban trac noise spectrum [9]. The A-weighted spectrum when plotted in one-third octave bands is approximately triangular in shape with a peak at 1 kHz and is 10 dB down at 160 Hz and 5 kHz. Subjectively this produces a reasonable simulation of constant trac noise. The masking trac noise was reproduced from an additional speaker placed adjacent to the ®rst speaker. The A-weighted level LAeq of the masking noise was adjusted to a constant SPL of 69 dB outside the listening room at a position 1 m from the centre of the closed window. This is approximately the average level of trac noise measured at a wide range of residential sites [10]. The sound signals fed to the loudspeaker were of identical voltage amplitude and they were adjusted so that the 0.4 s tone of the Swedish sound (sound 2) produced an Aweighted level at the same level as the masking trac noise i.e. 69 dB. Because of the dierent duration and frequency content of the signals the maximum output levels at the loudspeaker produced by the test sounds varied. Inside at the listener's ear position the masking noise level LAeq was 33.5 dB with the windows closed and 50.7 dB with the windows opened. Without the masking noise present the maximum Aweighted levels of the sounds varied from 26.3 to 36.5 dB with the window closed
Fig. 2. Experimental arrangement for annoyance and loudness assessments.
G.R. Watts et al. / Applied Acoustics 62 (2001) 15±27
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and from 40.3 to 59.2 dB with the window open. Thus the level of the masking noise under each condition was within the range of the maximum levels of the test sounds. Adult listeners were recruited through advertisements placed in local newspapers. An audiometer calibrated to ISO 389-1991 [11] was used to check hearing ability and it was found that only 2 had hearing diculties likely to interfere with the required assessment task. The criterion used was the ``referral level'' given by age in the Health and Safety Executive's guideline [12]. This indicates a level of hearing loss (without hearing aid) based on age at or below which further medical investigation is recommended. The results from these participants were not used in subsequent analysis and further participants were recruited to reach the target number. Thirty-six listeners took part; 18 listening to the sounds with the window open and 18 with the window closed. During each session the 18 sound signals were ®rst presented without masking trac noise and then with masking noise present. The description of these test sounds is given in Table 1. The presentation was balanced to avoid order eects. Their task was to rate the noise nuisance (assuming they have been exposed for several hours to the sound) under both conditions and then the procedure was repeated but on this occasion the loudness of the sounds were rated. The following 10 point scales were used: Noise annoyance scale: ``Not annoying at all''
0123456789
Loudness scale: ``Very quiet'' 0123456789
``Very annoying''
``Very loud''
For rating loudness an 11±12 s signal duration was used, however, for the more dicult task of assessing annoyance the sound duration was lengthened to 20 s. An 8 s period without the signal present was used for the assessment period. It was considered important to include ratings of loudness since the sounds will almost certainly be perceived to have dierent levels of loudness and this will in¯uence judgements of annoyance. Clearly quiet sounds will tend to be less intrusive and, therefore, less annoying than loud sounds. In a practical installation volume can be controlled so the aim was to identify sounds which are particularly annoying for their level of perceived loudness. 3.1. Results Eighteen men and 18 women took part in the study. Ages ranged from 18 to 73 the average age being 43 years. None of the listeners were wearing a hearing aid. The average annoyance and loudness ratings for the test signals were calculated for each condition i.e. with and without masking noise and with the window open and closed. The overall annoyance and loudness ratings were then calculated by averaging over all listening conditions. Fig. 3 shows the average annoyance score plotted against the average loudness score together with the regression line drawn through
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G.R. Watts et al. / Applied Acoustics 62 (2001) 15±27
Fig. 3. Average annoyance by average loudness.
the origin. Sounds falling below this line are to be preferred since the average annoyance they cause is relatively small when their loudness level is taken into account. As expected loudness was an important determinant of annoyance as can be seen from the excellent relationship between average annoyance and average loudness. The Pearson correlation coecient is 0.94 which is highly signi®cant (0.1% level). Reducing the noise level should reduce the loudness ratings of the sounds and hence limit annoyance. However, the signal needs to be detectable against trac noise so there is a limit to how quiet the signal can be made and yet remain viable as a locating signal. It is important that annoyance is as low as possible, whatever the chosen level of loudness. The six sounds with the lowest and highest overall annoyance to loudness ratio are tabulated in Table 2 together with the range in the ratio calculated for each of the four listening conditions. In general the ratios for the preferred sounds were consistently low across listening conditions although there was some tendency for the ratios to be higher with masking noise present. In general the Swedish and Australian signals, composite sounds ending with a chirp, and the chirp sound alone have the lowest ratios whereas the TRL double tick sound and those containing a pink noise pulse appear to be the most annoying. For example,
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Table 2 Sounds with lowest and highest overall annoyance to loudness ratio Sound number
Description
Average annoyance/average loudness (range of annoyance/loudness ratio)
Lowest 12 3 13 2 9 15
Long square wave/chirp Swedish Short square wave/chirp (short gap) Australian Chirp Short square wave/chirp (long gap)
0.93 0.96 0.98 0.99 1.03 1.04
(0.86±1.04) (0.82±1.03) (0.64±1.25) (0.89±1.14) (0.75±1.12) (0.90±1.12)
Highest 14 6 5 16 17 7
TRL2/chirp Short pink pulse TRL2 double tick Short square wave/short pink pulse Short square wave/long pink pulse Long pink pulse
1.41 1.33 1.27 1.26 1.21 1.21
(1.19±1.95) (1.08±1.79) (1.07±1.90) (1.13±1.55) (1.06±1.56) (1.04±1.42)
the result for the TRL2 chirp indicates a 50% higher degree of average annoyance relative to the loudness than the Swedish chirp. Based on these tests the six sounds with the lowest ratios were taken through to the next series of tests. 4. Sound location and confusion tests These further tests employed visually impaired people to assess the ease with which the sounds could be located and additionally to rate the risk of confusing the sounds with the standard UK crossing signal and a typical lorry reversing alarm. Thirty-six listeners were recruited from local groups for visually impaired people. Listeners were required to undergo an audiometric test on each ear identical to that used for the nuisance study. None were found to be wearing a hearing aid at the time of testing. Details such as age and gender were recorded as well as the extent of visual impairment. The TRL anechoic chamber was used for these studies. The ¯oor and one wall were arranged to be sound re¯ective to simulate a footway and building facade as shown in Fig. 4. All other surfaces were sound absorptive. A swivel chair was placed in the middle of a circle of loudspeakers of radius 2 m. There were six equi-spaced speakers along the perimeter of the circle with the loudspeaker cone axes at a height of 1.0 m directed towards the centre. Above the six speakers were placed a further six speakers with axes at a height of 1.18 m through which the simulated urban trac noise (referred to in the description of the nuisance study) was played at a constant level. This was intended to partially mask the test signals in order to reproduce to some extent the conditions existing near a junction with trac passing in all directions.
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Fig. 4. Experimental arrangement for sound location and confusion tests.
4.1. Sound location Each listener was seated in the swivel chair and was required to point towards the speaker from which the test sounds were played. The test sounds were played out of the speakers in a balanced order. This was carried out by storing the test sounds in a digital sampler, which was programmed to reproduce the sounds in a predetermined order. This order was balanced so that each sound was reproduced at each speaker the same number of times but the order was suciently complex that the listener would not have been aware of any pattern which would allow prediction of the position of the sound source. The speci®cation of the six selected test sounds are described in Table 3. Prior to the commencement of testing the LAeq level of the masking noise was set at 68 dB at the listening position. The levels of the test sounds were adjusted so that the subjectively judged loudness of the sounds on replay were similar as judged by a panel of listeners and were clearly audible against the masking noise. The maximum A-weighted levels of the test sounds at the listening position ranged from 65.6 to 80.8 dB. The results from 36 visually impaired people were used. Some listeners were totally without sight and others had partial sight. The range of impairment was
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Table 3 Speci®cation of test sounds for sound location study Description
Pulse duration (s)
Pulse separation (s)
Repeat interval (s)
Frequency (kHz)
Wave type
2. Australian 3. Swedish 9. Chirp 12. Long square wave/chirp 13. Short square wave/chirp 15. Short square wave/chirp
0.025 0.400 0.120 0.185/0.120 0.012/0.120 0.012/0.120
± ± ± 0.100 0.014 0.360
1.8 2.0 1.8 1.8 1.8 1.8
1.0 0.9 1.0 1.8/1.0 2.1/1.0 2.1/1.0
Square Square Chirp Square/chirp Square/chirp Square/chirp
considered to cover broadly that found in the population of the visually impaired. During each session the six test sounds were presented six times (once at each speaker position) in the balanced order described above. On hearing the test sound the listener was required to turn the swivel chair so that it faced towards the sound and to point towards the perceived source of sound. Each test sound lasted for about 10 s and there was a further 8 s before the next sound started. An experimenter scored an error if the direction of pointing was greater than 30 from the correct line i.e. to the centre of the speaker from which the sound was reproduced. In this case the direction of pointing would be closer to a speaker other than the correct one. Radial lines placed on the ¯oor were used to aid error assessment. If the listener could not decide on the correct direction he or she was asked to raise their arm. This was also scored as an error. Following this test, listeners were then asked to rate each of the test sounds in turn. For this purpose the sounds were replayed from a single speaker only. As before the test sound lasted for 10 s and following that there was a period of 8 s in which to make the assessment. The rating scale ranged from 1 (``not at all dicult to locate the sound'') to 5 (``very dicult to locate the sound''). 4.2. Sound confusion Tests were also conducted to assess the risk of confusing the test sounds with the standard ``cross'' signal used in the UK (shown in Fig. 1) and a typical lorry reversing alarm which had been recorded for this purpose (slow ``beep-beep'' sound). For evaluating the confusion with the cross signal, each test signal (replayed at the same level as before) was paired with a 10 s sample of the crossing signal leaving a 1 s gap. The maximum A-weighted level of the crossing signal was adjusted to a subjectively assessed realistic level of 76 dB. The pairs were presented in a balanced order through a single loudspeaker. The masking trac noise used in the direction tests was played throughout the test. There was an 8 s gap following the pairs in which the listener was required to rate the risk of confusion. The rating scale ranged from 1 (``not at all likely to confuse the two sounds'') to 5 (``very likely to confuse the two sounds'').
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An identical procedure was used for assessing the risk of confusion with the reversing alarm sound although a dierent balanced order was used. The maximum A-weighted level was adjusted to 77 dB which was considered realistic. For both direction and rating tasks listeners were given a period of practice before the main tests began. 4.3. Results The results from 36 listeners were analysed. Ages ranged from 16 to 82 with a mean age of 44 years, 19 women and 17 men took part. Eight listeners had hearing impairment levels below the referral level [12] but their results were included as both hearing and vision impairments are present in the pedestrian population and especially in the older age groups. For each test sound the number of direction errors made, N, were accumulated over the 36 subjects. Each subject was presented with each test sound six times and so the total number of presentations of a sound was 216. The error rate (expressed as a percentage) was calculated as: Error-rate 100
N 216
Table 4 gives the error rate by test sound. It can be seen that sound 13 (the short square wave signal followed by the chirp) had the lowest error rate of approximately 1.4%. This can be compared with an error rate of nearly ten times that for the chirp alone (i.e. sound 9). This dierence was found to be statistically dierent at the 1% level using the non-parametric sign test [13]. In terms of perceived diculty of locating the sounds, the highest average rating of 2.8 (greatest perceived diculty) was given for the chirp (sound 9) which agrees with the results of the error analysis. The Swedish sound (sound 3) had the lowest rating of 1.4. Table 4 also lists the overall confusion rating for the test sounds based on the average rating over all listeners. In terms of the risk of confusion with the crossing
Table 4 Summary of results Average rating (risk of confusion) Description
Error rate (%)
Average rating (location)
Crossing signal
Reversing alarm
2. Australian 3. Swedish 9. Chirp 12. Long square wave/chirp 13. Short square wave/chirp 15. Short square wave/chirp
7.41 6.94 12.5 5.56 1.39 6.02
2.17 1.42 2.83 1.75 2.61 2.78
1.39 1.58 1.39 1.86 1.50 1.64
1.39 1.61 1.31 1.56 1.28 1.36
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Fig. 5. Time history of sound 13 containing pulse of square waves and chirp.
signal the sounds were rated in a similar way. The average ratings were low ranging from 1.39 to 1.86. The Australian sound (sound 2) and the chirp (sound 9) were judged on average to be least likely to be confused with the crossing sound and the long square wave and chirp (sound 12) was judged most likely to be confused. These dierences were statistically signi®cant at the 5% level or better. Average ratings for confusing the sounds with the reversing alarm were also low and similar ranging from 1.28 to 1.61. In this case the Swedish sound (3) had the highest rating and the short square wave and chirp (sound 13) the lowest rating. However, these dierences were not statistically signi®cant. Testing with further groups of listeners would be required to con®rm this ®nding. 5. Discussion and conclusions Taking all the results into account it can be seen that the short square wave combined with the chirp (sound 13) performed best in the tests. For the subjects tested it had a very low direction error rate though in terms of perceived diculty of location it was not rated the most favourable. However, it can be argued that because this assessment is subjective and was based on listeners' memories of diculties encountered in the previous test, it was not such a reliable indicator as the objective test. Previous studies have indicated that sounds with a relatively wide frequency
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G.R. Watts et al. / Applied Acoustics 62 (2001) 15±27
content are more easily localised than narrow band sounds or pure tones [4,7]. For this reason square waves which generate odd harmonics are likely to be more easily localised than pure tones of the same fundamental frequency. The chirp (sound 9) that contained a single sinusoidal frequency of 1 kHz was on average more dicult to locate than the other sounds containing square waves. In addition composite sounds containing pulses with dierent fundamental frequencies should also provide advantages. Fig. 5 gives the time history of this preferred sound signal (sound 13). It can be seen that before the four tone bursts which produces the chirp there is a pulse of length 12 ms which contains square waves at a frequency of 2.1 kHz. There will be a signi®cant harmonic at three times this frequency (6.3 kHz) which will be less than 10 dB down on the fundamental. However, because the loss in hearing sensitivity with age tends to increases with frequency it is likely that for the elderly the higher harmonics would not contribute signi®cantly to localisation. The chirp following the square wave will add a further frequency (1 kHz ) to aid localisation. Sound 13 was least likely to be confused with a typical vehicle reversing alarm and was fourth most likely to be confused with the crossing signal of the six sounds tested. Although these ratings of confusion are low the results need to be con®rmed. Also there may be other sounds in the real environment apart from those tested that may cause confusion. It is planned to mount a street trial in a further phase of the study where visually impaired users of a crossing equipped with the signal can be questioned about the usefulness of the locating sound and about the potential for confusion. It should be noted that in the test of the annoyance of 18 sounds, sound 13 was the third least annoying sound when the judged loudness of the sounds was taken into account. It is possible that sounds containing a chirp are more acceptable as a consequence of producing a more ``natural'' sound than that produced by pulses of constant amplitude tones or pink noise. It is dicult to obtain reliable information about annoyance in an experimental situation and because the eects on annoyance of dierent levels of masking trac were not fully investigated in the present experiment, the results of this part of the study must be viewed as tentative and needing con®rmation. In a further phase of the project it is planned to test sounds in real street environments and to interview both users of the crossing and residents and shop workers in the vicinity. It is expected that tests will take place at a range of sites, both town centre and residential, with dierent levels of trac noise and with a range of window types and conditions (e.g. single and double glazed, closed, partly and fully open). Acknowledgements The study was commissioned by the Mobility Unit of the Department of Environment, Transport and the Regions. The co-operation of the Natural History Museum, London in providing a time history of a typical ®eld cricket chirp is gratefully acknowledged.
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References [1] Edwards MC, Savill TA. Experimental road crossing features for visually-impaired pedestrians. TRL Report 141, Transport Research Laboratory, Crowthorne, 1995. [2] Clark P, Watts GR, Savill TA. Review of audible locating signals used at pedestrian crossings. Trac Engineering and Control, October, 1997. [3] Wightman FL, Kistler DJ. The dominant role of low-frequency interaural time dierences in sound localization. J Acoust Soc Am 1992;91:1648±61. [4] Wightman FL, Kistler DJ. Sound localisation. In: Yost WA, Popper AN, Fay RR, editors. Human psychophysics. New York: Springer-Verlag, 1993. p. 155±92. [5] Blauert J. Spatial hearing: the psychophysics of human sound localization. Cambridge, MA: MIT, 1983. [6] Abel SM, Banerjee PJ. Accuracy versus choice response time in sound localization. Applied Acoustics 1996;49:405±17. [7] Withington D. The quest for better ambulance sirens. Ambulance 1996;11:20±1. [8] Reynolds WJ. Private communication, 1977. [9] British Standards Institution. BS EN 1793-3:1998, Road trac noise reducing devices Ð test method for determining the acoustic performance Ð Part 3: normalized trac noise spectrum. British Standards Institution, Chiswick, 1998. [10] Watts G. Vibration nuisance from road trac Ð results of a 50 site survey. Transport Research Laboratory Report LR 1119. Crowthorne, Berkshire, UK, 1990. [11] International Organization for Standardization. ISO 389-1991: acoustics: standard reference zero for the calibration of pure tone air conduction audiometers, 1991. [12] Health and Safety Executive. Audiometry in industry: a discussion document. London, HMSO, 1978. [13] Siegel S. Nonparametric statistics for the behavioural sciences. London: McGraw Hill, 1956. (p. 68± 75)