Botteldooren, Dick

De Boeck, E

Dragonetti, Laurent

Thomas, Pieter

Van Renterghem, Timothy

English

Ghent University Academic Bibliography

Creating an urban street reverberation mapP. Thomas, E. De Boeck, L. Dragonetti, T. Van Renterghem and D. BotteldoorenPieter.Thomas@intec.ugent.beDepartment of Information Technology (INTEC), Ghent University, St. Pietersnieuwstraat 41,B-9000 Gent, BelgiumSummaryReverberation increases sound levels in streets and aversely influences the perceived sound fieldquality. To assess this, it is studied if "reverberation maps" could be useful. Knowledge on streetreverberation could be used as a valuable alternative to physical methods such as ray tracing, beamtracing etc. to obtain an estimate of expected noise levels and sound quality indicators. But also onitself, a map showing the reverberation times of the streets could be very informative.In this work a reverberation map of some neighborhoods in Ghent, Belgium, is produced. To enablefast determination of EDT and T30 on different positions in a multitude of streets, a dedicatedmeasurement set-up is mounted on the roof of a car. It is investigated to what extend interpolationbetween measurement points and extrapolation to other areas is possible on the basis of the geometryand architectural aspects of the street and surrounding buildings, features that are available fromgeographical information layers.PACS no. 43.50.Rq, 43.55.-n1. IntroductionLarge percentages of the urban population areexposed to high traffic noise levels, causingannoyance[1], sleep disturbance[2] and other negativehealth impacts[3].To estimate noise exposure, noise maps are a usefultool. Typically, noise maps are estimates of thelong-term averaged noise levels and are commonlybased on numerical calculations using geometricalacoustics. In a highly complex city lay-out, a highnumber of sound reflections occur, which have tobe taken into account when accurately predictingthe noise level. However, the maximum number ofreflections is chosen to be low and fixed to limitcomputation times. In this way accuracy is lost asthe noise level is only calculated based on the firstfew reflections. To have an estimate of the influenceof later reflections, knowledge of the reverberationtime EDT and T30 could be of help. Furthermore,as these parameters describe the sound energydecay, a map depicting the reverberation time in thecity-streets, combined with knowledge on the trafficpass-by and noise sources in those streets, is mostuseful to identify problem cases.Other parameters related to room acoustics couldalso be used. Examples are center time Ts, which(c) European Acoustics Associationindicates the point of gravity of the impulse responseenergy, and clarity C80, which gives the ratio of earlysound energy to reverberant sound energy[4].Measurements of such parameters are based on anin-situ measurement of the impulse response[5]. Inorder to obtain a high number of impulse responsemeasurements in a reasonable time, a dedicatedmeasurement set-up is proposed, which allows fastand mobile measurements of the impulse responseand reverberation time in streets. However, eventhe measurement of the impulse responses withthe mobile measurement system is time consuming.It thus remains difficult to measure the impulseresponse of a high number of locations in a multitudeof streets, prior to the construction of a noise map.Therefore it will be checked if a correlation can befound between different geometrical and architecturalproperties and acoustical parameters.2. Measurement set-upThe determination of the acoustic parameters is basedon in-situ measurements of the impulse response. Forthese impulse response measurements an omnidirec-tional loudspeaker is needed, which reproduces an ex-citation signal. The response to this signal is thenrecorded by one or more omnidirectional microphones.As these measurements have to be conducted on amultitude of different positions, while the urban traf-fic should be disturbed as little as possible, the set-upP. Thomas et al., Creating an urban street reverberation mapFORUM ACUSTICUM 201127. June - 1. July, AalborgFigure 1. Mobile measurement set-up. An omnidirectionalspeaker is mounted on the roof of a car. An omnidirec-tional microphone Type 4189 of Bruël & Kjær is mountedin front and behind the speaker at 2.5m distance.should enable both mobile and fast measurements.For this reason it is decided to build the set-up withomnidirectional speaker and microphones on the roofof a car. The loudspeaker is placed near the middleof the roof, while two Bruël & Kjær omnidirectionalmicrophones Type 4189 are placed at a distance of2.5m at both sides of the speaker (fig. 1). In thisway, two impulse response measurements can be con-ducted simultaneously. The excitation signal used forthe measurements is an exponential sweep of 30 s andcovers the 63Hz to 16 kHz octave band. With thistest signal, each frequency is excitated at a differentmoment in time, while each octave band contains anequal amount of energy. This measurement cycle is re-peated minimum three times at each location. In thisway disturbed measurements can be filtered out.While performing the measurements, the position inthe street is logged and architectural aspects arenoted. The street geometry (length, width and height)was found in GIS databases.2.1. Signal processingFrom the recorded sweep the impulse response can beextracted by spectral division with the emitted sweep.The resulting impulse response is analyzed in octavebands and for each band filtered impulse response alevelfunction is calculated, describing the energy de-cay in function of time. Since the measurements ofthe impulse response are conducted in an urban en-vironment, they are likely to be contaminated withnoise. This results in a lower signal-to-noise ratio andthus limits the decay of the sound produced by theloudspeaker (and levelfunction) in the street. Choos-ing an appropriate test signal such as an exponen-tial sweep, high emission levels and repetitions canpartly counter this. Other techniques to lower the in-fluence of the background noise include subtraction ofa mean squared average of the noise from the band fil-tered response and truncation of the impulse response,of which the truncation point is determined as thecrossing point between the decaying slope and aver-aged noise level of the time-averaged band response[6].From the processed band levelfunctions the reverber-ation time can then be calculated by using the slope ofthe best fitted line between [0 dB,−10 dB] for EDTand [−5 dB,−35 dB] for T30.2.2. Influence of the carThe measurements of the impulse response and re-lated parameters are valid only if the influence ofthe car and set-up on the results can be ignored.Therefore, reference measurements are conducted inan open rural area with no reflecting boundaries, ex-cept for the soil. Five successive measurement cyclesare conducted and the reverberation time is analyzed,which should be significantly lower than the reverber-ation measured in the streets. Fig. 2 depicts the re-sulting mean reverberation time EDT and T30. Theseresults are calculated from an average of five measure-ment cycles. A comparison is made between resultsfrom the microphone at the back and front. As can benoted from fig. 2(a), the EDT is lower than 0.07 s atoctave bands higher than the 63Hz octave band. Thestandard deviation from the mean value is negligibleand the results from both the back and front micro-phone are alike. However, for the 63Hz octave band,the deviation and difference between microphones ishigher. Investigation of the used loudspeaker learnsthat these frequencies are not accurately reproduced.This octave band will thus be omitted.For the T30 (fig. 2(b)), somewhat higher values arefound. However, when comparing to street measure-ments, these are still sufficiently limited. The highestvalue (at the 125Hz octave band) is near 0.26 s. Thestandard deviation is larger than for the EDT , butstill acceptable. Some difference in T30 between frontand back can be observed, but the difference betweenboth microphone positions is limited (Δmax ≈ 0.09 sat 500Hz).In general the influence of the car and measurementset-up on the resulting reverberation time is negligiblecompared to reverberation times measured in moststreet configurations. However, more open streets,with gaps between buildings, could have very low re-verberation. Care should be taken to interpret suchresults, so a selection procedure will be built in todetect reverberation measurements which are not sig-nificantly different from the reference measurements.3. Urban street reverberation mea-surementsIn order to produce a reverberation map of a city andinvestigate whether the reverberation can be corre-lated to architectural and geometrical parameters, thereverberation is measured at 77 points in Ghent, Bel-gium. Streets of various width, building height, lengthP. Thomas et al., Creating an urban street reverberation mapFORUM ACUSTICUM 201127. June - 1. July, Aalborg63 Hz 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz 8 kHz 16 kHz00.10.20.30.40.50.60.70.80.910.150.06 0.05 0.04 0.02 0.01 0.01 0.01 0.010.30.06 0.060.03 0.03 0.03 0.01 0.01 0.01octave−bandEDT (s)frontback(a) EDT63 Hz 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz 8 kHz 16 kHz00.10.20.30.40.50.60.70.80.910.160.260.140.240.13 0.13 0.14 0.13 0.120.310.20.120.150.180.15 0.16 0.150.12octave−bandT30 (s)frontback(b) T30Figure 2. Measurement of the mean EDT and T30 for the reference situation, based on five measurement cycles. In bluethe results when using the front microphone, in red the results for the microphone at the back. The standard deviationfrom the mean value is also indicated.Figure 3. Geographical map indicating the different locations (77) measured in the city of Ghent. The marker at the leftindicates the reference measurement position. The colors indicate different measurement dates.and architectural styles were selected and for each ofthe locations, the reverberation EDT and T30 is mea-sured. Fig. 3 shows a map of Ghent with the differentmeasurement positions and also the location of thereference measurement position.Fig. 4 resp. fig. 5 shows a reverberation map of theEDT resp. T30. A difference is observed in the courseof EDT and T30. For the EDT a lower value is mea-sured in the city outskirts. Streets in these areas aretypically wider and detached housing is more com-mon. In the city-center, the reverberation increases.T30 is a less sensitive parameter than EDT , as lit-tle differences are observed between measurementsat center areas and outskirts. These maps howeveronly depict the reverberation at discrete measurementpoints within streets.P. Thomas et al., Creating an urban street reverberation mapFORUM ACUSTICUM 201127. June - 1. July, AalborgLegend0.07 - 0.1 s0.1 - 0.2 s0.2 - 0.3 s0.3 - 0.4 s0.4 - 0.5 s0.5 - 0.6 s0.6 - 0.7 s(a) EDT at 125Hz octave band.Legend0.07 - 0.1 s0.1 - 0.2 s0.2 - 0.3 s0.3 - 0.4 s0.4 - 0.5 s0.5 - 0.6 s0.6 - 0.7 s(b) EDT at 1 kHz octave band.Figure 4. Reverberation map of Ghent for EDT , measured at the 125Hz and 1 kHz octave band.Legend0.6 - 0.75 s0.75 - 0.90 s0.90 - 1.05 s1.05 - 1.20 s1.20 - 1.35 s1.35 - 1.50 s1.50 - 1.65 s(a) T30 at 125Hz octave band.Legend0.6 - 0.75 s0.75 - 0.90 s0.90 - 1.05 s1.05 - 1.20 s1.20 - 1.35 s1.35 - 1.50 s1.50 - 1.65 s(b) T30 at 1 kHz octave band.Figure 5. Reverberation map of Ghent for T30, measured at the 125Hz and 1 kHz octave bands.3.1. Correlation analysisRather then measuring the reverberation at a hugenumber of discrete locations, a time consuming work,it can be investigated to what extent the reverberationtime could be calculated in function of the geometri-cal and architectural parameters. If this would be thecase, a reverberation map with higher resolution couldbe created, based on simple parameters. Therefore,the reverberation time is plotted in function of streetwidth, height, length and height-width ratio. In fig.6 the correlation between EDT and the width resp.height-width ratio is shown for the 125Hz and 1 kHzP. Thomas et al., Creating an urban street reverberation mapFORUM ACUSTICUM 201127. June - 1. July, Aalborg0 5 10 15 20 2500.10.20.30.40.50.60.70.80.91widthEDT (s) @ 125 Hzy = −0.015x + 0.376R = −0.574p < 0.01(a) Influence of width at 125Hz octave band.0 5 10 15 20 2500.10.20.30.40.50.60.70.80.91widthEDT (s) @ 1 kHzy = −0.022x + 0.579R = −0.77p < 0.01(b) Influence of width at 1 kHz octave band.0 0.5 1 1.5 200.10.20.30.40.50.60.70.80.91height−width ratioEDT (s) @ 125 Hzy = 0.158x + 0.08R = 0.47p < 0.01(c) Influence of height-width ratio at 125Hz octave band.0 0.5 1 1.5 200.10.20.30.40.50.60.70.80.91height−width ratioEDT (s) @ 1 kHzy = 0.251x + 0.121R = 0.663p < 0.01(d) Influence of height-width ratio at 1 kHz octave band.Figure 6. Correlation analysis of EDT at the 125Hz and 1 kHz octave bands in function of width and height-widthratio. The fitted line is indicated, as well as the correlation coefficient and significance. The color scale indicates theamount of facade roughness, with yellow meaning many irregularities and red meaning little.octave band. It is seen that EDT correlates negativelywith the width. As the street width increases, energyis less contained in the street, so the EDT decreases.The EDT correlates positively with the height-widthratio. The higher this ratio, the more the street can beseen as a street canyon, the more the sound energy re-mains in the street. These correlations become higheras the frequency rises. The measurements show thatEDT is not correlated with the length of the street(not depicted).In fig. 7 the correlation between T30 and the widthresp. height-width ratio is given. The figures show asmall positive correlation with width and a small neg-ative correlation with height-width ratio. This corre-lation is however very small and furthermore not sig-nificant in most cases (p > 0.05). This is the case forall octave bands.In both figures the color of the points indicates theamount of facade roughness, with yellow meaningmany facade irregularities and ornaments and redmeaning only little. Quantification of the level of ir-regularities was based on visual evaluation of the fa-cades. At first sight, no obvious correlation is seenbetween the amount of roughness and both EDT andT30. However, further research should be conductedto incorporate a more thoroughly evaluation of thearchitectural details.4. ConclusionIn this paper a new method is designed to measure thereverberation time and other impulse response relatedparameters in a fast and mobile way. By putting themeasurement equipment on the roof of a car, streetP. Thomas et al., Creating an urban street reverberation mapFORUM ACUSTICUM 201127. June - 1. July, Aalborg0 5 10 15 20 2500.20.40.60.811.21.41.61.82widthT30 (s) @ 125 Hzy = 0.016x + 0.89R = 0.284p < 0.05(a) Influence of width at 125Hz octave band.0 5 10 15 20 2500.20.40.60.811.21.41.61.82widthT30 (s) @ 1 kHzy = 0.005x + 1.05R = 0.141p > 0.01(b) Influence of width at 1 kHz octave band.0 0.5 1 1.5 200.20.40.60.811.21.41.61.82height−width ratioT30 (s) @ 125 Hzy = −0.154x + 1.2R = −0.206p > 0.01(c) Influence of height-width ratio at 125Hz octave band.0 0.5 1 1.5 200.20.40.60.811.21.41.61.82height−width ratioT30 (s) @ 1 kHzy = −0.031x + 1.132R = −0.066p > 0.01(d) Influence of height-width ratio at 1 kHz octave band.Figure 7. Correlation analysis of T30 at the 125Hz and 1 kHz octave bands in function of width and height-width ratio.The fitted line is indicated, as well as the correlation coefficient and significance. The color scale indicates the amountof facade roughness, with yellow meaning many irregularities and red meaning little.reverberation measurements have been conducted in77 streets in the city of Ghent, Belgium. The result-ing EDT and T30 is analyzed in octave bands andplotted on a map to identify regions with low or highreverberation. In districts further from the city-centerthe reverberation time (EDT ) is typically lower dueto more openness of the streets.In an attempt to predict the reverberation in the city,a correlation analysis is conducted. It is found thatthe EDT correlates highly with the street width andheight-width ratio. However no significant correlationbetween the geometrical parameters and T30 can befound.In following work other acoustical parameters suchas center time Ts will be investigated and further re-search for the influence of facade geometry will beincluded.References[1] D. Ouis: Annoyance from road traffic noise: a review.J. of Env. Psych. 21 (2001) 101-102.[2] B. Griefahn, A. Marks, S. Robens: Noise emitted fromroad, rail and air traffic and their effects on sleep. J.Sound Vib. 295 (2006) 129-140.[3] L. Barregard, E. Bonde and E. Öhrström: Risk of hy-pertension from exposure to road traffic noise in apopulation-based sample. Occup. Environ. Med. 66(2009) 410-415.[4] D. Paini, A. Gade, J. Rindel: Is reverberation timeadequate for testing the acoustical quality of unroofedauditoriums? Proc IoA 28 (2006) 66-73[5] Acoustics - measurement of room acoustic parame-ters - part 1: performance spaces. ISO 3382-1:2009(E)(2009)[6] A. Lundeby, T. Vigran, H. Bietz, M. Vorländer: Uncer-tainties of measurements in room acoustics. Acustica81 (1995) 344-355.

Creating an urban street reverberation map

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Creating an urban street reverberation map

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