Statistical analysis of noise levels in urban areas

Applied Acoustics 34 {1991) 227-247

Statistical Analysis of Noise Levels in Urban Areas

A. Garcia & L. J. Faus

Laboratory of Acoustics, Applied Physics Department, University of Valencia, Valencia, Spain

{Received 3 December 1990; accepted 15 May 199l)

A BSTRA CT

Environmental noise measurements hare been carried out during recent years #t different cities and locations of Spain. The noise levels have been conthtuously sampled over 24 h periods using a noise level analyzer. The data contained b7 this paper represent a total of 4200 measurement hours. All the information has been used to investigate the time patterns of the noise levels under a wide range of different conditions and to study the relationships between several noise descriptors #z urban areas.

1 INTRODUCTION

Community noise surveys have been carried out in numerous countries over the past 30 years. 1 The type of noise measurements have been dependent on the purposes for which the surveys have been conducted: evaluation of noise exposure on urban populations, comparison of current noise levels with values specified in regulations, assessment of the impact of noise from planned developments, etc.

Most of the research carried out to date on environmental noise has been concerned with high levels of noise produced by specific sources such as road traffic and aircraft. A multiplicity of noise descriptors have been proposed in these studies to achieve the best correlation between noise exposure and human response. 2 However, more recently it has become clear that many of such descriptors are highly correlated with each other and that a single descriptor, for example Leq (24 h), could perhaps be used for predicting the

227 Applied Acoustics 0003-682X/91/$03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain

228 A. Garcia, L. J. FalLs

mean community response to a wide range of different sources without a significant loss in prediction accuracy.

The rapid increase in the number of motor vehicles in developed countries has led to a continuing increase in the noise levels due to road traffic. Many noise surveys conducted in order to investigate the magnitude of the problem in various cities through the world have revealed that road traffic is typically the largest contributor to recorded sound levels and the most important source of annoyance in metropolitan communities. 3

In general, the noise levels measured in urban areas show a wide temporal and geographical variation. The distribution of instantaneous noise levels measured in a given location can be explained as the sum of two different components: a distant process with a relatively low mean value and variance, which comprises an accumulation of many noise sources and it is represented by the lower percentiles, and a local process with a higher mean value and variance, related to a smaller number of noise sources relatively close to the measuring point, and responsible for the upper percentiles of the distribution. The relative contribution of both processes to the observed noise levels is different during day and night periods.

The best way to deal, in a consistent manner, with the temporal noise fluctuations is to make a statistical analysis of the time record of the noise. Ideally, such analysis would be made in bands of frequency and would distinguish between day and night, working days and weekends, winter and summer, etc. In practice, however, some concessions to economy are always necessary. The first simplification is usually to consider only the A-weighted sound levels, a step for which there is ample justification. Subsequent economy steps may involve a drastic reduction of the observation time periods (often restricted to diurnal periods) or the use of measurement equipment less complicated than statistical analysis gear (such as integrating sound level meters). The information obtained with these simple strategies is usually valid for general purpose studies (for instance, to measure the noise map of a city), but it is not sufficient for more specialized objectives (for instance, to provide a complete understanding of the existing sound environment in urban areas as a realistic basis for measuring an37 improvement or deterioration in the future or assessing the likely impact of new noise sources in a given neighbourhood).

In many respects, the increase of noise pollution in Spain during the last decades has followed a similar trend to other technologically advanced countries. In particular, it has been estimated that about 23% of the Spanish population is exposed to Leq > 65 dBA, being the second country in the 'world noise ranking' after Japan. '~ The noise measurements carried out in Madrid, Barcelona and Valencia have revealed that the diurnal equivalent sound levels in these cities are rather high, with mean values about

Statistical analysis of noise levels in urban areas 229

70dBA. 5-T On the other hand, the measurements carried out in some Spanish medium-sized cities have revealed that their noise climates are not significantly different to the biggest cities, s'9

As a further contribution to a greater understanding of this problem, the variation of noise levels over 24 h periods has been measured in a number of selected locations in several Spanish cities. Most of the measuring points are dominated by road traffic noise. The present survey covers a total of 4200 h of noise level recordings (175 complete days) carried out from 1980 to 1990. This paper describes the main results of the global analysis of all data obtained in these measurements.

2 NOISE LEVEL MEASUREMENTS

A-weighted noise levels have been measured continuously over 24 h periods in 50 different selected locations of seven Spanish cities: Valencia (population 710 000), Pamplona (185 000), Alcoy (66 000), Gandia (51 000}, Playa de Gandia (40 000), Burjassot (36 000) and Pobla de Vallbona (8000). About half of the measuring stations are located in quite noisy urban areas (Leq(24 h) > 65 dBA); the remaining are located in quieter areas (Leq(24 h) < 65 dBA).

All measurements have been carried out using a l /2in (12-5mm) condenser microphone (Brfiel & Kjaer, Noerum, Denmark, BK4165), a noise level analyzer (BK4426) and an alphanumeric printer (BK2312). In all cases, the instantaneous sound levels were sampled every 0-1 s, resulting in a total count of 36000 samples per hour. All hourly values of LI, L10, L50, L90, L99 and Leq were obtained through complete 24 h periods.

For practical reasons, the microphone was not mounted at street level, but in the balcony of a dwelling (generally, the homes of relatives or friends). About 42% of the measuring stations were located at the level of ground, first and second floors, and the remaining 58°/'o at the level of third floor and above. This condition should not be too significant, since, in many urban areas, the noise produced by street traffic is confined by the buildings just as in a closed room, and the attenuation rate of noise level with elevation above the ground is usually very low. In particular, when the 'canyon effect' is present (areas with high rise buildings, a common situation in most Spanish cities), it has been found that the sound attenuation is only about 0-5 dB for each 30m of elevation. 1°

The authors' present files include a total of 25 200 data (six different hourly noise parameters for 175 complete days of measurements). All this information has now been processed in a Tandon PCA computer using specific software developed in the authors' laboratory to obtain the

230 A. Garcia, L. Z Faus

variation of noise levels with sampling time and sampling days, the statistical distributions of instantaneous noise levels, and the relationships between different noise descriptors and equivalent sound level values.

3 RESULTS A N D DISCUSSION

The noise levels measured in the present survey show a wide dispersion, reflecting the considerable temporal and geographical variation of ambient noise in urban areas. Table I gives a summary of the values of different hourly noise levels obtained in all measurements.

Although these results cannot be considered as fully representative of the general sound environment existing in Spanish urban areas (as the measuring locations included in this survey were not strictly random selected), they show clearly that a large proportion of the Spanish citizens are exposed to high noise levels which interfere with a number of vital daily activities and affect their quality of life. ~t

3.1 Variation of noise levels with sampling t ime

Figures 1-3 show three representative examples of the results obtained in the 24 h measurements in urban areas with high, medium and low traffic volume. In general, the time patterns of the noise levels measured in a given location are related to the corresponding 'activity level' and, in particular, to the traffic volume variations.

The relationship between traffic volume and noise level is so close (as illustrated in Fig. 4) that all the efforts intended to evaluate theoretically the noise levels produced by road traffic are based in this variable. For example,

TABLE I Hourly Values of L1, LI0, L50, L90, L99 and Leq Obtained in All

Measurements Carried out in the Present Survey a

Descriptor Maximum Minimum Average SD

LI 96.3 28.3 72-3 7-4 LI0 90.0 26.7 65.3 7.7 L50 77.8 24.2 58.6 8.7 L90 73-3 22.8 53.3 8.9 L99 71-3 21-8 50-1 8-8 Leq 83-7 26.8 63.0 7.4

° 4200 data corresponding to 175 complete days in 50 different locations of seven Spanish cities covering a wide range of typical urban conditions.

Statistical analysis o f noise let, els in urban areas 231

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Variation of some community noise descriptors through 24 h at an urban area with high traffic volume (A. March, Valencia).

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Variation of some community noise descriptors through 24 h at an urban area with medium traffic volume (E. Piquer, Valencia).

232 A Garcia, L. J. Faus

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Variation of some community noise descriptors through 24 h at an urban area with low traffic volume (Barranco, Gandia)

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Fig. 4. Relationship between equivalent sound level Lcq and traffic volume Q measured continuously through 24 h at a residential-commercial location in the city of Gandia.


in an investigation carried out in Valencia some years ago t2 to develop an empirical formula capable of predicting noise levels in urban areas, the authors deduced the equation:

Leq = 48.6 + 8"1 log Q (r = 0"790)

Q being the traffic density in vehicles/h. The agreement between predicted and measured noise levels improved substantially (r = 0"932) when other additional variables such as width of street (reverberation field), percentage of heavy vehicles and mean velocity of the vehicles were considered.

In some cases, the continuous noise level measurements carried out in the present survey covered several consecutive days in a given location in order to study the day effects (Fig. 5). In general, the authors have not found any significant differences between the noise level patterns measured from Monday to Friday; as a trend, noise levels decrease slightly on Saturday and decrease further on Sunday. Of course, there are some exceptions to this general behaviour; for instance, in zones with an abundance of discos, pubs or restaurants, the differences between day and night or between working days and weekends could even appear reversed.

Table 2 shows the maxima, minima and mean values of hourly equivalent sound levels found in all the measurements carried out in this survey (4200 data corresponding to 175 complete days in 50 different locations of seven Spanish cities). According with the wide range of measuring location characteristics, the values of hourly equivalent sound levels show a wide

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Variation of equivalent sound level Leq measured continuously through seven consecutive days at a residential-commercial location in the city of Gandia.

234 A. Garcia, L. J. Faus

TABLE 2 Values of Hourly Equivalent Sound Level Found in All Measurements Carried

Out in the Present Survey

Time o fchty 1t0 Maximum Mininuon Acerage SD

0-1 79-4 32-6 61-4 7'10 I-2 75"2 31"0 60"5 7.18 2-3 73.9 30-5 58"9 7.46 3-4 72.4 29-0 58"0 7.66 4-5 72-2 26"8 56'8 7"51 5-6 75.7 27"3 57-4 7-43 6-7 74-5 32-5 58'6 7"06 7-8 79"4 43-0 61-9 6"64 8-9 79-4 43"2 63"5 6'96 9-10 79"6 41-5 64.2 6'51

10-I 1 78"8 40-5 64"0 7"83 I 1-12 79-4 43.9 65"2 5"79 12-13 78-6 45.5 65"9 5'56 13-14 80"0 39-1 66" 1 5.78 14-I 5 81"8 38"6 65"9 6"09 15-16 83"7 44'2 64'7 6.07 16-17 79"2 39"7 64"6 6"l 1 17-18 79"5 38-5 65"3 5"82 18-19 78"5 39"9 65"8 5-67 19-20 79.1 32'2 66-I 5-78 20-21 78"6 39.4 66"0 5"86 21-22 78.0 34-5 65"5 6"09 22-23 77-I 31"9 64"3 6-48 23-24 75"0 33'7 62.4 6-52

dispersion, ranging from 27 dBA (measured in the middle of the night at a very quiet location) to 84 dBA (measured in the afternoon at a very noisy location).

In general, the time variation of the diurnal noise levels (from 07:00 to 22:00 h) is small, especially in the high traffic volume locations. Obviously, the lowest values of the noise levels are usually found at night hours. The noise levels measured in working days show a rapid increase from 06:00 to 09:00h, reaching a maximum at 14:00h, followed by a small decrease at 16:00-17:00 h, a new maximum at 20:00 h and a regular decrease period to reach the night's lowest values at 5:00 h. The time patterns are quite different for Sundays (Fig. 6). In this case, for example, after reaching the lowest value at 07:00 h, the noise levels increase slowly up to 13:00 h.

All the above trends are consistent with those found in other community noise measurements carried out in Spain 5-9 and other Mediterranean countries, 13'~'* but they can show some significant differences with the

Statistical analysis o f noise le~'els in urban areas 235

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Time variation ofthe hourly values of equivalent sound levels Leqmeasured in a noisy location (P. Reig, Valencia), (a) on working days, and (b) on Sundays.

observations made in other areas due to the differences in activity and rest hours. On the basis of these differences, the length of day and night periods (and perhaps the definition of noise descriptors such as Ldn) should be carefully specified in the corresponding noise regulations.

3.2 Statistical noise level distributions

The instantaneous noise levels measured in urban areas fluctuate appreciably with time. Many investigations on road traffic noise have sampled the instantaneous sound levels and have grouped the measured values into channels to form a histogram of noise levels. In locations exposed to heavy and steady traffic, it is common to assume that such histograms closely approximate to the Gaussian distribution, zS't6 Using this assumption, relationships have been derived to link the statistical indices, Lx and Leq, with the standard deviation, d, of the noise level distribution. Two such relationships are:

L 1 0 - Leq = 1 .28d-0.115d 2

Leq - L50 = 0.115d 2

For freely flowing traffic, values o f d are often in the range 2-5 dBA and the above equations can be approximated as

L10 - Leq = 3


and

Leq - L50 = 1"4

which have been found to hold experimentally for a variety of traffic flow conditions. However, the road traffic in urban areas rarely flows freely: stop-start conditions imposed by traffic lights and intersections can cause the traffic flow to be pulsed or slow-moving. Consequently, the assumption ot'a Gaussian distribution may be invalid. The departure from the Gaussian curve can be measured through the values of skewness and kurtosis, two parameters related respectively with the asymmetry and peakedness of the experimental noise level distributions. ~ ~

As examples of the histograms recorded in urban areas, Figs 7 and 8 reproduce the noise level distributions measured respectively in a very noisy

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Fig. 7, A representative example of the instantaneous noise level statistical distributions measured in urban areas. This statistical distribution corresponds to a very noisy location (E. Bosch., Valencia). Equivalent sound level Leq = 79.5 dBA, mean sound level ( L ) = 77.5 dBA,

standard deviation d = 4.1, skewness s = 0.2, kurtosis k = 3"2.


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Fig. 8. A representative example of the instantaneous noise level statistical distributions measured in urban areas. This statistical distribution corresponds to a relatively quiet location (Trinitarios, Valencia). Equivalent sound level Leq = 62-9 dBA, mean sound level

( L ) = 55"3 dBA, standard deviation d = 6.3 dBA, skewness s = 1.2, kurtosis k = 4-5.

location and in a relatively quiet location. The instantaneous noise levels were sampled every 0-i s over 1 h periods. Road traffic in the first location was quite intense and practically stationary, while in the other location traffic volume was insignificant. Values of standard deviation, skewness and kurtosis are given for each distribution.

Actually, the instantaneous noise level distributions have not been measured in all locations considered in the present survey. As it has been previously explained, the measuring equipment was preprogrammed in such a way to provide the hourly values of descriptors L1, L10, LS0, L90, L99 and Leq over 24 h periods or several consecutive days. Although some statistical methods for estimating the noise level probability distributions from the observed values of Leq and specific Lx levels have been recently proposed, la

238 A. Garcia. L. J. Faus

this possibility has not been fully explored with the data obtained in the present survey• However, the authors have investigated the general shape of • typical' noise level distributions for urban areas, evaluating the asymmetry of these distributions from all the measured L x hourly values•

The asymmetry of a given noise level distribution has been expressed through a parameter A defined conventionally by the following equation:

A = ( L I 0 - L 5 0 ) - ( L 5 0 - L90)

Obviously, for a Gaussian distribution, A = 0. When A > 0 the distributions are skewed towards the lower level values (skewness s > 0 ) , while A < 0 represent distributions skewed towards the higher level values {skewness s<0 ) .

Figure 9 shows that statistical noise level distributions of the present noise survey, obtained in urban areas under a wide variety of conditions (4200 hourly data), display a wide variety of shapes. Most of these distributions are heavily skewed toward the lower level values: the average value of

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Statistical analysis of noise lerels in urban areas 239

asymmetry parameter A = 1.657, with a standard deviation of d = 2-76. Nevertheless, the asymmetry of the distributions decreases when the mean noise level increases and for Leq > 70dBA the distributions are approxi- mately normal.

3.3 Corre la t ions be tween noise descr iptors

The precise determination of noise level distributions and percentile noise level values L x is usually based in the use of quite sophisticated and expensive instruments (tape recorders, statistical analyzers, etc.). The use of modern integrating sound level meters provides only the values of Leq for a given time period. Therefore, it is interesting to investigate the L x - L e q

relationships in order to obtain information on the main features of instantaneous sound level distributions under the different experimental conditions that are usually found in urban areas.

The analysis of all the information collected in the authors' measurements

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240 A. Garcia, Lo J. Faus

(4200 data) has given the following regression line equations (Figs 10-14):

LI = 0-965 Leq + 11.5 L 1 0 = 1.031 Leq +0-6 L50 = 1.087 Leq - 9-9 L90 = 1-028 Leq - 11"5 L99 = 0-957 Leq -- 10.2

(r = 0-959, d = 2"12) (r = 0-980, d = 1"56) (r = 0-924, d = 3-33) (r = 0"851, d = 4"68) (r = 0"804, d = 5"22)

The best correlation coefficients r and the most accurate estimations (lowest standard deviations d) correspond to the LI and LI0 equations and the worst to the L90 and L99 equations• This result can be expected since the background noise levels (L90 and L99) observed in the absence of nearby noise sources are predominantly influenced by the general set-up of a given location, while the L1, LI0 and Leq values depend upon the specific noise sources truly existing in its immediate vicinity•

The authors have not found any significant differences among the regression equations obtained for the different cities covered in the present survey. Therefore, it could be concluded that the above equations have a

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Sta t i s t i ca l analys is o f noise levels in urban areas 24l

general validity for any prediction of correlations between noise level parameters in a wide variety of urban areas. The values predicted by these equations are comparable to those obtained with similar expressions deduced in other urban noise surveys.tg-2'

The analysis of the regression equations calculated on an hourly basis proves that the correlation coefficients between LI0 and Leq do not show any significant variation through 24h of day (from 0-95 to 0.99): the corresponding equation parameters and standard deviations show only minor changes through the 24 h period. However, the correlation coefficients between L90 and Leq range from 0-65 during the night to 0-90 at day. These results coincide with those found in an investigation carried out some years ago in England. 22 In this case, the equation parameters showed quite a regular variation through the 24 h of day; the standard deviations range from 2.5dBA during the evening hours up to 6"0dBA at 04:00-06:00h. Thus, the equivalent sound level Leq is an excellent predictor of L10 at any time, but it is rather a poor predictor of Lg0, especially during night hours.

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TABLE 3 Values of Ambient Noise General Descriptors Obtained in All

Measurements Carried Out in the Present Survey

Descriptor Maximum Minimum Acerage SD

Leqi24 h) 77.5 44.8 64.8 5-3 Leqld) 79.1 46.7 65"8 5.3 Leqln) 73"5 34.2 61"3 6"4 L~b~ 80"6 45'9 69"0 5"7

Table 3 shows the average values of a number of ambient noise general descriptors and their ranges for the 175 complete day measurements considered in the present survey• There are only minor differences in the values of these descriptors• This result is not unexpected, because the standard deviations of hourly values of noise levels are almost constant throughout the 24 h periods, with a tendency to be slightly higher during the

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night time, as indicated by the values for Leq corresponding to day and night periods• It has been also found that all general predictors are highly intercorrelated, with correlation coefficients ranging from 0"82 to 0.98. In view of such high correlations, it is unlikely that any one descriptor would prove significantly better than another in the prediction of community response•

The precise determination of general (long time) ambient noise descriptors in urban areas (such as the equivalent continuous sound level defined over the entire 24 h period) is very expensive and time-consuming. Therefore, the prediction of these descriptors based in short time measurement techniques could be very useful in many general applications, such as assessing the probable subjective response of a community towards some given levels of environmental noise• The authors have carefully explored this possibility using all the information obtained in the present noise survey. In particular, when the hourly values of Leq measured from 17:00 to 18:00h are used as reference variables, the relationships existing

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between general ambient noise descriptors and these Leq hourly values are:

Leq(24 h) = 0-995 Leq - 0-1 Leq(07 :00-22 :00 h) = 0"986 Leq + 1.4 Leq(22 :00-07 :00 h) = 1.380 Leq - 28-8

Ldn = 1" 159 Leq - 6"6

(r = 0"913, d = 2"37) (r = 0"927, d = 2-18) (r = 0"802, d = 3"48) (r = 0-849, d = 3-08)

The proposed method involves three successive steps:

(1) measurement of hourly equivalent sound level in a given location at any time of the day;

(2) evaluation of the hourly Leq value corresponding to the period 17:00-18:00h using the data shown in Table 2 to estimate the possible time variations; and

(3) computat ion of the desired ambient noise general descriptors, such as they are predicted by the above equations.

In the present analysis, each one of the 24 hourly values of equivalent sound level Leq has been considered as a possible reference variable for use

"~ s o

70

S O

50

- j . /"

...:~ ": r J . j

~ . ' " "

i I i J

SO 60 70 80

L 2 4 C e ~ o ~ r e d )

Fig. 15. Comparison between predicted and measured values of Leq(24 h). The predicted values of this general ambient noise descriptor have been calculated using the equation Leq(24 h)= 0"995 Leq -0-1 , where Leq is the hourly Leq value measured in each case from

17:00 to 18:00h.


in the above equations for prediction of the general ambient noise descriptors. Concerning the prediction of Leq(24h), the best results are obtained using the hourly values of Leq measured from 17:00 and 18:00 h (see above equations). For this noise descriptor, the correlation coefficients of the relationships deduced using all other Leq hourly values decrease from 0.91 to 0.77, while the standard deviations increase from 2.4 to 4.9. Not surprisingly, the best predictions of Leq (07:00-22:00h) and Leq (22:00-07:00 h) values are obtained using the hourly Leq values measured at diurnal and nocturnal periods, respectively; the corresponding correlation coefficients are r > 0-9 in all cases.

It should also be noticed that although the reference Leq measurements should be strictly carried out during one complete hour, the adoption of shorter measurement times (perhaps about 15-20min) usually provides good results. In order to improve the accuracy of predictions, the measurement of short time noise levels should be made during the daytime period. The use of integrating sound level meters is most suitable.

As an example of the potentiality of the proposed method, Fig. 15 compares the predicted and measured values of Leq(24 h). In this case, the correlation coefficient is r = 0-913 and standard deviation d = 2-4. Briefly, this result shows that the proposed method allows the prediction of Leq(24 h) values with an error less than 3"0dBA (90% confidence interval) from a single short time Leq value measured at any time over a 24 h period under a wide range of conditions.

4 CONCLUSIONS

A tentative extrapolation of the results obtained in this investigation would suggest that in order to reduce the economic cost of general purpose noise surveys in urban areas (for example, those related to the measurement of the noise map of a city or the prediction of general annoyance produced by the noise on a community), the complete sampling schedules can be substituted by much simpler short time measurement techniques using an appropriate noise descriptor (Leq), without producing a serious loss of any relevant information. The use of the equations given in this paper (or other similar) can afford a sufficient basis for predicting the noise descriptors and noise level distributions actually observed in most conditions usually found in urban areas.

ACKNOWLEDGEMENTS

This research has been financially supported by the Direcci6n General de Investigaci6n Cientifica y T~cnica (project no. PA86-0292) and the


Institucion Valenciana de Estudios e Investigaci6n.(IVEl). The authors would also like to thank M. Fajari, J. Romero, V. Alamar, A. M. Garcia and M. Arana for their collaboration in the realization of measurements.

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Statistical analysis o]" noise levels in urban areas 247

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Statistical analysis of noise levels in urban areas

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