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Applied Acoustics 43 (1994) 5%65 © 1994 Elsevier Science Limited

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Technical Note

A Study of Noise in Various Modes of Transport in Delhi

Krishan K u m a r & V. K. Jain*

School o f Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India

(Received 12 March 1993; revised version received 4 January 1994; accepted 14 January 1994)

ABSTRACT

A survey of traffic noise in the city of Delhi was carried out in order to examine the nature and levels of noise inside various types of vehicle. The study involved measurements of average A-weighted levels and power spectra of noise inside buses, auto-rickshaws, cars and trucks from which L~o, L50, L90 and Leq levels were estimated. It is found that noise levels in auto-rickshaws are the highest, followed by trucks, buses and cars. The power spectra o f a l l four types of vehicle exhibit rather similar behaviour.

1 I N T R O D U C T I O N

In recent years, noise has come to be recognized as an important area of research, along with other forms of pollution, in determining the quality of our environment. This is evident from a number of studies which have been carried out in various parts of the world (see, e.g., Refs 1-6). Singhal 7 has reviewed the noise surveys carried out in various cities in India. Vehicles on roads have been identified as a major source of noise, causing concern to the commuting passengers and people in residential and commercial complexes in the vicinity of traffic lanes. As a result, traffic noise has attracted a great deal of attention from environmentalists. Most of the measurements of noise have been made at a point of observation that is

* To whom correspondence should be addressed.

57

58 Krishan Kurnar, V. K. .lain

at some distance away from the line of traffic flow. However, very few studies to date have examined the nature and levels of noise inside various vehicles. Early studies of the spectral distribution of noise levels inside vehicles were conducted by Tempest and Bryan 8 and Williams and Tempest. 9 To our knowledge, no study concerning the noise levels inside various types of vehicle has been done in the context of metropolitan cities of India prior to this. In this paper we address this problem with a view to finding out the nature and levels of noise to which drivers and persons travelling in such vehicles are exposed.

In Section 2, materials and methods employed in the present study are given. Results are discussed in Section 3.

2 T R A N S P O R T SAMPLING AND M E A S U R E M E N T TECHNIQUES

The instrument used for our field survey was a CRL 2.37A precision (i.e. type-I) sound-level meter. The instrument has the facility of making measurements with 'A' or 'C' weighting and in the linear (Lin) mode. It can be used as an octave analyser in the frequency range 31.5 Hz- 16 KHz. The noise levels were measured in different modes of transport (buses, trucks, cars and auto-rickshaws) in the city of Delhi. Sampling was done inside two types of bus which ply on the roads of Delhi, namely, Delhi Transport Corporat ion (DTC) buses and privately owned buses. Readings were taken on seats situated approximately in the middle of the bus and the instrument was held in the hand at a height of about 2.5 ft from the floor of the bus. For integrated A-weighted levels, read- ings were taken at intervals of 15 s for a period of 12 min. Readings for the spectral distribution of noise levels were taken in linear (Lin) mode during the periods when the bus was (i) in first gear and (ii) in fourth gear. For first gear, the maximum reading was recorded while the bus was still accelerating. The readings in fourth gear were taken when the bus was running at a uniform speed. The buses chosen for sampling were moderately-to-densely packed with passengers and covered almost all of the major bus-routes of Delhi. A similar procedure was followed for sampling noise in auto-rickshaws, trucks and cars. In the case of trucks, measurements were made in the driver's cabin. Cars belonging to three different models, (i) Ambassador, (ii) Fiat and (iii) Maruti, were chosen for our study. Ambassador and Fiat cars have 4-cylinder engines of 1200 cc and 900 cc capacities, respectively. The Maruti cars, on the other hand, have 800 cc engines with three cylinders. Among the three categories of car, Maruti is smaller in size and relatively light in weight when

A study of noise in various modes of transport in Delhi 59

compared to the other two models. All of the readings were recorded on the back seat of cars with windows closed. The only difference was that the readings which were affected by outside disturbance were omitted. The dB (Lin) readings for power spectra were noted when the vehicle had attained a uniform speed in fourth gear.

3 RESULTS AND DISCUSSION

All of the data recorded during the field investigation were stored in a computer. The noise levels were then sorted out for each vehicle in ascending order of magnitude through a simple computer program. From the sorted data, various percentile indices, i.e. LI0, Ls0 and L90, were calculated using the following method. For the ungrouped data consisting of N readings arranged in ascending order of magnitude, if L i represents the noise level which is exceeded by i% of the readings and iN/lO0 = N' (say), then Li is equal to the mean of the (N - N')th and (N - N' + 1)th readings. Leq levels were then estimated using the expres- sion 10

Leq = Ls0 + d2/60 where

d-- Ll0 - L90

It was observed that the DTC buses have slightly higher noise levels in comparison to the private buses (Fig. l(a) and (b)). This may perhaps be attributed to the fact that private buses are maintained in relatively better condition than the state-owned DTC buses. Although engine noise is expected to be higher in first gear in comparison to fourth gear, the rattling of window panes and the road surface-tyre contact noise con- tribute appreciably to overall noise levels in fourth gear (i.e. at higher speeds). Typical 12-min Zeq values for DTC and private buses are found to be 83.95 dBA and 81.29 dBA, respectively (Table 2).

Table 1 shows the minimum and maximum levels of various indices observed in various types of vehicle. The noise levels in auto-rickshaws are found to be the highest among the types of vehicles surveyed, with Leq levels in the range 81-96 dBA In the case of trucks, the Leq levels lie between 83 dBA and 90 dBA. Among the three major types of car, the Zeq levels are found to be the highest in the case of the Ambassador and the lowest for the Maruti models. Figure 2 shows a histogram of Zeq levels inside auto-rickshaws, buses and taxis. The taxis include both the Ambassador and Fiat models. It is clear from the figure that Leq levels inside auto-rickshaws are highest, followed by buses and cars, respec- tively.

60 Krishan Kumar, V. K. Jain

130"

120"

110

100

90

bO

~7o

"=60

40

° I 120

110 l

100

t 90.

I ao "C

37o.

m

50-

z,O

ao

Fig. 1.

'\'0

FREQUENCY (Hz) ) ~ DTC BUSES

PRIVATE BUSES

(a)

c:x.~

\

FREQ.UENCY (Hz) ) D T C BUSES --o-- PRIVATE BUSES

(b)

Spectral distribution of dB (Lin) levels in (a) first gear and (b) fourth gear inside DTC and private buses.

A study of noise in various modes of transport in Delhi 61

TABLE 1 Minimum and Maximum Values of Llo, Lso, Lgo and Leq

Vehicle Levels for Various Types of

Type of vehicle Lw Lso L9o Leq (dBA) (dBA) (dBA) (dBA)

Min. Max. Min. Max. Min. Max. Min. Max.

Buses 79 99 76 94 68 89 77 92 Auto-rickshaws 83 96 81 95 77 89 81 96 Trucks 85 94 81 89 74 85 83 90 Maruti cars 67 74 65 71 61 68 65 71 Fiat cars 70 82 67 78 63 72 67 79 Taxis 75 82 72 79 67 76 72 80

Figure 3 compares the spectral distribution of linear noise levels inside buses, auto-rickshaws, taxis and trucks. Though the general trend of noise levels as a function of frequency in all types of vehicle is rather similar, there are certain notable features. Whereas noise levels fall with increase in frequency from 31-5 Hz onwards in other vehicles, they rise to

99

97

95

93

91

89

87

85

83

79

77

75

73

71

69

65

T

20 15 10 5 (

Fig. 2.

E

E

_

AUT0- RICKSHAWS BUSES TAXIS

0 25 20 15 10( 5 30 25 20 15 10( 5 0

NO. OF VEHICLES

Leq levels inside auto-rickshaws, buses and taxis.

62 Krishan Kumar, V. K. Jain

120

8O

• ~ 70

m ,o

6 0

5O

40 ̧

30 . . . . . . . . . . . . . . . ~3 . . . . . . . . . . . . . 1~ 101 102 ~ s

FREQUENCY (Hz) --e-- BUSES - -~ - - AUTO-RICKSHAWS - ~ - ~AXIS --~-- TRUCKS

Fig. 3. Spectral distribution of dB (Lin) levels inside buses, auto-rickshaws, taxis and trucks.

125 Hz in auto-rickshaws. From 500 Hz onwards, auto-rickshaws have the highest noise levels among all the types of vehicle, although these are very closely followed by trucks. In fact, the behaviour of the spectral curves of auto-rickshaws and trucks is very similar, i.e. a very small decline in noise levels from 500 Hz to 4 kHz. Thereafter the behaviour of all four spectral curves is similar, with noise levels falling very sharply up to 16 kHz.

A comparison of spectral distribution of dB (Lin) levels of Maruti, Fiat and Ambassador cars is shown in Fig. 4. It is clear that noise levels in Ambassador cars are appreciably higher than in the other models of car. The lowest noise levels prevail inside Maruti cars, although at the lower end of the spectrum (31.5 Hz, 63 Hz and 125 Hz), the difference in noise levels is very small between Maruti and Fiat cars. However, at 31.5 Hz the noise levels inside Maruti cars are closer to those inside Ambassador cars than to those inside Fiat cars.

For periodic manual sampling of noise to be useful in environmental noise control, some assurance on the accuracy of our estimates of average L~0, Lso, L90 and Leq levels must be given. In earlier works, the procedure involves fixing the degree of accuracy in advance by choosing a tolerance limit (usually _+ 1 dBA) at an acceptable level of confidence. However, we have deviated from this procedure by fixing the number of readings in

A study of noise in various modes of transport in Delhi 63

Fig. 4.

lOO

9o

8O

70

a ~ 60

~ 50

30

101 , , , . . . . ~1 2 , . . . . ,,,;107~ , . . . . , , ,hlo z~ , , , , ,,,,.J1o 5

F r e q u e n c y ( H z ) ~ - A r n b a s s a d o t C a r s - o - F i a t ~ , - M a r u t i

Spectral distribution of dB (Lin) levels inside Ambassador, Fiat and Maruti cars.

advance and then have calculated the tolerance limits at an acceptable level of confidence (99%). This has been done by applying a Student's t-test based on the normality of the parent population. Although the normality of the population in our case is not confirmed, we still went ahead with the test because these tests are usually quite reliable and are known to be applicable even for considerable departures from normality However, there might still be errors in our results for non-sampling reasons. For instance, there is always some correlation between succes- sive meter readings. Another possible error in the results might be due to reflection and standing waves from the objects (fixed and moving) near the microphone. Average values of the indices L~0, Ls0, L90 and Leq a r e

given in Table 2. Examination of Table 2 shows that among the various modes of trans-

port, noise levels are greatest in auto-rickshaws followed by trucks and

TABLE 2 Average Values of Llo, Lso, L90 and Leq Levels for Various Types of Vehicle at 1% Level

of Significance

Type of vehicle Lto Lso Lgo Leq (dBA) (dBA) (dBA) (dBA)

DTC buses 85.98 _+ 1.88 81.96 + 0.74 75.39 + 1.41 83.95 + 2.02 Private buses 83.75 + 1.27 79.58 + 1.16 73-88 + 1-08 81.29 + i.40 Auto-rickshaws 90.42 + 1.40 87.71 + 1.47 82.37 + 1.09 88.89 + 1.64 Trucks 88.88 + 1.00 84.76 + 0.94 78-57 + 1-18 86.52 + 1.00 Maruti cars 70-52 + 0.94 67-36 + 0.76 64-12 + 0.81 68.02 + 0.78 Fiat cars 74.78 + 1.26 71-40 + 1.16 67-46 + 1.03 72.32 + 1.24 Taxis 78.38 + 0.87 75.12 + 0.92 70.78 + 0.96 74.62 + 0.95

64 Krishan Kumar, V. K. Jain

Subjective rating of noise inside vehicles

TABLE 3 The Salford Criterion

Noise level not exceeding (dBA)

Quiet 67 Noticeable 73 Intrusive 79 Annoying 85 Very annoying 91

buses. The noise levels in cars are appreciably lower when compared to other modes of transport.

A criterion for noise inside various categories of vehicles, known as the 'Salford Criterion '~ is given in Table 3. Using this criterion, it may be concluded that the nature of noise in auto-rickshaws is annoying, whereas in trucks and buses it is intrusive. A quiet environment is found in Maruti and Fiat cars.

According to the environmental noise criterion recommended by WHO, permanent hearing loss is likely to occur if Leq (8 h) > 75 dBA for periods up to 40 years. One carries the same risk if subjected to an hour 's exposure at Leq -> 84 dBA. This implies that the drivers of buses, trucks and auto-rickshaws, and frequent users of auto-rickshaws, will be prone to hearing loss.

A C K N O W L E D G E M E N T

One of the authors (K.K.) would like to thank Jawaharlal Nehru Univer- sity for providing the financial support during the course of this work.

R E F E R E N C E S

1. Webster, J. E., Community noise survey in Medford, Massachusetts. J. Acoustical Sot:. Am., 54 (1973) 985 95.

2. Cannelli, G. B., Traffic noise pollution in Rome. Applied Acoustics, 7 (1974) 103.

3. Attenborough, K., Clark, S. & Utley, W. A., Background noise levels in United Kingdom. J. Sound Vibration, 48 (1976) 359-75.

4. Burgess, M. A., Noise prediction for urban traffic conditions--Related to measurements in Sydney Metropolitan Area. Applied Acoustics, 10 (1977) 1-7.

A study of noise in various modes of transport in Delhi 65

5. Ko, N. W. M., Traffic noise in a high-rise city. Applied Acoustics, 11 (1978) 225-39.

6. Bjorkman, M., Maximum noise levels in road traffic noise. J. Sound Vibra- tion, 127(3) (1988) 583-7.

7. Singhal, S. P., Noise: A giant environmental pollutant. J. Scientific and Industrial Research, 45 (1986) 544-57.

8. Tempest, W. & Bryan, M. E., Low frequency sound measurements in vehicles. Applied Acoustics, 5 (1972) 133-9.

9. Williams, D. & Tempest, W., Noise in heavy goods vehicles. J. Sound Vibration, 43(1) (1975) 97-107.

10. Magrab, F. B., Environmental Noise Control. John Wiley and Sons, New York, USA, 1975, p. 69.

11. Bryan, M. E., A tentative criterion for acceptable noise levels in passenger vehicles. J. Sound Vibration, 48(4) (1976) 525.