Journal of Sound and Vibration (1974) 37(1), 87-96

A LABORATORY STUDY OF NUISANCE DUE TO TRAFFIC

NOISE IN A SPEECH ENVIRONMENT

C. G. RICE, BRENDA M. SULLIVANt, J. G. CIIARLES~ AND C. G. GORDON

Institute of Sound and Vibration Research, University of Southampton, Southampton S09 5Ntt, England

AND

J. A. JOHN

Department of Mathematics, University of Southampton, Southampton S09 5NH, England

(Received 15 February 1974, and in revised form 13 May 1974)

A laboratory study was carried out to measure the levels at which various traffic noises start to interfere with the ability to relax and enjoy listening to the spoken word. The intrusive noises represented traffic as heard within dwellings and near roads with a high flow of light vehicles and a varying percentage of heavy vehicles.

The acoustic rating scale units which predicted the subjective responses with least scatter were those based on the Llo concept, the most successful being L~o dB(A). The levels equivalent to those measured "outside" the dwelling correlated somewhat better than those measured "inside".

For units based on peak values alone the "inside" measures performed better than the "outside" measures, and those taking account of low frequency noise factors (e.g., "B" and "D" weighting networks) gave results more in agreement than did those with the "A" weighting.

Of the more recently proposed units for noise assessment PNdB and Lea appeared to be as good as the peak measures, whereas LNp, NNI and TNI were apparently less successful.

In terms of Llo values subjects on average set the intrusion level to 6 dB(A) below the speech level. When the speech level is set to 54 dB(A), a commonly occurring everyday level, the intrusion level judged to be just unacceptable was 48 dB(A). This suggests that an acceptable indoor noise le~,el for listening to speech would be of the order of 45 L:o dB(A).

1. INTRODUCTION

The present index of traffic noise assessment used in Britain, and the one which was recom- mended by the Noise Advisory Council to be used in new legislation, is the "10-percent level" (Llo) in dB(A) measured at the facade of the building under consideration [1]. Whilst the research and social surveys which led to this decision have shown that correlation exists between the Llo index and general annoyance caused by noise, in the multi-dimensional context of the real (home) environment it is very difficult to separate the various acoustic and non-acoustic parameters from each other. It is often accepted however, that it is with regard to speech intelligibility and communication that people most critically judge their noise environment.

The study reported here was designed to investigate the effects which a variety of traffic

? Now at MAN-Acoustics and Noise, Inc., Seattle, Washington, U.S.A. ~: Now at Bickerdike/Allen/Bramble, 16 New Road, London NW3 1JA, England.

87

88 c.G. RICE ET AL.

noise situations would have on the appreciation of speech in a controlled environment. A laboratory study was chosen in which subjects were asked to adjust the intensity level of an intruding time-varying traffic noise signal, until they considered it to be just "unacceptable" for relaxed listening to speech. A criterion of speech interference was not used; rather subjects were asked to select the level at which the traffic noise just began to be noticeably unacceptable. The speech and noise signals were fed into an anechoic test room via two separate loudspeaker systems. Synthesized traffic noise was used to allow sufficient variation and control of the major noise parameters (level, variance and cycle-time).

2. TEST SIGNALS

It was assumed that major intrusion due to traffic noise occurs close to roads with high traffic flows, and it is related strongly to the noisy vehicles, usually commercial vehicles, rather than to the mean traffic noise. The signals were therefore produced to simulate the sound from roads with varying percentages of heavy vehicles, and with vehicles of varying noise level. A high flow of light vehicles was represented by a steady level of frequency-shaped broad band noise (called the background noise), against which the occasional passing of a heavy vehicle was simulated 13y superimposing pass-by recordings of a real lorry.

To reduce the number of possible traffic conditions, three main variables were considered: the percentage of heavy vehicles (assumed to travel at uniform speed and uniform vehicle separation); the distance of the observer from the carriageway which, with the speed of the vehicle, controls the duration of the pass-by; and the intensity of the pass-by relative to the steady flow level (a function of observer distance and magnitude of the light traffic flow).

The intervals [I] between the peaks in the test signals were chosen to be, respectively,

5 seconds (equivalent, for example, to 12Yo heavy vehicles in a 6000 v/hr light traffic flow), or

15 seconds (4~o heavy in a 6000 v]hr light flow), or 40 seconds (1.5 ~o heavy in a 6000 v]hr light flow).

The intensities of the peak noise levels (P) relative to the steady traffic noise were chosen to be 5 dB or 20 dB, respectively. These reflect the range of amplitude differentials occurring in practice; the normal range for continuous steady traffic noise is of the order 0-20 dB.

The third parameter was the duration (D), or width, of the signal pulse of each heavy vehicle

TABLE 1 Description o f traJfic noises

Amplitude of peak (P) Fluctuation of peak (I) (above background) Peak duration (D)

1 2 3 4 5 6 7 8 9

10 11 12 13

Steady (light traffic) 1 peak every 5 seconds

1 peak every 15 seconds

1 peak every 40 seconds

"Background" noise 5 15 5 5

20 15 20 5 5 15 5 5

20 15 20 5 5 15 5 5

20 15 20 5

NUISANCE DUE TO TRAFFIC NOISE 89

pass-by. Signals from a heavy commercial vehicle travelling at 74 km/h (46 mile/h) measured .tt both 15.25 and .30 m (50 and I00 ft) were used, and these were assumed typical with respect to vehicle type, speed and position for road situations where nuisance due to traffic noise could occur. The durations of the two pass-bys between the 20 dB points, were edited to be 5 and 15 seconds, respectively. Loops were made which repeated each of these signals at 5, 15 and 40 second intervals.

Repeating the 15 second duratio~ signals at 5 second intervals produced an overlap of ,~djacent pass-by profiles which resulted in reduction of the relative amplitude differentials. l-he steady background noise and heavy commercial vehicle pass-bys were separately recorded md mixed together at the required relative levels. Time histories of the test signals are shown in Figure 1, which correspond to the description given in Table 1.

I '~~". ( I ) (2) (4)

(6) (7)

(10)

l f f I I I ~, 112) -K) 0 § Time (seconds)

(5}

I

: I

(131

Figure I. Time history of test signals.

The speech signals were obtained by recording a male voice reading short stories in an anechoic chamber. The assessment of the mean level of the speech signal caused some prob- lems, as the intensity of the speech varied over a large range, not least because of pauses between sentences, etc. The long-term averaging techniques (viz., 6 minute estimates) of the statistical distribution or octave band average energy (Lcq) values produced results that did not vary much between the recordings used, except for overall intensity differences due probably to different positions of the recording microphone relative to the reader. As these long term measures were relatively insensitive, it was decided to line up the speech recordings for presentation to the subjects on the basis of equal Lso dB(A). Subsequent statistical analysis showed this to be a satisfactory measure as no significant experimental differences between the speech recordings were found.

The speech signals were set within +1 dB at 42 Lso dB(A), which was equivalent to 54 Llo dB(A), the level found in a preliminary study to be comfortable listening level in the quiet environment of the test facility. During the study, subjects were first required to set up the speech signal at their own preferred level, although the level of 54 Llo was used in the rest of the study. Analysis of the speech levels chosen by the subjects showed that on average they chose a level 5 dB below the test level: i.e., 49 L~o dB(A) (standard deviation 5 dB).

Typical frequency spectra of the two components of the test signals are shown in Figure 2, together With an average speech recording spectrum.

The test signals and the speech signals were fed separately into the subjective listening

90 C. G. RICE E T AL.

Bond numbers Frequency (Hz)

- - I 0

ioI g

- - I n

._.= o

r r

I

Over~l level t . ~

I 0 I I 12 1314 1 5 1 6 1 7 1 8 1 9 2 0 2 1 ? - - 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1 3 2 3 3 3 4 3 5 3 6 3 " 7 3 8 5 9 4 0 4 1 4 2 4 5 4 4 4 5 4 6 l , I l l I I , I I , I I l I I l I I I I I I I I I I I I I I l I I i " l

t 0 2 0 5 0 I 0 0 2 0 0 5 0 0 I 0 0 0 2 0 0 0 5 0 0 0 I 0 0 0 0 2 0 0 0 0 4 0 0 0 0 I I I I I I I I I I I , I

~ . . / N /Lorry pass-by noise ot peok "--' \ , , ~ A

.~. \,. /~t \ J ,; -,....I ~. \vJ /// St e~!"-'~,"" Speech-'~/ background " ~ ?

x

I I I I I I I " I" I

~-Octove ond octave bond centre frequencies (Hz)

Figure 2. Subjective study test signal spectra.

Ferrooroph 7 t o p e recorder

I O � 9

Nogro IV lope recorder

Anechoic chamber

_MM MM -3dB~oct,,,e fi,,e, ~Sobie~, I,

o U

- - J -

Figure 3. Schematic diagram of the equipment used.

facility at I.S.V.R., which consists of two loudspeakers situated in a small chamber (20 m a) lined with polyurethane foam wedges, i

The subject sits in a foam chair placed 1.83 m (6 ft) in front of the loudspeakers (see Figure 3) and the equalized pure response of the vehicle noise loudspeaker measured at the subject's head position (with the subject absent) is within 6 dB over the frequency range 50 Hz-10 kHz.

As the recordings of the lorry pass-bys were made in the open, it was decided to reproduce

NUISANCE DUE TO TRAFFIC NOISE 91

internal sound conditions by weighting the mixed traffic noise signal by means of a - 3 dB/ ~ctave filter.

The experimental design used comprised two 13 x 13 balanced Graeco-Latin squares, tchich required 26 subjects. Each subject received all 13 test situations, which were presented in two sessions, given on different days to reduce subject fatigue, each of which lasted ap- proximately 45 minutes. The Graeco-Latin square design is shown in Figure 4, subjects 1-13

Subject Presentation order no. 1st 2rid 3rd 4th 5th 6th 7th 8th 9th 10th llth 12th 13th

I I m 21 13a 3k 12b 4j l l e 5i 10d 6h 9e 7g 8f II 2a 3m l b 41 13e 5k 12d 6j l i e 7i 10f 8h 9g

I I I 3b 4a 2e 5m l d 61 13e 7k 12f 8j l l g 9i 10h IV 4c 5b 3d 6a 2e 7m I f 81 13g 9k 12h 10j l l i V 5d 6c 4e 7b 3f 8a 2g 9m l h I01 13i I l k 12j

VI 6e 7d 5f 8c 4g 9b 3h 10a 2i l l m l j 121 13k VII 7f 8e 6g 9d 5h 10e 4i l i b 3j 12a 2k 13m I1

VIII 8g 9f 7h 10e 6i l l d 5j 12c 4k 13b 31 l a 2m IX 9h 10g 8i l l f 7j 12e 6k 13d 51 l e 4m 2b 3a X 10i l l h 9j 12g 8k 13f 71 l e 6m 2d 5a 3e 4b

XI l l j 12i 10k 13h 91 l g 8m 2f 7a 3e 6b 4d 5e XII 12k 13j 111 l i 10m 2h 9a 3g 8b 4f 7c 5e 6d

XIII 131 l k 12m 2j l l a 3i 10b 4h 9c 5g 8d 6f 7e

1-13--13 test signals a-m--I 3 speech recordings

I-XlII--13 subjects

Figure 4. Graeco-Latin square design.

receiving treatments in rows as indicated; subjects 14-21 received the reverse order of treat- ments within rows, corresponding to subjects 1-13, respectively. The allocation of noises, speech conditions and subjects to the design was done at random.

3. SUBJECTIVE STUDY

All of the 26 subjects used were given routine pure-tone audiometry to ensure they had hearing within +20 dB of ISO normal. They were employees or students of the University of Southampton aged between 18-59 (average age 24), 16 being male and 10 female.

Each subject was required to read the instructions before the test. These were as follows.

"INSTRUCTIONS

We would like you to help us study a problem concerned with traffic noise. We want to find out how it interferes with people talking or listening to radio or T.V. inside their homes without having to raise the volume.

We will play you a variety of such sounds while you are listening to a recorded talk, and we want you to adjust the level of the background noise until it just starts to interfere with your ability to relax and enjoy listening.

To start with, we will play the speech recording only and we want you to adjust its level, using the control knob by the chair, until it is at a level at which you would like to listen to it. When you have reached that level, signal to the experimenter.

Then we will play the fluctuating noise as a background to the speech. We want you to listen to it first and, when the light on the control box goes out, to adjust the level of the noise using the control box knob, until you reach the just unacceptable level. Tell the experimenter when you have made your decision.

92 C. G. RICE ETAL,

We will then play some more fluctuating noises. Each time you have found the right level, tell the experimenter. Wait for the light on the control box to go out before starting your adjustment. The sounds will be played for as long as you need to make your decision. Listen carefully and do not hurry your judgement."

4. RESULTS

The settings of the attenuator controlling the traffic noise level, chosen by each subject as his "just acceptable" level for each test situation were noted. These were related to physical measures of the test signals measured by a microphone in the chamber (in the absence of a subject), and analysed by using the real time analysis and computational facilities of the I.S.V.R. Data Analysis Centre. Measurements of some units were made both as the subject heard the sound (in the "inside" environment) and as a noise survey might measure them (in the "outside" environment--simulated by removing the - 3 dB octave filter).

Over eighty rating scale units were evaluated to see which "best" related the physical characteristics of the noises to the judged subjective responses. The criterion o f "bes t " is not easy to define and is still the subject of research debate; however, in the present context it is felt that it is not unreasonable to expect the "ideal unit" to be one which would give the same numerical value for all 13 traffic noise signals when lined up at the average levels chosen by subjects.

The data were analysed by using statistical analyses ofvariance (see, for example, the books by Snedecor and Cochran [2] or Cooper [3]). The purpose of an analysis of variance is to partition the total variation in the data into components due to ascribable causes, such as noise effects and differences between subjects, and a component unascribable to any individual cause--a residual or error component. An F-ratio test may be used to test whether a component due to a particular cause contributes significantly to the total variation. This test is carried out by calculating the ratio of the variance due to the component and the error variance.

In this experiment the F-ratio was used to determine the relative effectiveness of each rating scale unit. For each unit this was the ratio of the variation attributable to the 13 different noises to the residual variation remaining after differences between subjects, speech recordings presentation orders and noises had been removed. A summary analysis of variance table, together with a partial breakdown of the test noise component, is given in Table 2 for some of

TABLE 2 Summary analysis of variance tab&for a selection of weighted values measured inside

F-ratios ),

Degrees dB(A) Source of of .Llo Peak Max. Lcax Lm,2 NNI TNI variation freedom (dB(A)) dB(A) �89 s

Subjects 25 78.8 78.8 78.8 78.8 78.8 78.8 78.8 Order 12 4"1 4"1 4.1 4.1 4.1 4.1 4"1 Speech 12 1-I 1.1 1.1 1"1 1-1 1"1 1-1 Noise 12 5.54 9-24 9.60 6.55 21.8 58.2 590.6

Interval (I) 2 5.5 43.8 40.5 0.8 15-3 1 1 0 " 4 461.8 Peak (P) 1 9.6 1"7 5"0 27.5 71.4 0.4 2957-3 Duration (/9) 1 25.7 4"1 4.8 24.1 9.7 0-1 98.0

Residual 276 Total 337

Levels of significance: 5 70 F (25,276) = 1.6 F(12,276) = 1.8

1 70 F (25,276) = 1.9

NUISANCE DUE TO TRAFFIC NOISE

TABLE 3

F-ratios for selected units

93

dB(A) dB(B) dB(D) PLdB

Measured as heard inside Llo Statistical distribution analyser 5.54 Peak-level recorder r.m.s, maximum value 9.24 Maximum integrated �89 second by computer 9'60 Lso dB(A) 69'70 L,qt--Energy mean dB(A) by computer 6,55 L, q2--Dosemeter 7.91 Lcq3--Lso + (Llo - - L9o)2/57 36"50 Ll~rl--Leq3 -I- (Llo - - L9o) 30.0 L~v2--L~q3 + 2"56a 21"75 L~ra--L, a2 + 2"56a 34.94 NNI--PNLm,. + 151ogN - 20 58"17

720 where N = ( I+ 1)

TNI~I.,9o + 4(Lxo - L9o) - 30 590"55

7-53 7.26 7-67 7.25 8"12 7.61 7.84

9.00

l~Ieasured outside Llo.,~ Statistical distribution analyser 4"54 5"19 5"30 Peak-level recorder r.m.s, maximum value 9.54 8"95 9.13

Levels of significance: 5~ F(12,276)= 1.8 1% F (12,276) --- 2.3

Results indicate that no unit satisfactorily rates the subjective judgements.

�9 he units. In this table, the F-ratios for subjects, order and speech are the same for all units. listing of the F-ratios for a further selection of the units is given in Table 3. The results of the statistical analysis show that subject differences (as expected) are highly

ignificant, that differences between speech recordings are not significant, and that the order -fleet is just significant. This latter effect shows the trend that subjects became less tolerant as the experiment proceeded, although only the first noise presented was set at a statistically ~ignificantly higher level than any of the 12 subsequent noises,which did not differ significantly. l'he mean levels also showed that there is some decrease in tolerance throughout the test, with a "'recovery" between presentations 7 and 8, which was the point at which the test was broken into two sessions (see Figure 5).

8 m

o 4

I I I I I I | I I I I I I

x

x.x, I | I I 5 | 1 I I I t iii I I

t I 2 5 4 6 71 i B 9 IO 12 13 I

I$! S~$iCq~ 2 ~ ~SS i~g

Presentation order

Figure 5. Presentation order effect.

94 C. G. RICE E T A L .

TABLE 4 Rank order of selected units

Standard Unitt F-ratio deviation

Llo dB(A)~ 4"5 1"6 L1o dB(B):~ 5"2 1.7 Llo dB(D):~ 5"3 1"7 L~o dB(A) 5"5 1"8 L~ql dB(A) 6"6 1"9 Peak dB(D) 7.3 2.0 Lto dB(D) 7-3 2-0 PNdB max 7"5 2.1 Llo dB(B) 7.5 2.1 dB(D) max 7"6 2.1 Peak dB(B) 7.7 2.1 PLdB max 7.8 2.1 Leq2 dB(A) 7"9 2"1 dB(B) max 8"1 2-2 dB(D)2 max 8.2 2.2 dBLin max 8.5 2.2 Peak dB(B):~ 9.0 2.3 L~ql dB(D) 9"0 2"3 TPNdB 9"1 2"3 Peak dB(D)~ 9.1 2.3 Peak dB(A) 9.2 2.3 Peak dB(A)~t 9.5 2.3 dB(A) max 9.6 2.3 dB(D)3 10.2 2.4 LNP~ 21.8 3.5 LNPI 30'0 4"1 LNPa 34"9 4"5 Leq3 dB(A) 36"5 4"5 NNI 58 "2 5"7 Lso dB(A) 69"7 6"28 TNI 590'6 18"3

t See Table 3 for description. :~ Noise measured in equivalent outside

position, all others measured as heard inside.

The F-ratio criterion shows that none of the units investigated rated the noise in the same way as did the subjects: i.e., all gave F-ratios that showed statistically significant differences between noises. It is useful to rank order a selection of units in terms of both the F-ratio and the standard deviations of the average equality levels chosen by subjects. These are shown in Table 4. For peak-value units, the "inside" measures give better correlation with the subject's assessments than do the "outside" measures, although this trend is reversed for the Llo measures.

Composite units such as TNI and LNp were considerably less effective (in terms of the criterion stated) in predicting the equality of subjective intrusion than were the less complex units, and of the rating scale units in use, none performed significantly better than the L~o dB(A) measure as evaluated in the "outside" environment. Use of approximated formulae for L,q and Lr~p based on the assumption that the test noises are normally distributed are not too satisfactory. Figure 6 shows in fact that such noises are not necessarily so distributed, and caution is therefore advised with the use of such relationships in real life situations.

NUISANCE DUE TO TRAFFIC NOISE 9 9 9 9 I I t I [ i i I

9 9 9 5 999 998

995

99O

9 5 0

9OO

�9 ~ 8 0 0

.o 70{ -E -o 60c / ( I )

=~ 30C

o zoc ~ /191 /~(ll)

IOC 5C

0.I 1 0 . 1 I I I I i I I I I

0 2 5 5 0 7 5 1 0 0 1 2 5 1 5 0 2 0 0 25 '0 30"0

Relative level (dB)

Figure 6. A-weighted statistical distribution of four of test noises aligned for subjective equality.

95

In terms of L1o values, subjects set the intrusion level to 6 dB(A), on average, below the ~peech level. With the speech level set at 54 dB(A) Llo, the intrusion level judged to be just L1nacceptable was 48 dB(A) Llo, suggesting a probable acceptable indoor noise level for listening speech of the order of 45 dB(A) Llo.

5. CONCLUSIONS

Examination of the rank order shown in Table 4 suggests that Llo dB(A) measured at the outside of the building is the most appropriate unit to use for the assessment of the indoor intrusion caused by traffic noise. No particular measurement advantage appears to be gained by emphasizing the lower frequencies (e.g., by using PLdB (Stevens Mk VII) "B" and "D" weighting networks) unless peak or maximum integrated one-half second x;alues are used and the measurements are made indoors. This appears to place an unnecessary restriction on traffic noise prediction schemes.

Within the limits and applicability of this study, therefore, it would appear that, whilst ;till not ideal, the physical rating scale unit that best predicts the intrusion of traffic noise within the simulated indoor speech environment is the Llo dB(A). Other factors, however, ~hould be borne in mind when assessing the efficiency of a particular rating scale unit. These include, for example, the ability of the unit to discriminate between different types of sound characteristics, and whether or not account can be taken of the rate at which noise peaks occur.

96 C.G. RICE ETAL.

ACKNOWLEDGMENT

The authors acknowledge the support given by the Transport and Road Research Laboratory of the Department of the Environment, for whom the study described was under- taken by the Wolfson Unit for Noise and Vibration Control, of the Institute of Sound and Vibration Research of the University of Southampton.

REFERENCES

1. DEPARTMErqT Or THE ENVIRONMENT 1972 New Housing and Road Traffic Noise, a Design Guide for Architects. London: H.M. Stationery Office.

2. G. W. SNrDrCOR and W. G. COCrmAN 1967 Statistical Methods. Iowa State University Press. 3. B. E. COOPER 1969 Statistics for Experimentalists. Oxford: Pergamon Press.