Survey of the exterior noise of some passenger cars

Journal of So,rod and Vibration (1973) 29(4), 483-499

SURVEY OF

OF SOME

THE

EXTERIOR

PASSENGER

NOISE CARS

E.J. RAa'H~ blterkeller A G, CH-8052 Z~rich, Switzerland F. CASULA Fiat,/-10135 Torino, Italy H. HARTWIG Volkswagenwerk AG, D-318 IVolfsburg, Germany AND H. MALLET Renault, F-92 Rueit-Mahnaison, France

(Received 9 April 1973) The results of extensive measurements made on 58 different models of European passenger cars are used to show the influence of speed and driving conditions on noise emission. The range of levels covered by different driving conditions around a speed of 50 km/h has median values of 66.4 to 79.5 dB(A). The spread in levels measured for all car models in any single driving condition is smaller than the spread due to different driving conditions.

1. INTRODUCTION With the constant increase of traffic volume in the past years, the noise caused by it has become an important aspect of environmental conditions in built-up areas. The public in general is getting to be more aware of noise emission, and official bodies are extending laws and regulations for improved noise control. The automobile industry is keenly interested in good regulations, so that the efforts spent for noise reduction may result in a m a x i m u m benefit to the public. It is actively studying means of noise reduction. The present study has been sponsored by a number of European car manufacturers in an effort to obtain better insight into the current noise production of passenger cars. A selection of the major results is presented in the form of statistical distributions.

2. SAMPLE OF CARS USED The evaluation of noise emission was carried out on a total of 58 car models produced by 14 manufacturers. Based on the official count of new car registrations in Europe during the year 1971, these models account for 57"2770 of all the cars registered. The importance of some major groups of evaluated cars in terms of market share is shown in Table 1. 483

E. J. RATH~ET AL.

484

TABLE 1

Characteristics and respective market shares o f cars tested Car type

No. of models tested

Fraction of market represented

48 10

42.23 Yo 15-04 Yo

58

57.27 Yo

47 11

41.00 16-27 Yo

58

57.27 ~o

39 9 10

51.23 Yo 2.04 Yo 4.00

58

57.27 ~o

3 2 43 7 3

1.6 0-07 Yo 48.26 Yo 0.62 ~o 6.72 Yo

58

57.27 %

Front engine Rear engine

Water-cooled engine Air-cooled engine

4-speed gear box 5-speed gear box Automatic gear box

Diesel engine Wankel engine Four-cylinder petrol engine Six-cylinder petrol engine Two-cylinder petrol engine

TABLE 2

Mechanical parameter ranges o f cars tested Parameter

Lowest value

Median value

Highest value

Engine power Engine capacity Max. engine torque Max. engine rpmt Engine power/capacity Stroke/bore Car weight/power Bore Weight

18 435 3 4000 27-3 0.65 7.4 54-5 520

75 1570 12 5600 50 0.9 13.8 80 1010

180 3270 25 7000 74.4 1.47 28-9 701.8 1634

Units HP cm 3 m kg rev/min HP/litre kg/HP mm kg

It is also important to have a good cross-section of the current c~r market with respect to the mechanical parameters of the cars. For some parameters the range of valu.es encountered is indicated in Table 2. For each of the parameters a considerable range could be included. 3. METHODS OF TESTING 3.1. MEASURINGCONDITIONS All the tests were carried out on measuring sites that were free of any other traffic or interference. A dry smooth asphalt test track was used. Each single test was performed in t Throughout this paper rpm = rev/min.

EXTERIOR NOISE OF CARS

485

both directions of travel with respect to the microphone, and an average figure for the noise emission of the two sides o f each vehicle was used as a result. In several instances repeated measurements were performed, in order to increase the accuracy of measurement.

Rolling noise For the evaluation of rolling noise the vehicles were accelerated to the desired speed, and then allowed to coast past the measuring plane with the engine stopped. The speeds when passing the microphone were selected to be 50, 75 and 100 km/h.

Stationary measurements The noise levels of the car engines in no-load condition were determined in measurements with stationary vehicles. The sound levels were determined in two lateral microphone positions on both sides of each car at a distance of 7 m from the car body and at a height of 1.2 m above the ground. For these tests the engine speed was chosen to be the same as for approach conditions defined by ISO for the acceleration tests.

Constant speed tests Several driving conditions at constant speed were examined. In passages at 50 km/h in second gear the engine noise can be expected to dominate. In fourth gear passages at a constant speed of 50, 75 and 100 km/h were evaluated.

Acceleration tests A driving condition of primary interest is the acceleration test defined by ISO Recommendation R 362. For cars with up to four gear speeds the second gear must be used. Cars with five gear speeds are tested in third gear. The approach speed is defined as either the speed with an engine speed of 3/4 of the speed at which the engine develops its maximum power, or at 50 km/h, whichever is the lowest. 3.2. MEASURING SYSTEM The measuring system consisted of a recording section to obtain calibrated analog magnetic tape recordings of all noise signals, and a data reduction section for subsequent analysis of all recordings in the laboratory with the aid of a computer. All measured noise signals were recorded on full track 1/4 inch magnetic tape.

Microphone positions Four precision condenser microphones were used. Three of them were located at a distance of 7.5 m from the center line of the test track and at heights of 1-2, 4 and 7 m above the ground. The fourth microphone was located at a distance of 25 m from the center of the track and at a height of 4 m above ground.

Vehicle speed The speed of the test vehicles was established by means of an array of eleven photoelectric sensors placed at intervals of 5 m along the test track. Two additional sensors of the same type served to trigger the automatic start and stop functions of the complete recording system.

Enghze speed The engine speed o f each test vehicle was monitored within the vehicle by means of a direct reading instrument. The ignition signal used for this purpose was also transmitted by FM-radio link to the measuring van. There, a second direct reading instrument and audiomonitoring were provided. The signal was also recorded on magnetic tape, together with the signals from the speed measuring sensors.

486

E.J.

ET AL.

RATHE

Overall accuracy The overall accuracy of the measuring system depends on the individual tolerances of all the elements involved in the system. Since there are many of them, and each single noise source has a different time varying spectrum that makes different sections o f the tolerance range important for each test, it is very difficult to assess an overall accuracy of the measurements. The primary task was to provide comparable data treated in exactly the same manner throughout. The median level difference found between direct sound level readings on site and the data reduction was 0.1 dB(A). The cumulative distribution of level differences determined in that way gave a spread o f - 0 . 3 dB(A) as 16th percentile and +1-2 riB(A) as 84th percentile values.

Repeatability The repeatability of the tests, which also includes influences of the driver and the vehicle, gave equally good results. For three cars repeat measurements on three different days were carried out. The spread of the results for the acceleration tests was 0.1 dB(A) for one car, 0.2 dB(A) for the second car and 0.8 dB(A) for the third car. The time dependence of the A-weighted sound level for consecutive acceleration passages of one car are shown in Figure 14 in section 4.4. We feel that the small spread in levels found in these cases would indicate that an adequate repeatability was achieved within the limits of the available equipment.

4. RESULTS OF THE SURVEY 4.1.

ROLLING

NOISE

Weighted levels For all models of cars tested, the rolling noise was measured at speeds of 50, 75 and 100 km/h on a dry smooth asphalt test track. The cumulative distribution of the maximum A-weighted sound level for these passages is given in Figure 1. This representation allows the easy determination of the fraction of cars with levels below or above any given limit chosen on the abscissa scale. It also indicates the spreads of levels determined. The median levels found for these three speeds are 64.3, 69-5 and 74.5 dB(A), respectively. The standard deviations are of the order of 2 dB(A). The results are in good agreement with earlier data of Mills [I] and Rath6 [2]. 99

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Figure 1. Cumulative distribution of the values of maximum A-weighted sound level for coasting passages on smooth dry asphalt test track. The microphone position was at 7-5 m from the center of the test track and 1.2 m above ground (ISO-microphone position after Recommendation R 362).

487

EXTERIOR NOISE OF CARS

Speeddependence The increase in sound level with speed was determined for each car on the basis of six individual measurements (three speeds, two directions of travel). The resulting distribution in Figure 2 has a median value of 10 dB(A) per doubling of speed. 99

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Figure 2. Cumulative distribution of the speed dependence of the maximum A-weighted sound level in coasting passages.

Spectra For each passage, the I/3 octave spectrum at the time of maximum A-weighted sound level was determined. The general trend associated with these spectra can be shown by treating the levels measured in each single 1/3 octave band statistically and determining the median level. In this manner a set of smoothed spectra has been drawn for the speeds of 50, 75 and 100 km/h in Figure 3. The general shape of these spectra corresponds to a fiat section up to 1500 Hz, and then a - 6 dB/octave slope. The comparison of the curves for the three speeds shows an even increase in sound pressui-e level over most of the frequency range. It corresponds.to the differences in A-weighted sound levels mentioned above. There is a slight characteristic peak at 500 Hz for 50 km/h which goes up to I kHz for 100 km/h and which is probably due to a typical tread pattern dimension. 80

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Figure 3. Spectra of rolling noise at the moment of maximum A-level from coasting passages. ]SO microphone, dry smooth asphalt test track. The curves were obtained by statistical analysis of the levels for all cars in erich 1[3 octave band and by plotting the median values for each band.

488

E.J. RATHE E T A L .

Car weight A distinction between light cars and heavy cars within the sample of cars tested shows the heavy ones to be slightly more noisy at 50 km/h, but shows a small difference at I00 km/h. Additional measurements made with a small group of cars and with some variations in car load resulted in the conclusion that the weight of a passenger car has little influence on rolling noise radiation.

Type of tyre The median value o f the maximum A-weighted sound levels for the cars with cross-ply tyres was nearly 2 dB(A) above'that for the cars with radial tyres. Cars with wide tyres (> 103 ram) were less noisy by 1 dB(A) at 50 km/h than cars with narrow tyres (<96 mm). At the speed of 100 km/h there was no difference. 4.2. NOISE OF STATIONARY CARS

The measurements on stationary vehicles were carried out with the measuring microphone at a distance of 7 m from the exterior surface of the cars, and at a height of 1.2 m above the ground. They were located at either side of the middle of the car.

Weighted levels The distribution of levels derived from these positions is plotted in Figure 4. The full line with a median value of 70 dB(A) corresponds to the average sound level o f both sides. It was measured at a constant engine speed equal to that required on the approach for the acceleration test after ISO. The level differences between the two sides are small. This is evident from the dashed curve of Figure 4, where only the higher level of the two microphone positions was used. 9Cd

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Figure 4. Cumulative distribution of the values of A-weighted sound level for stationary cars with the

engine speed corresponding to the approach condition of the ISO acceleration test. Microphones at lateral position at 7 m from car body. Dry smooth asphalt ground surface. A, Mean level for both sides of car; B, higher level of the two sides.

Enghle speed dependence The influence of engine speed is given in Figure 5. It was determined from measurements made at engine speeds differing by -t-500 rev/min with respect to the above measurements. A median value o f 14 dB(A) per doubling of engine speed was found. This dependence is strong, and in order to obtain reproducible results, the engine speed has to be controlled accurately.

EXTERIORNOISEOF CARS 99

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Figure 5. Cumulative distribution of the values of engine speed sensitivity of the A-weighted sound level for stationary cars. Derived from measurements at the engine speed corresponding to the approach condition for ISO test and variations of +500 rev/min in speed.

4.3. PASSAGESAT CONSTANTSPEED

Weightedlevels The distributions o f A-weighted sound levels for the speeds of 50, 75 and 100 km/h for passages in fourth gear are given in Figure 6. The median values are 66.4, 72.6 and 78.2 dB(A), respectively. The standard deviations are all of the order o f 2 dB(A). The corresponding distribution for measurements at 50 km/h in second gear has been drawn in the same figure as a dashed curve. The median value is the same as for the speed of 75 km/h in fourth gear. I I I l l l

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Figure 6. Cumulative distribution of the values of maximum A-weighted sound level for constant speed passages. ISO microphone, dry smooth asphalt test track. , Passages in fourth gear; . . . . , passages at 50 km/h in second gear. Although a considerable amount of data is available on noise from constant speed passages o f cars, it is in most cases taken from natural road traffic with quite a spread of speeds and can hardly be compared with the present data. Frietzsche's [3] levels for constant speed passages are similar in speed dependence for the fourth gear passages. The influence of the road surface would probably account for his levels being slightly higher, as is shown also by Ullrich [6]. The results o f Rath6 [2] for fourth gear at 60 km]h agree very well, as do also those for the second gear test where these were done close to the speed of 50 km/h and are comparable. A theoretical speed dependence o f 30 dB(A) decade o f speed is proposed by Galloway [4]. This is based on noise emission being proportional to the power expended by the vehicle, or to the cube o f vehicle speed. The absolute level obtained with his formula

490

R A T H I ~ E T AL .

E.J.

agrees well with the present data at medium speeds. The reason for the measured slope being greater lies in the contribution from engine noise that starts contributing more to the A-level as its components increase in frequency and are less attenuated by the A-weighting curve.

Averaged spectra The curves resulting from the statistical analysis of spectra measured at the moment of maximum A-weighted sound level o f each passage are shown in Figure 7. The full lines were determined for the three speeds of 50, 75 and 100 km/h in fourth gear. The lower frequency parts of all these spectra are determined by engine noise components. In the spectrum for the speed of 50 km]h there are peaks due to engine noise at 63 and 125 Hz. With increasing speed the frequency of these peaks moves to 125 Hz and 250 Hz for the speed of I00 km/h. The medium and high frequeficy parts of the spectra show a gradual increase with speed. For the dashed curve, which was measured at a speed of 50 km]h in second gear, the engine noise peaks are similar to those for I00 km]h in fourth gear. The general shape of the spectra is very similar to that forrolling noise in Figure 3, with the exception of the influence of engine noise at the low frequency end. ~2

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Figure 7. Spectra of constant speed passage noise at time of maximum A-'level. ISO microphone, dry smooth asphalt test track. The curves were obtained by statistical analysis of the levels for all cars in each 1/3 octave band and by plotting the median values. Passages in fourth gear: A, 50 km]h; B, 75 km/h; C, 100 km]h. Passages in second gear: D, 50 km/h.

bldividual spectra The wide spread of engine speeds found between the different car models tested under the conditions shown in Figure 7 leads to a very pronounced smoothing of the noise spectra. Individual spectra have much more distinct engine noise peaks. In order to find typical cases, and to estimate the importance of engine noise for the overall A-weighted sound level a different approach can be taken. On the basis of the distribution of maximum A-weighted sound levels o f all cars, the individual spectrum that gave the median A-weighted level was determined. After applying a level correction according to the A-weighting curve, these individual spectra were plotted in Figure 8. It can be seen that the medium frequency components determine the A-level for passages in fourth gear at all the three speeds examined. For the passage in second gear at 50 km/h the engine noise peak dominates. The irregularity of the spectra is even more pronounced, if the individual spectra are drawn for those cars whose A-weighted maximum sound levels were closest to the 84 ~o percentile of the A-level

491

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Frequency(Hz) Figure 8. A-weighted spectra of constant speed passage noise at time of maximum A-level. ISO microphone, dry smooth asphalt test track. The curves were obtained by statistical analysis of the maximum A-weighted sound levels of all cars and by plotting the individual spectra for the cars with median A-level. Passages in fourth gear: A, 50 km/h; B, 75 k m / h ; C, 100 km/h. Passages in second gear: D, 50 km/h. 80

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Figure 9. A-weighted spectra ofcortstant speed passage noise at time of maximum A-level. ISO microphone, dry smooth asphalt test track. The curves were obtained by statistical analysis of the maximum A-weighted sound levels of all cars and by plotting the individual spectra for the cars closest to the 84th percentile of the A-level distribution. Passages in fourth gear: A, 50 k m / h ; B, 75 kin/h; C, 100 km/h. Passages in second gear: D, 50 km/h.

distributions. This is shown in Figure 9. In all the spectra of these cars with higher noise levels the engine noise determines the overall level. 4.4. ACCELERATION PASSAGES

Weighted levels The distributions of sound levels measured under conditions as laid down in ISO Recommendation R 362 are plotted in Figure 10. The full line was obtained by using data from all cars tested. For the dashed line all cars with automatic gear box, five-gear box as well as the cars with Diesel or Wankel engine were left out. This hardly changes the distribution. The median level is 79.5 dB(A). The levels measured for acceleration runs can best be compared with the results of Betzl [5]. The agreement is within l dB for all categories of cars where the sample used is sufficiently large.

492

E. J. R A T H E E T A L .

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Figure 10. Cumulative distribution of the values of maximum A-weighted sound level for acceleration passages after ISO. ISO microphone, dry smooth asphalt test track. A, all cars; B, all cars except those with Diesel or Wankel engine, and cars with automatic or 5-speed gear box.

Speed dependence The tests were repeated for all cars but with approach speeds being used that differed from the conditions specified by the ISO Recommendation by 4-500 rev/min of engine speed. The results for each car were used to compute the dependence of maximum sound level on the engine speed at the moment of maximum sound level. The distribution for these values is shown in Figure 11. The median value is 9 dB(A) per doubling of rpm. The spread is rather large and probably due in part to the large range of approach engine speeds under ISO conditions (range of 2200 to 5250 rev/min, median = 3620 rev/min). 99

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Figure 11. Cumulative distribution of the values of engine speed sensitivity of the maximum A-weighted sound level for acceleration passages after ISO. ISO microphone, dry smooth asphalt test track.

Averagedspectra The median values and estimates of the respective standard deviations of the levels in each l]3-octave band are plotted in Figure 12 for the time of maximum level during the acceleration passages. The general nature of the spectra can be described by a 9 dB/octave rise at lower frequencies to about 125 Hz, and then a drop of 3 dB/octave throughout the mid- and high-frequency range.

hzdividualspectra For three cars picked from the distribution of ISO sound levels, the individual spectra after applying the A-weighting are shown in Figure 13. The engine noise peaks are again

493

E X T E R I O R NOISE OF C A R S 90

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Figure 12. Spectra of noise from acceleration passages after ISO at the time of maximum Aolevel. ISO microphone, dry smooth asphalt test track. The curves were obtained by statistical analysis of the levels for all cars in each 1/3 octave band and by plotting the median value and the 16th and 84th percentile points in each band. 8O

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Figure 13. A-weighted spectra of noise from acceleration passagesafter ISO at time of maximum A-level. /SO microphone, dry smooth asphalt test track. The curves were obtained by statistical analysis of the maximum A-weighted sound levels of all cars and by plotting the individual spectra for the car with median A-level and for the cars closest to the 16th and 84th percentiles of the A-level distribution.

very m u c h m o r e p r o n o u n c e d than in F i g u r e 12. T h e engine-related noise sources are m a i n l y responsible for the overall A-weighted s o u n d level.

Time dependence of level In Figure 14, the time d e p e n d e n c e o f the A-weighted s o u n d level has been r e p r o d u c e d for a t o t a l o f eight passages with the same test vehicle. T h e u p p e r g r o u p was o b t a i n e d for one direction o f travel a n d the lower g r o u p for the o t h e r direction o f travel. In spite o f some slight differences in the shapes o f the curves, the m a x i m u m levels show a very small spread. This p a r t i c u l a r vehicle shows some differences in the curves for the two directions o f travel a n d little difference in m a x i m u m level, b u t other cars showed still greater changes between the two sides due to a s y m m e t r i c p l a c e m e n t o f noise r a d i a t i n g c o m p o n e n t s .

494

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Figure 14. Time dependence of the A-weighted sound level for acceleration passages after ISO. ISO microphone, dry smooth asphalt test track. Repeated tests for one car. (a) Passage with microphone on right-hand side of driver; (b) opposite direction of travel. The standard deviation is 0-26 dB for (a) and 0.44 dB for (b). The mean difference between direction of travel is 0-35 dB.

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Figure 15. Plots of the A-weighted sound level as a function of the position of the front of the car with respect to the microphone plane for acceleration passages after ISO of four cars. ISO microphone. Curve A w/is obtained for a car With rear engine, the other curves relate to cars with front engine.

The sound level does not reach its maximum value at the same vehicle position with respect to the microphone plane for all cars. The measured sound levels are plotted in Figure 15 for four different cars as functions of the position of the front end of the car with respect to the microphone plane. Curve A for a car with rear engine reaches the maximum value when the car has passed the microphone plane, because the noise is radiated mainly from the rear parts of the car. The other three curves obtained with front engine cars show that in this case the maximum sound level can occur on either side of the microphone plane. The distinction depends on the relative importance of directly radiated engine noise and the exhaust noise emission. It is also affected by the engine speed increase during the passage of the car. 4.5. SUMMARYOF MEASUREMENTS The main results of the measurements carried out for the microphone position defined by ISO Recommendation 362 are given in Table 3. Based on the statistical distributions of measured levels, the average levels and an estimate for the standard deviation are tabulated. Some average levels obtained from a study on rolling noise characteristics that was carried out with the same measuring and data processing equipment on a smaller sample of cars have been added to the table.

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EXTERIOR NOISE OF CARS TABLE

3

Average maxhnum A-weighted sound levels measured at the ISO microphone position

Passage type

Acceleration (after ISO) Noise level sensitivity with engine speed Constant speed 50 km]h, 2nd gear 50 km]h, 4th gear 75 km]h, 4th gear 100 km/h, 4th gear Sensitivity with speed, 4th gear Engbte noise (stationary) No load, at ISO approach engine speed Sensitivity with engine speed Rolling noise Radial tyres, dry asphalt track, 50 km/h 75 km/h 100 km/h Sensitivity with speed Crossply tyres, dry asphalt 50 km/h Winter tyres, dry asphalt 50 km]h Radial tyres, wet asphalt :: '50 km/h Radial tyres, dry cobbles 50 km/h Spiked tyres, dry asphalt 50 km/h

Sound level dB(A)

Standard deviation dB(A)

79.5 9.0/doub.

2"0 4.5/doub.

72.6 66.4 72.6 78.2 11.5/doub.

4"0 2"0 2"0 2"0 1.2[doub.

70-5 14.0]doub.

3"0 3"5/doub.

64.3 69.5 74.5 10.0/doub.

2.0 2.0 2.0 1-5/doub.

66.2 67"0 71.0 74.0 75.0

TABLE 4

Average maMmum A-weighted sound levels measured at 25 m distance front the test track and a height of 4 m above the ground Passage type Acceleration after ISO Constant speed 50 km/h in 2nd gear Constant speed 50 km/h in 4th gear Constant speed 75 km/h in 4th gear Constant speed 100 km/h in 4th gear Coasting at 50 km/h Coasting at 75 km/h Coasting at 100 km/h

Sound level dB(A) 67"9 63"1 56"3 61 "3 66"3 54"1 58"7 63 "3

496

E.J. RATHt~E T ,elL.

4.6. NOISEEMISSIONVALUESFOR LAND USE PLANNING Noise levels measured at a distance of 25 m from a street are often used for purposes of land use planning. It can be of interest to see how these levels are affected by different driving conditions. For this purpose a corresponding microphone position was used for all the tests. The average values for the maximum A-weighted sound level are given in Table 4 for all standard measurement types that have been carried out. The relationship among these levels is the same as for the ISO microphone and the comments given in section 5.1 are equally applicable.

5. PARAMETERS INFLUENCING NOISE EMISSION 5.1. DRIVINGCONDITIONS The driving conditions of the acceleration test after ISO R 362 can be considered to be representative o f the greatest noise radiation. More noise can be produced under certain conditions, but this data is not available and would also be very extreme.

Acceleration hlfluence I f a driver does not accelerate his car near the microphone as specified by ISO, then this results in a passage at a constant speed of 50 km/h in second gear for all cars with the exception of cars with five-speed gear boxes. Since the latter are a minority in this survey, the difference in m a x i m u m A-weighted sound levels between the ISO acceleration test and the test at a constant speed o f 50 km/h in second gear is a good indication of the driver's influence on noise levels by accelerating the car. The distribution for this difference is given on Figure 16 with a dashed curve. It ranges from negative values that would be expected for some cars with five-speed gear boxes to nearly 10 dB(A), with a median value of about 5 dB(A). 99

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Figure 16. Cumulative distribution of the differences in maximum A-weighted sound level between

acceleration passages after ISO and constant speed passages. ISO microphone, dry smooth asphalt test track. , Passages in fourth gear; . . . . , passages at 50 km/h in second gear.

Speed hlfluence For uniform driving conditions the driver would tend to use a higher gear and thus further reduce the exterior noise levels. This is a goal aimed at by m a n y systematic improvements in traffic handling by better roads and synchronized traffic lights, or if possible the elimination

497

EXTERIOR NOISE OF CARS

ofstreet crossings. Driving conditions in fourth gear are also compared with the ISO acceleration test in Figure 16. The median level differences are as follows: ISO acceleration--constant speed, 4th gear 50 kin/h: 12 dB(A); ISO acceleration--constant speed, 4th gear 75 km/h: 7 dB(A); ISO acceleration--constant speed, 4th gear 100 km/h: 2 dB(A). Six small cars, six cars with five-speed gear boxes and two cars with automatic gear boxes have higher levels at 100 km/h than in the ISO tests.

Spectral differences In Figure 17 the median spectral differences based on the statistical evaluation in each I/3-octave band have been plotted for the different driving conditions. The reference condition is again the ISO acceleration test.

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Figure 17. Spectral differences between ISO acceleration run and constant speed passages in fourth gear for passage noise at time of maximum A-level. ISO microphone. The curves were obtained by statistical analysis of the level differences for all cars in each 1/3 octave band and by plotting the median value. A, 50 kin/h; B, 75 kin/h; C, 100 km]h; D, constant speed of 50 km/h in secor~d gear; E, coasting passage at 50 km/h.

For the passages at a constant speed of 50 km]h in second gear a dashed curve shows a rather uniform median level difference of 5 dB(A) over a wide frequency range. The acceleration of a car does not only affect the low-frequency end of the spectrum. The three full lines were obtained for passages at constant speed in fourth gear. The lowest of these three curves for the speed of 100 km/h indicates that the median spectral difference compared to the ISO acceleration test is small. In the low-frequency range up to about I00 Hz the level differences are negative. This is due to the average car running at a slightly higher engine speed at 100 km/h than for the ISO test. The difference in spectra is of similar nature for the speed of 75 km/h shown by the middle curve. For the range above 500 Hz up to high frequencies the difference is close to 5 dB(A). The mid-frequency range between 100 Hz and 500 Hz is affected by higher harmonics of engine noise. The uppermost full line for the speed ofS0 km]h in fourth gear had a fairly uniform level difference of about 12 dB(A) over the whole range of frequencies from 125 Hz to 8 kHz. This result again shows that the acceleration affects a wide frequency range in its noise level.

498

E.J. RATtlI~E T

AL.

The uppermost curve drawn in a fine dotted line in Figure 17 was obtained as spectral difference between the rolling noise of coasting runs at 50 km/h and ISOacceleration runs. Since all these curves are based on the same reference condition, it is also possible to compare them with each other. The difference between curve E for coasting passages at 50 km/h and curve A for the constant speed passage at 50 km/h in fourth gear is of particular interest, since it shows the influence of engine noise. This influence is smallest in the range between 500 Hz and 4 kHz and reaches a maximum of 13 dB at 125 Hz and approaches l0 dB at 8 kHz. The differences in A-weighted sound levels between these two driving conditions tend to be small because of the small spectral differences in the range of I kHz. 5.2. MECttANICALCHARACTERISTICSOF CARS The measuring conditions described above were the same for all cars tested. The differences in maximum sound levels found have therefore to be attributed to differences in the mechanical characteristics of these cars9 It would be interesting to find out which parameters permit an optimization of the noise radiation without undue loss of driving performance. Unfortunately, a great number of parameters is involved. I

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Figure 18. Correlation plot for measured rersus calculated maximum A-weighted sound levels for ISO acceleration passages. Some examples of quantizable and non-quantizable parameters are the following: engine capacity, engine power, cylinder bore, maximum engine torque, maximum engine rpm, car weight, car length, car width, maximum speed, engine position in car, position of driven wheels, material of engine block, degrees of damping of body, quality of engine mounting, degree of screening of engine, effectiveness of intake and exhaust mufflers plus a whole series of composite parameters such as stroke/bore ratio, specific engine power (HP/liter), engine power/car weight (HP/kg). The restriction to treating quantizable parameters only is not sufficient. These parameters are very strongly related to each other. A change in one parameter would therefore cause changes in other parameters above and beyond the influence required from the point of view of noise radiation. For example, it is not possible to distinguish the influences of engine capacity and engine power on noise radiation without specific experiments on appropriately modified cars.

EXTERIOR NOISE OF CARS

499

Any variation in operating condition must also be taken into consideration. One way of doing this is the definition of noise levels for standardized operating conditions. Extensive calculations have been done in which this approach was used. They involved a careful selection o f four basic parameters with low interparameter correlation, and were used to calculate noise levels by using relationships derived by multiple regression procedures. The relationships describe trends found in the measured values, but do not allow the establishment o f causal connections between mechanical parameters and noise emission. A comparison of measured and calculated maximum sound levels for the ISO acceleration tests is shown in Figure 18. Although some trends seem to have been taken care of, the remaining scatter proves that additional parameters (such as, for example, the degree of shielding provided by the engine compartment) have a marked influence on the acoustic performance of a car. One direction of the trends indicates that cars with large, slow turning engines are quieter than those with small, fast turning engines for the same maximum rated power. ACKNOWLEDGMENT The survey was supported by Citroen, Fiat, Peugeot, Renault, Volkswagen and the Unikeller Group o f Companies. The authors wish to thank their colleagues within all the above firms for their contributions and helpful discussions.

REFERENCES 1. C. H. G. MILLS 1970 MIRA Bldletin No. 3. Noise emitted by coasting vehicles. 2. E.J. RA'rH~ 1966 Acustica 17, 268-227. Ueber den L~irm des Strassenverkehrs. 3. G. FRIETZSCHE1971 Kampfdem L&m 18, 94-103. Ger/iuschmessungen an Kraftfahrzeugen unter versehiedenen Betriebszust~inden. 4. W. J. GALLOWAY1971 National Cooperatire Highway Research Program Report. Highway noise measurement, simulation and mixed reactions. 5. W. BETZL1971 KampfdemL&m 18, 122-127. Kritisehe Betrachtung der bei der TyppriJfung von Kraftfahrzeugen gemessenen Ger~iuschwerte im Hinblick auf die zu e~vartenden internationalen Vorschriften. 6. S.'ULLRICtl 1972 Kampfdem Liirm 19, 131-136. Zum Verh~iltnis Rollger~iusch zu Fahrger.:iusch bei Personenkraftwagen.