Influence on tire/road noise emission by vehicles of different construction

VTIratt"

357A

₁₉₉₀

Influence on tire/road noise

emis-sion by vehicles of different

con-struction

Jerzy A. Ejsmont and Ulf Sandberg

w V g-OC/l

Statens vé'g- och trafikinstitut (vm - 581 01 Linképing

l/TIran

357A

1990

Influence on tire/road noise

emis-sion by vehicles of different

con-struction

Jerzy A. Ejsmont and Ulf Sandberg

_Vag-00/)

_{Statens ve'g- och trafikinstitut (VTI/ - 58 1 0 1 Linkb ping}

CONTENTS 4.1 4.2 4.3 4.4 4.5 6.1 6.2 6.3 6.4 6.5 6.6 6.7 PREFACE ABSTRACT REFERAT SUMMARY SAMMANFATTNING INTRODUCTION

PURPOSE AND LIMITATIONS

POTENTIAL CAUSES FOR VEHICLE INFLUENCE ON TIRE/ROAD NOISE EXPERIMENTAL DESIGN General Test surface Test tires Test vehicles Measuring set-up RESULTS DISCUSSION

Difference between cars in A-weighted sound levels Differences between cars in frequency spectra

Differences due to a change in body-to-road clearance Differences due to transmission

Differences in recorded time histories Computer simulations of time histories Investigations by other researchers

CONCLUSIONS

REFERENCES

VTI RAPPORT 357A

II III VI m k m -P P ll 19 19 20 21 22 22 23 23 25 27

PREFACE

This report is written jointly by Jerzy A. Ejsmont and Ulf Sandberg. Jerzy A. Ejsmont is the main author. Both authors have contributed to all chapters; however, with emphasis by Dr. Ejsmont on technical descriptions and results and by Dr. Sandberg on more general chapters. The time history simulations were made by Dr. Ejsmont solely.

The work reported was part of a deve10pment of standardized tire/road noise measuring methods in an ad-hoc group under the ECE/WP 29/GRRF for which Dr. Sandberg was chairman.

The authors would like to thank two persons at this institute especially

for their contribution to this work:

0 Mr. Jan Wenall, who assisted in the experiment and who also made most of the spectral analyses afterwards.

0 Ms. Eva Gustavsson, who has given many useful comments and suggestions regarding the report.

This report has been reviewed according to the normal VTI common practice regarding reports.

In uence on Tire/Road Noise Emission by Vehicles of Different Con-struction

by

Jerzy A. Ejsmont

Technical University of Gdansk PL-SO 952 Gdansk, Poland

and

Ulf Sandberg

Swedish Road and Traffic Research Institute

5-581 01 Linkoping, Sweden

ABSTRACT

The report presents an investigation of test vehicle influence on the results of tire/ road noise measurements. The experiments were conduct-ed by testing the same set of tires on four different, compact cars and another set of tires on four bigger cars. The cars were selected to be different in age, condition, type, etc. Despite a great variation in car type and construction the differences of the measured A-weighted sound levels were very small. Some interesting differences in noise spectra were, however, noticed for cars of unusual construction and/ or in slightly impaired condition.

II

Inverkan p5 dack/véigbanebulleremissionen av valet av provfordon

av

Jerzy A. Ejsmont

Tekniska hogskolan i Gda sk

PL-SO 952 Gdar lsk, Polen

och

Ulf Sandberg

Statens vag och trafikinstitut 581 01 Linkoping

REFERAT

Rapporten presenterar en undersokning av hur mycket matningar av dack/vagbanebuller péverkas av provfordonets egenskaper och kondition.

Forsok utfordes genom att samma dack testades pa fyra olika smébilar

och en annan typ av dack pa fyra st'orre bilar. Bilarna valdes ut for att vara olika betraffande alder, skick, typ etc. Trots stora variationer i biltyp och konstruktion var skillnaderna mycket sma betraffande de uppmatta A-viktade ljudnivaerna. Emellertid noterades en del intres santa skillnader i frekvensspektra for bilar med speciella tekniska

losningar och/eller i forsamrad kondition.

III

Influence on Tire/Road Noise Emission by Vehicles of Different Con struction

by

J erzy A. E jsmont

Technical University of Gdansk PL SO 952 Gdansk, Poland

Ulf Sandberg

Swedish Road and Traffic Research Institute

5 581 01 Linkdping, Sweden

SUMMARY

The Group of rapporteurs on brakes and running gear (GRRF), within the United Nations Economic Commission for Europe (ECE), established some years ago an ad hoc group with the task to develop standardized methods for tire/road noise measurement. One of the methods proposed in 1986 was the so called coast-by method, in which the tires are tested by coasting an ordinary vehicle equipped with the test tires along a road or a test track to the side of which a microphone is placed. The question has been raised whether the vehicle itself may influence the measured noise, which is supposed to originate from the tires. So far, the vehicle influence on tire/road noise has been considered negligible. However, in recent years it has been suggested that this may not be correct.

This report presents an investigation with the purpose to study the

influence of the test vehicle on the tire/road noise emission.

The experiments were conducted by testing the same set of tires on four

different compact cars (the "light car" group). Another set of tires was

tested on four bigger cars (the "heavy car" group). The cars were selected to be as different as was possible (within the time and budget restraints) in age, condition, type, etc. Coast-by runs were made at 30, 50, 70 and 90 km/h during which the maximum A-weighted sound level as well as the frequency spectra were measured.

Despite a great variation in car construction, condition, etc., the cars caused little influence on the tire/road noise when A-weighted sound

IV

levels were concerned. Interesting differences occurred only for a car with damaged front wheel adjustment and a car with very stiff suspen-sion in relation to "normal" cars. These differences were so small, however measurable, that they are of questionable practical importance. See the figure on page III for a summary of the results.

At 70 km/h, which is the recommended speed for such tests, no differences were measured of such a size as to be detectable by normal hearing.

A study of the frequency spectra revealed that, outside the frequency range which is of the greatest importance to the overall assessment, there were a few interesting differences. One car had brakes which emitted high frequency noise, mainly at the two lowest speeds. Another car, with a hydro-pneumatic suspension and also low air resistance, emitted less noise in relation to the others at very low frequencies. We speculate that the latter might be due either to a suspension or a

turbulence effect.

There were indications that some minor discrepancies between cars occurred at the fundamental tire tread impact frequencies. These cases happened to coincide to the cars with extremely stiff, respectively extremely soft suspension.

The speed for which the influence of the car type onnoise appeared to be smallest was 70 km/h. This is in line with the recommendations given in the proposed measurement standard.

Even if no important differences in the noise from the tires traceable to the test cars appeared at 70 km/h, a few recommendations concerning

the choice of test cars for tire/ road noise measurements are warranted:

o The cars should be in good general condition. Especially important is that the front wheel adjustment and the suspension are all right.

0 The problem with brake noise must be observed, especially at speeds

of 50 km/h and lower.

0 Cars with un-conventional suspension types, e.g. hydro-pneumatic, should be avoided until their effect on tire/ road noise has been better

established.

The main conclusion is that the measurements do not appear to be influenced by the choice of test car to such a degree that it is of practical importance, provided that the recommendations in the mea surement procedure are full-y followed.

60 ~ dBiA) 68 - dB(A) dBiA) 78 i dB(A)

o\\ 73 " \\\\\ rib-D Piaf W ~~~H~ _ _ ~ _ _ _ --__"'D" ~ <> Honda 1 77 -59 67 Groupi Tire: 72 4 o 1SSSR13 O~ \ K ~ K />~( 1.0 ' \ \VC // \:_ A Mazda // NC Golf /// 7 q / -1 6 .-58 A 66 ... no... ., ...

... --- "

71 -

...

Volvo 765

...:::.'.'.':.l... ... . . . . n . I I . ' . ' . ' ' ' ' . . ._ Volvo 142

I ... ...u... ...~--...-..I_',-,o.-.-...-.-,-.- ...:..:Lit: --.-...:::..-...I.-.. I. . -. . . I .~ . . I. . . ' .. . '. - . . . - "H

... ~41 GroupZ ...-- Tire: "-... . 16SSR15 57 - 65 ~ d "'1 Citroen _/ 30km/h 50 kmlh 70 km/h 90 km/h

A weighted sound levels in dB(A) for each Speed and each vehicle. Note

that differences of less than 0.5 dB(A) are not statistically significant (p

5 5%). Differences of less than 1 dB(A) are generally not considered as

audible.

VI

Inverkan pa dack/véigbanebulleremissionen av valet av provfordon

av

Jerzy A. Ejsmont

Tekniska hogskolan i Gdansk

PL-8O 952 Gdansk, Polen

Ulf Sandberg

Statens véig och trafikinstitUt 581 01 Linkoping

SAMMANFATTNING

"Rapportorgruppen for bromsar och hjulstéill" (GRRF) inom Forenta

Nationernas Ekonomiska Europakommission (ECE) bildade i mitten p21 80

talet en ad-hoc grupp med uppgift att utveckla standardiserade metoder for dack/vagbanebullermatningar. Ett tardigt forslag redovisades 1986 i vilket en av de foreslagna metoderna var den 5 k forbifartsmetoden déir dacken testas genom forbifart (frirullning) med ett vanligt fordon utrustat med provdack pa en vag eller provbana vid vars sida en mikrofon placerats. De ovriga tva metoderna som foreslagits anvéinder ej normala fordon vid provningarna. For den forstnamnda metoden har frégan uppkommit huruvida fordonet sjalvt kan paverka det uppmatta ljud som formodas komma fran dacken. Hittills har fordonets paverkan pa dack/vagbanebullret ansetts forsumbart, men under senare at har vissa utlandska matningar antytt att detta kanske inte éir riktigt.

Denna rapport presenterar en undersokning med syfte att studera prov-fordonets péverkan pa déick/véigbanebulleremissionen.

Experimenten utfordes genom bullermatning med forbifartsmetoden var dvid man utnyttjade samma dackuppsattning pa fyra olika smébilar

(gruppen kallad "latta fordon"). En déickuppsattning av annan typ testades

pa fyra storre bilar (gruppen kallad "tunga fordon"). Bilarna valdes ut for

att vara 5a olika som mojligt betréiffande alder, skick, typ etc. Forbi-fartsmatningar gjordes vid 30, 50, 7O och 90 km/h varvid den maximala A-viktade ljudnivén och frekvensspektrum uppmattes.

VII

Trots stor variation 1 bilkonstruktion, skick, etc, hade bilarna IIten Inverkan pa dack/vagbanebullret betréiffande A-viktade Ijudnivaer.

In-tressanta skillnader intraffade endast med en bil med felaktig

fram-hjulsinstéillning och en bil med mycket styva fjadringsegenskaper i forhallande till "normala" bilar. Dessa skillnader air 53 sma - fast

matbara att de saknar praktisk betydelse. Se figuren pa sid VIII for en

sammanfattning av resultaten.

Vid 70 km/h, vilket éir den rekommenderade hastigheten for sédana

tester, upptacktes inga beaktansvarda skillnader.

En studie av frekvensspektra avslc jjade att det fanns ett fatal intressanta skillnader utanfor det frekvensomréde som 5r av storst vikt for den totala bedomningen. En bil hade bromsar som avgav hogfrekvent Ijud, huvudsakligen vid de tva Iagsta hastigheterna. En annan bil, med hydro-pneumatisk fjadring och samtidigt lagre luftmotsténd, avgav mindre buller i forhallande till de andra vid mycket Iaga frekvenser. Man kan spekulera om det senare skulle kunna bero antingen pa en fjadrings- eIIer

en turbulenseffekt.

Det fanns indikationer pa att négra smarre diskrepanser meIIan bilarna intraffade vid de dominerande dackmonsterfrekvenserna. Det rékade vara sa att dessa fall overensstamde med de fall da bilarna hade mycket styv respektive mycket mjuk fjadring.

Den hastighet déir biltypen péverkade bullret minst tycktes vara 70 km/h. Detta 5r I linje med rekommendationerna i den foreslagna matstandar den. Aven om inga signifikanta skillnader i buller fran bildacken kan harledas till provfordonen vid 70 km/h Iéimnas nagra rekommendationer angéende val av provfordon for dack/vagbanebullermatningar:

o Provfordonen skall vara i gott allma'nt skick. Séirskilt viktigt ar att framhjulsjusteringen och fja'dringen at tillfredsstallande.

VIII

0 Problemet med buller frén bromsarna méste observeras, speciellt vid

50 km/h och légre hastigheter.

o Bilar med fjédringstyper som inte éir sé vanliga, t ex hydro

pneuma-tisk, bo'r undvikas vid provningen tills deras effekt p5 déiCk/végbane

buller har séikerst'éllts.

Den viktigaste slutsatsen éir att méitningarna inte tycks péverkas av val av provfordon i sédan grad att det har négon praktisk betydelse, forutsatt att den foreslagna métningsproceduren foljs till fullo.

60 '4 dB(A) 68 o dB(A) dam) 78 * dB(A)

/ / <>\\ " D\ C//// \\\\\ \ 0/, k x ~\ \\\ ,. 0 FIG? f ~hu _ ~ _ ~0 ~:>~ ; ;: % ~~ I D ~ 0 Honda ! i 59 67 77 q : . Group1 72 4 F Tire: C r .x

O~\\ /A_ _ _ _ _ __ "_A, o\\ i ISSSR.)

N 45 ___ \ § I // ~~O \VC \ I // \ :N:A Mazda I ,// 0 Golf J A // 6 76 q / 6 - . 58 . o - ' ' ' ' - . ' ..-.... ..-"'-.-.,_ « 71 d VOIVO 760 El ' ' ' ' ' - ... In Volvo-15,2 I _{...n... ...-...,,.-..,-...'...-.:'.:..:it.. .1 .- ~ ov ~ '} .. . I . - i I I _ . ' . . ~ . I . _ _ _ _ . ,. . . - - - ~"" SUOD b .1 ... a ...." -. ' . iGroupZ ...u Tire: I 16581215 57 6S " -" E Citroen J 30km/h SO kmlh 70 kmlh 90 kmlh

A-viktade ljudnivéer i dB(A) for varje hastighet och fordon. Observera

att skillnader mindre éin 0.5 dB(A) inte éir statistiskt signifikanta (p _<_

5 %). Skillnader mindre éin l dB(A) anses vanligtvis inte horbara.

I INTRODUCTION

In 1983 the Group of rapporteurs on brakes and running gear (GRRF),

within the United Nations Economic Commission for Europe (ECE),

established an ad hoc group with the task to develOp standardized

methods for tire/road noise measurement.

Three methods have beenseriously considered in this work; the coast by,

laboratory drum and trailer methods (Refs. 1, 2). In the coast by method

the tires are tested on an ordinary vehicle which is coasting on a road or test track to the side of which a microphone is placed. Then it is possible that the vehicle itself influences the noise emission from the tires. However, this influence has, traditionally, been considered to be negli gible, though it has not been carefully tested - at least no such study has been published.

In 1985, however, some French data were reported which indicated that

there could be up to 4 dB(A) of difference in tire/road noise between two vehicles which were using the same tires (Ref. 3). Also Steven (Ref. it)

reported differences as high as 2.5 dB(A). The differences were most pronounced on a smooth-textured road surface. Recently, it has also been suggested that for some modern tires with very stiff sidewalls, vibrations from the tire tread may be transmitted via the axle and suspension to the vehicle body and may contribute as emission from the

vehicle body to the overall noise (Ref. 8).

It can also be mentioned that Ref. 9 claims that the air turbulence around a truck body may cause noise that is recorded as "tire noise". It is suggested to drive both upwind and downwind and record the differences in noise level as a function of vehicle speed in order to calculate the

"true" tire noise level.

When developing standard measuring methods for tire/road noise it is important to consider the possible impairment of the representativity of the measured values due to the vehicle influences and to avoid such influence by a proper design of the method.

2 PURPOSE AND LIMITATIONS

This experimental study was conducted in order to investigate the potential problem of vehicle influence on the noise emission from a certain tire type. It was aimed at detecting differences in tire/road noise emission due to different vehicles in the order of 0.5 dB(A); therefore the investigation needed to be made with the highest obtainable

preci-sion.

A limitation of the experiment was that only tires of "common" construction were used. Tires with extra stiff sidewalls were thus not included. Partly, the authors did not consider this important when the experiment was planned, partly such tires are generally used mostly on sport cars.

Also, only cars were used in the tests.

3 POTENTIAL CAUSES FOR VEHICLE INFLUENCE oN

TIRE/ROAD NOISE

The following items are listed as potential factors influencing tire/road

noise due to the test vehicle (see Fig. l): 1 Transmission noise (during coasting)

2 Brake noise (brakes are not 100 % released)

3 Body shape, i.e. aerodynamic noise around the vehicle body ll Dzo, from extra equipment, such as radio antennas, etc. 5 Excessive tire/road noise due to wrong wheel adjustment

6 Influence of suspension, e.g. by vibrations transmitted to the body or by suspension influence on tire dynamics

7 Influence of tire load and inflation

8 Body-road clearance (shadowing and reflecting properties)

9 Influence of rim tvne

Fig. l The vehicle parameters which may possibly influence tire/road noise

4 EXPERIMENTAL DESIGN

#.1 General

Two tire types and sizes were chosen for the tests. Eight vehicles were selected and grouped into a "heavy car" group and a "light car" group . Each tire was tested on the cars in one of the groups at the speeds of 30,

50, 70 and 90 km/h. The vehicles were chosen to be as different as

possible; the main difficulty being to find very different vehicles on which the same tire could be fitted. The finally chosen vehicles varied in age (1-13 years), in accumulated driving distance (7500-85000 km), in weight (160 kg within each group), in rim dimension (#.5 5.5"), in physical dimensions and body shape, in equipment such as radio antenna, side mirrors and spray flaps, in body-road clearance, in brake type and finally in suspension type (from soft hydro-pneumatic to very stiff mechanical

of a sports-car type).

The road surface was chosen to be a smooth-textured ar specially quiet

type. This was based on an analysis of the potential problems listed in Chapter 3, from which it appeared that if they are important they should

be accentuated when tire/road noise is low. When tire/road noise is high

it will mask noise originating from turbulence, brakes, body etc.

As stated above, the speeds ranged from 30 to 90 km/h. In the proposed

standard in (Ref. 1) 70 km/h is the nominal speed, which would be

sufficient for classifying tires; but here other speeds were included due to the desire to investigate the speed-dependent disturbing noise.

All tests were made on the same test track, undisturbed by other noise, on the very same day. The wind was very weak (less than 1 m/s) but the temperature varied from 150C in the morning to 20°C in the afternoon

and 100C in the late evening. The air humidity was very high in the

morning and late evening but low in the afternoon.

For each test condition 5-6 acceptable runs were made. The measure-ments and analyses were made according to the prOposed standard (Ref. 1), except for the intentional variation in vehicle condition, load and rim size. Also, the smooth test track surface did not exactly meet the

requirements as it was by intention somewhat more quiet than a normal

road surface.

Two additional tests were made: one with varying body-road clearance (the pneumatic suspension of the Citro'én was utilized) and one with front wheel or four wheel drive (the Honda Civic Shuttle 4WD). Besides the mentioned experimental program, a computer simulation of possible influence of car wheel base, wheel track, length and width was per-formed (Ref. 10).

4.2 Test surface

The test surface was an asphalt concrete with chippings max. 12 mm. It was a dense surface, however with a relatively open texture. It was located on the Mantorp Race Track, was more than 10 years old and had not been exposed to normal traffic and thus not compacted. Its texture was measured according to Ref. 1 and is indicated in Ref. 2 (as surface No. 3 in Fig. 4 there). It can be described as being approximately "midway" between a smooth and a rough reference surface (Ref. 1), or perhaps somewhat closer to the smooth surface.

This surface has been found to give approximately 4 dB(A) lower noise

level than normal, trafficked, smooth asphaltic concrete, for the type of

tires used here.

4.3 Test tires

The following two types of tires were utilized (please refer to Fig. 2):

0 Size 155SRl3, steel radial, type Gislaved Speed 216, driven less than 17000 km.

0 Size l65SRl5, steel radial, type Michelin XZX, driven less than 10000 km.

The rolling direction of the tires as well as the position of the tires on the car were preserved for all conducted tests. This is very important

Fig. 2 The two test tires: Gislaved Speed 216 used on the cars in the "light car" group and Michelin XZX used on the cars in the "heavy car" group

since it is the experience of many researchers that the rolling direction of tires may influence the noise (see further Chapter 6.7)

4.4 Test vehicles

All relevant vehicle data are given in Table l and the cars are shown in Figs. 3 and 4. The weight of the unladen cars varied by 250 kg for the "light car" group and 420 kg for the "heavy car" group. This was more

than desired so the lighter vehicles within each group were extra loaded.

Despite this, a load difference of 120 160 kg was preserved. In this way the proposed standard (Ref. 1) was violated slightly as the load per tire in the most extreme case was 68-69 % of the maximum allowed tire load. The standard specifies a load of 50-65 % (nominally 60%) of the maximum load.

Due to the very different vehicles it was not possible to use the same rims on all cars. This was an advantage in the light of the purpose of the experiment, i.e. to detect various influences.

inwmmw -. -ww.»,i».w.. - y x \1Q>~r w -, -..*wu"z¢n .9 .. .r, A~9 w 7m»

VTI RAPPORT 357A

Fig. 3. The test cars

in the "light" group

VW Golf CL = VW Rabbit

Honda Civic Shuttle

Mazda 323 Combi

WM - A~ ' -., . ) -.-.v¢a-~.vv~.-V . .

VTI RAPPORT 357A

Fig. 4. The test cars

in the "heavy" group

Volvo 142

Citro'én GSA 1220

Saab 900 GL1

Table 1 Test vehicle data

Type VW Golf Honda Mazda 323 Polski Fiat SAAB VOLVO VOLVO CITROEN CL 82 Civic Kombi 125 P 900 GL1 760 GLE 142 GSA 1220 Shuttle 85 Sedan 81 Pallas 8 ; Parameters Year model 1982 1985 1981 1976 1984 1983 1972 1980 Total height (mm) 1410 1490 1375 1440 1420 1434 1460 1350 Total length (mm) 3835 3990 3955 4226 4739 4785 4630 4200 Total width (mm) 1620 1660 1640 1630 1690 1760 1710 1630 Wheel base (mm) 2400 2450 2365 2506 2517 2770 2620 2550 Front wheel track (mm) 1390 1400 1390 1298 1430 1460 1350 1380 Rear wheel track (mm) 1360 1415 1395 1275 1440 1460 1350 1330 Ground clear-ance (mm) 210 240 200 260 200 210 190 220-260 Distance front/ front axle (mm) 770 570 780 600 950 750 720 800 Front/rear wheel drive F F/4WD F R F R R F Brakes front/

rear disc/drum disc/drum disc/drum disc/disc disc/disc disc/disc disc/disc disc/disc Type of front

suspension indep. indep. indep. indep. indep. indep. indep. hydropneumatic

Type of rear

suspension stiff axle stiff axle indep. stiff axle stiff axle stiff axle stiff axle hydropneumatic Curb weight (kg) 930 970 940 1180 1270 1450 1290 1030 Extra load (kg) 130 150 220 0 5O 0 O 290

All cars were tested and adjusted at a test station for front-wheel alignment. In one case no adjustment for poor alignment was made (the Honda).

This car had been involved in a slight accident and its right front was a little damaged. As a consequence, the toe in had been influenced. It was found to be 11 mm which is clearly outside the tolerances for this front-wheel driven car. This car was tested with this faulty toe in which would help to detect any influence of such mis alignment, although it is clearly required in the standard that the front wheels must be properly adjusted.

10

4.5 Measuring set-up

As stated already in 4.1, all the tests reported here were performed (essentially) in accordance with the proposal for tire/road noise measurements by the coast-by method (Ref. 1). The test vehicles were coasting by a roadside microphone which was placed 1.2 m above the road level and 7.5 m from the center line of the vehicle's path.

The sound was recorded by the following system:

1. Microphone B&K #165 with windscreen

2. Precision sound level meter B&K 2209

3. Tape recorder Nagra IV SJ

Additionally, an acoustical calibrator, BcScK pistonphone 4220, was used for calibration and an electro optical speed measuring device utilizing a

pair of sensors 20 m apart (centered around the microphone location) was

used for measuring the actual speed of the vehicles.

All the recordings were analyzed in the laboratory using a third octave band spectrum analyzer, B&K type 2131. Whenever necessary, results were corrected for speed deviations from the nominal speeds according

to Ref. 1.

ll

5 RESULTS

Table 2 gives the overall A-weighted values. The pooled standard deviation for separate runs is 0.32 dB(A) which means that the least

significant difference between two cars is 0.6 dB(A) (95 % confidence

level, paired t-test). Differences smaller than this should be neglected. The systematic errors are very small in this case as all measurements were made with careful calibrations, the same equipment, the same day at the same location by the same personnel. The values in Table 2 are illustrated in Fig. 5.

Table 2 The measured A weighted tire/road sound levels

Car A weighted sound level at Load Rim the speed of:

Tire Car' (kg) width 30 50 70 90 km/h Comments

8

N VW Golf GL 1060 5 58.6 66.4 71.9 76.2

'C . .

8 Honda Civic Shuttle 1120 4.5" 59.5 67.3 72.5 77.4 Front wheel adjustment not OK. loe'u

CL was mn

: Mazda 323 Combi 1160 4.5 58.0 66.6 71.8 76.3

35: Fiat 125P 1180 5" 59.4 67.7 72.6 77.2 Very stiff suspension. Disc brake

2% noise audible at 30 km/h. High humidity

SEE

Peak-peak deviation 120 0 5 1 5 1.3 0 8 1 2 Volvo 142 1290 5 57 6 65.5 70.8 75.6

E Citroen GSA 1220 1320 4 5 57 5 65.1 70.7 74.8 Normal road Clearance : Saab 900 GLI 1320 5 5 57 7 65.6 70.7 75.5

:52 Volvo 760 GLE 1450 5 57.6 65.8 71.2 75.6 High load

2%_ULD

E3 Peak peak deviation 160 1 O 2 0.7 O 5 0 8

Frequency spectra, in third-octave bands, for speeds of 30, 50, 70 and 90 km/h are given in Figs. 6 9. Here, differences between cars that are less than 1 dB are not statistically significant (1.5 dB for frequencies

lower than 500 Hz).

The effect of using four-wheel drive, compared to normal front-wheel

drive was negligible (see Table 2). The transmission noise might

theore-tically be slightly altered but in this case the effect was not noticeable.

Figs. 10 and 11 show the measured time histories of the A weighted

sound level for the light and heavy car groups. The time scale has been

converted to a distance scale assuming a speed of 70 km/h. The zero distance was set at a point where the very front part of the car passes the point which is closest to the microphone.

VTI RAPPORT 357A r l

60

dB

(A

)

68

'

dB

(A

)

(I

BM

)

78

dB

(A

)

\ \ Fi of \ A --D 0 Ho nd a

59

-67

77

d

5G ro up 1 To e:

72

_O

\

1SS

SR1

3

/

/

\

C

A

Mo

zd

o

,/

0

Gol

f

J

58

-A

66

~

76

.

Vo lvo 76 ? Vo lvo 14 2 Sa ab Gr oup Z > W e :

'...

....

..

16

5$

R1

S

S 7 -_ .. .. .. . 0. . '0 o

I

Ci

fr

oén

J

#

30

km

/h

50

km

/h

70

km

/h

90

km

lh

Fi g. 5 A-we ig ht ed so un d le ve l me as ur ed fo r th e "h ea vy" an d "l ig ht " ca r gr oup s. 12

l3 SO UN D PR ES SU RE LE VE L dB SO UN D PR ES SU RE LE VE L 80 W W W W W W W W 1 W W W W W W WW W WW 1 W W W W W W WW WW W L i J

-

-

FIRT

70_ L ... .. J --- uu GOLF

J

-

---» HONOR

J

60.1L J ,E J 504 J J J

401J

J

J

J

30 L J 1 J

20 1L1lthLWl ff%i:%% f%EF%%f%%f%% %%fi#ff%

15 31.5 53 Hz 125 250 500 (Hz 2K 4K 8K 16K 32K FREQUENCY 80 W W W l T 1 W I W 1 1 1 W 1 T W T W 1 T T 1 W 1 W W W 1 1 W I W J- _{UOLUO 760} J 70 L ... .. UOLUO 142 J

1

--- -- sans 900

1_

--- -- CITROEN

J

60%.

J

E

J

S()_+ J a J 40.v J 1L J 30 L ' J

W

J

J

20 %%f%%f%%f%%f+}f%uff%f%.f%%f%%ifr 16 31.5 53 Hz 125 250 500 IKHz 2K 4K 9 16K 32K FREQUENCY Fig. 6 Frequency spectra for the "light car" group (above) and the

"heavy car" group (below). Speed: 30 km/h

14

80 m WW W W WWWI W W I W W W W WI WW I I WWT j I I W I WIWT W W W W I U _l " .J g FIRT :3 701, ... .. 92133 J a --- -- UH GOLF a w --- -- HONDH (/7 E a 60. J D Z 3 o J. U) 50-. J J J 40_. J J J 30.V J ? J 20 k l L L L L L L L_. L L JLL PL LJL L L L L L 1 IL L L L L I I T W I r I I r T T l' I W l' I I l' I I l I I I I I [ T I r I W 16 31 s 63HZ 125 250 500 IKHZ 2K 4K 8K 16K 32K FREQUENCY 80 W W I W I F1 W WI WV W I W I I W i T W I 1 W W j j W W T W 1 W on '0 . J

A

J

UOLUO 760

g 70 b ... .. UOLUO 142 J

g

7

--- - $998 900

g 9 --- -- CITROEN J D 3 E 60%. J D g T J O m

sea.

_J

T J

40 J

j J

T

J

30a.

J

J

J

20 1 1 f %%fhtfttft+ff§ftff§tfttf%%itf 16 31.5 63 Hz 125 250 500 (Hz 2K 4K 8K 16K 32K FREQUENCY

Fig. 7 Frequency spectra for the "light car" group (above) and the

"heavy car" group (below). Speed: 50 km/h

15 80 m 1 1 j I 1 1 j I l j FT 1 1 1 1 1 w j fj j 1 1 1 j j j 1 j 1 '1 j 0 .t J .J g LIJ DC 3 r (/7 LLJ

E 601.

J

C) Z

3

J

O T (f) 501. J

J

J

40 _

J

30.1; J T J 20 L L L L L l l L L L L L L L L L L L L l L L L L L L L L L L L L 1 j r V V I U I r V V 1 V I I I I I r I I r I I r I I I I I 16 31.5 63 Hz 125 250 500 IKHZ 2K 4K 8K 16K 32K FREQUENCY m 1 ' 1 1 j T I 1 U 1 V j 1 T T Y j 1 j T I V 1 j j 1 1 j j U

V

_{UOLUO 750}

_J

E ... .. UOLUO 142

E: 70%.

_{--- sane 900}

m

--- CITROEN

m L d :3 T (/7 U7 2% 60.» J C3 2 D .1- J O (/7 504. _l ,t J J 401.. 1L J 30-, J

J

J

20 t%f%%f%tf%tft%ft%f%%ittft%f%%ft% 16 31.5 63 HZ 125 250 500 (HZ 2K 4K 3K 16K 32K FREQUENCY

Fig 8 Frequency spectra for the "light car" group (above) and the "heavy car" group (below). Speed: 70 km/ h

16 80 11111111 1 1 j 111 1 j 1 11,111 1 1 1 1 11 1' 1 1 1 1 1 1 m t) v _J d a 70_L J A m x F D '1 m m '5 ! 50.... a Q D Z 3 0 J O m

50,.

₄

+ J 401. a ' -- FIRT V ... MRZDR

30

_~h

---u uu GOLF

_{---~ HONDR}

_d

1. J 20 :¢f+%fr%f..,.%fr%f:%fref:%f:ef:: 16 31.5 63Hz 125 250 500 IKHZ 2K 4K 8K 16K 32K FREQUENCY 80 1 1 1 1 1 1 1 1 1 T 1 1 1 1 1 1 1' j 11 1' 1 1 1 1 1 1 1 1 1 1' 1 -_ P LIJ E 701L 4 LEE]

3

+

3

L

g 607 D Z 3 n J

.o

m

l

50%.

J

J WL 401L 4

+

UOLUO 760

30qL . . . u UOLUO 142 A

-- $998 900

,L --- CITROEN

J

20

ref:rfrefrrferf:rf:¢ierfrrfrrwa.

16 31.5 63HZ 125 250 500 IKHZ 2K 4K 8K 16K 32K FREQUENCY

Fig. 9 Frequency spectra for the "light car" group (above) and the

"heavy car" group (below). Speed: 90 km/h

VTI RAPPORT 357A

TH

;

TI

RE

S:

13

",

Gi

sl

ave

d

21

6,

-70

km

/h

;

Sur

f.

1

75

[d

B(

A)

]

\

,-,

,m

,A

72

"

/'/

x

\\

\

eg

r

,'

¢\

66

-\I

z

FI

AT

Izs

P

r

/

~-MA

ZD

A

32

3

-/

I

-vu

co

r?

I

/

__

._

HO

ND

A

Ci

vi

c

Sh

ut

tl

e

4wD

63

'

17 l I l l I J I

50

I

I

I

I

I

I

I

-2

0

~1

5

1

0

S

0

5

10

15

DI

ST

AN

CE

[m

]

Fig. 10 Ti me hi st or y of th e Awe ig ht ed so un d le ve l fo r th e "l ight ca r" gr oup .

VTI RAPPORT 357A

TH

;

TI

RE

S:

15

",

Mi

ch

el

in

XZ

X,

70

km

/h

;

Sur

f.

1

75

LA

[d

B(

A)

]

₇₂

-

₆

9

-

₆

3

~

V O L V O 7 5 0 -V OL V O 14 2 A ut o m at i c -S A A B 9 0 0 -C I T R O E N G S A . -l l l

l

l

i

l

I

l

-%

C)

S

O

S

10

15

DI

ST

AN

CE

[m

]

Fig. 11 Ti me hi st ory of th e Awe ig ht ed so un d le ve l fo r the "h ea vy ca r" gr oup

20

18

l9

6 DISCUSSION

6.] Difference between cars in A weighted sound levels

The difference between the cars seems to be lowest at 70 km/h. At lower speeds the errors might increase due to low tire/road noise level in relation to disturbances. Especially, brake noise can appear here. At 90 km/h there is a risk for influence of air turbulence noise around the vehicle body. Also, the coast by is so fast that the accuracy might suffer

(e.g. faster signal changes and difficulties to drive).

At 70 km/h, i.e. the most important speed, the maximum noise differ

ence between the cars is 0.8 dB(A). This is too high to be explained by

random errors but it is so low that it has little practical importance. No such differences can be detected by normal hearing.

For the "heavy car" group, no differences at any speed are so high that they are important.

For the "light car" group there are statistically significant differences at 30, 50 and 90 km/h, but it is questionable whether they are of any practical importance as they are audible only under excellent indoor circumstances. However, as indicators of vehicle influence from a measuring point of view, they are interesting.

It can be seen that the differences occur between the Honda and Fiat on

one hand and the Golf (Rabbit) and Mazda on the other. It is obvious

that one could expect that the Honda with its faulty front-wheel adjustment would emit higher noise than normal. It is rather surprising that it did not emit even higher noise than that measured. The faulty toe in of 11 mm results in a slip angle of 0.9 deg. for each front wheel. Experiments, in another study, performed in a laboratory on a road wheel covered with replica road surface NEl (Ref. 2) showed that the increase

of the A weighted sound level for such an angle compared to 0 deg. can be as high as 2-4 dB(A) at 50 km/h.

20

The Fiat was extreme (intentionally) in two ways: Firstly, it had the most loaded tires; secondly, its suspension was not original but a very stiff "sports car" type. In addition, it was tested in the morning when the humidity was very high, although the road surface was judged to be dry. Considering all these factors, it was not surprising that the Fiat gave

higher tire/road noise levels.

6.2 Differences between cars in frequency spectra.

With a few exceptions not only the A-weighted overall levels but also the spectra are very similar. However, it is interesting to discuss the discrepancies even though they are small.

First, concentrate on the "heavy car" group. The only major discrepancy between the cars is that the Citroen has a slightly different spectrum

shape at 90 km/h than the other cars (less pronounced at 70 km/h). One

hypothetical explanation could be that the noise below 300 Hz, which by the way is not important for the A weighted sound level, is to a great degree influenced or caused by turbulence around the cars. The Citroen is unique in the way that it has 10 20 % less cross-section area than the other cars and a very low drag coefficient which could explain the lower low frequency noise. As such turbulence effects are highly speed-dependent they would be much less pronounced at speeds lower than 90 km/h which fits the observation. Another hypothetical explanation could be that the soft suspension of the Citroen allows no significant struc-ture borne sound transmission to the body, and consequently the noise emission from the body can be neglected, while the stiffer suspension of the other cars perhaps, but not very likely, might result in some noise emission from the body. This mechanism has always before been consi-dered as negligible. It is also difficult to fit this hypothesis to the observed speed influence.

Both these hypotheses are somewhat inconsistent with the spectra for the Volvo 760, the Volvo 142 and the Saab which are very similar to each other despite the fact that they have somewhat different aerodynamics.

21

It is difficult to find any explanation for the discrepancy at 90 km/h of ca 3 dB at the fundamental tread impact frequency 800 Hz (shown in Figure 10) between the Citro'én and the other cars. One speculation is that the suspension stiffness influences the mechanical impedance seen

from the tire tread.

Now, turn to the "light car" group. There is one very apparent difference at frequencies above 2 kHz between the Fiat and the other cars. This is more pronounced the lower the speed is. It is no doubt that this is due to noise from the brakes which was clearly audible and identifyable during the measurements at 30 km/h. However, this brake noise has very small

influence on the overall A-weighted sound level. At 70 and 90 km/h it

increased the levels about 0.1 dB(A), at 30 km/h about 0.5 dB(A).

The Honda emits higher noise levels than the Mazda and VW Golf

(Rabbit) at high frequencies and at the fundamental tread impact

frequency. This could be due to the extremely high toe-in which would excite an increased movement in the tire tread elements in the plane tangential to the tire tread. Such a conclusion is in line with the investigation on slip angle influence by Ejsmont 6c Sandberg reported in Ref. 5 and with observations made by Pope and Reynolds Ref. 6.

A third observation is worth mentioning, namely that the Fiat with its very stiff suspension appears to have higher noise emission than the Mazda and the Golf at the fundamental tread impact frequencies. This is consistent with the speculation earlier concerning the Citro'en .

6.3 Differences due to a change in body to road clearance

Fig. 12 shows the influence of different body to-road clearance (220 resp. 260 mm) which also means that the screening of the tires is slightly different. It is only at 160 and 200 Hz that this has an effect on noise. The reason is probably some type of reflection or resonance effect. The A weighted value is not influenced at all.

So un dpr es sur e leve l 22

80 I j I I W T I l 1 I I T 1 1 1 I T a j I W T I W j i F1 ' r dB T- J 70 .

_T

J T J 60 HIGHEST J 1" 4 "NORMAL" SO 1. J 1L J

4 o .1.

J

._ J 30 .1. J

.L

J

20 L l L L L L k L L k L L L L l L L L L L LL J_ L L L I I I' I Tj I I l 1 I l I I l I I T I I I I I l I I I l I r I 16 31.5 63 1'25 250 500 (Hz 2K 4K 8K 16K 32K Frequency

Fig. 12 The effect of increasing the vehicle body road clearance. Test object: Citro'én , settings "normal" and "highest"

6.4 Differences due to transmission

When the four-wheel drive, instead of the normal front-wheel drive, was active on the Honda the noise increased 0 2 dB at a few low frequencies only, which was of no practical importance. (Both cases concerned coast

like

by with engine switched off and clutch disengaged, in all

measurements reported here.

6.5 Differences in recorded time histories

For the "heavy cars", the differences in time histories are not higher

than 1.5 dB(A) within the whole recorded distance and around 1.0 dB(A)

at or close to the moment of maximum sound level. For the "light cars",

a difference of nearly 3 dB(A) between the Honda Civic and the Mazda

323 is visible, as seen in Fig. 10. It seems obvious that the highest

23

influence of front wheel alignment should be observed when the car is approaching the microphone. On the other hand, about 5 m before the point closest to the microphone, the most distant front wheel becomes screened by the car body which could result in the local peaks and

valleys recorded for the distance around -6 m (before the nearest point).

6.6 Computer simulations of time histories

A computer simulation of time histories for cars with different wheel

base, wheel track and body dimensions was performed (Ref. 10). In the

simulation, the screening effect of tires and body are considered and the contribution from each tire is calculated separately. The simulation showed that the above-mentioned car parameters should not influence the maximum A-weighted sound level more than 0.1 to 0.2 dB(A). There can be bigger differences at the distances at which the tires become screened by other tires or the car body. In Fig. 13, results ofa simulation performed for three very different Fiat cars are presented (ranging from

a sub compact Fiat 126 to a big luxury Fiat Argenta).

6.7 Investigations by other researchers

Steven (Ref. 4) found that differences as high as 2.5 dB(A) could be

attributable to the vehicle. This is considerably higher than in the investigation reported here. However, according to personal communi-cation, Steven did not control the positioning of the tires on the vehicles and neither the rolling direction of the tires. This could significantly increase the measured differences since in some cases it was observed that noise emission is not symmetrical to the tire plane. Also, for tires rolling in different directions, the trailing and leading edges of the tread elements are worn in different ways depending on the rolling direction. A change of rotation direction may change the local interaction between

tire and road surface.

Ref. 7 tested the influence of chassis height (body road clearance) on

tire/road noise at coast-by measurements, using a "Citro'én GS Break" car equipped with Michelin ZX MSSRlS tires. With this car, the chassis height was set at "low", "intermediate" and "high", covering the mini VTI RAPPORT 357A

24

|26

Ritmo

Argenta

--- -- Fiat

- --Fiat

~12

-9

6

-3

O

3

6

9

12

DISTANCE [m]

Fig. 13 Simulated time histories of the A weighted noise level for Flat cars of different dimensions and body shapes. The time scale has been converted to a distance scale assuming a constant speed.

mum-maximum variations by using its hydro-pneumatic suspension regulation. The tests were done at 70 km/h and they showed a difference in noise level of only 0.1 dB(A) between the extreme chassis heights. In the frequency spectra, there is a noticeable difference only in the range of 160 250 Hz (high clearance gave higher noise level). These results are thus amazingly consistent with the results reported in Fig. 12 above.

25

7 CONCLUSIONS

Despite a great variation in car construction, condition, etc., the cars caused little influence on tire/ road noise when A-weighted sound levels are concerned. Interesting differences occurred only for a car with damaged front wheel adjustment and a car with very stiff suspension in relation to "normal" cars. These differences are so small, however measurable, that they are of questionable practical importance.

At 70 km/h, which is the recommended speed for such tests, no differences were measured of such a size as to be detectable by normal hearing.

However, even though the noticed differences are unimportant for the A weighted noise levels, they indicate that the vehicles influence the noise which is assumed to be externally radiated tire/ road noise. A study of the frequency spectra revealed that, outside the frequency range which is of the greatest importance to the overall assessment, there were some interesting differences. One car had brakes which emitted high frequency noise, mainly at the two lowest speeds. Another car, with a hydro pneumatic suspension, emitted less noise in relation to the others at very low frequencies. We speculate that the latter is due either to a suspension or a turbulence effect.

There were indications that some minor discrepancies between cars occurred at the fundamental tire tread impact frequencies. It happened that these cases coincided with the cars with extremely stiff, respect-ively extremely soft suspension. If tires with lower sidewalls (or which for other reasons are stiffer) are used, then it is wise to further investigate whether the suspension may interact with or transmit the tire vibrations causing externally radiated noise.

The speed for which the influence on noise of the car type appeared to be smallest was 70 km/h. This is in line with the recommendations given in the proposed measurement standard (Ref. 1).

26

Even if no important differences in the tire/road noise from the cars appeared at 70 km/h a few recommendations concerning the choice of

cars for tire/road noise tests are warranted:

o The cars should be in good general condition. Especially important is that the front wheel adjustment and the suspension are all right. 0 The problem with brake noise must be observed, especially at speeds

of 50 km/h and lower.

0 Cars with suspension types that are not very common, e.g. hydro pneumatic, should be avoided until their effect on tire/road noise has been better established.

0 Even though it was not investigated in this study, the authors believe that the tires used for testing should not be run in another direction than the one used for their conditioning.

The main conclusion is that the measurement procedure in Ref. 1 does not appear to be influenced by the choice of test car to such a degree that it is of practical importance, provided that the recommendations in the procedure are fully followed. A test speed of around 70 km/h should be preferred.

27

REFERENCES

1. ECE/GRB Ad-hoc Group on Methods for Measurement of Tyre/Road Noise: "Methods for Measurement of Tyre/Road

Noise - Proposed Methodology". TRANS/SCl/WP29/GRB/R.85

(1986)

2. Sandberg, U.; Ejsmont, J.A.: "DeveIOpment of Three Methods for Measurement of Tire/Road Noise Emission: Coast-By, Trailer and Laboratory Drum". Noise Control Engineering

Journal, Vol. 27, No. 3, pp. 68 88 (1986).

3. Letter to U. Sandberg,, Swedish Road and Traffic Research

Institute, 1985-03 06 from J. Martin, U.T.A.C., Montlhéry,

France.

4. Steven, 1-1.: "Investigations on a Measuring Method for the Tyre Road-Noise of Passenger Cars". Proceedings of a WorkshOp on

Rolling Noise Generation, October 10/11 1989, Inst. for

Technical Acoustics, Technical University of Berlin.

5. Ejsmont, J.A.; Sandberg, U.: "Cornering Influence on Tire/Road Noise". Proceedings of INTER-NOISE 88, pp. 13413 1346, Noise Control Foundation, Poughkeepsie, N.Y., USA (1988).

6. Pope, 3.; Reynolds, W.C.: "Basic Studies of Automobile Tire Noise". Technical Report No. TNS l, Stanford University, USA

(1978).

7. Unpublished data from IFM Akustikbyran AB, Stockholm. Mea surements in 1975 by O. Bennerhult.

8. Personal communication with Mr. E. Kafka, Goodyear Technical

Centre, Luxembourg (Aug. 1990).

9. Oswald, L.II.: "Exterior-radiated Aerodynamic Noise of Vehicles at Highway Speeds". Research Publication GMR-2622, General Motors Research Laboratories, Warren, Michigan 48090, USA

(1978%

10. Ejsmont, J.A.: Zastosowanie Komputerow W Badaniach Halasu Opon Samochodowych. Auto-Technika Motoryzacyjna 6/1990, Dodatek Naukowo Techniczny.