Road surface characterization with respect to tire noise : A proposed recommendation

ores evsPruive

No. 114A - 1976

Statens vag- och trafikinstitut (VTI) - Fack - 581 01 Linkoping

National Road & Traffic Research Institute - Fack ' 581 01 Linkoping - Sweden

Road Surface Characterization

with Respect to Tire Noise

A Proposed Recommendation

FOREWORD

This report is essentially a translation from a

chap-ter of an earlier report written in Swedishx. The work was originally started as part of the project "Tire noise - recommendation regarding a measurement method" financed by the Swedish Board for Technical Develop-ment and run by IFM Akustikbyran in cooperation with

the National Swedish Road and Traffic Research

Insti-tute. The following text was also included as part of the final report for that project (in Swedish)XX

x Sandberg, U.: vagbanekarakterisering med avseende

pa dackbuller. Report No 92, Statens vag och

trafikinstitut (National Swedish Road and Traffic Research Institute), Linkoping 1976.

xx Gadefelt, G., Bennerhult, O.,Sandberg,[L: Forslag

till metod for matning av externt dackbuller fran motorfordon. Report No 74 4746 a+b, Styrelsen for

teknisk utveckling (Swedish Board for Technical

DevelOpment), Stockholm 1976.

N N N N N N M N N N N M N N N N N N N @ d a c n m a > ~u m wi t h m ¢> + b g U J I Q H P A P A H CONTENTS SUMMARY SAMMANFATTNING INTRODUCTION

CHARACTERIZATION OF ROAD SURFACE BY

PROFILO-METER MEASUREMENT

Road surface characteristics Roughness

Recommendation

Remarks

Units

Spatial frequency range of interest

Frequency resolution

General

Recommended octave bands

Total power and root mean square value

General

Recommendation Measurement tracks

Statistical significance Instrumentation

Method of measurement and analysis

The profilometer Calibration

Presentation of results

(RMS)

CLASSIFICATION OF ROAD SURFACE BY SUBJECTIVE RANKING

REFERENCES

VTI REPORT NO. 114A

Page o o xo o o o o o o q m c n o xm wm m wwwm w +4 H }_ J U. ) 16

H

SUMMARY

Road Surface Characterization with Respect to Tire

Noise - A Proposed Recommendation

In order to get well defined test conditions for tire

noise, a method for characterization of the road

sur-face with respect to tire noise generation is presented. The method implies that the power spectral density

(PSD) of a measured road profile is computed for spatial frequencies between 2 and 1000 cycles/meter (correspond ing to wavelengths l - 500 mm). Analysis with frequency resolution corresponding to octave or thirdoctave bands

is recommended.

The described method does not give a complete descrip

tion of the road parameters influencing tire noise, but can be considered as an important partial solution.

II

SAMMANFATTNING

Végbanekarakterisering med avseende p5 déckbuller -Férslag till rekommendation

I avsikt att erhélla Valdefinierade testférhéllanden fer déckbuller presenteras en metod fer karakterisering

av végbanors egenskaper med avseende p5 dackbuller-generering. Metoden innebér att profilkurvor som upp métts fer den aktuella Végbeléggningen frekvensanalyse ras, sé att ett effekttéthetsspektrum fbr beléggningens

skrovligheter erhélls. Dérvid har spatiala frekvenser mellan 2 och lOOO perioder per meter (motsvarande vag

léngder l - 500 mm) medtagits, och den erforderliga frekvensupplbsningen fareslés motsvara oktav- eller tersoktavband.

Den beskrivna metoden ger inte négon komplett beskriv-ning av de végbaneparametrar som paverkar déckbullret, men bér utgéra en betydelsefull dellésning.

INTRODUCTION

As a consequence of the need for tire noise research the question of adequate and standardized measurement methods has become urgent. The noise measurement in

itself brings no overwhelming difficulties. However,

it is not sufficient to measure the noise, but it is

also necessary to characterize the ambient factors influencing the level and the frequency content of the noise. It is commonly agreed that the texture of the road surface affects the generation of tire noise. In fact, many investigators suggest that the tire and the road surface should be considered together and not the tire separately. The influence of the road sur face is one of the reasons for the variations in

mea-sured noise level and spectrum between different test

sites. Comparison of tire noise measured at different

test sites and/or different occasions is meaningless,

if the condition and characteristics of the pavement

is not at the same time described.

In order to get well defined test conditions for tire

noise, it is consequently of great importance that the qualities of the pavement influencing tire noise can be adequately described. The recommendation presented in this report has that purpose, but does not consti-tute a complete method for road surface characteriza-tion. Neither does it constitute an official standard in any way.

The recommendation is based on theoretical and practi cal studies supporting the importance and evident

influences of the road surface macrotexture profile on tire noise. Comparison of frequency spectra from the

profile with tire noise spectra, has shown that, for some of the tested tire/road combinations, profile

spectra has a strong influence on noise spectra. It is, however, important to observe that characterization

by the macrotexture profile spectrum alone not seems to be sufficient, but only a partial solution to the problem. These studies are reported in Swedish in re-ference /55/. Rere-ference /55/ also contains a

compre-hensive reference list, but after it was written,

some new reports - important as an additional back-ground for this recommendation - has come to the authors' knowledgex.

CHARACTERIZATION OF ROAD SURFACE BY PROFILOMETER MEASUREMENT

Road surface characteristics

BQESDE§§§

Knowledge of the relative importance of the different

generation mechanisms is too limited to determine with

confidence which parameters should be used to

charac-terize the noise influencing qualities of the road. Probably the most important road-depending mechanism is tire vibrations caused by the roughness of the

pavement. "Roughness" here means macroscopic roughness

of wavelengths approximately 1 250 mm.

The amplitude and frequency distribution of the rough-ness is important, and can be expressed by means of the fourier transform of the surface profile, whose absolute value is squared to obtain a power density

spectrum.

x In this report the complete updated reference list

is given. It is interesting to note that one of the new references (/57/) has used a very similar

technique to the one utilized here to obtain a pro-file spectrum, and that reference /56/ has analyzed data by a new technique showing evidence of radially excited tire vibrations due to the road macrotexture.

Recommendation

It is recommended that the roughness of the pavement is included as a parameter influencing tire noise, and is expressed as the power spectral density (PSD) of the road surface profile as a function of the spatial frequency. The profile of the road surface is measured

in the direction of vehicle travel.

Remarks

Power spectral density is a function of frequency; in this case the designation "spatial frequency" is used to indicate that it is not a conventional oscillation but a periodicity, or length distribution, in the longitudinal profile of the road surface.

A power spectral density description does not give a complete characterization with respect to parameters

influencing tire noise, but should, when further

know-ledge has been obtained, be completed by information about for example friction prOperties, sound absorp tion and drainage capacity.

Units

The vertical displacement in the profile, here denoted

h, has the unit /m/ (alternatively /mm/). h is a

func-tion of horizontal displacement, 1.

For the mean square (MS) of 11(1), also called power

or variance, here denoted S, the unit is /m2/. The reference plane is assumed chosen so that the mean value of h equals 0 for the desired length interval, eliminating any {XI component in the spectrum.

The root mean square value (RMS) of the profile is

denoted hr_ms and the unit is /m/. It is equal to the

standard deviation.

Spatial frequency is here written fSp and has the unit

/m_l/. It is the inverse to wavelength. An alternative

unit is (c/m] (cycles per meter).

Power spectral density is written G(fsp) or, where risk for mixing withanxnu szequivalents is negligib le, G(f). The dimension is power per frequency inter

1)

val, and the unit in this case will be (mZ/m- or Qua/c/mj.

Due to the wide deviation in the values of G(f) and the desirable equivalence with noise measures, it is convenient to relate the actual power Spectral densi ty to a reference level, thereby establishing a rela tive measure called power Spectral density level (PSD

level):

LG: lO-lOlog 3 )

ref

where LG==PSD level in dB relative to the reference

Gref. An apprOpriate value for Gref lS prOposed

_ -12 2 Gref lO Hl/b/m

With this reference actual profile PSD levels will be in the range O-80 dB. In a similar manner power level,

LS, is defined:

L = lO-lOlog S

Spatial frequency range of interest

In order to cover the important frequency range for vibration excited tire noise, we recommend the

analy-zed spatial frequency range to be at least 8-500 c/m and preferably 4- lOOO c/m. In the latter case corre-sponding wavelengths are l-250 mm.

Note 1: If vibration included tire noise is considered

the range of 4- lOOO c/m covers, at vehicle speeds

between 36 and 108 km/h, at least the noise and vibra-tion frequencies l20-lOOOO Hz.

Note 2: At a vehicle speed of v m/s across a roughness of periodicity fSp c/m, corresponding vibration- and noise frequency will be

Frequency resolution

General

Investigated pavements have shown a power spectral

density of random nature without marked narrow band frequency components. Therefore it is not necessary to

Lu abetter frequency resolution than octave bands. Exceptions are characterization of periodically groo

ved pavements, or when only a short, well defined

distance of the road is considered for narrow band comparison with tire noise. Special notice should be given to statistical prOperties. Int mmxacases where it is easier to perform analysis in l/3 octave bands, this could be accepted, provided that the frequency range of interest is covered and the band limits coin cide where possible.

2.4.2

_{Bessmms9§s§_992§ys_b§2§§}

Total power and root mean square value (RMS)

General

Under many circumstances a summarizing measure repre senting the road surface roughness is desirable, be cause it makes possible fast and convenient compari sons. Information thus obtained is of course more limited in value depending on the lack of frequency

information.

Recommendation

As a summarizing measure of the roughness, it is for the time being recommended to use the total power

(variance) level in the spatial frequency range 4- lOOO c/m, here denoted LStot.

As a secondary basis the corresponding linear quanti

ty, Stot, or hrmS (=u/StOtH=RMS value,or standard deviation), could be used.

After gaining better knowledge about the effect of the road surface on tire noise, it is perhaps possible to determine a weighting function for the spectrum of the roughness, that is a function of spatial frequency proportional to the importance of the different fre-quencies of the roughness, and thereafter to compute the power level. This will then be a counterpart to the A-weighting curve (among others) in acoustics.

Measurement tracks

Certain minimum requirements of the measurement tracks should be fulfilled to get sufficient representativity of the result.

1. At least 4 tracks, each of 0,5 m or longer, should

be analyzed. The power spectral density of the diffe-rent tracks should be averaged.

2. The tracks chosen shall be as representative as possible of that part of the road where the tires are rolling.

3. The pavement shall be free from loose particles

such as stones.

Note 1: Experience has shown that the profile can have different characteristics in transverse direction re

lative to the longitudinal direction due to wear.

Determination of the transverse profile can give mis leading results because of "wear tracks" and is not recommended. Instead it is suggested that the position of the different longitudinal tracks is varied some-what in the transverse direction to get a reasonable average of that part of the road where the tires are rolling.

Note 2: The minimum length of the tracks, 0,5 m, is

caused by the interest in wavelengths as long as 0,25 m (4 c/m) and the desired frequency resolution (the lowest octaVe band). To get better statistical signi ficance, longer tracks should be analyzed if possible.

Statistical significance

Investigations made so far indicates that, due to the limitations in primary data (four measurement tracks

each of 0,5 m), the statistical uncertainty is of the

following order of magnitude:

:1 dB

Octave band level: i2 dB

Narrow band levels (bandwidth 2 c/m) i3 dB Total power level:

This is expressed as maximum standard deviation, i e

the confidence level is 68%. For 99,9% confidence the

values should be multiplied by 3, provided that the

distribution is Gaussian (normal).

Instrumentation

M2229§_9£_m§§§2£§m§22_§2§_§a§ly§i§

A profilometer with electrical output signal and a tape recorder (or other corresponding apparatus) is used to register the profile of the pavement. The power spectral density can then be obtained in essen tially three ways:

a. Calculation of the auto correlation function of the

profile and fourier transformation thereof.

b. Fourier transformation of the profile curve, and

squaring of the absolute value of the transform. c. Spectral analysis by a parallel filter bank, or by

a sweeping analyzer (in reality a hardware version of the method in b.). In practice, a very fast

scanning of the profile is required, possibly rea-lized by speed transformation on tape recorder or by means of digital technique (time compression).

None of the methods is recommended relative to the

others, but we would like to point out the danger of aliasing distortion if sampling is used.

Ibs_p£9£ll9msés£

A profilometer with electrical output and a measure-ment range covering 0,5 m or more is required. The resolution longitudinally must be better than 0,5 mm to fulfil the sampling theorem at a desired upper

spatial frequency limit of 1000 c/m.

Vertically the profilometer should have a measuring range of at least 15 mm and an accuracy better than

i0,2 mm.

To make possible frequency analysis of the profile, it is usually necessary for the profilometer to have a constant scanning speed, or a sampling unit giving samples at a constant length interval.

Note: According to the low power content in the high-est octave band on current pavements, it is, if neces sary, possible to decrease the measurement range to 4-500 c/m which will require at least 1 mm

resolu-tion oh the profilometer. The total power level will

suffer'insignificantly.

lO

Calibration

A careful calibration of the profile measuring device

(both the vertical and horizontal-coordinate) shall be

made on every measured pavement. It is desirable that the complete measuring and analysis system is included. This can be accomplished by scanning a profile of

sinusoidal-, step- or pulse shape with accurately known dimensions, and using its frequency spectrum

(which can be calculated) as a reference.

Presentation of results

The following presentation of the result is recommen-ded:

I. General information

a. Date and location of measurements.

b. Rough description of the road surface with avai lable data or by a visual estimation.

c. Number of tracks and their length.

d. Type of profilometer. Resolution and accuracy (briefly).

e. Method of analysis.

II.Results

a. Octave band diagram or table. Alternatively third

octave band diagram. Scales.

b. If necessary: Narrow band spectral diagram. Also RMS value, h

c. Total power level, LStOt-

_rms

(not necessary).

d. In special cases it can be practical, as a comple-ment, to transform the spatial frequency scale to

the corresponding noise frequency scale for the

vehicle speed in question. This will facilitate comparison of noise and profile spectra.

11

Example of presentation of results:

General information

Measurement made 1976 09-25 on national trunk road 55,

nas.

6 km south of crossing with E3 outside

Strang-Surface treatment Yl, constructed 1975. 4 tracks of 0,5 m.

Mechanical stepping profilometer with stylus sensor. aggregate size 12-l8 mm,

Electrical output through linear potentiometer. Step length 0,4 mm. Stylus diameter 1 mm. Calibra tion by scanning a step 7,4 mm high, 10,2 mm long. Narrow band analysis, nominal bandwidth 2 c/m. Time

compression spectrum analyzer (Federal Scientific UA-SOO).

bands in computer (HP 2100 S).

Summation of narrow bands into octave

Result

See next page.

P S D l e ve l 12 50 40 30 20 {d B re l 10 1 2 mé/ c/ m] 10 1000 4 8 16 32 64 126 250 500 Spatial frequency (c/m)

Total power level:

12 2

1: 66,7 dB rel 10 m

Range 4-1000

l3.

CLASSIFICATION OF ROAD SURFACE BY SUBJECTIVE RANKING

In cases where the apprOpriate instrumentation is not

available and can not be obtained, another

characteri-zation method should be used. If a profilometer is accessible but frequency analysis is impossible to make, it is suggested that the profile of the road

surface is recorded on paper. In other cases the fol-lowing method for subjective ranking based on Visual inspection of the pavement is recommended. It should, if possible, be completed by other available data of

interest for noise generation.

Typical dimensions of the roughness (expressed as

wavelengths) in transversal and longitudinal direction

is estimated, as well as the degree of periodicity. The height of the asperities is also estimated. The air permeability of the pavement is estimated from a visual inspection of the macrostructure of voids and channels in the surface. Finally the harshness (micro roughness) of the surface is estimated.

An evaluation according to table 3.1 is made. Then the pavement is characterized by a configuration of figures, for example:

2 1 2 30/41/8lx RXX

X Only intervals where there seems to be a periodic

structure, or distinct size distribution, are in

cluded. If there is no such interval, the roughness is considered random ("R").

xx If roughness is the same in transversal direction as in longitudinal, the symbol is omitted.

14

Which means:

2 The height of the asperities is on the

average l-2 mm.

1 The surface has a medium air permeability.

The surface is harsh.

30 There is a longitudinal roughness,

perio-dicity weak, of approximately 4-8 mm. 3

41 As above, 8-l6 mm distinct periodicity. 81 As above, 128-256 mm distinct periodicity.

R Transversally the roughness is random.

Of course this method has all the serious disadvanta ges of subjective rating, but it gives some indication of what is felt to be important for tire noise

gene-ration, and it should be better than no characteriza-tion at all.

Table 3.1. Proposal for subjective ranking method for road surface classification. Height of asperities (2-texture depth) Period (wave-length) of Degree roughness, of transversally periOdiCity Period (wave-length) of Degree roughness, lon- of gitudinally periodicity x . Micro-roughness Air permea _ bilityxx V T I R E P O R T N O . 1 1 4 A mm mm 16 0, 16 5 l Small Medium 1 High 0 Smooth 0 (polished) Medium 1 2 Harsh 2 (unpolished)

sand patch method.Estimation of peak to-peak-value. Can be substituted by (texturedepth)-2 measured by the Here: The capability of the pavement to remove air enclosed in the contact patch between tire and road; drainage through the surface as well as between the tire contact patch and the road surface.

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