38 IRJ April 2010

U NTIL the mid-1980s much workon wheel/rail noise wasundertaken in the United States,

but since then Europe has beenprominent in developing bothunderstanding and standards. Efforts inthe next decade will probably be moreevenly shared between China, the US inthe wake of a passenger rail resurgence,Europe, and Japan. Britain, as ever, mightmaintain research whose excellence farsurpasses that of its railway.

In the 1970s, Mr Paul Remington andhis colleagues from Bolt, Beranek andNewman proposed that wheel/railnoise comprised essentially threeelements: rolling noise, impact noiseand squeal. The first two depend onwheel and rail irregularities, and aretherefore related to one another. Impactnoise results from discrete irregularities,such as joints, whereas rolling noisederives from “broad-band”irregularities, commonly known asacoustic roughness. Squeal is caused bya stick-slip oscillation excited by largeangles of attack of the wheelset andwheel/rail friction that decreases withsliding speed: this is similar to the

mechanism that causes a crystal glass toring by dragging one’s finger aroundthe rim.

This article is concerned only withnoise resulting from wheel and railirregularities, and in particular acousticroughness. A model for this mechanismof wheel/rail noise (which is treated ingreater detail in the text, Railway Noiseand Vibration by Prof D J Thompson)comprises essentially three elements(Figure 1):� excitation, which is the combinedroughness of rails and wheels� the mechanism by whichirregularities excite vibration of thecomponents (wheels, rails, sleepers)that radiate noise, and� propagation of noise from thesecomponents to the observer.

A change in any one of these threeelements can reduce noise. For example,reprofiling wheels and rails reducesexcitation, provided it is done to asatisfactory standard. Dampingtreatments can be applied to wheels or

rails to cut vibration. The propagationpath can be influenced by noise barriersor natural features such as trees, bushes,grass and snow.

The effect of rail irregularities onnoise is shown roughly in Figure 2:noise increases by 5-10dBA for a0.050mm-depth corrugation. Theincrease is greater for disc-braked thantread-braked rolling stock as wheelroughness is lower with disc brakes, soan increase in irregularities on the railresults in a greater relative increase inthe combined wheel and rail roughness.

Irregularities clearly have asignificant influence on wheel/railnoise, and their control is a vital andrelatively simple component of wheeland rail maintenance that has manyother benefits, such as the maintenanceof an appropriate transverse profile.

Because irregularities are criticallyimportant to noise, equipment has beendeveloped to measure and therebycontrol them. One of the earliestexamples is a trolley profilometer made

Track

As transit systems proliferateand high-speed railwaysexpand, noise will not onlybecome more important butmay in some cases even inhibittheir growth. It is thereforedesirable for the railwayengineer to have at least apassing familiarity with thefactors that influence noise andhow they can be controlled,says Dr Stuart Grassie*.

Controlling irregularitiesin rail to reduce noise

*Dr Stuart Grassie has worked in thearea of corrugation, damage andmaintenance of rails for 34 years. He isdirector of RailMeasurement (RML),which manufactures equipment tomeasure rail irregularities and acousticroughness from walking speed to over150km/h, including the CAT.

Roughness: excitationWheel roughness Rail roughness

Contact filterContact

receptancesRoughness input

Wheelreceptances Rail

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Dynamics:response toexcitation

Wheel/rail interaction

Propagation: how noisegets from wheel and railto listenerPropagation

Noise measured at wayside

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Rail response

Wheel radiation

Wheel response

Rail radiation

Wheel vibration Rail vibration

Noise from wheel Noise from rail

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Figure 1: a model for wheel/rail rolling noise

at Cambridge University in the 1970s.Since the 1990s the instrument of choicehas been a straight-edge profilometer,which can be extremely accurate, but isusually heavy, slow to use, and limitedfundamentally by the length of thestraight edge that is used as a datum.Typically two operators are requiredand a van is needed to transport theequipment.

The corrugation analysis trolley(CAT) has been developed from theCambridge trolley to measure at firstcorrugation, primarily for qualityassurance (QA) of rail grinding, andlater acoustic roughness. While anaccuracy of microns is satisfactory forQA of rail grinding, fractions of amicron are significant for acousticroughness. CAT, which can be carried tothe site and operated by one person,measures the rail at 1m/s forcorrugation, 0.5m/s for acousticroughness. The length of track that canbe measured is essentially unlimited.

Results are available for reviewimmediately, which is useful ifsomething of interest is noted. I usedCAT to discover and then check brokenrails and spalling of occasional rails ondifferent metros, which would havebeen difficult if not impossible toobserve with other equipment.

It has been demonstrated that CATcan give substantially identicalmeasurements of rail roughness totraditional straight-edge instruments .Figure 3 is an example taken from a“road test” of the measuring protocolthat is now contained in EN 15610:2009,which shows the so-called one-thirdoctave spectrum of rail roughness on atest site, measured by severalinstruments and measuring teams. Theone-third octave spectrum is a graph ofamplitude of irregularity as a functionof wavelength, often shown withwavelength decreasing along the x-axis.The amplitude of irregularity is theroot-mean-square (RMS) amplitude in a

wavelength band in which the longerwavelength is 21/3 times the shorterwavelength. The logarithm of RMSamplitude is graphed ie:

In Figure 3, CAT is instrument H. Thestraight line is the limit spectrum forthe acoustic technical specification forinteroperability (TSI), which is similarto that in EN ISO 2095:2005. This isessentially rail on which irregularitiesare sufficiently small that acoustic typetesting of vehicles can be undertaken

without the effect of the vehicle beingobscured by wheel/rail noise.

Many CATs are in use worldwide andit is interesting to compare roughnessfrom different types of railway (Figure4). On metros - the example shown isfor track just before grinding - it iscommon for corrugation to develop invery distinct wavelength ranges: this isa consequence of the uniformity ofrolling stock and speeds.

Rolling stock and speeds are alsouniform on light rail systems, andlikewise give rise to corrugation at

40 IRJ April 2010

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CAT is used to measure rail corrugation and acoustic roughness.

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Effect of rail irregularities on wheel/railrolling noise.

Results of the EN ISO 3095:2005 “road test”: a comparison of results from severalinstruments and measuring teams. H represents CAT.

well-defined wavelengths. Light railsystems differ from the other types ofrailway insofar as short wavelengthroughness (<30mm) is high and doesnot reduce with traffic. This results fromroutinely dumping sand on the track toensure sufficient adhesion to achievehigh traction and braking rates.

On mixed passenger and freightrailways, where speed and traffic vary,corrugation is spread over a broaderwavelength range, even if it is apparentto the naked eye and occurs as a resultof the same “wavelength-fixingmechanism” that causes corrugation onmetros.

Surprisingly, a well-maintainedheavy-haul railway can have lowerlevels of roughness than the smoothestpassenger line, probably because thehigh axleloads flatten short wavelengthirregularities, and so-called pinned-pinned resonance corrugation developsslowly if at all because of the lowrunning speeds. Perhaps suppliersshould undertake acoustic-type testingof their vehicles on well-maintainedheavy-haul lines!

Whereas the EN ISO 3095 referencespecifies limits on rail irregularities toensure that wheel/rail noise is low, itdoes not say how such irregularitiescan be achieved. This gap is filled bythe European standard on reprofiling(EN 13231-3:2006), the current versionof which states three methods to decidewhether residual longitudinalirregularities are acceptable. It is shownhere that the method stated in terms ofRMS amplitudes of irregularity iscomparable with the limit spectrumstated in EN ISO 3095:2005. During the

42 IRJ April 2010

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drafting of this standard in the 1990s,measurements of grinding by Loramand Speno were undertaken using CATto show that the stated levels of residualirregularities were achievable.

The allowable irregularities increasewith increasing wavelength, but thewavelength bands are considerablybroader: 10-30mm, 30-100mm, etc. Eachof these comprises essentially five one-third octave bands, since by a quirk ofmathematics, 21/3 = 101/10 = 1.26 (tothree significant figures). If it isassumed that the logarithm of RMSamplitude of irregularity in these one-third octave bands varies linearly with

the logarithm of wavelength, and thatthe combined limit for the five one-third octave bands is the same as inEN 13231-3:2006, the limit curve shownin Figure 5 is obtained for wavelengthsof more than 30mm. This is almostidentical to the limit line required bythe standards that have been derivedfrom considerations of noise, ie prEN15610 and EN ISO 3095. For shorterwavelengths, typical reprofilingoperations cannot achieve the lowlevels of irregularity required by theacoustic standards, but traffic usuallyreduces roughness at these wavelengths(Figure 4).

Seldom if ever can two broad-basedgroups, with no common membershipand with different objectives, haveworked entirely independently on twodifferent standards and producedessentially the same standard:EN 13231-3 to limit irregularities thatcan exist on rails after reprofiling, andEN ISO 3095/prEN 15610 to state adesirable level of roughness for what isessentially a quiet or quieter railway.This unique success should beapplauded, even if it was achieved byaccident, and act as an example to thosebodies that are responsible forstandards to ensure coincidence of whatis necessary, what can be measured, andwhat can be delivered.

Finally, I am grateful to Prof DavidThompson and Dr Chris Jones of ISVRat Southampton University, who havecontributed greatly to the subject ofrailway noise, for the use of figures 1, 2and 3. IRJ

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Figure 5: comparison of roughness limits

Figure 4: roughness measured on different railways