19 - Noise

Question 1

Context

Answer 1

Noise energy produced by rolling wheel is of the order of 1 millionth of the energy taken to drive the train
Enough to be an annoyance and subject of legislation
Noise limits set through EU Technical Specifications for Interoperability and UK’s Rolling Stock Noise National Technical Specification Notice

Question 2

Rail-wheel interface noise

Answer 2

Straight track - ‘rolling noise’ with broad frequency content
Discrete features such as rail joints and wheel flats generate impulsive noise
Curves - loud ‘curve squeal’ often dominated by single frequencies
All originate at wheel-rail interface, but noise not always emitted from interface itself
For curve squeal, modes of vibration of whole wheel are responsible for noise
For both rolling and impact noise, vibration of whole wheel structure and considerable length of track involved in noise generation
At low frequencies, vibration is transmitted from track through ground and may be experienced as vibration (4-80Hz) or low-frequency rumbling noise (30-350Hz)

Question 3

Sources of railway noise

Answer 3

Aerodynamic noise (high speeds)
Traction noise (dominated by noise from fans or engines)
Wheel-rail interface is origin of most important sources of noise

Question 4

Noise and vibration basics

Answer 4

Sound consists of audible fluctuations in air pressure which propagate through air as waves with speed of 340m/s at 20C
Range of pressure variations perceptible by human ear is between 20 microPa and 200,000,000 microPa
To express sound magnitude, root mean square is used

Question 5

Pressure basics

Answer 5

Logarithmic unit of the decibel (dB) scale is used
Not an absolute unit of measurement but specifies relationship to audio threshold (i.e. tells us by how much a sound exceeds audio threshold)

Question 6

Sound power basics

Answer 6

Total sound emitted by a source can be quantified by its power, W
Directivity index (DI) depends on how directional sound is
Radiates in all directions (e.g. bursting balloon) with no concentration in any direction - DI = ln(1) = 0
Radiates as a hemisphere (e.g. sound source close to ground) so sound above ground will be double normal level - DI = ln(2) = 3
Proportion of mechanical power converted to sound is typically 10^-7 to 10^-5 - acoustic efficiency is very low

Question 7

Vibration basics

Answer 7

Vibration of a structure can be expressed in terms of its: displacement; velocity; acceleration and frequency
Radiating efficiency depends on structure’s shape and size

Question 8

Rolling noise - surface roughness

Answer 8

Causes noise as wheel and rail running surfaces are not perfectly smooth
Rail and wheel roughness interact, cause vibration, both vehicle and track radiate noise
Wavelengths of 5-250mm produce vibrations in audible frequency range
Amplitudes of roughness sufficient to generate noise are in range from tens of microns at long wavelengths to less than a micron for short wavelengths
Generally, amplitude is around 10^-4 times roughness wavelength - often not visible on surface

Question 9

Reducing rolling noise

Answer 9

‘Contact filter’ implies that rail-wheel contact will itself filter out some sources of vibration
Roughness wavelengths that are short relative to contact size (10-15mm) excite vibration of the system much less than larger wavelengths
Rolling noise covers frequency range of approximately 100-5000Hz, with peak in range 500-2500Hz

Question 10

Noise radiation - track or wheel?

Answer 10

Wheel and track both vibrate as result of roughness excitation
Systems act as amplifiers to noise, radiating sound over much larger area than contact itself
Relative importance of wheel and track component of sound radiation depends on details of design as well as train speed and roughness spectrum
Noise radiation depends on combined roughness of wheel and track
Possible that a rough wheel causes significant noise to be radiated mainly by track vibration or vice versa

Question 11

Radiation of noise - wheel

Answer 11

Railway wheel is lightly damped, resonant structure
Can vibrate freely at series of resonant frequencies or ‘natural’ frequencies
Associated vibration patterns are mode shapes
Rolling noise caused by excitation in vertical direction
Modes with large radial motion in tread area are most important
FE is effective at calculating modes

Question 12

Radiation of noise - track

Answer 12

Behaviour of track is characterised by waves propagating away from excitation point
Vibration peaks are caused by whole track vibrating on stiffness of ballast
Rail may vibrate on stiffness of rail pad
Periodic support by sleepers can produce vibration where half wavelength of rail matches sleeper spacing - important for corrugation growth, less important for noise
Above frequency of rail-on-pad ‘resonance’, bending waves propagate in rail and can be transmitted over large distances
Waves decay with distance due to damping effect of pads and fasteners - significant effect on noise radiation
Longer section of rail vibrating for each wheel, more noise is radiated
Decay rate in dB/m is used to describe how quickly vibration of rail decreases with distance from excitation point (wheel contact)
Stiffer rail pads cause more rapid decay rate between around 300 to 2000Hz

Question 13

Noise radiation - breakdown

Answer 13

Breakdown changes with frequency, depending on resonance of wheels, rail pad stiffness etc.

Question 14

Most important sources of noise

Answer 14

Sleepers at low frequencies
Rails in mid-frequencies
Wheels at high frequencies
As speed increases, noise spectrum peak shifts towards higher frequencies, giving greater importance to wheel in total sound level

Question 15

Reduction of excitation

Answer 15

Smooth wheels and rails

Question 16

Reduction of vibration and sound radiation

Answer 16

Optimised wheel and track designs

Question 17

Reduction of sound transmission

Answer 17

Shielding
Barriers
Shrouds

Question 18

Tread brakes

Answer 18

Brake blocks running on wheel tread rather than a brake disc are more noisy than disc braked vehicles
Tread brakes produce wheel corrugation of 40 and 80mm wavelength, caused by thermal input and wear
Fewer passenger vehicles now use tread brakes, but many freight vehicles do
Cast iron brake blocks on freight are being replaced by composites which can prevent corrugation but increases costs

Question 19

Smooth rails

Answer 19

Rail grinding can remove corrugations and large scale roughness from the rail
Conventional grinding uses rotating grinding stones which leave a 10-20mm roughness or undulation in the rail surface
Roughness can mean that freshly ground track remains noisy
Special oscillating grindstones can avoid this problem - usually applied at sites with noise problems, lines close to houses etc.
Grinding is not one-time solution
Surfaces deteriorate and further grinding needed

Question 20

Wheel optimisation

Answer 20

Wheel noise is caused by resonance - one means of reducing noise is to increase wheel damping
Absorber can be added
CSA of wheel can be varied, changing resonances to ones which are not excited by rail and wheel roughness at design vehicle speed
Reducing wheel diameter
Smaller wheels have higher resonant frequencies
Moving frequencies above approximate 5kHz limit at which rail-wheel contact excites resonance will reduce noise

Question 21

Optimisation of track

Answer 21

Increase rail pad stiffness
To increase decay rate of vibrations without increasing rail pad stiffness, various types of rail damper exist
CDM rail damper has separate elements with a steel plate, elastic intermediate layer and 2 rubber blocks
Schrey & Veit rail dampers are separate elements of steel plates and elastomer wafers, built up like a sandwich and bonded using clamping bolt assembly below and on web of rail
Tata rail damper consists of steel strips, enclosed by a polymer, which is bonded to rails - mechanical properties can be described as a mass-spring system

Question 22

Shielding measures

Answer 22

Aims to restrict sound propagation
Local shielding may be close to wheels and rails in form of shrouds around bogies or low barriers close to rails
Larger shields at trackside are also possible - care needed to avoid wheel-rail noise being reflected up and over barriers