Noise Vibrations and Harshness analysis

Vehicle noise sources

Aero- acoustics („wind noise”)

Tire noise

Vehicle interior noise is a combination of:• engine noise;• road noise;• intake noise;• exhaust noise;• aerodynamic noise;• noise from components and ancillaries;• brake noise;• squeaks, rattles and ‘tizzes’.

Vehicle noise sources

CROLLA, D. 2009. Automotive Engineering e-Mega Reference, Elsevier Science.

The interaction with the structure can be either as:• An airborne noise path – airborne noise

from outside the passenger compartment leaking in to cause airborne noise inside.

• A structure-borne noise path – vibration from outside causing the surfaces of the passenger compartment to vibrate and radiate noise.

Idealised models of acoustic sources

• Monopol – results from unsteady volumetric flow, (for example in exhaust pipe of piston engine). In automotive the Monopol source is reduced by muffler. Occur for low Mach number, therefore valid for most automobiles. This source is dominant for cars

• Dipol- caused by unsteady forces caused by Karman vortex. For high Mach number, almost not valid for cars

• Quatropole- caused by two fluids elements colliding with one another, causing internal stress on the fluid (for example in jet). For very high Mach number, therefore usually ignored.

1. At low speed (50 km/h) theengine noise is predominate

2. At speed of 50-100 km/h tirenoise is getting more intensive

3. Wind noise constantly increaseswith respect to the speed. Thewind noise becomes lauderthan tire nosie at approx. 140km/h

Wind noise

• Car with low aerodynamic drag low noise generation

• The sound intensively increases with the 6th power of the car speed.

• The flow around a vehicle causes aerodynamic noise; In almost all cases the physical reason is periodic flow separation from certain elements of the surface - gutters, mirrors and the radio antenna.

• Aerodynamic (wind) noise is a significant source of interior noise for many vehiclestravelling at higher speeds. It is easily confused with road noise, being generallybroadband in nature but with a strong low-frequency bias.

• Aerodynamic noise is caused through a variety of mechanisms:

Aerodynamic excitation of the so-called ‘greenhouse surfaces’ on the car (the glass-work and the roof panel) causing structure-radiated noise in the interior

Airflow over the underside of the vehicle causing transmission of airborne sound to the interior, particularly in the wheel-arch areas.

Noise transmission through door and glass seals due to aspiration (leakage) or due to aerodynamic excitation of doors and glass caused by disturbed airflowover the seals.

Vortex shedding over protrusions in the bodywork (such as aerials, roof bars, etc.) causing tonal airborne noise.

Cavity flows through partially open windows and sunroofs causing intense low-frequency (below 25 Hz commonly) noise and buffeting.

Wind noise sorces types

Leak noise

(„aspiration noise”)

• Occurs when there is a thin path connecting outside with the inside of the car.

• An annoying Whistle tone arises

• The leak noise always dominate other wind noise source

• Arises mostly on door seals, moveable glass seals, fixed glass seals

Cavity noise

• Occurs due to cavity outside a vehicle .

• Arises on the gaps around a the doors, exposed gaps on the outside mirrors, gaps in the radiator grille

• Cavity noise is generated by trailing edge impinging the rear surface of the cavity.

• The noise is on high frequency, hence human can only feel it more then hear it

Wind rush noise

• Caused by fluctuating

pressure outside the car.

•Flow is mostly turbulent,

hence laud.

•It is impossible to avoid

the wind rush noise.

• most intensive noise is in

the region of separation

and vortices A-pillar

HUCHO, W. H. 2013. Aerodynamics of Road Vehicles: From Fluid Mechanics to Vehicle Engineering, Elsevier Science.

The following types of air flow are

observed:

• Attached flow

• Quasi-two-dimensional separation

• Reattached air flow

• Three-dimensional vortex separation

Wind noise generation

• Attached flow along the entire length ofthe cuboid can be generated by means of arounded edge atcontour point A.

• A sharp edge at this point brings about aseparation

• bubble between A and B with subsequentreattachment of air flow

• Within the separation bubble, a distinctioncan be made between area A with side airflow and area B with clearly reversed flow.

HUCHO, W. H. 2013. Aerodynamics of Road Vehicles: From Fluid Mechanics to Vehicle Engineering, Elsevier Science.

HUCHO, W. H. 2013. Aerodynamics of Road Vehicles: From Fluid Mechanics to Vehicle Engineering, Elsevier Science.

There are, in addition, two extremelyundesirable effects to be dealt with, which occur especially in motor vehicles:• Booming occurs with open side windows or

an open sliding roof. Effects are inducedwithin the air column inside the vehicle bythe exterior air flow, causing the passengercompartment to become a resonator.

• leaking door seals. leaking door joint is undesirable for two reasons: Damping of the intensive noise from the

A-pillar vortex decreases and the open gap acts as a noise path.

A high-frequency jet air flow is set up from the passenger compartmentthrough the gap because of the great pressure difference. The door clearance becomes an additional source of noise

In a quasi-two-dimensional separation bubble,maximum noise intensity occurs in the area ofreattachment of air flow. The level measured withinattached air flow decreases as length of attached airflow increases

Wind noise sources

The A-pillar

1. High flow ferocity at the A-pillar causes greater level of noise.

2. The reattachment of the A-pillar vortex is a main source of interior noise

3. Mirror located near the A-pillar also affect the flow and generates noise

HUCHO, W. H. 2013. Aerodynamics of Road Vehicles: From Fluid Mechanics to Vehicle Engineering, Elsevier Science.

Radio antena

Strouhal number (constant for given shape). Describes oscillating flow mechanics

S=fd/U

f frequency, Hz

d diameter, m

U velocity, m/s

Noise is a consequence of Karman vortex.

Warping helical strake around the antenna prevent creating the Karman vortex, hence reduce noise

roof luggage rack creates similar noise. The solution is similar.

Karman vortex,

HUCHO, W. H. 2013. Aerodynamics of Road Vehicles: From Fluid Mechanics to Vehicle Engineering, Elsevier Science.

• In the region of periodic flowseparations, vortices are shed fromboth sides of the body in alternatingsequence.

• These vortices move downstream inthe wake and they can be observedover a long distance.

• In a coordinate system movingdownstream with the vortices, aregular pattern of these vortices isfound, which is called a vonKarman's vortex street.

• At a certain point of the flow field, all flow quantities change with the frequency n of the vortex separation from the body.

Mirrors Reduce noise by moving mirrors backwards

Avoiding paddle shape and sharp corners.

Eliminating any gaps in mirror housing.

With mirror

Without mirror

Sou

nd

leve

l, d

B(A

)

Frequency, Hz

Underbody

•Rough underbody generated turbulence and cavity and in consequence noise.

•The underbody noise is less significant and can be confused with road noise.

•As the underbody gets rougher, the larger noise will be produced

HEISLER, H. 2002. Advanced Vehicle Technology, Butterworth-Heinemann.

Reduction of the wind noise

•Stiff body and door frames with good sealing

•Create optimal A-pillar design

•Optimize the location and shape of the

mirror

•Create smooth underbody

•Reduce any protruding objects.

Rolling noise

Generation and amplification effects related to tire/road noise

Driver /vehicle

• Driving speed

• Wheel torque

• Vehicle load

Tires

• Tread pattern

• Carcass

• Material

• Pressure/ temperature

• Dimension

• Degree of absorption

Roadway

• Coating material

• Roughness

• Weather condition (moisture)

• Porosity

• Elasticity

white noise - the response of the vehicle to a stochastic road with energy density uniform over the spectrumpink noise - the energy density is biased towards lowfrequencies

Actual ISO roads are therefore particular cases of pink noise.

Noise map of the wheel1. Air pumping effect-As the tire rolls, air is trapped and compressed in the voids between

the tread and the pavement. Noise is generated when the compressed air is released athigh speed to the atmosphere at the exit of the contact patch

2. Tread element vibrations-Tread elements impact the pavement as the tire rolls. Whenthe elements leave the contact patch, they are released from a highly stressed state.These induce vibrations of the tread, which form a major source of tire noise. Carcassvibrations and the grooves and lug voids in the tread acting like resonating pipes alsocontribute to noise radiation from the tire.

Tire / road noise generationVibration-related mechanisms of

tire/road noise generationAerodynamically related generation and

amplification mechanisms of tire/road noise

Tire „horn effect”

• The horn shape amplifies the sound pressure.

• The horn effect is a major factor in the radiation of tire-road noise.

• Two kinds of sound sources:1. quasi-monopole sources

2. vibration modes of the surface

• The horn effect and displacement mechanics can be reduced by using thick pours pavement

GILLESPIE, T. D. 1992. Fundamentals of Vehicle Dynamics, Society of Automotive Engineers.

Different tires, different noise

Tire type CPX Noise Level dB(A)

ASTM Slick 91,2

UniRoyal Tiger Paw 92,4

ASTM 501- Ribbed 93,8

Firestone FR 380 93,9

MasterCraft Glacier Grip 94,7

Goodyear Aquatread 94,9

Michelin Rain Forest 95,2

Tire/road noise (rolling noise)influence on volume

Tire • The harder rubber the higher

noise, especially tread patterns

• With of a tire; noise increases withtire with

• The type of a tire; radial is lauderthan bias, increase of carcassstiffness results in reduction ofnoise

• Non-uniformities; (tire runout,unbalance) can cause a noise levelincrease

Tire/road noise (rolling noise)

Surface roughness

• Roughness depth

• Roughness wavelength

• Shape

• Air flow resistance

• Skid resistance

Porosity

• Layer thickness

• Air void content

• Air flow resistance

• Tortuosity

Elasticity

• Stiffness

• Loss factor

Tire/Road Noise

• Speed of the vehicle 25 dB (increase from 30 to 130 km/h)

• Pavement (incl. extremes) 17 dB (from the best porous asphalts to the worst stone setts)

• Pavement (conventional) 11 dB (if stone setts are excluded)

• Truck tire type (conventional, one size) 10 dB (same size; a couple of decibels more if different sizes)

• Car tire type (conventional) 10 dB (including width range 145–255 mm)

• Car tire type (conventional) 8 dB (same width)

• Studs in tire (relative to no studs) 8 dB (for new studs at low speeds; less at higher speeds)

• Load and inflation 5 dB (for ±25% variation around nominal recommended values)

• Road condition (wet/dry) 5 dB (increase in heavy rain; light rain gives lower influence)

• Temperature 4 dB (over range 0–40◦C; mostly decreasing noise with increasingtemperature)

• Torque on the wheel (normal driving) 3 dB (over range 0–3 m/s2; increasing noise with increasing acceleration)

Examples of rolling noise reduction (tires)

1. The flow is disturbed bythe dense perpendicularserrations resulting innoise reduction .

2. Leading air through thegroove without distortion.This creates a whistlingsound known as “piperesonance”.

3. Silence ring. Whichminimises vibration

Rolling noise reduction (road)

Engine noise generation

•Combustion

•Mechanical dynamics

Forcing functions

•Structural response to input force

Transmission path•Vibrating surfaces

•Radiation efficiency

Exterior sources

NOISE

Large forces are generated within the engine. The forces produces vibration in structure, and the vibration is transmitted to external components Hence, the amount of noise is depended on upon the material ability to transmit vibrationReducing the noise can be also performed by reducing the force input

Engine noise causes

Direct suggestion for noise

1. gas change procedures 2. cooling air fan3. generator air fan4.belt drives

Indirect suggestion for noise

(impact sound procedures)

Combustion

Impact excitation

1. Changing power transmission over playful components

pistonpiston rodcrankshaft

2. Piston secondary movement impulse opening and closing3. Pprocedures

valvesinjection

4. Gear wheel drive

Suggestion

Mechanics

Suggestion for change force

1. Gas forces combustiongas change

2. Mass forces (reciprocating and rotary engine components)3. Mass forces and spring tension of valve control4. Gear wheel

Auxiliaries

1. Fuel injection2. Steering booster pump 3. Generator4. Climatic compressor5. Ssecondary tire pump6. Oil pump7. Water pump

Which engine is lauder (diesel engine vs gasoline engines)

• Most engines produces approximately this same amount of noise at their max speed and load, although noise levels compared at a given speed can vary by up to 30 dB

• Considering typical sound pressure spectra for two engines operating at maximum speed and load, one can notice that the overall noise level is not very different

Combustion noiseThe direct excitation of the engine structure(piston and cylinder head) by cylinder pressure

Indirect combustion noise

• Develops when construction elements fastening relatively to each other after passing plays

• Indirect combustion is part of mechanical noise

Direct combustion noise

• Arises as effect of combustion pressure on the walls of the combustion chamber and from the noise portions arriving into the crankcase over the internal impact sound

Combustion noise• The step increase in cylinder pressure produced by fuel combustion causes the

broadband increase in cylinder pressure spectrum of the diesel

• If the cylinder pressure trace is smooth, there will be very high amplitudes of low-frequency excitation to the engine structure and little high-frequency content

• The step increase of pressure in a cylinder causes the more high-frequency excitation of the engine structure.

Reducing of combustion nose

Gasoline engines

Ignition (spark) timing. Retardation of ignition towards TDC reduces noise

Spark plug location. Centrally located plug will provoke combustion more rapidly.

Number of sparks plugs. Two spar plugs are even more efficient in terms of combustion provoking .

Swirl and tumble of the air–fuel mixture. Higher swirl will ensure better mixing, hence faster combustion.

Diesel engines

Injection timing. Retarding injection timing reduces combustion noise, unless the start of combustion is pushed after TDC.

Boost pressure. Greater boost pressure causes the fuel evaporation, and in consequence, shortening the ignition delay.

Intake manifold temperature. Enlarging the intake temperature reduces the ignition delay and ensures easier combustion

Compression ratio. Increasing compression also has influence on reducing ignition delay and , accordingly the combustion.

Injection characteristics. Reducing the amount of fuel injected during the ignition delay drastically reduces noise

•Fuel cetane. Higher cetane fuel reduces ignition delay

In all engines the combustion noise is controlled by adjusting the cylinder pressure i.e. the heat release (combustion)

Mechanical noise

• Occurs due to clearances within the engine. The mechanical noise arises as a

consequence of surfaces impacting to each other. For example, piston slap is

caused by the piston moving laterally or rocking in the cylinder and impacting

against the cylinder wall.

• Identifying source of the noise pure tone is rather easy. The frequency of the

measured noise at a certain speed can be compared to calculations of potential

source frequencies.

• Impact noise identification is more difficult due to the fact that impact noise is

broadband. Usually modified parts, design to magnify or eliminate particular

clearance, are introduce in order to detect the source of sound. For example,

Teflon-padded piston are used for determination of the piston slap noise level

Reduction of the mechanical noisePiston skirt profile, both top to bottom and around the circumference. This

reduces the gap between the piston and the cylinder, hence it reduces the possibly to slap.

Bore distortion. Reducing bore distortion, controlling thermal expansion, modifying the skirt profile, and using coatings are all means of allowing a tighter fit without risking scuffing and engine seizure.

Piston pin offset. This changes the way piston rotate when crossing the bore.

Piston coatings to reduce friction or to allow a tighter fit.

Piston inserts to control thermal expansion.

Piston skirt stiffness.

Applying hydraulic valve lifters, which eliminate the lash in valve trains and thus reduce noise.

Carefully design cam profile eliminates force spikes in valve train and, thus reduces noise.

Mounting accessories (oil pumps, power steering pumps, alternators, air-conditioning compressors etc.) in way that avoids mounting bracket resonances caused by engine firing frequency .

Intake and exhaust system

• The exhaust and intake noise arises due to the pressure pulsation cased by the engine operation and additional generated noise.

• The exhaust noise is partially cancelled by means of mufflers ,which are design to reduce sound levels at frequencies of 0 600Hz.

• The tailpipe length has great influence on the muffler performance. The tailpipe acts as a resonant cavity that couples with the muffler cavity

• The speed of exhaust gas has detrimental influence on muffler performance.

• The effect of gas flow is dependent on the internal design of the muffler due to the fact that is related to the interaction of sound with turbulence

Exhaust silencers

NUNNEY, M. 2015. Light and Heavy Vehicle Technology, Routledge.

The purpose of a motor vehicle exhaust silencer is therefore to suppress this noise to a legallyacceptable level. It does this by breaking up the intermittent stream of high velocity exhaust gases intonumerous small streams, which has the effect of reducing both their velocity and their temperature sothat they leave the tail pipe of the exhaust system in a more nearly constant flow at low velocity.

a. the exhaust gases expanding into the silencer were compelled to pass through aseries of chambers separated by perforated flat or conical plates, before leavingfrom the other end of the silencer.

• Effective in suppressing exhaust noise• too much back-pressure in the system, which could further increase if the small perforations became

partially obstructed with carbon deposited by the exhaust gases

Exhaust silencersthe expansion method, the incoming gases from the downtake pipe are initially allowed to expand into the main chamberof the silencer to reduce both their pressure and their temperature. Following this their direction of flow is changed bypassing them through side-by-side perforated tubes, in the process of which more of their energy of vibration is convertedinto heat energy by frictional loss, before they pass into the tail pipe. This method of silencing is best suited to suppressinglow and medium-frequency exhaust noise.Helmholtz chamber - In the same way that particularly troublesome peaks of induction noise can be suppressed bymaking the air cleaner and silencer act as a Helmholtz resonator. For this purpose the silencer incorporates a separatechamber of calculated volume that has a narrow neck or inserted tube, across which the pulsating flow of exhaust gasescan pass. When the resonant frequency of vibration created in the Helmholtz chamber abstracts sound energy fromvibration in the main silencer, the latter will be much reduced and the noise suppressed.the absorption method, the sound is damped through the medium of an acoustically dead material of a soft and porousnature. As applied to an exhaust silencer, a heat resistant material such as glass fiber mat or stainless steel wool is packedinto the annular space between a perforated or a louvred central tube and the expansion chamber through which itextends. Sound waves leaving the central tube and striking the absorbent material have their energy of vibration partiallyconverted to heat energy. They are mainly effective in damping out high-frequencyexhaust noise

NUNNEY, M. 2015. Light and Heavy Vehicle Technology, Routledge.

Mufflers designs

Dissipative muffler • Dissipative mufflers reduces noise by

means of sound absorbing material, which transfers the acoustic energy mainly in to heat as it propagates through the muffler. Typical sound absorbing materials used are rock wool, glass wool and plastic foams

Reactive silencers • Reactive silencer reflects the

sound waves backwards andprevent the sound fromtransmitting along the pipe. Thedesign is based either on theHelmholtz resonator or anexpansion chamber.

Noise measurementaccording to DIN ISO 5130

Noise measurementpass-by SAE J1470

• The vehicle noise is measured by means of two microphones, one on eitherside of the vehicle

• The vehicle under test drives into the testing route at a speed of 50 km/h. 10 m in front of the microphones, the vehicle then accelerates until the end of the vehicle is 10 m away from the microphones. (Drivers usually use special tools that assist them in choosing the correct gear and entry speed)

WEBER, J. 2009. Automotive Development Processes: Processes for Successful Customer Oriented Vehicle Development, Springer Berlin Heidelberg.

• To be able to separate engine and exhaust noise from tire/road noise, the ISO 362 test can also be carried through on a dynamometer

• This setup then requires 36 pairs of microphones in a semi-free field

• An additional advantage of such facility is that tests can be carried through at standardized climate conditions

Noise measurementISO 362

WEBER, J. 2009. Automotive Development Processes: Processes for Successful Customer Oriented Vehicle Development, Springer Berlin Heidelberg.

Other standardized test procedures that relate to vehicle noise emission

are the coast-by test (tire noise test) according to ISO 13325, and the

exhaust noise test according to ISO 5130

Literature1. GILLESPIE, T. D. 1992. Fundamentals of Vehicle Dynamics, Society of Automotive

Engineers.

2. HAPPIAN-SMITH, J. 2001. An Introduction to Modern Vehicle Design, Butterworth-Heinemann.

3. HEISLER, H. 2002. Advanced Vehicle Technology, Butterworth-Heinemann.

4. HILLIARD, J. C. & SPRINGER, G. S. 1984. Fuel Economy: In Road Vehicles Powered by Spark Ignition Engines, Springer.

5. HUCHO, W. H. 2013. Aerodynamics of Road Vehicles: From Fluid Mechanics to Vehicle Engineering, Elsevier Science.

6. PACEJKA, H. 2005. Tire and Vehicle Dynamics, Elsevier Science.

7. MORELLO, L., ROSSINI, L. R., PIA, G. & TONOLI, A. 2011. The Automotive Body: Volume I: Components Design, Springer Netherlands.

8. WONG, J. Y. 2001. Theory of Ground Vehicles, Wiley.

9. CROLLA, D. 2009. Automotive Engineering e-Mega Reference, Elsevier Science.

10. BONNICK, A. 2008. Automotive Science and Mathematics, Taylor & Francis.

11. NUNNEY, M. 2015. Light and Heavy Vehicle Technology, Routledge.

12. WEBER, J. 2009. Automotive Development Processes: Processes for Successful Customer Oriented Vehicle Development, Springer Berlin Heidelberg.