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WIND NOISE SOURCES AND REDUCTION

1. Introduction

1) As an automobile travels down the road, the air in front of it is displaced which interacts

with the surface of the car body to generate aerodynamic noise or wind noise.

2) High wind noise can make it difficult to converse with other passengers or listen to the

radio. It can add to driver fatigue on long high-way driving. Hence, manufacturers pay

close attention to wind noise and try to minimize it. Fortunately, wind noise can be

controlled by careful attention to design and assembly.

3) Noise reduction algorithms can be used to alleviate the effects of wind noise, road noise

and engine noise on the sound quality of the communication system.

4) Wind noise is generally proportional to the sixth power of velocity. At lower speeds,

wind noise levels are very low and are easily masked by the tyre noise, powertrain noise

and ambient noise. At speeds > 160kmph, wind noise is the dominant noise source.

2. Sources of Wind Noise

1) Turbulence through holes, which is a function of the seal between and around doors,

hood, windshield is a common source

2) Exterior varying wind conditions, such as cross – winds on a highway

3) Very low frequency (10-20Hz) beating noise occurring when either a rear-window or

sunroof are open. Aerodynamic noise due to exterior varying wind conditions and

beating noise is referred to as wind buffeting or wind gusting noise.

4) The frequency spectrum of steady wind noise is typically broadband and heavily biased

towards the low frequencies (31.5-63Hz).

5) Gusting noise due to cross wind has frequency varying upto 300Hz or above.

2.1 Idealized Models of Acoustic Sources

These models are used to relate actual sources in order to predict which source will be

dominant.

2.1.1 Monopole

1) It results from unsteady volumetric flow.

2) Most efficient at low Mach numbers.

3) Sound intensity produced is proportional to V4, where V is the flow velocity.

4) Primary sources are sound from the exhaust pipe of an unmuffled piston

engine, engine intake and exhaust.

2.1.2 Dipole

1) It results from unsteady pressures acting on a rigid surface.

2) Sound intensity is proportional to V6.

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3) Sources are unsteady forces caused by Von Karman vortex shedding on a

radio antenna.

4) Idipole/Imonopole is proportional to M2, where M is the Mach number.

2.1.3 Quadrupole

1) Caused due to the collision of two fluid elements, causing unsteady internal

stresses in the fluid.

2) Intensity is proportional to V8.

2.2 Physical Wind Noise Sources

2.2.1 Leak Noise

1) It is also known as aspiration noise and is caused by the existence of a direct

flow path connecting the exterior of the automobile to the passenger

compartment.

2) Leak noise is caused by both the monopole and dipole mechanisms. Since

the monopole mechanism is quite efficient at generating sound and since the

sound may be tonal in nature and may fluctuate in time, the leak noise will be

noticeable and annoying.

3) It must be eliminated first; else wind noise reduction measures will not be

effective in lowering the interior wind noise levels.

4) Sources are door seals, moveable glass seals etc.

2.2.2 Cavity Noise

1) The presence of cavities can cause wind noise in a region of high flow

velocity, such as the A-pillar or around the outside rear mirrors.

2) Model: - One model assumes that wind noise is caused by the trailing edge

wake impinging on the rear surface of the cavity. Since the shear layer flow is

turbulent, there is no preferred frequency and the resulting cavity noise is

broadband in nature.

3) The other cavity model involves a feedback and resonance phenomenon. A

disturbance is shed from the front edge of the cavity and is connected at the

local flow velocity. This disturbance impinges on the rear edge of the cavity

and generates an acoustic wave that propagates in all directions.

t = L/U + L/Co, where

L: - dimension of cavity in the local flow direction

U: - local flow velocity

Co: - speed of sound in air

2.2.3 Wind Rush Noise

1) This is generated by the fluctuating pressures on the exterior of the

vehicle caused by air flow over the surface. The airflow is turbulent over the

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majority of automobile surface. Thus, the presence of pressure is fluctuating at

the surface.

2) It is also produced even if the flow is attached everywhere on the vehicle

surface because the flow is turbulent over most of the car. If the flow is

separated, the wall pressure fluctuations are more intense by a factor of

approximately ten and hence much more wind noise is generated.

3) The most serious wind noise problems are associated with the A-pillar

area flow. The air-stream velocity around the A-pillar corner can be 60%

higher than the free stream velocity and hence the wind noise will almost be

17dB louder than a source exposed to free-stream velocity.

2.3 Underbody Wind Noise

1) Underbody wind noise sources are complex flow structures involving separation,

vortex convection and reattachment, with strong dependence on the detailed

underbody geometry. The largest underbody noise sources typically originate

from flow separations at the wheels, wheelhouse, engine/exhaust system,

suspension, and structural cross-members.

2) The resulting pressure fluctuations excite the underbody floor pan structure and

radiate into the vehicle interior as low frequency noise. Accurate simulation of these

noise sources requires solution of the time-varying flow structures and resulting

wall pressure fluctuations (WPF) on the underbody surfaces.

3) Moreover, the ability to accurately capture energy-containing anisotropic structures

and convection of cascaded turbulent structures over a wide range of length scales is

necessary.

Fig 1. Underbody wind noise

2.4 Greenhouse Wind Noise

1) For a vehicle traveling at highway speeds, turbulent flow provides a distributed

force excitation on the greenhouse panels (such as the side windows and

windshield), generating an acoustic field that also acts on the panels.

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2) The greenhouse noise sources are complex transient flow structures produced by

flow separations and vortices resulting from various exterior geometry features such

as the cowl, A-pillar, or mirror assembly.

3) Accurate prediction of greenhouse noise sources requires predicting the time-varying

flow structures and resulting wall pressure fluctuations (WPF) on the greenhouse

panels, including effects of small geometric details. The turbulent excitation, panel

vibration, and acoustics are of widely varying length scales (wave number

spectrum), providing a significant technical challenge to predict accurately over

the important frequency range.

4) Sound transmission of the exterior acoustic field to the interior is particularly

important near the acoustic/structure coincidence frequency, and appears to be

significant even though the turbulent wall pressure amplitudes far exceed those of the

acoustic pressures.

2.4.1 Evaluating Vehicle Interior Noise from Greenhouse Wind Noise Sources

1) The initial step (Computational Fluid Dynamics) simulates the fluctuating

pressure loads on the exterior panels of the vehicle at speed. These

transient pressures are analyzed in the frequency domain to develop load cases

for a structural acoustics model of the vehicle panel dynamics and interior

acoustics. The interior acoustic frequencies are simulated using Statistical

Energy Analysis with the interior acoustic cavity subsystems indicated in

Figure 2.

2) The baseline test condition, tow mirror at 120 kph and zero degree yaw, has

been duplicated in all the experiments to maintain consistency between wind

tunnel tests. Tape was added to the exterior of the vehicle at seal locations and

any other area that could allow wind noise into the vehicle. The taped

condition ensured that the interior noise measurements were not affected by

other noise pathways into the vehicle that were not included in the simulation.

The vehicle was placed on a circular rotating platform in the wind tunnel

to allow measurements at positive and negative yaw angles. To measure

negative yaw angles, the rotating platform is moved so the right side of

the vehicle faces the oncoming wind flow, as seen in Figure 3.

Fig 2. Interior acoustic cavities in the vehicle sea model

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Fig 3. Rotating platform

2.4.2 Conclusions

1) The noise sources on the driver side glass are higher for −10 yaw for all

frequency bands. But this trend is reversed for the passenger side glass due to

significant reduction in WPF levels for −10 yaw. The difference in dB levels

between these yaw angles increases with frequency above 250 Hz for both

glasses.

Fig 4. Driver front side glass

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Fig 5. Passenger front side glass

2.5 Sunroof/Window Buffeting

1) Vehicle buffeting due to an open sunroof or side window was identified as a

significant contributor. Buffeting (also known as wind throb) is an unpleasant,

high-amplitude, low-frequency booming caused by flow-excited Helmholtz

resonance of the interior cabin. Vortex shedding in the shear layer over the

cavity opening (sunroof or side window) couples with the cabin acoustics, leading

to a self-sustained oscillation of shear layer and cabin pressure.

2) Accurate prediction of the vehicle buffeting phenomenon requires capturing the

bidirectional coupling between the transient shear layer aerodynamics (vortex

shedding) and the acoustic response of the cabin.

3) Quantitatively correct prediction of the flow-excited resonance requires an accurate

prediction of complex transient behavior including turbulent vortex formation,

shedding, and convection through the shear layer; and direct capture of the acoustic

interaction requires a compressible solver.

4) The acoustic response of the vehicle cabin must also be correctly modeled. In

addition, small geometric details can significantly impact the shear layer/acoustic

coupling behavior, requiring a high degree of accuracy with fully detailed geometry.

These considerations make simulation of vehicle buffeting a challenging problem.

2.5.1 Optimization of Mirror Angle for Front Window Buffeting and Wind Noise

1) Buffeting is low frequency pressure pulses (below 20 Hz. in larger vehicles) of

high amplitude caused when grazing wind across the opening excites the

acoustic cavity modes inside the vehicle. This in turn generates vortices from

the leading edge of the opening and sets up a feedback loop. The compliance

of the air in the vehicle cavity and compliance of the walls act as a spring

(stiffness) and the air in the cavity opening as the mass.

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2) Buffeting is felt in the vehicle as pressure on the occupant’s ears to the point

of discomfort. In addition, there is a typically an audible harmonic associated

with the buffeting as well creating a booming/beating noise in the vehicle

cabin.

3) Typically for sunroof buffeting, comfort stops and/or sunroof deflectors

are used to reduce or eliminate the phenomenon. For side window

buffeting, venting the other windows has shown to reduce the issue in the

past. For the front windows, mirror angle in relation to the side glass can

be used to reduce the severity of the buffeting. This must be balanced with

the overall mirror wind noise heard in the vehicle however, making the

tuning of this angle critical.

Fig 6. Angle of mirror head in relation to side glass

4) Buffeting was evaluated for a variety of mirror angles (θ) ranging from 0 to

30 degrees. 30 degrees was deemed the maximum realistic angle that

could be used in production. The mirror angle was varied by hand with the

cut lines sealed with tape, and then the mirror angle measured with respect to

the glass. Data was taken in the DaimlerChrysler Aero-Acoustic Wind Tunnel

(AAWT) at 0 degrees of yaw at speeds from 40-87 MPH. Based on subjective

vehicle evaluations the primary area of concern for front window buffeting

was between the speeds of 40-87 MPH.

5) The driver door window was in the fully down position, which based on

testing was the worst case scenario for front window buffeting. Measurements

were made using an Aachen head located in the driver’s seat the ear located

820 mm from the base of the instrument cluster. Data was analyzed using

Artemis 8.0 software with 1 Hz. resolution and linear weighting.

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2.5.2 Conclusions

Fig 7. Front window buffeting with 7.5 degrees mirror angle

Fig 8. Front window buffeting with 30 degrees mirror angle

1) Based on the buffeting results, 30 degrees was the only angle that reached the

objective of no greater than 110 dB for front window buffeting. A mirror angle of

0 degrees proved to be the best for wind noise, but 30 degrees was comparable to 0

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degrees in most frequency ranges. Therefore, 30 degrees was deemed to be the

best mirror angle for a balance between buffeting and wind noise.

2.6 A-pillar

1) Major source of wind noise. Many components are joined - windshield, door, outside

rearview mirror and the front quarter panel – so there are many opportunities for fit

problems and poor sealing.

2) The flow velocity around the A-pillar is relatively high, so any exposed cavity or

sharp edges will cause a high wind noise.

3) Also, since the A-pillar is closest to the front seat occupants’ ear, the noise can be

easily heard.

4) The radius of the A-pillar should be as large as practically possible to keep the

flow velocity down and to keep the intensity of separated flow turbulence down

as well.

5) An auxiliary seal is needed to seal the A-pillar gap on doors with fully framed

windows. This seal must extend down to the A-pillar to the point where the door

meets the body.

2.7 Outside Rearview Mirror

1) Since the mirror consists of a bluff body with surface irregularities located in the

region of local high speed flow, it can cause high wind noise.

2) The mirror should be mounted some distance rearward from the front edge of the

door, rather than at the curve point of door. This removes the mirror out of the

maximum flow area which reduces wind noise levels.

3) Mirrors with rounded housing are generally quieter. The housing and attachment must

be shaped for minimum noise generation. Sharp edges should be eliminated and a

radius of 3-5mm should be used in all corners and the trailing edges of the

mirror housing.

4) Holes and gaps in the mirror housing should be eliminated. Folding outside rear-view

mirrors are more common, both for crash safety reasons and to reduce the overall

width of car to facilitate loading onto car carriers.

5) Mirror attachment to the door is a major concern, as there is a potential for gaps that

may cause high wind noise level. Thus, a foam gasket should always be used between

the mirror base and the door to seal off potential gaps.

2.8 Windshield Wipers

1) The high speed flow through the exposed structure of the wipers can cause high wind

noise levels.

2) The only solution to hide the wipers is by tucking them behind the rear edge of

the hood or putting them behind some sort of flow detector.

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2.9 Radio-antenna

1) The noise source is the periodic shedding of the Von Karman vortex street.

2) The fluctuating lift force is the source of the dipole noise generated.

3) The method to reduce this noise is to disturb the flow so that the unsteady flow is no

longer correlated along the cylinder axis so that the correlated vortices and forces

cannot form.

4) The classical method is to wrap a helical strake around the antenna. This strake

disturbs the flow sufficiently so that no tonal noise is generated.

2.10 Doors

1) The exterior surface gaps between the body and doors can be a cavity noise source,

generally broad band.

2) The gap between the door and the body at A-pillar is critical. The other vertical door

gaps at the B and C pillars are also potential wind noise sources.

3) Shingling means that the door downstream would be tucked in somewhere inboard of

the door or body panel in front of it, which would eliminate wind noise and eliminate

the need for auxiliary door seals. However, this does not happen as the flow merely

reattaches slightly farther downstream, and the fluctuating pressure excites modes in

the cavity and cause noise.

4) The door itself must not have any leak paths. Drain holes for anti-corrosive coatings

should be sealed with plugs if these holes are not needed for water drainage. Water

drainage holes should be carefully designed to ensure they do not serve as a leak

path or as a transmission path for noise.

2.11 Roof Racks

1) The cross members of the roof rack are exposed to high-velocity airflow and may

generate strong Von Karman street tones.

2) Adding helical strakes to the cross members would impair the function of roof-rack.

Additionally, the roof rack is a styling element. Some manufacturers have attempted

to address the roof-rack wind noise problems by making the cross-members with a

airfoil shape. However, this does not generally work.

a. A true airfoil has a small thickness-chord ratio (0.15). Since the roof rack members

must carry the weight of luggage, it must be rather thick. Thus, to achieve a thickness-

chord ratio of 0.15 the cross members would end up being atleast 100mm wide

which may not be acceptable from styling point of view.

b. A true airfoil has a very sharp trailing edge. This is not allowed on roof racks for

safety reason. Thus, the angle of attack of airflow into the airfoil will change rapidly

wit time.

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