Wang X. Vehicle Noise and Vibration Refinement

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286

Vehicle interior noise reﬁ nement – cabin sound

package design and development

D. VIGÉ, Centro Ricerche Fiat, Italy

Abstract: The topic of this chapter is the analysis of the main

vibroacoustic problems related to the sound package design and

development of a vehicle. This chapter presents an overview of the noise

paths for vehicle interior noise, followed by a description of the basic

acoustic principles for analysis of the performance of commonly used

acoustic materials and for solutions currently applied to modern vehicles,

including a short description of the available simulation tools and the

design process for cabin sound package development.

Key words: sound package design, vehicle interior noise, acoustics

materials, sound absorption, sound isolation.

13.1 Introduction

In today’s market the perceived quality of a vehicle is one of the main

aspects that customers take into account in choosing the vehicle to buy. A

very important component of perceived quality is the noise inside the cabin.

This is conﬁ rmed by ‘buyer selection criteria’ surveys, which show that the

ranking of noise is quite high among all the other parameters playing a role

in the customer’s vehicle choice. For these reasons the design of the sound

package is a key feature in vehicle development. To improve this comfort

factor related to vehicle acoustics, products such as absorption and damping

materials are currently available on the market but, unfortunately, they are

very heavy and expensive. Moreover, the acquired noise and vibration

beneﬁ ts have penalties in terms of additional cost and weight, whereas the

fuel consumption requirements push towards low-weight solutions and the

need for reducing costs requires low-priced solutions. All these aspects

make the sound package design of a vehicle an important challenge driven

by a big effort of optimisation to ﬁ nd a good balance between acoustic

performance, weight and cost.

13.2 Internal noise sources in a vehicle

The sound package design of a vehicle for the reduction of internal noise

requires knowledge of the different sources of noise, in order to ﬁ nd and

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Vehicle interior noise reﬁ nement – cabin sound package design 287

analyse the different transmission paths: the sound isolation of the vehicle’s

panels should be balanced according to the strength and positioning of the

different sources.

There are many sources of noise in a vehicle: in the past the engine was

the most important, and the ﬁ rst NVH studies were applied to reduce noise

and vibrations generated by the engine and powertrain. As a result current

engines and powertrain systems show a strongly reduced level of noise, and

consequently other sources of noise such as road noise become very signiﬁ -

cant. Moreover, the increase of the speed of vehicles has strongly increased

the importance of aerodynamic noise (George, 1990). Below is a short

description of the main noise sources in vehicles.

13.2.1 Engine and powertrain

The engine plays a very important role for internal noise of vehicles.

Internal combustion engines are generally strong sources of noise and

vibration, whereas electric engines are often much quieter. The vibrations

from the engine are mainly generated by the reciprocating and rotational

masses such as pistons, connecting rods and shafts; for this reason balancing

of these masses is very important and an increased number of pistons allows

one to reduce the effect of the lower orders of the engine: for a four-

cylinder engine the second order is the most important, whereas for a ﬁ ve-

cylinder it is order 2.5 and for a six-cylinder it is the third order. Other

sources of vibration come from the gearbox, the differential and the struc-

tural modes of the exhaust system. For airborne noise there are many

acoustic sources in the powertrain system, like the vibrating panels of the

engine body itself, pumps, electric generators, valves, belts and chains, but

the most important are the intake and exhaust tailpipes, which act as very

efﬁ cient acoustic sources as they are almost monopole sources (Roger,

1996). The intake, the exhaust and the number of cylinders play important

roles in the typical sound of a car and allow for brand-noise customisation

by car makers.

13.2.2 Suspensions

The vehicle suspension system is located in the structure-borne transmis-

sion path between the road–tyre interaction and the vehicle body, so the

design of this system must be done carefully taking into account its behav-

iour in vibration transmission. The suspension geometry, the stiffness of the

bushings connecting different components of the system, the ﬂ exibility and

the mass distribution of brackets and other suspension components are very

important factors for the dynamic behaviour of this system and its capabil-

ity to ﬁ lter or amplify the vibrations coming from the wheels to the vehicle

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288 Vehicle noise and vibration reﬁ nement

body. The suspension system can also self-generate noise: the ﬂ ow noise of

the oil inside the shock absorbers and the maximum displacement bump of

the bushing in high-load conditions are frequent examples.

13.2.3 Tyres

The tyres are the vehicle parts in contact with the rough road surfaces. They

have a dual role in road-noise generation and transmission. The geometry

of the tread and the vibrations of the tyre surfaces in contact with the road

are important airborne paths for noise emission; air pumping into the tread

is also an aerodynamic source of noise. For the structure-borne path the

tyres and their dynamic behaviour are very important, as they are respon-

sible for the transmission of the forces between the road and the wheels;

the air cavity inside the tyre and its acoustic modes must be taken into

account as these resonances increase the transmission of forces.

13.2.4 Aerodynamic sources

The importance of aerodynamic noise in cars has considerably increased.

In the 1970s this source of noise was only perceived at over 160 km/h,

but in today’s cars it reaches the same level as other sources already at

100 km/h. At higher speeds it quickly becomes one of the most important

noise sources, as the intensity of the aerodynamic noise in a car follows the

behaviour of a dipole source, which is proportional to the sixth power of

the speed:

1 ∝ V

(13.1)

There are different aerodynamic sources in a vehicle:

• Global ﬂ ow separation, with big vortexes in the low-frequency range

• Local ﬂ ow separation, with local vortexes in the mid- to high-frequency

range (the most important regions for a car are the A-pillar, the wind-

shield, the underneath ﬂ ow, the accessories like rear mirrors and wind-

shield wipers, the radio antenna, etc.)

• Turbulence and boundary layer noise (typically a broadband noise)

• Leakage noise from seals, with or without net ﬂ ow (see Fig. 13.1, Vigé,

1999).

13.2.5 Squeaks and rattles from interior dashboard

and trimmings

These represent an indirect source of noise in a vehicle, as they are gener-

ated by the dynamic displacement of the surfaces of the dashboard and

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Vehicle interior noise reﬁ nement – cabin sound package design 289

internal trimmings, which is caused by the sources of vibrations in a car

(mainly the road and powertrain excitations). Squeaks are given by stick

and slip phenomena, where surfaces in contact are switching between

contact and slipping conditions. Rattles are generated by impacts of adja-

cent surfaces. The noise generated by these sources is generally in the

high-frequency range (500–5000 Hz for squeaks and 200–2000 Hz for

rattles), even though the road and engine excitations responsible for these

phenomena are in the very low frequency range. Both squeaks and rattles

can be reduced by control of tolerances and an appropriate choice of

materials.

13.2.6 Other sources

Besides the above main sources of noise in a vehicle there are many other

secondary sources, such as brakes (responsible for the annoying brake

squeal noise), electrical and mechanical accessories, etc.

13.3 Vehicle noise paths

Internal vehicle noise does not depend only on the acoustic and vibration

sources; very important roles are also played by the different transmission

paths between the sources and the receivers (i.e. the driver’s and passen-

gers’ ears). In a vehicle there are two different categories of transmission

paths: structure-borne and airborne paths. They are related to completely

different mechanisms of energy transmission. In a common vehicle (like

a car) experience shows that very often the structure-borne noise

Seal

Aerodynamic

unsteady

pressures

Aerodynamic

unsteady

pressures

Seal vibrations

Leakage

Noise

Turbulent flow

Transmission

with net flow

Transmission

without net

flow

13.1 Noise from seals with and without net ﬂ ow (Vigé, 1999).

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290 Vehicle noise and vibration reﬁ nement

transmission path dominates at low frequency (<200 Hz) while the airborne

noise transmission path dominates above 500 Hz. In the mid-frequency

range, both transmission paths have commonly the same level of

importance.

13.3.1 Structure-borne noise transmission paths

The structure-borne path is a vibration transmission path, in which noise is

emitted by the panels surrounding the internal cabin. A simple example is

the case of the engine of a vehicle: if we consider the engine when running

as a source of vibrations (Fig. 13.2), the energy associated with this vibra-

tion will be transferred to the vehicle structure through the engine mounts,

then the vibration will propagate over the whole vehicle structure onto the

panels facing the passenger compartment. The vibration of these panels will

transfer the energy to the air cavity inside the cabin, generating the noise

perceived by the occupants. The reduction of structure-borne noise is there-

fore focused mainly on the structure and the isolation of the vibrations: in

this example the engine mounts play an important role in the attenuation

of the level of vibration, the vehicle structure should have a low sensitivity

to the forces coming from the mounts, and the panels should have a low

radiation efﬁ ciency and possibly be damped to reduce their levels of

vibration.

13.3.2 Airborne noise transmission paths

The airborne path is a completely different transmission mechanism, as it

is an acoustic transmission path. A simple example is still the engine in

Force

Vibration of body panels

Noise

Force

13.2 Structure-borne paths.

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Vehicle interior noise reﬁ nement – cabin sound package design 291

running conditions (Fig. 13.3): if we consider the engine as an acoustic

source (i.e. the noise generated by the vibrating panels of the engine body),

in this case the noise propagates through the air, through holes and across

the surfaces of the structure. In the latter case the acoustic propagation will

involve also the vibration of the panels of the structure but we are still

dealing with an airborne path (the noise outside the panel makes it vibrate,

and the panel will generate noise inside the passenger compartment). For

these reasons the reduction of airborne noise is more focused on the acous-

tic path (particular attention should be paid to avoid holes and leakages),

on the structure (which should work in acoustic isolation) and on acoustic

absorption inside the cabin (this could be done also for structure-borne

noise, but the common acoustic materials show better performance in the

high-frequency range, where the noise is dominated by the airborne path).

The cabin sound package design, which is the central topic of this chapter,

is then mainly related to the airborne path of noise transmission.

13.4 Basic principles

Control of airborne noise transmission is obtained by two different actions:

sound isolation and sound absorption of the vehicle cabin. The ﬁ rst action

is focused on keeping acoustic energy out of the cabin, whereas the second

acts to absorb the acoustic energy in order to reduce the residual noise

inside the passenger compartment. In this section the basic principles of

sound isolation and absorption are underlined, including the description of

the measuring facilities and the material properties related to these actions.

13.4.1 Sound transmission loss

The sound isolation capability of a given partition (window, panel, wall,

etc.) is very important for airborne noise transmission in a vehicle as most

Noise

13.3 Airborne paths.

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292 Vehicle noise and vibration reﬁ nement

of the acoustic sources are external to the cabin: a good level of isolation

is then a good starting point to reduce the interior noise. This acoustic isola-

tion capability of the structures is measured by the deﬁ nition of the sound

transmission loss (TL):

out

(, ) log

(, )

= 10

(13.2)

where:

β is the angle of incidence of the sound wave over the surface of the given

partition

f is the frequency of the considered sound wave

is the sound power incident at the source side

out

is the sound power radiated at the opposite side of the partition.

One important fact is that the TL depends on the angle of incidence of

the sound. The simplest situation is the case of acoustic plane waves normal

to the surface: in this case we can deﬁ ne the transmission loss for normal

incidence:

out

() log

()

= 10

(13.3)

The typical shape of the TL curve is shown in Fig. 13.4; there are different

ranges of frequency with different behaviour of the TL. For very low fre-

quency, below the ﬁ rst resonance mode of the structure, there is the ‘stiff-

ness controlled region’, where the stiffness of the plate inﬂ uences the level

of the TL. The second region is the ‘resonant region’, dominated by the reso-

nances of the structure and their level of damping. Increasing the frequency,

the modal density increases and the effect of individual modes is no longer

noticeable. In the ‘mass-controlled region’, the TL is dominated by the mass

per unit surface area of the structure. The ﬁ nal region shown in Fig. 13.4 is

controlled by the ‘critical frequency’ (f

), due to the coincidence of the

wavelength in the structure and in air (see below); the level of the response

in this region is controlled by the damping of the structure.

Usually at low frequency the internal noise is dominated by the structure-

borne path of transmission; for this reason the most important regions of

the TL curve in common vehicle applications are the high-frequency regions

(where the airborne noise path is dominant): the mass-controlled region

and the coincidence frequency region.

Mass law

In the mass-controlled region the TL follows the so-called ‘mass law’. In

the simple case of normal incidence waves on an inﬁ nite uniform plate, it

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Vehicle interior noise reﬁ nement – cabin sound package design 293

is possible to ﬁ nd an analytical solution to calculate the TL. In this case the

normal incidence mass law (valid for compression wave lengths Ⰷ plate

thickness) is given by:

() logf

⎛

⎝

⎜

⎞

⎠

⎟

10 1

(13.4)

where:

is the mass per unit surface area of the plate

is the characteristic impedance of the gas (air) of mass density ρ

and

speed of sound c

It is easy to see that in the mass law region the TL increases by 6 dB

when doubling the frequency or doubling the mass per unit surface area.

Also, Equation 13.4 shows that for a given gas (usually air) the TL in the

mass law region depends only on the mass per unit surface area; any modi-

ﬁ cation of the plate’s stiffness or damping cannot change the level of the

TL in this region. For better sound isolation without increasing the weight

too much, it is possible to adopt speciﬁ c solutions, such as double wall bar-

riers (see Section 13.5.1).

In practical applications it is more common to deal with incident sound

ﬁ elds where the angle of the acoustic waves varies randomly from 0° to 90°

with about the same level of probability. For this reason, it is usual to deﬁ ne

a ‘random incidence transmission loss’ (TL

random

). Comparison of the pre-

dicted values of TL with the measured value (see Section 13.4.3) shows that

better agreement is achieved by considering a random ﬁ eld with incident

angles in the range 0° to 78° (deﬁ ned as ‘ﬁ eld transmission loss’, TL

ﬁ eld

)

Stiffness

Resonances

Mass

Coincidence

Frequency (Hz)

Transmission loss (dB)

13.4 Typical behaviour of the TL for a real partition.

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294 Vehicle noise and vibration reﬁ nement

rather than between 0° and 90°: the reason is that in practice it is very dif-

ﬁ cult to obtain a real diffuse ﬁ eld in all directions. The relationships between

these values in the low-frequency range (f Ⰶ f

) are as follows:

random

= TL

− 10 log(0.23TL

) (13.5)

ﬁ eld

= TL

− 5 (13.6)

Coincidence frequency

Wave coincidence is a phenomenon that occurs when the length of an

acoustic wave in air is coincident with the wavelength of the structural

bending wave in the panel: in this situation the panel is driven by a strong

excitation in a resonance condition, and vibrates with an amplitude very

close to the amplitude of the displacement of the air particles associated

with the acoustic wave. The TL of the panel falls to a very low value, which

would be zero theoretically in the case of no structural damping. The

minimum TL in this region is then controlled by the damping value of the

structure, as in the dynamic resonance phenomenon.

There is a wave coincidence when the wavelength of the bending struc-

tural wave is equal to the wavelength of the sound wave in air projected to

the panel (see Fig. 13.5):

sin

(13.7)

Incident waves

Panel

Flexural waves

on the structure

13.5 Wave coincidence.

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