DAMPING OF
STRUCTURE-BORNE NOISEIN AUTOMOBILES

Design guidelines for development
and practical applications
Heike-Ursula Obst, Christian Hardt,
Bernd Fuhrmann

CONTENTS

This book was produced with the technical collaboration ofSika Automotive Frankfurt-Worms GmbH.

Acoustics as an element of comfort

4

Fundamentals of acoustics

8

Structure-borne sound compared to airborne sound ...........................

9

Physics of structure-borne sound ............................................................

13

Perception and assessment of structure-borne sound..........................

15

Structure-borne noise in vehicles

17

Excitation of structure-borne noise ..........................................................

17

Transfer paths in the vehicle’s interior and sound radiating surfaces...

22

Technical concepts against structure-borne noise ...............................

24

Acoustic comfort requirements ...............................................................

29

Structure-borne noise damping

33

General information on materials ...........................................................

34

Overview of damping systems ................................................................

36

Single-layer systems..................................................................................

40

Multi-layer systems....................................................................................

42

Sandwich systems ....................................................................................

46

Quantification of the performance

51

Measurement methods for characterization of products .....................

51

Relevant frequency and temperature ranges for acoustic problems© 2019 All rights reserved withSZ Scala GmbH, 81677 Munich
www.sz-scala.deFirst published in Germany in the series
Die Bibliothek der Technik
Original title: Körperschalldämpfung in Personenkraftwagen© 2017 by SZ Scala GmbHISBN 978-3-86236-116-8
Figures: Nos. 12, 13, 14, 40 iStock by Getty Images;
all others Sika Automotive, Worms
Typesetting: JournalMedia GmbH, 85540 Munich-Haar
Printing and binding: optimal media GmbH, 17207 Röbel/Müritz
Printed in Germany 236667

Faist_eng_Inhalt.indd 2-3

from radiating surfaces ............................................................................

60

Design of damping concepts

63

Design on different car segments ...........................................................

64

Analysis of vibrations in body-in-white systems.......................................

65

Selection of appropriate damping measures ........................................

69

Outlook72

Technical terms

75

Sources and literature

77

The company behind this book79

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4

ACOUSTICS AS AN ELEMENT OF COMFORT5

ACOUSTICS AS AN ELEMENT OF COMFORT

Acoustics as an elementof comfort

Goals of theacoustic design

Faist_eng_Inhalt.indd 4-5

At all times, developments in automotive technology were aimed at increasing the functionality of the vehicle as well as increasing driver andpassenger comfort. At the beginning of series
production, however, vehicle acoustics was
only of limited importance. In fact, the focus
was on basic manufacturing concepts to makevehicles affordable to a broader economic class.
The primarygoal of acoustic optimizations wasto reduce or eliminate undesirable noise which
could negatively affect passenger comfort andtraffic safety. Nowadays, targeted developmentof vehicle acoustics presents one of the many
opportunities for an automobile manufacturer
to define a vehicle’s value, place it in specific car
segments, and target specific groups of buyers.
With car segments representing lowerpriced vehicle models, customer requirements
regarding passenger comfort are less important
than satisfying mobility needs at a relatively
low purchase price. In contrast, buyers ofhigher-priced vehicles have pronounced comfort expectations that include the acoustic optimization of their car. Door slam, for instance, is
one of the first acoustic customer assessments
while at a car dealership. A solid door slamming sound gives the impression of quality,
soundness, and security, even before the customer drives the first yards. Criteria of quality
rating also include the sealing-off of external
noise and the possibility of conversing easily
while inside the running vehicle. Acoustic expectations vary widely dependent on the indi-vidual car segment, although they are often not
articulated explicitly. Instead, depending on the
car segment there are some implicit expectations, for instance: It is more acceptable forsubcompact and compact cars to sound rather
tinny and noisy. Sports cars should sound harshand loud also inside the car while luxury carsmust exhibit a calm atmosphere while giving
the impression of seclusion (Fig. 1).
When buying a new car, today customers
are willing to spend more money for optionalcomfort features than in the past. Following
the desire for comfort, expectations in vehicle
acoustics are increasing even in the lower andmiddle car segments. The rapid development ofcommunication systems and their use throughout all car segments requires additional measures that improve interior acoustics. Voicecontrolled and hands-free equipment require aquiet environment with minimal noise.

Fig. 1:
Typical damping
package for asedan

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6

Vehicle classand target
group communication

Faist_eng_Inhalt.indd 6-7

ACOUSTICS AS AN ELEMENT OF COMFORTACOUSTICS AS AN ELEMENT OF COMFORT

Passive acoustic measures achieve special
significance in interior vehicle acoustics. Prior
to the development of more efficient sounddamping and absorbing materials, acoustic
measures in car interiors were limited to reducing the sound emission of the car body throughthe use of insulating films. Even prior to WorldWar II, bituminous felt mats were used forsound reduction in car bodies.
The vibrating and sound-radiating car bodystill remains at the center of vehicle acoustic
research and has moreover increased in significance. The reasons for this are the increasing
acoustic requirements as a function of passenger comfort on the one hand and technicaldevelopments in car body construction on theother hand. More stringent legal requirements
on vehicle emissions are forcing vehicle manufacturers towards a more lightweight mode of
construction. This results in the use of newmaterials for car body construction. Multimaterial systems based on various steels as wellas the use of aluminum, die-cast light alloys,
fiber composites and other plastics, however,influence the vibration behavior and the resultant noise emissions.
Requirements for an increase in comfort
and pressure to make structures lighter posenew challenges when designing sound damping
concepts. They need to be integrated in generalacoustics concepts to meet the demands ofautomobile manufacturers for specific target
customer groups and car segments.
This book is dedicated to the damping ofstructure-borne noise as part of the passive
acoustic measures in vehicle technology and isintended to provide design guidelines for prac-

tical application in vehicle design. It describes
problems and possible solutions in the main
application area of automobiles and provides
an overview of the most important car bodynoise reduction products. This information
conforms with state-of-the-art technology
regarding quantification of the acoustic efficiency of these products and their applicationin car structures. The design of noise damping
concepts will increase in importance in the
future in order to tailor the acoustic comfort
to specific car segments in a targeted manner.

7

17.12.2018 11:35:29

8Sound waves

Fig. 2:Vibration model fora flexible medium

Faist_eng_Inhalt.indd 8-9

FUNDAMENTALS OF ACOUSTICSFUNDAMENTALS OF ACOUSTICS

Fundamentals of acoustics

tion of the original excitation, resulting in alongitudinal wave. In the case of transverse
waves, displacement is perpendicular to the direction of propagation. They can only develop
in the lattice structure of a solid.

Acoustics as a branch of physics deals with theemission and perception of sound. It examines
vibrations caused by pressure and density
variations in the frequency range audible forhumans.
Unlike electromagnetic waves, sound waves
can only propagate in a medium, not a vacuum.
Acoustic vibrations in gaseous flexible media,
such as air (airborne sound), can be distinguished from acoustic vibrations in solid or
liquid flexible media (structure-borne or fluidborne sound). Along with sound waves, energy
is transmitted, but not matter.
The occurrence of local and temporal density variations in a flexible medium is presented
in Figure 2. Due to binding forces, particles intheir neutral position keep the required distances from their neighbors. Medium density
is constant. If an external force deflects theparticles from their neutral position, the neighboring particles are also moved through theexisting bonds. The resultant compression anddecompression zones propagate in the direc-

9

Structure-borne sound comparedto airborne soundStructure-borne soundStructure-borne sound should be understood asvibrations propagating in a solid, such as vibrations in cars and machines, and the transfer ofvibrations in buildings (impact sound, soundpower in walls), as well as vibrations and movements of the earth’s crust caused by earthquakes. One only talks about structure-borne
sound, however, specified when the vibration
frequency exceeds 15 Hz.
Structure-borne sound is perceived byhumans as vibrations. Structure-borne soundalso radiates from surfaces as airborne soundand then becomes audible. Excitation is either
mechanical or through airborne sound. In afrequency range of 15 Hz to 100 Hz, structureborne sound causes pressure on the ear drum
and anxiety. Particularly unpleasant is therange between 60 Hz and 100 Hz.
Two types of sound waves can occur insolids: Longitudinal waves and traverse waves.
With longitudinal waves, the solid particles
oscillate around their neutral position parallelto the direction of propagation of the sound, resulting in local compression sections and zones
with reduced density. In comparison, solid particles oscillate perpendicularly to the directionof propagation around their neutral position in

Structure-borne
sound waves

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FUNDAMENTALS OF ACOUSTICS

Sound pressure in Pa

loud

Neutral position

quiet

Longitudinal wave

Fig. 3:
Wave types in structure-borne sounds

transverse waves (Fig. 3). Sound velocity depends on the propagation medium, frequency,
wavelength, and temperature. Structure-borne
sound of very low frequencies can propagate
almost undamped over distances of many miles.
Airborne soundAirborne sound propagates as longitudinal
waves in the air. It is perceivable to the humanear within the frequency range of approx. 20 Hz
to 20 kHz which is referred to as the hearing
range.
Figure 4 depicts the auditory sensation areaof a human being. Speech perceptibility ranges
from 200 Hz to 5 kHz, and the range of music
perceptibility is broader. The actual hearing
range of humans depends on their age and isreduced by ear damage. While a child is normally able to perceive frequencies up to approx.
18 kHz, the actual hearing range of a 65-year-

Faist_eng_Inhalt.indd 10-11

200

140

Pain threshold

20

120

2

100

0.2

80

Music
Speech

0.02

60

0.002
0.0002

40

Auditory
threshold20

0.00002

InfrasoundTransverse wave

11

FUNDAMENTALS OF ACOUSTICS

Sound pressure level in dB

10

0
20

50

100

200

500

1000

2000

Frequency in Hz (pitch)

old is limited on average to 5 kHz in frequency.
Frequent exposure to noise, such as in discosand concert halls or when failing to use hearing
protection at the workplace, can even decrease
the upper hearing range limit in young people toa maximum 4 kHz. Because of the ear’s anatomy, the frequency range between 1 kHz and
2 kHz is transmitted directly. Between 2 kHz
and 4 kHz an amplification takes place resulting in a particular sensitivity, such as with ababy’s cry and signals of alarm systems.

5000 10 000 20 000 Ultra-

soundFig. 4:
Auditory sensation
area of a human

Acoustic levels

Level data are common in acoustics. The level ofa physical quantity is calculated as the logarithmof the ratio of an effective value to a reference
value; the common unit of level is called decibel(abbreviated as dB). If the measured effective
value and the reference value are identical, use of
the logarithm results in a level value of 0 dB.
For airborne sound, a sound pressure levelof Lp applies:

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12Sound pressurelevel

FUNDAMENTALS OF ACOUSTICS

Lp = 10 · log10pp2dB = 20 · log10
dB
p0
p02

where
p effective value of airborne sound pressure in Pa
p0 reference value of airborne sound pressure:p0 = 2·10–5 Pa – corresponds to the auditory
threshold at 2 kHz (cf. Fig. 4).
The airborne sound pressure level range between 0 dB and 50 dB is perceived by humans
as pleasant, while levels between 50 dB and90 dB are discerned as annoying. Levels from
90 dB to 120 dB may present a health hazard,
whereas levels higher than 120 dB lead to thepain threshold being exceeded.
For structure-borne sound, accelerationlevel La applies:
Accelerationlevel

La = 10 · log10

a
dB
a0

where
a effective value of vibration acceleration in
m/s2
a0 reference value of vibration acceleration in
m/s2.
For acceleration levels, acceleration ofgravity (g = 9.81 m/s2) is usually used as a reference value of vibration acceleration. Contrary
to sound pressure, the reference value is notdetermined in a normative way.
Levels of separate (incoherent) sound sources
cannot be summed but rather need to be added“energetically”.
Lges = 10 · log10

Faist_eng_Inhalt.indd 12-13

(

n

)

100,1 · Li dBා

i=1

FUNDAMENTALS OF ACOUSTICS

13

where
Lges total level in dB
Li level of the i-th separate sound source indB.
From a level difference of 10 dB, the lower
level has no significant influence on the total
levels.
The energetic mean value of many soundlevels is calculated in the following manner:
Lges = 10 · log10

(

n

)

1
100,1 · Li dB
n i=1
ාPhysics of structure-borne soundLongitudinal and transverse waves, as well as
their combinations, can occur in a solid. Wave
forms occurring in a solid depend on its geometry or dimensions in relation to the wavelength(Fig. 5).
In bounded media, such as bars or plates,with non-negligible shear stress bending waves
can occur. These are transverse waves whosesound velocity is a function of frequency. In thisway, dispersion occurs, meaning that bending
waves of different frequencies can traverse thesolid at a different propagation time. In the caseof broadband excitation, as with white noise
generated by an electrodynamic shaker, waves ofcertain frequencies hit their response point sooner
than waves of other frequencies. Bending waves
carry much of the sound energy, and they are asignificant cause for airborne sound radiation.
So-called Rayleigh waves which only have alimited penetration depth into the solid can
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14

Fig. 5:
Waveforms occurring in solids

FUNDAMENTALS OF ACOUSTICS

Longitudinal
wave (density
wave)

Transverse wave
(shear wave)

Torsional wave

Quasi-longitudinal wave
(expansion
wave)

Rayleigh wave
(surface wave)

Bendingwave

boundaries. Acoustic surface waves are neither
purely transverse nor purely longitudinal.
Surface vibrations of individual surface areas
can differ significantly even though they arelocated on the same item.
The propagation velocity of longitudinal
waves is basically higher than that of transverse

Faist_eng_Inhalt.indd 14-15

FUNDAMENTALS OF ACOUSTICS

15

waves. Thus, point mechanical vibration excitation of a system can result in various
responses to this excitation being detected at a
distant point at different times. This phenomenon does not occur in airborne and fluid-borne
sound, as only longitudinal waves can exist inthese media.
For quantification of wave propagation ina solid, its elastic properties must be known.
Sound velocity is determined by density with
Poisson’s ratio μ, bulk modulus K, as well as
shear modulus G (transverse waves), or Young’s
modulus E (longitudinal waves).

Perception and assessment ofstructure-borne soundStructure-borne sound can be sensed in a tactile
way by humans. Under certain circumstances,
however, it is also discernible because of bone
conduction to the inner ear. It is only airborne
sound emitted from vibrating solids that is perceived by the outer, middle, and inner ear.
Mechanical vibration energy that reaches
the ear in the form of airborne sound alongwith structure-borne sound (bone conduction),is translated in the inner ear as electricalimpulses. These are transferred to the acoustic
center in the brain through neural networks
and are further differentiated there into speech,
audio sound, and noise.
Subjective assessment of vibroacoustic comfort is oriented around the above-mentioned
sensations caused by structure-borne sound.
During an analysis, these sensations are directly inquired into and their influence on
concentration abilities is researched.

Psychoacoustic
assessment

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16

FUNDAMENTALS OF ACOUSTICS

Frequency in Hz
125

250

500

1000

1500

2000

3000

4000

8000

12 000English (UK)
FrenchChinese (Mandarin)
GermanSpanishEnglish (US)
Italian
Dutch
RussianJapanese
Portuguese
Fig. 6:
Dominant frequency
ranges in various
languages

17

STRUCTURE-BORNE NOISE IN VEHICLESThe cultural background also influences
acoustic perception. People born in Asia prefera different frequency distribution to thoseborn in Europe. The dominant frequencies invarious languages that influence acoustic impressions from childhood on are presented in
Figure 6. This results in culturally adapted
noise designs. In a globalized world, however,such characteristic hearing patterns are becoming more and more uniform.

Structure-borne noisein vehicles
Vehicles are complex multi-component systems. On the one hand, vehicle acoustics deals
with the exterior noise that is subject to legal
regulation for homologation, and interior noiseon the other hand. The latter is decisive for the
comfort sensation of passengers. Structureborne noise, as structure-borne sound is usually referred to in the context of automotiveengineering, is a major cause for interior noise.

Excitation of structure-borne noise
Essentially, structure-borne noise in vehicles
has three root sources (Fig. 7):» Tire-road contact» Wind load
» Powertrain.

Fig. 7:
Sources of vehicle
interior noise
Excitation through rain and wind

Excitation throughthe powertrain

Excitation through road surface
via tires and axle suspension,stone chips

Faist_eng_Inhalt.indd 16-17

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18

Excitation
through tirevibration

STRUCTURE-BORNE NOISE IN VEHICLES

In most driving scenarios, whether within city
limits or on highways, excitation throughtire-road contact dominates. Aerodynamic excitation only becomes a dominant contributor
to interior noise when traveling at very highspeeds. Excitation through the powertrain
dominates in automobiles only when constantly driving in a low gear or when accelerating.
Both sources for structure-borne noise andsources for airborne sound (air pumping) contribute to noise generation when a tire rolls
against a road’s surface. The roughness of thetread pattern and the road‘s surface contributeto exciting radial and tangential vibrations inrolling tires (Fig. 8).

Fig. 9:Impact excitation
on tiresments of tread blocks on the road’s surface.
These stick-slip effects also contribute to theexcitation of structure-borne noise in the tire’s
structure (Fig. 10).
Fig. 10:
Stick-slip effectduring tire-road
contact

Fig. 8:
Excitation of a rollingtire on a roller drum
test rig, portrayal ofstructural amplitude(IBNM, Leibniz University Hannover)Vibrational excitation of a tire mostly includes impact excitation through tread blocks
hitting the road’s surface at the run-in of theflattened tire. In the contact zone, tread blocks
become stressed when the two-dimensionallycurved tread is pressed against the contact surface (Fig. 9). This results in small slide move-

Faist_eng_Inhalt.indd 18-19

19

STRUCTURE-BORNE NOISE IN VEHICLESThe resultant shear stresses are reduced inoutlet zones of the tire by pulsed “snappingout” of the tread blocks lifting off the road’s
surface. This leads to tread block vibrations
that are further transferred into the tire carcass. This excitation is especially pronounced
on flat roads where good contact between thetire and the road’s surface leads to increased
tangential friction forces in the contact area
(Fig. 11).
Significant tire deformations occur throughtread blocks hitting and lifting at tire-road contact and through road unevenness. Throughthese mechanisms, vibrations with specific17.12.2018 11:35:30

20

STRUCTURE-BORNE NOISE IN VEHICLESFig. 11:Adhesion at tire
lift-off

frequency distributions are introduced throughthe chassis to the car body.
Excitation resulting from tire-road contact
is strongly determined by the mechanical andstructural properties of the tire; for example,
excitation increases significantly with tire
hardness. A larger tire width also leads to anincrease in tire-road excitation; therefore, thepredominant tendency of using wider tires
leads to a significant increase in tire excitation.
Moreover, favorable profile formation is ofgreat importance. In the middle of the tire,
where no drainage capacities are required,longitudinal profiles are favorable. Generally
speaking, irregular profile divisions are used to
avoid tonal noise components.
Tire vibrations are transferred into the car
body through the chassis. The trend towards
self-supporting tires with fail-safe properties(run-flat technology) considerably boosts theimportance of this source of structure-borne
noise. Structure-borne noise constituents oflow frequencies significantly affect the noiselevel in the interior of a car.

Faist_eng_Inhalt.indd 20-21

21

STRUCTURE-BORNE NOISE IN VEHICLESWind load excitation in vehicles only becomes significant at high speeds (Fig. 12). It
influences the roof and large side surfaces and,indirectly, the car body. The excited vibrations
are transferred by the car body as structureborne noise. A structure-borne noise component dominating interior noise lies in the

Aerodynamic
excitation

frequency range between 40 Hz and 1500 Hz.
Within this range, this share in structure-borne
noise is also decisive for the sound pressurelevel in a car’s interior and is perceived ashumming.
The trend towards turbocharged engineswith limited cylinder capacity and higher
speeds leads to a significant shift in the noise
spectrum of the internal combustion engine.
This has immediate consequences for struc-Fig. 12:Wind load excitationin an automobile

Excitation
through thepowertrain

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22

STRUCTURE-BORNE NOISE IN VEHICLESSTRUCTURE-BORNE NOISE IN VEHICLES

ture-borne noise excitation through the powertrain.
Electrification of a car’s powertrain alsochanges the acoustics. The contribution of thepowertrain to vibration changes and as a resultof this, high-frequency sources of structureborne noise constituted by the auxiliary power
units are no longer masked. The tire-road contact gains special importance in cars with anelectrified powertrain.

the engine bearings, the mounting of the exhaust system or, indirectly, via the drive shaft
and wheel suspension system. Another significant transfer path is the tire-road excitation via
chassis components (Fig. 13).
Structure-borne noise is transferred fromexcitation points along the transfer paths into
the car’s interior. Surfaces facing the vehicle’s
interior are thus excited into vibration. Surface
vibrations of the excited panels lead to periodic
compression and decompression of adjacent
air layers; i.e. to airborne sound radiation thatcan be perceived as annoying noise by passengers. To achieve a reduction in vibration contributors, it is thus necessary to analyze transfer paths in detail and identify their sources.
With the contributions to specific paths wellidentified, spots for efficient optimization also
become apparent.
In order to meet comfort requirements forthe interior, either the structure-borne noise
has to be avoided at its source or its transfer
along the transfer paths has to be cut off
or dampened. Moreover, resonance effects ofsurfaces along these paths as a source of structure-borne noise amplification have to beeliminated.
From an acoustic point of view, today’slightweight bodies present a challenge. The
ongoing reduction of panel thickness for the
purpose of weight optimization increases boththe sensitivity towards structure-borne noisetransmission and the resulting airborne soundemissions. Above all, large surfaces such as
vehicle roof, doors, and side walls start to resonate and contribute to the negative perceptionof interior noise.

Transfer paths in the vehicle’s
interior and sound radiating
surfaces
Because motor vehicles are complex structureswith a number of components, vibrations thatare perceived in specific places in the passenger
cabin may originate from distant sources ofstructure-borne noise. Vibration energy frompowertrain components is transferred to theinterior via different paths such as, e.g. throughFig. 13:
Transfer paths oftire-road excitation

Faist_eng_Inhalt.indd 22-23

23

Lightweight
structures as achallenge

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24

STRUCTURE-BORNE NOISE IN VEHICLESSTRUCTURE-BORNE NOISE IN VEHICLES

25

Technical concepts against
structure-borne noisePrimary andsecondarymeasures

Faist_eng_Inhalt.indd 24-25

Technical concepts for a reduction of structureborne noise can be basically divided into twogroups: measures close to the source (primary)and distant from the source (secondary).
Whereas primary measures are intended toprevent the creation of structure-borne noise,secondary measures are aimed at damping
structure-borne noise (energy transformation)or minimizing its effects (prevention of airborne
sound emissions).
At the beginning of this section, major reasons for structure-borne noise in vehicles were
described and their transfer path into the car’s
interior were explained. In the following text, it
will be explained which structural approaches
can be utilized to prevent structure-borne noisetransfer into the vehicle body as much as possible. As described, the powertrain presents asignificant source of structure-borne noise excitation in a vehicle. Depending on motorization concepts, various problems are brought tothe foreground.
Vibrational masses of a piston engine together with pressure peaks during work phasesof the piston induce a broad frequency spectrum. Contemporary piston engines are usually
optimized for vibrations such that free massforces and mass moments are minimized withsometimes very expensive measures such ascounter-rotating balance shafts. With the goalof avoiding structure-borne noise, however,this is usually insufficient.
In electric engines, vibrations during operation usually occur as a result of commutation.

Energy densities generated through a magnetic
field variable in time and space produce forces
in radial and tangential direction and produce
harmonic and cogging torques that generate thecommutation noise that is typical for electricmotors. Additionally, imbalances and fit tolerances in bearings generate noise that is transferred together with commutation noise as
structure-borne noise to the car body (Fig. 14).
Manual or automatic gearboxes with various coupling types and connections to the drive
wheels in the form of drive shafts also contribute to the transmission of structure-borne noise
to the body structure.
Vibrational decoupling of the engine, gearbox, and drive shafts through passive and active bearing damping contributes significantly
to reducing structure-borne noise. While passive bearings are limited in their efficiency
spectrum, active bearings offer broader possibilities. The following bearing types havebecome popular:
As representatives of passive bearings, static
rubber-metal bearings enable decoupling of theFig. 14:
Undercarriage ofan electric vehicle

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26

Two approaches
to secondary
measures

Faist_eng_Inhalt.indd 26-27

STRUCTURE-BORNE NOISE IN VEHICLESSTRUCTURE-BORNE NOISE IN VEHICLES

drive unit only in a limited frequency range thatis determined by the structural design of the
bearing (geometry, choice of materials).
Hydraulic bearings enable damping dependent on frequency and amplitude. This is implemented through the integration of a hydraulic damping system into the bearing. Hydraulicbearings enable damping and stiffness properties to vary dynamically depending on theload condition of the engine and thus provide
optimal decoupling of the drive unit.
Classic vibration absorbers are also used invarious versions to suppress critical resonanceeffects in a targeted manner. As a rule, we are
dealing with freely vibrating masses that are
adjusted in such a manner that their eigenfrequencies may eliminate a given frequency
share of the arising vibration. Depending on
the application, vibration absorbers as well as
hydraulic bearings can be designed as passive(narrow frequency spectrum), semi-active (activation only after exceeding a defined threshold value) or active (controlled) elements.
When discussing measures for the dampingof structure-borne noise (secondary measures),two approaches can be distinguished. The primary goal is to prevent structure-borne-noise
induced emissions of airborne sound fromvibrating car body surfaces or attachments or
at least limit their intensity, as this airborne
sound is ultimately, in the majority of cases,
perceived as annoying noise by car passengers.
The first approach aims at mechanically
preventing the surface from emitting airborne
sound. This can be achieved by influencing
the surface geometry by local stiffening, e.g.
through corrugations or indentations, local

increases in mass, material stiffness, or a combination of these individual solutions (Fig. 15).
These measures influence the local distributionof a car body’s natural vibration modes and thepoint emissions of airborne sound. Vibrationenergy is thus redirected to other car bodyareas. If these measures are efficient, however,there is the risk that the same problems arise inother places because the vibration energy ismerely moved to another place in the car body.
Additionally, there is a possibility of using composite panels. These are sandwich panels with aplastic foil inserted between two metal top panels. This foil decouples the metal elements. Optimally designed, such composite structures arecharacterized by very good internal damping.
Therefore, they enable good damping of structure-borne noise. In practice, however, such
solutions are not applied very often in vehicle
construction because of the limited deform-

27

Fig. 15:
Damping measures
on a body in white:geometric measuresand damping coatings are shown.

17.12.2018 11:35:41

28

Classic damping of structureborne noise:
active and passive measures

STRUCTURE-BORNE NOISE IN VEHICLES

ability, joining technology, and stiffness, as wellas the unbalanced cost-to-benefit ratio.
The second approach mostly aims at the
maximal reduction of vibration energy of thestructure-borne noise by transforming it into
heat. This classic damping of structure-borne
noise can again be divided into active and passive measures.
Abbreviated ASAC (Active Structural
Acoustic Control), active measures focus on
eliminating vibration with exact phase-shiftedvibration (Fig. 16). In theory, this works wellfor narrow-band vibrations, but in practicefails with complex mechanical problems. On
the one hand, usually in structure-borne noise,
we are dealing with a broadband mixture ofmany stochastically distributed individual
frequencies and amplitudes that depend on
many operational conditions of the vehicle. On
the other hand, vibrations transverse the bodystructure in all possible directions and are deflected and amplified or reduced by superpositions. Vibration reduction with ASAC would
need to generate exactly the same frequency
spectrum phase-shifted in the same directions.
Today, neither the necessary sensors nor the

resulting real-time actuators are available tomeet such requirements to a sufficient degree.
For these reasons, passive measures constitute the preferred solution. Vibration energy ofthe body is transformed mechanically into heatthrough a damping layer on the panels.

The subjective perception of vibrations in vehicle acoustics is just as important as the subjective perception of airborne sound. Individual
perception depends on the vibration frequency(Fig. 17). The tactile sensitivity of a human, withregard to vibrations, is the highest within a frequency range of 4 Hz to 10 Hz. At bodily contact, low-frequency vibrations up to 50 Hz areperceived by humans directly as vibrations.
Vibrations in the frequency range of approx.
1 kHz are relevant in vehicles as structureborne noise. Vibrations can be both heard andperceived by touch between approx. 20 Hz and
100 Hz.

Old person
Amplitude

Dog
Cat
Time

Dolphin

102

103

85

1100

85

1100

Faist_eng_Inhalt.indd 28-29

Phase-shifted vibration

Ultrasound

104

16

105

106

20 000

16
450

5000
1100

15

50 000

760

1500

60
7000
150

Bat
Hedgehog

Fig. 17:
Human ranges ofaudibility and voicecompared to otherliving creatures

Frequency in Hz
Audible soundInfrasoundYoung person

Vibration

Passivemeasures asa preferredsolution

Acoustic comfort requirements

101

Fig. 16:
Destructive
inference

29

STRUCTURE-BORNE NOISE IN VEHICLES

1000
220

Vocal range

65 000120 000

150 000
10 000 120 000
120 000
60 000Hearing range

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30

Requirements
for interior noise

Faist_eng_Inhalt.indd 30-31

STRUCTURE-BORNE NOISE IN VEHICLESSTRUCTURE-BORNE NOISE IN VEHICLES

Vibrations generated through tire-road
contact, wind load and powertrain that aretransferred to the car interior as structureborne noise can be immediately perceived as
annoying. Airborne sound excited by structure-borne noise is heard. Moreover, some vibrations can also be perceived by touch, such as
through vibrations in bottom panels felt by thefeet, vibrations in the seat rail transferred into
the seats are felt by the buttocks, and vibrationsof the steering wheel or the gearshift lever can
be felt by the hands. Low-frequency natural
vibrations of the car body cause resonance ininner organs. As a result of such resonance, passengers may experience discomfort or feelingsof nausea.
Requirements regarding interior noise in
vehicles vary a lot depending on the vehicle category. In the automobile sector there is a specific demand for acoustic comfort in each car
segment. In sub-compact and compact cars, thepassengers’ level of tolerance towards annoying contributions to internal noise is estimatedto be higher than in mid-size and luxury cars
with premium requirements. Special requirements are set for vehicles used as rolling offices.
In these vehicles, a feeling of seclusion isdesired so that a private atmosphere could becreated. The same applies to vehicles marketed
as being comfortable for long-distance travel,such as crossover SUVs and minivans, thatshould provide a stress-free space.
The car segment of full-size vans can be divided into two parts. There are vehicle superstructures that in the literal sense of the word
operate as pure transporters. They consist of adriver cabin and usually a closed loading space.

Generally, in such vehicles, only limited resources are used to increase acoustic comfort.
However, increased resources for acoustic
comfort are quite common in full-size vans thatare used as mini-buses because of their design.
In this way, high-value interior lines of equipment in the premium segment, such as thoseoffered for VIP shuttles for example, are absolutely comparable with mid-size limousineswith respect to their acoustic comfort.
In recent years, heavy trucks experienced atransformation into a comfortable workplace
for the driver. On the one hand, the driver cabin
serves as a living space during rest periods ofthe obligatory driving pauses. This should bewell insulated from external noise to enable relaxation for the driver. On the other hand, theload on the driver during work should be kept
to a minimum. Additionally, there should besuperior acoustic comfort in this section of
the vehicle. Structurally, driver cabins are decoupled from the chassis with hydro-pneumatic
bearings that per se keep the structure-borne
noise transfer relatively low. To account for
inputs from the remaining transfer paths,
coatings for the damping of structure-borne
noise are applied to the large panel surfaces ofdoors and side walls and on the cabin floor.
Buses are used as a means of public transport and travel. When used for public transport,
the primary concerns are as low as possible
fuel/power consumption, as high as possible
transport capacity, and good accessibility forpassengers. In overland buses, the demands
for passenger comfort must be added to thislist. The structure of buses often comprises aload-bearing ladder frame with a decoupled31

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STRUCTURE-BORNE NOISE IN VEHICLES

frame set on top. This frame carries the interior
superstructures, glazing, and external paneling.
This principle significantly reduces the problems of structure-borne noise when comparedto the self-supporting safety passenger compartment of an automobile. Acoustic measuresin buses are thus aimed at reducing airborne
sound rather than damping structure-borne
noise.
Thus the main application area for measures towards the damping of structure-borne
noise are automobiles from all car segments.
The difficulty lies in targeted design for the
special features of the specific car segments andin requirements of the vehicle manufacturers
concerning vehicle positioning within the car
segments.

Faist_eng_Inhalt.indd 32-33

33

STRUCTURE-BORNE NOISE DAMPING

Structure-borne noise
damping
Damping of structure-borne noise is of outstanding importance for the automobile industry in order to cope with customers’ comfort
expectations regarding NVH behavior (NVH:
noise, vibration, harshness). Through technological advancement towards powertrain electrification and autonomous driving, new challenges also emerge in the area of damping thestructure-borne noise.
Damping means that mechanical energy iswithdrawn from a vibrating system. Usually
the withdrawn energy is converted into heat.
As a consequence, waves in the structure become weakened and amplitudes in resonanceareas are reduced. A common method involves
the application of materials which provide
excellent damping characteristics. In-depth
knowledge of the vibrational system of a vehicle and the damping behavior of the appliedmaterials in frequency and temperature range
is essential for design optimization as well as
for simulations targeted at the reduction ofvibrations.
The application of damping material on vehicle body panels constitutes an essential tool
to improve acoustic comfort and provide a subjective assessment of vehicle value. Damping
materials are developed with the focus on theconversion of vibration energy of the body inwhite. Depending on vehicle class and the resulting customer expectations regarding comfort, damping packages of up to 30 kg in weightare integrated into a car body.

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STRUCTURE-BORNE NOISE DAMPING

STRUCTURE-BORNE NOISE DAMPING

General information on materials

heat at the molecular level. At a macroscopic
level, the addition of appropriate fillers andmaterials increases internal friction.
A vibration-damping coating exhibits maximal damping at the exact glass transition temperature of the material, meaning the temperature at which the material transfers from thesolid into a viscous state. Damping capacities
decrease both below and above the glass transition temperature. Most vehicle manufacturers
want maximal damping at room temperatureor slightly above.
Bitumen naturally exhibits very good
damping properties at room temperature,
whereas as the result of inner and outer plasticization, the glass transition of polymers has tobe put at around room temperature. Therefore,
and for economic reasons, bitumen as a raw
material has become very popular among the
manufacturers of sound-damping coatings.
Today, polymer films have been widely replacedby films with bitumen content and extrudable damping compounds based on aqueous
acrylates or cross-linkable natural rubbers.

The theoretical basis for structure-borne noise
damping was developed in the early 1960s by
Dr. Hermann Oberst. Within the framework of
the development of vibration-damping materials, new materials, including polymers on an
acrylate and polyvinyl acetate basis as well as
already known materials such as, e.g. bitumen
and self-adhesive cardboard were analyzed fortheir suitability at that time. The suitability of amaterial is expressed by the loss factor determined in a standardized procedure: the Oberst
Method (Table 1). The loss factor relates to apercentage value of the vibration energy thathas been transformed into heat through internal friction in a test bar (cf. Quantification of
the performance, p. 51).
The transformation of vibration energy
into heat takes place in damping materials bothat microscopic and macroscopic levels. At amicroscopic level, long chains of branchedpolymer groups or asphaltenes in bitumen are
subjected to expansion and compression stress.
Through relaxation and a new grouping of
chains, vibration energy gets transformed into

Table 1:
Loss factors ofcommon materials

Faist_eng_Inhalt.indd 34-35

Material

Loss factor

Steel

0.0006

Aluminum

0.0001

Copper

0.002

Brass

0.001

Lead

0.02

Bituminous ﬁlm

0.2–0.6

Sandwich system

0.2–1.035

Maximaldamping

Fig. 18:
Single-layer andsandwich systems

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STRUCTURE-BORNE NOISE DAMPING

STRUCTURE-BORNE NOISE DAMPING

Today, damping materials manufactured
from suitable raw materials are used in various
fields of car body construction as stamped
parts or extrudable compounds (Fig. 18).

white construction, after cataphoretic coating,or as late as after applying a topcoat. In addition, the damping of structure-borne noise
can be influenced to a high degree. Thus on theperformance scale, multi-layer systems can beplaced above single-layer systems.
Sandwich systems mostly differ from multilayer systems in that their performance can begreatly influenced by the combination of layerswith various moduli of elasticity (E moduli)
and adhesives. Very thin systems, very light systems and systems acting over a very wide range(high performance in broad temperature andfrequency ranges) can be implemented as wellas various mixed forms in between these extremes. This puts sandwich systems at the topof the performance scale of systems for thedamping of structure-borne noise. These systems are able to satisfy diverse requirementswith respect to vehicle acoustics (Fig. 19).

Overview of damping systems
Materials for the damping of structure-borne
noise which are applied to the body panels asclassic secondary measures are subdivided into
three basic system structures:
Three system
structures ...

» Single-layer systems» Multi-layer systems» Sandwich systems.
Single-layer systems are applied directly to thesubstrate without another intermediary layer.
Adhesion is achieved through adhesion of thedamping material to the car body. The acoustic
efficiency of single-layer systems lies at thelower end of the performance scale of classic
sound damping systems.
The group of multi-layer systems differsfrom single-layer systems in that the laminateof real damping materials is joined to thesubstrate with an additional adhesion layer.
Self-adhesive and hot-melt adhesive multi-layer
systems can be distinguished. For example,
with the damping materials, fleece lamination
can be applied as an additional surface layer.
It takes over such functions as absorption of
airborne sound or stiffening in the case of
hybrid structures.
Multi-layer systems generally offer the possibility of tailoring the application of structureborne noise damping to the customer’s process
sequence. They can be applied in the body-in-

Faist_eng_Inhalt.indd 36-37

37... with different
performancecapacities

Sandwich systems

Performancegg

36

Multi-layer systems

Single-layer systems

Fig. 19:
Classification of
damping systems
according toperformance

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38

STRUCTURE-BORNE NOISE DAMPING

39

STRUCTURE-BORNE NOISE DAMPING

H3 Cover plate
Damping material

h2

E2/d2

Panel

h1

E1
d = f(d2, E2/E1, h2/h1)dLoss factor for a combination of panel and

H2 Viscoelastic coating
H1 Base plated=d2 Y g
1 + (2 + Y) g + (1 + Y) (1 + d22) g2

Y, g = f (G2, H1, H2, H3, E1, E3, B1, B3, f)

damping material

Fig. 20:
Influences on theloss factor of asingle-layer system

Fig. 21:Single-layer system
consisting of abituminous film

Faist_eng_Inhalt.indd 38-39

d2

Loss factor for damping material

E1

Modulus of elasticity – paneld2

Loss factor of the intermediate layer

E2

Modulus of elasticity – damping material

Ei

Modulus of elasticityh1

Panel thickness

Hi

Material thicknessh2

Thickness of the damping material

Bi

Bending stiffness

Single-layer systems and special extrudable compounds are preferred for the floor
areas in the body. Damping efficiency reaches
its maximum at or near room temperature,and the damping level is moderate and limited
in temperature range (Figs. 20 and 21, seealso Fig. 24, p. 45). In comparison, sandwich

G2 Shear modulus of the intermediate layer

systems and also multi-layer systems haveconsiderably broader temperature ranges
for efficiency and a significantly higher damping effect (Figs. 22 and 23, see also Fig. 26,
p. 47).
We will explain various systems in more detail below. The presented materials provide an

Fig. 22:
Influences on theloss factor of asandwich system

Fig. 23:
Aluminum-butyl
rubber sandwich
system

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STRUCTURE-BORNE NOISE DAMPING

STRUCTURE-BORNE NOISE DAMPING

insight into the wealth of various design possibilities of usual damping elements in theautomobile industry. Through combination andlamination, multi-layer systems or sandwich
systems can be produced based on single-layer
systems. Through a combination of coverings
with a lot of tensile stiffness, such as aluminum,
glass fibers or organic fibers, an additional
stiffening effect can be generated. This meetsthe needs of the increased complexity in car
body construction, where hard and soft steels,
aluminum alloys, magnesium alloys, and fiberreinforced plastics are used in various combinations.

tic efficiency. Typical densities of magnetic
bituminous films lie in the range of 1.7 g/cm3
to 2.5 g/cm3, whereas non-magnetic bituminous films only exhibit a density of 1.0 g/cm3
to 1.8 g/cm3. A lot of weight can be saved byusing non-magnetic materials as at a thicknessof 2 mm, for example, the typical surface
weight is only 3 kg/m2 instead of 5 kg/m2. An
achievable loss factor with bituminous films
varies between 0.1 to 0.2. With single-layer
film systems, limited storage and transportcapacities are a disadvantage. When the thermal load is high, single-layer systems display atendency to block. Single-layer film systems
exhibit deficits in adhering at low temperatures as there is no adhesive layer.
Apart from film systems, extrudable compounds are the second largest group of singlelayer systems. These are extrudable, solventbased materials that are applied to the car bodyby a machine. A significant advantage of extrudable compounds lies in the possibility of
automatic application. However, these materials can only be applied to the body-in-white
construction and in the paint shop because adrying oven temperature is required for theirdrying. Chemically, three types of extrudable
compounds can be distinguished based on
epoxy, natural rubber, and acrylate. Bituminous
compounds can also be applied as pumpable
damping systems. They do not require drying,as hot-applied compounds cool down at ambient temperatures. Above all, the acrylate-based
system has gained popularity because it exhibits the best damping properties. The acoustic efficiency of these damping coatings depends
on their thickness; for example, at a surface

Single-layer systems

Bituminous films

Faist_eng_Inhalt.indd 40-41

Single-layer systems (materials consisting onlyof a single homogeneous layer) constitute abasic framework of materials for the dampingof structure-borne noise in the car industry.
Typical examples include bituminous films
without an additional adhesive or sealing application as well as extrudable compounds.
Bituminous films are used as an economical damping material in the car industry. Depending on the mineral filling materials used,magnetic and non-magnetic bituminous films
can be produced that vary in density. The advantage of magnetic materials lies in the simple
application in the car body production process, as the bituminous film adheres to the steel
background due to magnetic forces. Aluminum substrates do not offer such an application advantage. Disadvantageous is thehigh surface weight that results from a highmaterial density necessary for a high acous-

41

Extrudable
compounds

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42

STRUCTURE-BORNE NOISE DAMPING

STRUCTURE-BORNE NOISE DAMPING

weight of 4 kg/m2 it achieves a maximal loss
factor of 0.3.

corrosion problems, it is better to use a magnetic bituminous product to guarantee a tightseal with complete adhesion.
In the paint shop, mostly self-adhesive magnetic films are used because they can be easily
positioned, and they melt well in a drying oven.
In critical positions, such as roof areas, selfadhesive magnetic materials are used for safety
reasons. In the floor area, non-magnetic meltfilms are also used to reduce weight.
In the assembly shop, only self-adhesive
films can be applied because the body is nolonger subject to temperature influences. Thesefilms can be magnetic or non-magnetic.
Thus, there are many bituminous multilayer systems that can be adjusted optimallydepending on the application. Typical self-adhesive bituminous films reach maximal acoustic efficiency in the temperature range between
10°C and 30°C, whereas the maximum for hotmelt adhesive bituminous films lies between
30°C and 50°C. The exact position of the maximum depends, apart from material thickness,on the adhesive used. With self-adhesive systems,
adhesives based on bitumen, natural rubber,and above all acrylate, are used. In contrast,
hot melts are mostly based on ethylene-vinyl
acetate (EVA).
Non-magnetic bituminous multi-layer systems achieve a loss factor of 0.2 to 0.3 with asurface weight of 3 kg/m2 to 4.5 kg/m2. Becauseof their higher density, magnetic materials withsimilar acoustic properties exhibit a surface
weight of 3.2 kg/m2 to 7 kg/m2.
Through the use of an approx. 0.5-mm-thinmagnetic bituminous film in combination witha non-magnetic bituminous film at a signifi-

Multi-layer systems

Self- or hot-meltadhesive multilayer systems

Faist_eng_Inhalt.indd 42-43

Multiple-layer damping materials with a higher
level of performance are created on the basisof the above-mentioned single-layer systems.
Depending on specific customer requirements
for low-temperature impact strength, a layer ofadhesive or hot-melt adhesive can be required.
The combination of two different layers leads
to a decrease in weight. If layers exhibit different moduli of elasticity, however, the acoustic
efficiency can additionally be increased, forexample, by laminating a soft bituminous filmwith a stiffer one.
Multi-layer systems can be equipped with
self-adhesive or hot-melt adhesive. Self-adhesive materials exhibit immediate adhesion atroom temperature, whereas hot-melt adhesivematerials only adhere to the substrate andmelt after applying heat. The place of application – floor area, vertical structure (e.g. doors)
or overhead assembly (e.g. roof area) is pivotal for the choice of material. Moreover, theindividual step of the production process(body-in-white construction, paint shop, andassembly) needs to always be taken into
account.
In body-in-white construction, only selfadhesive materials can be used because before
the oven processes, the car body is driven
through a pre-treatment bath and cataphoretic
coating bath and hot-melt adhesive materials
are not necessarily stable during these processes. Because a lack of adhesion can lead to43Film multi-layer
system

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Foam multi-layer
systemHybrid multilayer system

Faist_eng_Inhalt.indd 44-45

STRUCTURE-BORNE NOISE DAMPING

cantly lower density, the application advantage
of magnetic materials is retained with a simultaneously minimized surface weight. A thinmagnetic layer is enough because only an areaof several tenths of a millimeter becomes
magnetized in the production process, and only
this lower part of the film has contact with thesubstrate. Due to the avoidance of a fully magnetizable film, the overall density drops, andwith the same surface weight, better acoustic
efficiency can be achieved when compared toconventional magnetic bituminous films. Using
this method, the loss factor can be increased
by 25%.
A multi-layer system consisting of bituminous foam with a density of 0.35 g/cm3 and a
top layer of stiff bituminous film combines
both advantages: weight reduction and an increase in loss factor. This is a lightweight product that can contribute to weight optimizationof the whole vehicle and thus to a reduction ofpollutant emissions. Through the variation in
thickness of the bituminous foam and bituminous film, multi-layer systems can be produced
that reach a loss factor of more than 0.25 witha surface weight of 2.4 kg/m2.
In Figure 24, the loss factors of foam multilayer systems with various surface weights
have been presented. With low surface weights,
they are characterized by their excellent acoustic properties. In comparison to hot-melt
bituminous films, a similar acoustic efficiency
is achieved with approx. 50% less weight(Fig. 25).
The lamination of bituminous films with
an epoxy layer leads to the creation of a
hybrid product that is both damping and stiff-

45

STRUCTURE-BORNE NOISE DAMPING

0.40

0.35

0.30

Loss factor [-]

44

0.25

0.20

0.15

0.10

0.05

0.00
–20

–10

0

10

20

30

40

50

60

Temperature in °C
1.9 kg/m2

2.1 kg/m2

ening. This material achieves a loss factor of
approx. 0.2 as well as exceptional stiffening
and is used as a standard in many areas of a carbody.

2.4 kg/m2

Fig. 24:
Temperature dependency of theloss factor in foam
multi-layer systems
with various surface
weights

Fig. 25:
Foam multi-layer
systems consistingof bituminous film
and bituminous
foam

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STRUCTURE-BORNE NOISE DAMPING

Sandwich systems

Aluminumbitumen sandwich system

Faist_eng_Inhalt.indd 46-47

Through the combination of a substrate, a bitumen-based or natural rubber-based compound,and a stiff cover layer, a sandwich system is created that is characterized by its significantly
increased acoustic efficiency. Vibration energy
is transformed into heat by shearing the film
between both stiff layers (cover material andsubstrate). With such systems, loss factors between 0.25 and 0.45 are achieved with low surface weights between 1.3 kg/m2 and 3.5 kg/m2.
Through lamination of self-adhesive bituminous film and aluminum foil with the aid ofa layer of adhesive, a very acoustically efficientsystem is achieved with a loss factor of 0.35
and surface weight of 2.5 kg/m2. The lower
adhesive layer is mostly responsible for the resistance to high and low temperatures, but theadhesive layer between the aluminum foil andthe bituminous film can influence the position
and maximum of the acoustic efficiency. Similarly, with single-layer systems, an increase of
thickness in the bituminous layer can also improve damping properties. Moreover, the loss
factor can be influenced by the thickness of thealuminum foil.
In order to transform this aluminum-bitumen sandwich system into a lightweight construction product, the bituminous film with aminimal density of approx. 1 g/cm3 is replacedwith bituminous foam at a density of only
0.3 g/cm3. Two adhesive layers are also required to bind the bituminous foam with thesubstrate and the aluminum foil.
At both low and high temperatures, the adhesives ensure secure joints and strongly influ-

47

STRUCTURE-BORNE NOISE DAMPING

ence the acoustic efficiency of the material. By
selection of adhesives and layer thicknesses, theposition of maximal acoustic efficiency can bevaried between 10°C and 40°C. The maximal
level depends above all on the thicknesses of
the adhesive layers used. The thickness of thealuminum foil is another factor that influences
acoustic performance. The loss factor isstrongly influenced as shown in Figure 26. An
increase in thickness of 0.1 mm to 0.3 mm,with otherwise identical material structure,leads to an increase of the loss factor from 0.25
to 0.4. Thus an increase in acoustic efficiency of
60% can be achieved with the surface weight
only increasing from 1.3 kg/m2 to 1.7 kg/m2, orby 30%. A tailor-made lightweight construc-Fig. 26:
Temperature dependency of the loss
factor of aluminumbitumen sandwich
systems with various
thickness of the aluminum covering

0.45
0.40
0.35
0.30

Loss factor [-]

46

0.25
0.20
0.15
0.10
0.05
0.00
–20

–10

0

10

20

30

40

50

60

Temperature in °C
Aluminum 0.10 mmAluminum 0.15 mm

Aluminum 0.30 mm

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Aluminum-butyl
rubber sandwich system

Faist_eng_Inhalt.indd 48-49

STRUCTURE-BORNE NOISE DAMPING

tion that exactly matches customer expectations can be created by appropriate selection of
not only the thickness of the bituminous foam
and aluminum foil – both of which significantly
influence the weight of the sandwich system –but also of the requisite adhesives.
Using a filling compound consisting of natural rubber (butyl rubber) instead of bitumen
has the advantage of being self-adhesive.
Therefore, no adhesives are necessary. Lamination with aluminum foil creates a highefficiency damping material. The self-adhesive
butyl compound can be used during all threephases of vehicle manufacturing (meaning thebody-in-white construction, paint shop, andassembly shop). It is especially suitable for the
body-in-white construction as the non-polar
adhesive sticks well to oily and fatty surfaces
and guarantees complete bonding. Apart fromthe composition of the rubber-based fillingcompound, the layer thickness and the thickness of the aluminum foil essentially determine
the acoustic efficiency.
Figure 27 shows the influence of layer
thickness of the butyl compound and aluminum foil. An increase of thickness of the butyl
compound from 1.6 mm to 1.9 mm causes anincrease of the loss factor from 0.28 to 0.35.
Further improvement in damping is achievedby utilizing an aluminum-butyl rubber sandwich system with an aluminum foil of 0.3 mm
thickness. It has a loss factor of 0.45 with a surface weight of 3.6 kg/m2. This product meets
top requirements for the damping of structureborne noise. Even door wobbling can be completely eliminated, i.e. the annoying excitationof the car doors at higher speeds.

49

STRUCTURE-BORNE NOISE DAMPING

0.50
0.45
0.40
0.35

Loss factor [-]

48

0.30
0.25
0.20
0.15
0.10
0.05
0.00
–20

–10

0

10

20

30

40

50

60

Temperature in °C
Aluminum 0.15 mm
Butyl 1.6 mm

Aluminum 0.15 mm
Butyl 1.9 mm

Bitumen and butyl rubber can be laminated with extensional stiff materials otherthan aluminum foils. Specifically, aluminumcompounds, glass fleece compounds, and glass
fibers work well with bitumen and butyl
rubber. Glass fibers also have the advantage ofbeing electrically non-conductive. Dependingon the assembly location, intended use andcustomer requirements, prepregs (mostly impregnated glass-fiber fabrics), cardboard, andnatural fiber compounds can also be used.
Whether or not a given material is suitable
as a cover layer for butyl or bituminous
compounds depends to a great degree on its
extensional stiffness.

Aluminum 0.30 mm
Butyl 2.0 mm

Fig. 27:
Temperature dependency of the loss
factor of aluminumbutyl rubber sandwich systems

Lamination withother materials

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50

Lamination withglass fiber fabric

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STRUCTURE-BORNE NOISE DAMPING

Lamination of a butyl compound withglass-fiber fabric generates a high-efficiency
acoustic element that can be used in the bodyin-white construction. Acoustic properties arestrongly influenced by the layer thickness of thebutyl compound and strength of the glass-fiber
fabric. Loss factors can be generated which
are comparable with those of aluminum-butyl
rubber sandwich systems.
Glass-fiber fabrics with bitumen-based
compounds yield a hybrid material characterized by excellent acoustic properties andacceptable stiffening effects. Because of theweight reduction in car bodies, the substratepanels are also becoming thinner and thinner.
Thus there is a need for additional stiffening of
the panels with light materials. So far, stiffening
elements have been used in addition to damping elements. The sandwich system comprising
glass-fiber fabric and bituminous compound
combines acoustic and stiffening effects. In thebody-in-white construction, where stiffening
elements are mostly used, the use of magnetic
bitumen layers is particularly advantageous.

51

QUANTIFICATION OF THE PERFORMANCE

Quantification of theperformance
Damping materials are used in the automotive industry as well as other industries to reduce vibrations and noise. These materials have viscoelastic
properties. In order to quantify their performanceboth at various temperatures and frequencies, it isnecessary to analyze these materials.

Measurement methods forcharacterization of products
Oberst Method

The Oberst Method is today’s standard forcharacterizing the performance of damping
materials (Fig. 28). With this procedure, a barof predefined thickness, length, and width, on
which the sample is applied, is rigidly clamped
in vertical position from the top end. On the top
end and on the lower, free end of the bar, there
are inductive actuators and sensors for excitation and measurement of bending waves.
Dependent on the resonance behavior of thetest bar with the sample under examination,
the damping behavior is characterized, forexample, by the loss factor.
The loss factor relates to a percentage valueof the vibration energy that has been converted
into heat in a test material through internal
friction. Measurement values for the loss factor
are determined for the relevant temperature
range. For products used in the automotive
industry, a range of –40°C to +80°C is used.
Depending on the system, loss factors up to
0.3 can be reliably determined by this proce-

Standard
method ...

... with limited
potential

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52

QUANTIFICATION OF THE PERFORMANCE

Fig. 28:
Test rig for thedetermination ofloss factors for
damping materials
according to theOberst Method

53

QUANTIFICATION OF THE PERFORMANCE

0.20
0.18

Loss factor [-]

0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
–40

–30

–20

–10

0

10

20

30

40

50

60

70

80

Temperature in °C

dure, however, above this threshold, the measurement results are imprecise. Exact values
can only be determined for resonant frequencies where the bar’s natural vibrations are excited. For other frequency values, loss factors
are calculated with linear interpolation or
extrapolation. Such linear damping behavior
relating to frequency is at best applicable tosingle-layer systems.
For a determined frequency (140 Hz or
200 Hz), the measurement results obtained

Faist_eng_Inhalt.indd 52-53

with the Oberst Method are usually presented
in a diagram versus temperature (Fig. 29).
However, this frequency usually does not correspond to the resonance frequency, and thepresented loss factor is thus interpolated.
Another kind of presentation contains realmeasurement values at the second-order andthird-order resonance frequency, but these shift
with temperature.
Because of the inductive sensors, only magnetic test bars can be used in the Oberst
Method. Non-magnetic sample materials canonly be tested after application of small magnetic plates. Moreover, it is hard to excite highly
damping samples to vibration with this contactless induction method, so that the measurement of their loss factors is not possible.

Fig. 29:
Measurement result
according to theOberst Method:interpolated loss factor (here at 200 Hz)
for all the measured
temperatures

OLF Method

OLF stands for “optical determination of loss
factor,” and this method (Fig. 30) enables highfrequency resolution over a wide temperature
spectrum and allows for exact determination of

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54

QUANTIFICATION OF THE PERFORMANCEQUANTIFICATION OF THE PERFORMANCE

Upper end of the bar

55

Fig. 31:
Bending wave dyingout along the bar,visualized withmeasurement of alaser vibrometer

Lower end of the bar

Fig. 30:
Test rig for opticalloss factor determination according
to the OLF Method

Faist_eng_Inhalt.indd 54-55

the damping behavior without limitations concerning the frequency or temperature range.
A broadband signal is used to excite thesample material applied to the measurement
bar at the clamped top end. Bending waves run
to the lower, free end of the bar with the amplitude reducing along with the corresponding
damping behavior of the applied material. Amplitude values are measured with single-point
lasers at two positions along the bar. The loss
factor of the material is calculated from thetransfer function resolved according to the frequency. Measurements are taken over temperature ranges standardized for the Oberst Method
which are from –40°C to +80°C
Contrary to the Oberst Method, which isbased on stationary waves, the bending waves
in the OLF Method are measured along the bar.
For this purpose, bending waves need to die out
completely before reaching the free end of thebar (Fig. 31); otherwise, reflection will occur at

the free end, and reflected waves would interfere with the original ones and interfere withmeasurement results. Therefore, the measurement bar needs to be significantly longer thanin the Oberst Method (approx. 2 m). With
broadband excitation and the loss factor calculation from transfer functions, a result with avery high resolution in the frequency range isachieved.
Figures 32 to 34 show representative 3D
diagrams for OLF measurements. The temperature (y-axis) is presented versus frequency(x-axis), and the color scale identifies the loss
factor. Figure 32 presents measurement results
for an aluminum-butyl rubber sandwich system. The material exhibits a high damping
level (loss factor > 0.4) with a slight frequency
dependency, comparable with the frequency
behavior of bituminous film in Figure 33.
However, it can be clearly seen that bituminous film with the same surface weight has

Exact and highresolution measurement result

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QUANTIFICATION OF THE PERFORMANCE

57

QUANTIFICATION OF THE PERFORMANCE

80

80

70

70

60

60

50

50

40

40

Temperature in °C

Temperature in °C

56

30
20
10

30
20
10

0

0

–10

–10

–20

–20

–30

–30

–40

–40
50

200

350

500

650

800

950

1100

1250

1400

1550

50

200

Frequency in Hz
0.60–0.64
0.56–0.60
0.52–0.56
0.48–0.52

Fig. 32:
Color-coded loss
factor of an aluminum-butyl rubber
sandwich system(surface weight
2.8 kg/m²) overfrequency and temperature, determinedwith the OLF Method

Faist_eng_Inhalt.indd 56-57

0.44–0.48
0.40–0.44
0.36–0.40
0.32–0.36

0.28–0.32
0.24–0.28
0.20–0.24
0.16–0.20

350

500

650

800

950

1100

1250

1400

1550

Frequency in Hz
0.12–0.16
0.08–0.12
0.04–0.08
0.00–0.04

significantly worse damping properties (lossfactor of approx. 0.2). In contrast, Figure 34
shows a clear dependency between frequency
and loss factor. The loss factor diminishes significantly at higher frequencies. This damping
characteristic is typical for the material classof special, lightweight sandwich systems. The
result from the measurement of a multi-layer
concept with a surface weight of 0.9 kg/m2 has
also been presented, that is only about a thirdof the surface weight of the materials discussed
before. Despite this, the concept achieves a loss

0.60–0.64
0.56–0.60
0.52–0.56
0.48–0.52

0.44–0.48
0.40–0.44
0.36–0.40
0.32–0.36

0.28–0.32
0.24–0.28
0.20–0.24
0.16–0.20

factor of approx. 0.3 in a frequency range up
to 800 Hz.
The curves in Figure 35 compare the values
for 200 Hz in the OLF measurements andOberst values of the corresponding materials
calculated for the same frequency. The curves
with the lowest loss factors are those frommeasurements with bituminous film. As expected for such a single-layer material with an adhesive, the curves from both methods lie one
above the other. In contrast, the loss factors forsandwich systems measured with the OLF

0.12–0.16
0.08–0.12
0.04–0.08
0.00–0.04

Fig. 33:
Color-coded loss
factor of a bituminous film (surface
weight 2.7 kg/m²)
over frequency andtemperature, determined with the
OLF Method

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58

QUANTIFICATION OF THE PERFORMANCE59

QUANTIFICATION OF THE PERFORMANCE

80

0.6

70
0.5

Loss factor [-]60
50

Temperature in °C

40
30

0.4
0.3
0.2

20

0.1

10

0.0
–40

–30

–20

–10

0

10

20

30

40

50

60

70

0

Temperature in °C–10

OLF aluminum-butyl rubber sandwich system (surface weight 2.8 kg/m2)
Oberst aluminum-butyl rubber sandwich system (surface weight 2.8 kg/m2)
OLF light-weight sandwich system (surface weight 0.9 kg/m2)
Oberst light-weight sandwich system (surface weight 0.9 kg/m2)
OLF bituminous film (surface weight 2.7 kg/m2)
Oberst bituminous film (surface weight 2.7 kg/m2)

–20
–30

80

–40
50

200

350

500

650

800

950

1100

1250

1400

1550

Frequency in Hz
0.60–0.64
0.56–0.60
0.52–0.56
0.48–0.52

Fig. 34:
Color-coded loss
factor of a lightweight sandwich
system (surface
weight 0.9 kg/m²)
over frequency andtemperature, determined with the
OLF Method

Faist_eng_Inhalt.indd 58-59

0.44–0.48
0.40–0.44
0.36–0.40
0.32–0.36

0.28–0.32
0.24–0.28
0.20–0.24
0.16–0.20

0.12–0.16
0,08–0.12
0.04–0.08
0.00–0.04

Method are higher than those determinedusing the Oberst Method. These results, as
described above, confirm that loss factors canonly be calculated for resonant frequencies
when using the Oberst Method. In the case ofhigh loss factors, however, resonance peaks are
too wide to enable detection. Therefore, themaximum for the lightweight product and thealuminum-butyl rubber sandwich system iscapped because of system-related limitations.
As the comparison shows, the materials are

significantly undervalued with the standardized
Oberst Method.
Center Impedance Method andPower Injection Method

With the Center Impedance Method, the damping behavior is determined based on the intensity of a response signal to an introduced force
signal. Both signals are measured directly with
an impedance head. Just as with the OLF
Method, the Center Impedance Method is not
subject to the limitations of magnetic sample
materials. Sufficient excitation is guaranteed byan electrodynamic shaker. The test bar is excitedin the center and both ends are free. With thistest structure, only odd-numbered vibration
modes occur. Temperature-dependent measurements are conducted in a climate chamber.

Fig. 35:
Comparison of
measurement
values according to
the Oberst Method
at 200 Hz withmeasurement
values according to
the OLF Method for
damping materials
from Figures 32 to 34

Direct measurement of force
and response
signal17.12.2018 11:35:52

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QUANTIFICATION OF THE PERFORMANCEQUANTIFICATION OF THE PERFORMANCE

The loss factor is determined for the occurringresonance frequencies from the transfer function, i.e. from the response of the system to theintroduced excitation.
The Power Injection Method (PIM) determines the damping behavior of two-dimensional structures (plates) with vibrations excitedby an electrodynamic shaker. In this process,the excitation point is located outside the symmetry axes of the item under investigation.
With acceleration measurements taken atseveral positions distributed on the item, thedamping characteristic is determined throughcomparisons of the dissipated energy with theintroduced excitation. With this method, theweight of the higher number of accelerometers
applied changes the vibrational behavior of thetested plate. This method also confirms thefindings in other procedures that the damping
properties of sandwich systems decrease withincreases in frequency.

fiber-reinforced plastics are used besides conventional body steels.
With thin high-strength steels, as well asaluminum and carbon-fiber-reinforced plastics, there is a tendency for vibration excitation
to occur within the frequency range exceeding
500 Hz. In thick, conventional body steels, theproblematic frequency range is rather low andis below 500 Hz.
With the frequency-optimized design of thedamping coatings, a damping material suitablefor each substrate material used can be prepared. Soft steels have a tendency to be dampened adequately with single-layer systems.
Special sandwich elements laminated withaluminum foil are used with aluminum andcarbon-fiber-reinforced plastics. In order not
to nullify the weight savings achieved by usingaluminum and carbon-fiber-reinforced plastics
with heavy damping coatings, lightweight construction damping coatings are being used
more and more frequently.
Vibration damping materials convert vibration energy into heat best at or around thematerials’ glass transition temperature. At
temperatures significantly below and above
the materials’ glass transition temperature(under 0°C and above 60°C), the loss factor
drops significantly. Therefore, when designing
vibration damping systems for higher-value
vehicles, the relevant temperatures of operation are determined. Individual areas in thevehicle can be provided with temperatureadjusted damping coatings and thus receive
targeted noise damping. For high-temperatureareas, such as over the catalytic convertor,
muffler, or over the gear tunnel, but also

Relevant frequency and temperature ranges for acoustic problems
from radiating surfaces
When designing a damping package for a car
body, one needs to take into account the mounting locations, the temperature to be expected inoperation, and the construction materials used
to achieve optimal damping efficiency. In thedesign department, fundamental changes in theselection of raw materials are currently taking
place in order to follow the trend towards
lighter vehicles. In the so-called hybrid structure, high-strength steel, aluminum, magnesium
alloys, and composite materials such as carbon-

Faist_eng_Inhalt.indd 60-61

61

Frequency
range depending on material

Efficiency atoperation
temperature

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62

QUANTIFICATION OF THE PERFORMANCEDESIGN OF DAMPING CONCEPTS

80

63

Design of damping
concepts

7060
50

Temperature in °C

40
30
20
10
0
–10
–20
–30
–40
50

200

350

500

650

800

950

1100

1250

1400

1550

Frequency in Hz
0.60–0.64
0.56–0.60
0.52–0.56
0.48–0.52

Fig. 36:
Color-coded
loss factor for a
damping element
optimized for hightemperatures overfrequency andtemperature, determined with the
OLF Method

Faist_eng_Inhalt.indd 62-63

0.44–0.48
0.40–0.44
0.36–0.40
0.32–0.36

0.28–0.32
0.24–0.28
0.20–0.24
0.16–0.20

0.12–0.16
0.08–0.12
0.04–0.08
0.00–0.04

towards the engine compartment, sandwichmaterials with a high glass transition temperature are preferred (Fig. 36).

The design of damping measures in a vehicle
takes place in the design and development
phase. It can either happen very early (in theconstruction phase) or very late (shortly before
the first prototype phase). Selection of appropriate materials takes place within the framework of development of the actual acoustic
package consisting of structure-borne noise
damping, insulation, and absorption measures.
Often, a well-proven surface allocation iscopied from the previous model and adapted
for the new one at the end of the development
phase.
Early in the development process, a package for the damping of structure-borne noise
can be determined through computer simulations that provide body-in-white data and thecharacteristics of damping materials. Hotspot
analyses of vibration amplitudes are performed
using laser-vibrometry, as described in the following sections. For this purpose, is it necessary to have a body-in-white available. The
analysis of the acoustic properties of a new vehicle ready for series production concludes thedevelopment phase. This takes place during adriving test, and all possible load and temperature conditions, ranging from arctic cold totropical heat, are extensively tested.
Because classic damping products are seldom critical with regard to tooling costs anddevelopment time, development results thatwere already approved are often re-worked
after finishing the test phase, and the detected

Simulation andoptimization

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DESIGN OF DAMPING CONCEPTS

DESIGN OF DAMPING CONCEPTS

weak points are eliminated. It happens timeand again even in the series phase that identified and described optimization goals in thedesign of a damping concept are changed or
improved.

With sub-compact and compact cars, theeconomic viability at a sufficiently good acoustic level is the predominant factor. The usual
amount of materials needed for the damping ofstructure-borne noise varies from 2 kg to 4 kg
per vehicle. As a rule, standard materials, single-layer systems, and extrudable compounds
are used.
With mid-size cars, the damping of structure-borne noise varies depending on thedesired level of acoustic comfort. Typically,
between 5 kg and 12 kg of materials are usedper vehicle. Here, the borders between standard
and premium vehicles are blurred. Acoustic
comfort gains importance and becomes decisive for perception of the vehicle’s high level ofquality. Besides standard materials, also multilayer systems, high-value sandwich systems,
and lightweight products are used to a greater
extent in mid-size cars.
With the exception of thoroughbred sports
cars (where different acoustic level standards
apply), between 15 kg and 30 kg of damping
materials are used in a full-size or luxury car.
A whole range and multitude of acousticallyefficient products can be found here that focus
on utilizing high-value and high-performancedamping materials.

Design on different car segments

Economic
viability

Faist_eng_Inhalt.indd 64-65

For the design on different car segments, issues
such as the temperature conditions during vehicle operation, the selected damping material,
and the acoustic problem in the respective component are decisive factors. For the damping
package as part of the general acoustic concept,
high loss factors even with low surface weightsof the damping material should be realized,even though a combination of sound deadening
and improvement of sound transmission is desired, especially in the floor area. Sound transmission from the exterior, such as road noise, isreduced only through the application of heavy
damping coatings. Thus the vehicle manufacturer will conduct a detailed analysis of theacoustic weak points of the car body to determine suitable configurations. Furthermore,
possibilities of automated application also
influence the selection of damping materials.
However, these possibilities are limited due topoor accessibility.
Beyond the technical possibilities, economic factors also play a decisive role whenchoosing materials. With the conflicting priorities of acoustically optimal and economically
acceptable acoustic package design, a big difference appears across all car segments fromsubcompact cars to full-size luxury cars, SUVs,
and minivans.

65

Analysis of vibrations inbody-in-white systems
Today, comfortable vibration behavior is expected from each mid-size car. In years past,this was the case for only luxury-class vehicles.
Such acoustic comfort depends essentiallyon the damping materials used, which are

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Hotspot
analyses

DESIGN OF DAMPING CONCEPTS

DESIGN OF DAMPING CONCEPTS

developed specifically for the purpose of
reducing car body vibrations. This material
needs to be applied to areas with thehighest vibration amplitudes (also known as“hotspots”) in order to achieve optimal efficiency. Detailed knowledge of the position ofthese weak points is a prerequisite for an
efficient damping concept.
Hotspot analyses are mainly performed on
the body in white with a laser Doppler vibrometer (Fig. 37). Excitation takes place with theuse of electrodynamic shakers at significant
points for the introduction of structure-borne
noise.
The functional principle of a laser Doppler
vibrometer is based on measurement of the velocities of vibrating surfaces with optical interferometry. Because of the Doppler effect, thevibrating surface causes a periodic shift in thefrequency of backscattered light of the normal
incident laser beam. This shift is determined by
the velocity of the surface vibrations and wavelength of the laser. The signal laser beam isequalized in an interferometer with a reference
laser signal of known frequency so that thevelocity information, as well as an amplitude of
the vibration at the measurement point, can beextracted. Numerous measurement points on asurface, such as the front floor driver’s side,
provide information on the behavior of thisarea with regard to structure-borne noise. To
prevent the structure measured from being

67

Fig. 38:
Hotspot in driver’s
foot area of a bodyin whiteFig. 37:
Laser Doppler
vibrometer at
hotspot analysis ona body in white

Faist_eng_Inhalt.indd 66-67

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68

Fig. 39:
Reduction in hotspot
amplitude throughapplication of
damping materials

DESIGN OF DAMPING CONCEPTS

DESIGN OF DAMPING CONCEPTS

influenced by the measurement procedure, thetest is contactless.
Once the hotspots are identified (Fig. 38),
the areas where damping materials need to be
applied to improve vibration comfort arelocated. The material used depends on manyfactors, including the required loss factor,
available space, as well as weight and cost
targets.
Figure 39 shows the effect of a damping
material applied as an example to a 700-Hz
hotspot in the driver’s floor area. The surface
response to the excitation with an electromagnetic shaker has been represented in frequency
resolution (transfer function). At the hotspot(marked red), the levels have been reduced
by approx. 10 dB. Subjectively, a reduction of
10 dB translates into cutting the structureborne noise perception in half.

The hotspot analysis is performed for all
relevant surfaces of the body in white, including the floor areas, tunnel, areas in the front
wall, heel areas, back seat, trunk, spare wheel
well, loading floor, wheelhouses, roof, sidewalls, and doors. Appropriate materials need
to be selected for each application area. For example, the materials for the tunnel are adapted
to suit higher temperatures. For the heel area,
the front wall, wheelhouses, roof, and doors,magnetic or self-adhesive systems for verticalor overhead assembly are used.
According to this procedure, a damping
concept for the body in white is developed
step-by-step by selecting appropriate damping
systems for optimal results under consideration of all aspects.

–45

Damping
concept

Selection of appropriate damping
measures
The basic damping concept is defined early inthe vehicle concept phase. Depending on thevehicle manufacturer, vehicle category, and thedevelopment philosophy followed, two totallydifferent approaches can be distinguished regarding surface pattern and acoustic efficiencyof materials.

Hotspot–50

Transfer function H1 in dB

69

–55
–60
–65
–70
–75

» Minimal surface allocation with maximal
material performance» Maximal surface allocation with minimal
material performance

–80
–85
–90
–95
0

200

400

600

800

1000

1200

1400

1600

Frequency in Hz
Foot area, front, left: empty

Faist_eng_Inhalt.indd 68-69

Foot area, front, left: with damping materialWith the first approach (minimal approach),
the basic concept provides few or even no
damping coatings. In line with this development
philosophy, damping measures are only taken

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Maximalapproach

Faist_eng_Inhalt.indd 70-71

DESIGN OF DAMPING CONCEPTS

DESIGN OF DAMPING CONCEPTS

when they are deemed necessary for vehicle development to achieve the desired car acoustics.
In this case, such systems are applied in a solution-oriented manner that allows the problem
to be solved. This approach allows for the greatest flexibility in material selection. However,the fact that the costs are difficult to predictshould be mentioned as a disadvantage.
In the second approach (maximal approach), a basic package of damping measures
is defined depending on the car segment andbased upon experience from previous vehicles.
Ideally, this basic package covers a large surface of the car body and reserves as much construction space as possible for the thickness.
Necessary costs are planned and approved forsuch a package. Over an extended period oftime used for vehicle development, the basic
package is only optimized if any components
are changed in the vicinity. This period ends assoon as the vehicle reaches the design level atwhich reliable noise measurements can be performed. If necessary, the findings will lead to an
adjustment of the damping package by deletingmeasures, adding them, or changing the materials used. The advantage of this procedure is
the extensive planning security, but there is the
disadvantage of reduced flexibility in concept
adjustments. This is particularly true when
automation concepts are included in the basic
concept.
For technical reasons, vehicle manufacturers using extensively automated production
styles prefer damping concepts with a high degree of automation. For example, extrudable
acoustic compounds are only used commercially when the basic package of maximal ap-

proach requires few changes. If some areas are
dropped, or alternative materials need to be
used, extrusion automation can lead to a financial disaster.
Automation concepts capable of bringing
conventional damping coatings into the vehicle
body do not have this disadvantage. They enable significant flexibility in material application because it is possible to apply different,
efficient material systems in key areas without
calling the whole automation concept into
question. At the same time, an opportunity
presents itself to use significantly more favorable material systems when compared to extrudable acoustic compounds.
Lightweight structure concepts are becoming more and more important to keep harmful
emissions and fuel consumption at a relatively
low level. The right choice is lightweight materials that enable great weight savings without
drawbacks regarding physical properties. This,
however, frequently leads to conflicts in thearea of vehicle acoustics. There are high-quality material systems available that exhibit only
25% of the surface weight compared with conventional systems and simultaneously have significantly better acoustic efficiency. In contrast,
lightweight body structures often require local
applications of two to four times heavier damping systems to compensate for the loss in airborne sound damping caused by the lighter
body materials. Because these measures are
only applied in a very focused manner, the minimal approach described above is best suited
for such challenges.

71

Lightweight
structure
concepts

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72

OUTLOOK73

OUTLOOK

Outlook

Faist_eng_Inhalt.indd 72-73

tic concepts are a necessity. The damping ofstructure-borne noise will continue to make asignificant contribution to this.
The combustion engine also contributes tothe excitation of structure-borne noise, but thisis not the case in all driving scenarios. At approx. 60 km/h, excitation from tire-road contact via the chassis exceeds the noise producedby the engine. Therefore, eliminating the com-

Fig. 41:
Contributions to
structure-borne
noise

90

Maximal sound level in dB

Cars in the
futureNo other industry has made technical developments or advances so dynamic as the car industry. For some time, autonomous driving andelectromobility (e-mobility) have not been
works of fiction but instead are becoming a partof reality.
Over the past few years, clear developments
towards improving driving comfort can be observed. Vehicle acoustics are being increasingly
perceived as a significant contributor to comfort. Likewise, the demand for acoustic measures in cars keeps increasing.
The need for continuous development andadjustment of damping concepts derives fromthe advancements in body construction. New
concepts in multi-material mixes, including theuse of high-strength and ultrahigh-strength
steels, aluminum, magnesium, and composite
materials, such as carbon-fiber-reinforced
plastics, also lead to a change of the parameters
for car acoustics.
With the trend towards more driver assistance systems and, as a result, noticeable relief
for the driver, the acoustic perception of carpassengers will change. Already, the increase inpopularity of speech control systems confirms
the need for acoustic improvement in vehicles.
Cars in the future will be partially, or evencompletely, autonomous, making drivers lessinvolved in the driving process (Fig. 40). Thus,
the car can be transformed into a mobile workplace or simply into a place for entertainmentand communication. Because of the growing
sensitivity towards annoying noise, new acous-

Fig. 40:
Autonomous
driving

85
80
75
70
65
60
20

30

40

50

60

70

80

90

100

110

120

130

140

Speed in km/h
Total noise

Tire-road noise

Drive unit noise

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74

Electric cars

Faist_eng_Inhalt.indd 74-75

OUTLOOKbustion engine can only have a positive influence on the excitation of structure-borne noise
at low velocities, for instance, in city traffic(Fig. 41). Thus, the need for measures that aim
at reducing structure-borne noise remains valid
also with increasing electrification. However,this poses completely new challenges for car
development.
The noise spectrum of an electric car is significantly different to that of conventionallydriven cars. Electrification of the powertrain
and elimination of the typical combustion engine sound leaves room for other noise sources.
Whether it is a door humming at higher speeds,stones hitting the wheelhouse and/or undercarriage, noise from the auxiliary units, or
simply rain pattering on the roof, all theseannoying noises can be minimized with thedamping of structure-borne noise.
An increase in demand for lightweight
structure solutions is associated with electrification of the powertrain. Today, lightweight
concepts are the biggest focus for car manufacturers simply because emission guidelines arecontinuously becoming more stringent. However, in contrast, the primary driving force
behind developments in electrified vehicles is
to increase their range. Because of this development, classic damping products are being
replaced more and more by high-performance
multi-layer systems or equally efficient lightweight structural products.

TECHNICAL TERMS

75

Technical terms
Adhesion is understood to be binding forces between an adhesive surface and thesubstrate surface generated through interactionof adhesive molecules with the substrate surface.
Amplitude The maximal deviation of a sinusoidal wave is referred to as amplitude. Amplitude is necessary for description of the vibration.
Bitumen Bitumen is extracted from petroleum
and is the portion of the individual petroleum
fractions with the highest boiling point. This isthus a raw material completely different fromtar, which is extracted from coal.
Butyl compound Butyl compound is based on
various natural rubbers, e.g. polyisobutylene
rubber.
Epoxy Describes materials which are based on
epoxy resins. These are reactive resins that generate hard plastics in combination with othercomponents.
Fit tolerance In machine construction, the fit
tolerance is the dimensional relationship between two components. It is derived from thedimensional tolerances of both parts.
Glass transition temperature The glass transition temperature corresponds to the temperature at which a polymer or glass makes a transition from solid to viscous state.
Imbalance Imbalance is the case of a rotating
body if the rotation axis does not correspondto the mass centroid axis. Imbalance leads tovibrations.
Impedance head Measurement sensor formeasuring resistance resulting from propagating vibration in a structure. Impedance is thequotient of force and velocity.
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TECHNICAL TERMS

Longitudinal wave A longitudinal wave is aphysical wave vibrating in the direction of
propagation. Longitudinal waves can propagate in any medium, regardless of whether it issolid, liquid or gaseous.
Loss factor The loss factor relates to the percentage of vibration energy that has been transformed into heat in a test material throughinternal friction. It is a measure to characterize
damping materials.
Low-temperature impact strength Property
of a material to adhere even at low temperatures down to –30°C and not to chip off after an
impact on the car body in winterPoisson’s ratio Material property in mechanics and strength theory, belonging to the elastic
constants of material and serving the purpose of
calculating lateral contraction – named after
Siméon Denis Poisson
Prepreg Abbreviation for pre-impregnatedfibers: this material is mainly used in the aviation industry.
Real-time actuators Actuators that transformsignals in mechanical and other physical factors
in real time, i.e. without time lag
Resonance frequency Frequency of excitation leading to the locally maximum deviation(resonance amplitude)
Transverse wave Transverse wave − also
called sliding or shear wave − is a physical wave
in which vibrations run perpendicular to thedirection of propagation. Transverse waves canonly propagate in solids.

Faist_eng_Inhalt.indd 76-77

SOURCES AND LITERATURE

77

Sources and literature[1] Biermann, Jan Welm: Geräuschverhalten
von Kraftfahrzeugen [Noise behavior of motor
vehicles]. University lecture transcript, Institutefor Automotive Engineering, RWTH Aachen,
1988[2] Tyre/road interaction. Last update 9 February 2012 by Patrik Andersson. Research in
the Vibroacoustics Group, Division of Applied
Acoustics, Department of Civil and Environmental Engineering, Chalmers University ofTechnology, Gothenburg, Sweden. http://www.
ta.chalmers.se/research.php?page=vibgrptyre,
accessed on 22.06.2017[3] Möser, Michael; Kropp, Wolfgang: Körperschall – Physikalische Grundlagen und technische Anwendungen [Structure-borne sound:
Physical basics and technical applications]. 3rd
updated edition. Berlin, Heidelberg: Springer,
2010 (originally published under: Cremer,
Lothar; Heckl, Manfred)[4] Eisele, Georg; Genender, Peter Josef;
Wolff, Klaus; Schürmann, Gregor: Electric vehicle sound design - Just wishful thinking? In:
Proceedings AAC 2010: Aachen Acoustics Colloquium, 23rd–24th November 2010, Aachen.
P. 47–59[5] Kollmeier, Birger: Physikalische Grundlagen der Akustik [Physical basics of acoustics].
Medical Physics, Faculty VI, Oldenburg University. http://medi.uni-oldenburg.de/download/
docs/lehre/kollm_phystechmed_akustik/aku1.
pdf, accessed on 22.06.2017

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78

SOURCES AND LITERATURE[6] Wikipedia – The Free Encyclopedia. https://de.wikipedia.org/wiki/Reifen-FahrbahnGeräusch [tire/road noise], accessed on
22.06.2017[7] Yu, Xiongying; Pang, Jian; Min, Fujiang;Wen, Wei; Gong, Shichao: Structural transfer
path analysis of automobile tire/road noise.
Inter.noise, Melbourne, 16.–19.11.2014[8] Zeller, Peter (ed.): Handbuch Fahrzeugakustik: Grundlagen, Auslegung, Berechnung, Versuch [Handbook of vehicle acoustics:
basics, design, calculation, tests] 2nd ed.
Wiesbaden: Vieweg&Teubner, 2012 (ATZ/
MTZ-Fachbuch)

THE COMPANY BEHIND THIS BOOK

Sika Automotive
Sika is a specialty chemicals company with a leading position in thedevelopment and production of systems and products for bonding, sealing,
damping, reinforcing, and protecting in the building sector and the motor vehicleindustry. With over 200 factories in more than 100 countries Sika employsaround 25,000 people worldwide. Sika’s international network of production andmarketing companies has now extended its operations all around the world. This
global market presence enables the group to adapt its highly specialized knowhow quickly and efficiently to the changing needs of local markets.
Within the industry section Sika concentrates specifically on the following
sectors:- Automotive- Automotive Aftermarket- Transportation- Marine- Facades & Fenestration- Industrial Lamination- Appliances & Manufacturing- Renewable Energy
In the Sika Acoustic Test facilities, Sika Engineers and Application Experts
perform virtual and physical testing, analyze the results, and propose solutions
that deliver optimal weight-to-performance ratio for each specific application.
The Sika Technology Centers have been designed to support the development,
validation and optimization of sealing, damping, bonding, and reinforcing
products and solutions. Sika brand names such as SikaDamp®, SikaBaffle®,
Sikaflex®, SikaPower®, SikaReinforcer®, SikaSeal® and SikaForce® are firmly
established in the international marketplace, where they are expression andembodiment of a unified, marketing-oriented corporate philosophy.

SIKA AUTOMOTIVEFRANKFURT-WORMS GMBH
Weinsheimer Strasse 9667547 Worms
GermanyTel.
+49 6241 301-0
Fax+49 6241 301-230
sikaautomotive.com

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