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New challenges in automotive experimental

modal analysis: Acoustic Modal AnalysisChoukri Mostapha, Business Development Manager Automotive Industries

Simcenter Symposium

zur Fahrzeugentwicklung

17-18, Oktober 2017

Unrestricted © Siemens AG 2017

2017.MM.DD Siemens PLM Software

.

1. Introduction and challenges

2. New excitation source and algorithm

3. Case study

4. Conclusions

Overview



Acoustic Modal Analysis

Interior car sound: very important attribute in vehicle engineering

• Predict the acoustic behavior with simulation models

• Role of testing and experimental acoustic modal analysis

• Understand modelling challenges and improve modelling know-how

• Troubleshooting

Force

[N]

Noise

[m3/s2]

Vibration

[m/s2]

Sound

Pressure

[Pa]

Excitation

res

po

ns

e

Structural

Dynamics

Modal Model

Acoustic

sensitivity

for structural

excitation

Acoustic FRF &

Modal Model

Structural

sensitivity to

acoustic

excitation

1000.000.00 Hz

120.00

30.00

dB

Pa/(

m3/s

2)

1.00

0.00

Real

/

F FRF 3_5:mic2:S/vvs:1:S

F FRF 3_5:mic2:S/vvs:2:S

B Coherence 3_5:mic2:S/Multiple



What are the challenges though?

• Automotive cavities are inherently coupled with the

flexibility of the body structure

• Acoustic modes are highly damped

• System need to be excited with many (>8) acoustic

sources to avoid distortions

• A good description of the cavity modes and the

interaction with the body requires many (few 100s)

microphones

• FRF matrix with many columns (sources) is

challenging for standard modal parameter estimation

algorithmsYoshimura et al., Modal analysis of automotive

cabin by multiple acoustic excitation, ISMA 2012



Some theoretical background

The governing equation for a 3D closed acoustic system with rigid boundaries is:

qpc

p 2

2 1 Sound pressure [N/m2]

Volume velocity per unit volume [1/s]

Speed of sound [m/s]

Density of medium [kg/m3]

c

q

p

qpCpBpA CBA ,,Acoustical mass,

damping, stiffness

matrices



.



3. Case study

4. Conclusions

Overview



New Excitation method

LMS Q-Sources Low Frequency monopole Sound Source (Q-MED)

Minimize cavity distortion

High levels at low frequency makes it

ideal for highly damped acoustic FRFs

in cavities

Omnidirectional monopole sound

source

Real-time sound source strength

measurement

315050 100 100070 200 300 500 700

Hz

dB

Directivity plot at 1 meter in semi anechoic conditions

100010 10020 30 40 60 200 300 500

Hz

dB

Pa/(

m3/s

2)

180.00

-180.00

°

100 100020 30 40 60 200 300 500

New source

Miniature source



New Modal Parameter Estimation SolverMaximum Likelihood estimation of a Modal Model (ML-MM)

New iterative frequency-domain solver:

• Two steps: 1) Initial estimation of modal parameters (using Polymax

estimator )

2) Error between modal model & FRF data is minimized

using a non-linear iterative optimization technique

(Levenberg - Marquardt technique)

Motivations behind introducing ML-MM

• Better fitting of an FRFs matrix with so many

columns (i.e. large number of references)

• High quality models while applying some desired

constraints on the identified model:

• FRFs reciprocity

• Real mode shapes

• Confidence bounds on all the modal parameters

(frequency, damping, part. factors) in case the FRFs

variance is available (see 𝜎2 in the equation)

Measured FRFs

Polymax1 5% Noise

Initial values:

• Poles

•Participation factors

Maximum Likelihood estimation of Modal Model

2 4 6 8 10 123000

4000

5000

6000

7000

Iteration

ML

-co

st f

un

ctio

nM

L-c

ost fu

nctio

n

Iteration

Levenberg - Marquardt optimization

Improved estimates for all the modal parameters

together with their confidence bounds

2

o i f

oi

N

o

N

i

N

k kH

koikoi HH

1 1 12

2ML

ML

)(

)(),()(

𝐻 𝜃, 𝑠𝑘 =

𝑟=1

𝑁𝑚𝜓𝑟𝐿𝑟𝑠𝑘 − 𝜆𝑟

+𝜓𝑟∗𝐿𝑟

∗

𝑠𝑘 − 𝜆𝑟∗ +

𝐿𝑅

𝑠𝑘2 + 𝑈𝑅



Acoustic Modal Analysis : an integrated approach rather than an

algorithm

Early FE analysis Test setup Modal Analysis



.



3. Case study

4. Conclusions

Overview



Case study : Acoustic Modal Analysis on Fully Trimmed Sedan



Preliminary Analysis

Many-reference vehicle cabin test

An acoustic FE model of the interior case-study-car was created in LMS Virtual.Lab

AcousticsRigid wall assumption

Frequency range: [0-200] Hz

Fro

m t

he N

um

erical M

od

el

• Number of modes and their

shape

• Proper distribution of sources to avoid nodal lines

• Test geometry (Wireframe) of

the measurements points

• Guideline for selecting

meaningful modes



Microphone positions

When the purpose is the validation, correlation and updating

of the FE numerical model, an accurate description of the

acoustic modes is necessary.

• Microphones were positioned:

• On roving arrays

• With a spacing equal to 20 cm

• Both at the boundary surface and inside the cavity

• Also in extreme positions (foot regions, hat shelf)

• 527 locations



S:02S:01S:04S:03

S:06S:05

S:08S:07 S:12S:11

S:10S:09

Source locations

• 12 sources spread over the cavity

• Geometrically symmetric positioning

• Close to the edge, corners and at the

maximum amplitude location

Source:02(Front Windshield RH)

Source:04(Rear Passenger Ears RH)

Source:06(Trunk RH)

Source:08(Front Foot Region RH)

Source:10(Front Seat Passenger RH)

Source:12(Rear Foot Region LH)



Sensor localization algorithm

Accurate and automated test geometry generation

The actual location of a sensor is critical for model

validation

In case of acoustic cavities, manually measuring

each sensor location can be extremely

cumbersome

Relies on few measured locations to detect all

sensors coordinates based on time of arrival of

acoustic wave

Optimal source signal design to maximize time

resolution and SNR

Validation example on 38 microphones using as reference CAD

locations




Measurement data

800.000.00 Hz

133.39

33.39

dB

Pa/(

m3/s

2)

180.00

-180.00

Phase

°

44.00 220.00

1 microphone / 10 sources




Limitations of existing algorithms

220.0044.00 Hz

100.00

60.00

dB

Pa/(

m3/s

2)

180.00

-180.00

Phase

°

FRF Cavity:1000506:S/Source:12:S 0 # Measured

Synthesized FRF Cavity:1000506:S/Source:12:S 1 # PolyMAX - Complex

Synthesized FRF Cavity:1000506:S/Source:12:S 1 # PolyMAX - Real

• Polymax cannot fit the data in an optimal

way

• When computing real modes, which better

represent standing waves, fit to data is

further deteriorated

• If less references are used, fit to data

improves but modes result distorted



The MLMM estimator

Optimize your modal analysis results



The MLMM modal parameter estimator

Results

220.0044.00 Hz

100.00

60.00

dB

Pa/(

m3/s

2)

180.00

-180.00

Phase

°

FRF Cavity:1000506:S/Source:12:S 0 # Measured

Synthesized FRF Cavity:1000506:S/Source:12:S 1 # PolyMAX - Complex

Synthesized FRF Cavity:1000506:S/Source:12:S 1 # PolyMAX - Real

Synthesized FRF Cavity:1000506:S/Source:12:S 1 # MLMM - Real

Polymax MLMM

Real Complex Real Complex

Mean

Fitting Error

[%]

27.7 20.9 9.6 5.6

Mean

Fitting

Correlation

[%]

79.3 84.6 92.1 95.2




Complex vs. real mode shapes

Real mode shape Complex mode shape




Real mode shapes



Vibro-Acoustic Modal Analysis

The need for balancing the vibro-acoustic FRF matrix

• Observation: very large

differences (>100 dB) in

numerical values of mixed

vibro-acoustic FRFs

• Need for balancing

• To avoid numerical problems

when analyzing all data

together

• To ensure that data from all 4

quadrants are used with the

same weight

76.0012.00 Hz

140.00

-100.00

dB

Pa/(

m3/s

2)

76.0012.00 Hz

140.00

-100.00

dB

Pa/N

76.0012.00 Hz

140.00

-100.00

dB

g/(

m3/s

2)

76.0012.00 Hz

140.00

-100.00

dB

g/N

Force

[N]

Noise

[m3/s2]

Vibration

[g]

Sound

Pressure

[Pa]

Excitation

resp

on

se

Structural

Dynamics

Modal Model

Acoustic

sensitivity

for structural

excitation

Acoustic FRF &

Modal Model

Structural

sensitivity to

acoustic

excitation



.



3. Case study

4. Conclusions

Overview



Conclusions

Increased interest in acoustic characterization of automotive cabins

• Improved modelling capabilities validated by test

• Vehicle development and troubleshooting projects

• Call for enhanced testing capabilities

New sound source

• Compact, omnidirectional and capable of generating high noise levels in the

low frequency range

• Detailed experimental study: new source excellently suited for automotive

cabin acoustic modal analysis

New modal analysis method

• Maximum Likelihood Estimation based on the Modal Model (MLMM)

• Deals properly with FRF matrices with many references

• Provides superior FRF synthesis results



Close the loop with simulation

Conclusions

A Complete testing solution closing the loop with simulation

800.000.00 Hz

133.39

33.39

dB

Pa/(

m3/s

2)

180.00

-180.00

Phase

°

44.00 220.00

FRF Cavity:1000106:S/Source:01:S










LMS Qsources Microphone Grid LMS SCADAS Data Acquisition Modal Analysis

Interior acoustics is an important brand value

• Managing cavity modes at an early stage using acoustic

simulation models

• Role of testing and experimental acoustic modal analysis

• Improve simulation know-how

• Vehicle development support and troubleshooting

Thank you!

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