LMS Webex - Supporting High Frequency Noise Analysis

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High frequency FRF testingTom knechten LMS Engineering Services


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Webex overview

Challenges with acoustic excitation

Noise level

Directivity Sensor freq response

Housing radiation

Challenges with structural excitation

Accessibility

Mass loading

Sensor freq response

Housing radiation

Reproducability


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LMS Qsources hardware completes LMS NVH test solution

LMS ES provides unique structural and acoustic excitation hardware &services used in NVH by engineering.

Focus on performance attributes: system dynamics, comfort/sound quality.

Offering:

Standard set of structural and acoustic exciters covering most of the typicalapplications. EMA, ASQ, TPA, body isolation testing,

Innovative customized solutions

Benefits:

Increased efficiency of Transfer Function (FRF) measurements by enablingreciprocal measurements.

Extended measurement capability - Excitation at difficult to reach locations.


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14.000.00 s

Time

6000.00

0.00

Hz

GEAR:-X(CH4)

0.00

-90.00

dB g

AutoPow er GEAR:-X WF 163 [152.07-949 .89 rpm]

Identify all the orders on the same waterfall diagram

Time Domain

Engine related

MG1 related

MG2 related

PSD related


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FRF information

FRF measurements on vehicle bodies enables various analyses:

Body sensitivity to dynamic structural or acoustic loads

Body isolation Mode frequencies

Input data for Transfer-Path-Analysis model(TPA), Airborne Source Quantification(ASQ)

In order to increase measurement efficiency reciprocal measurements are common.


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Reciprocal acoustic excitation

The reciprocity principle:

Vibro-acoustic system transfer Acoustic system transfer

Volume acceleration enables measuring vibro-acoustic FRFs without post-processing asmost common motion sensors are accelerometers which output an acceleration signal.

1

2

2

1

Q

a

F

p

&

=

1Q&

2x&&

1p

2F

2

1

1

2-F

p

Q

x=

&&&

Reciprocal FRF Direct FRF

2

1

1

2

p

p

p

p=


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LMS Qsources Mid Frequency Volume SourceWorking principle

1. An electrodynamic speaker to excite the structure

2. A reference signal to measure sound source strength

of the speaker.

Compared to a normal speaker:

High SPL output

Designed to behave like a point/monopole source

Internal sound source strength reference sensor

Electronic protection against overload

Comments on design:

A small nozzle to reduce diffraction effect of speaker. A flexible tube enabling fast & easy positioning

Reference sensor integrated in nozzle of sound source to define the excitation


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Challenges with high frequency acoustic excitationDirectivity of noise emission

Directivity becomes more relevantas frequency increases as ratio

between wavelength andhardware size decreases.

Directivity plot shows 3frequency ranges [dB]

1000Hz

4000Hz

10000Hz

Pressure measurement at 0.5meter distance, every 30

Mic frequency response chartFreefield-pressure @ 00 incidence


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Challenges with high frequency acoustic excitationDirectivity of sensors

The frequency response of the measurementequipment should be acceptable for high

frequencies.

20000100 Hz

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Log

Mic frequency response chartFreefield-pressure @ 00 incidence


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2238789 Octave 1/3

Hz

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dB

Pa

100.00

20.00

dB

[0-20480Hz]

Pa

A L

Challenges with high frequency acoustic excitationHousing radiation

The monopole source should excite the acoustic environmentwith the noise that is emitted at the nozzle only.

Noise radiation from tubing or housing should be avoided

Compact driver design

Reinforced tubing

Double sealed driver connection

Nonlinear tube acoustics make radiating

noise uncorrelated and therefore not critical


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Challenges with high frequency acoustic excitationNonlinear tube acoustics

At low source output level, the emitted noise is symmetric.

At maximum output level, the time signal of the reference sensor is deteriorated. The

pressure in the tube is in the range where nonlinear acoustics apply.

Symmetric waveform

Asymmetric waveform


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Challenges with high frequency acoustic excitationStable sound source strength measurement

Volume acceleration as a quantity for sound source strength is more independent of acousticenvironment, compared to sound power calculation based on pressure measurements.

Volume acceleration reference sensor infree-field and in an engine bay showconsistent source strength quantification.

Pressure reference sensor in free-field andin an engine bay show variable soundsource strength in function of acousticenvironment.

Q& p


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Challenges with high frequency acoustic excitationHigh frequency reciprocal FRF measurement

As isolation performance increases with frequency, so do the noise levels of the sourceneed to increase. This triggered LMS to develop a special version of the current sourcewhich allows a higher noise level at frequencies above 3kHz.

2 versions exist:

A normal mid high frequencysource with 4 meter tube

A wide frequency range mid highfrequency source with a 2 meter tubethat can be extended to a 6 meter tube.

200-10000Hz 150-10000HzHigher noise level


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Challenges with high frequency acoustic excitationNoise level - Q-MHF vs Q-MHF-WIDE(long&short tube)

1500050 100 1000 1000060 70 80 90 200 300 400 500 600 700 800 2000 3000 4000 5000 6000 7000

Hz

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dB

(m3/s2)2/Hz

F PSD VOLACC:S SHORT TUBE MAX SPECTRA 500-10kHz MAX AMPLI RUN 2

F PSD VOLACC:S LONG TUBE MAX SPECTRA 150-2kHz MAX AMPLI RUN 2

F PSD VOLACC:S STANDARD TUBE MAX SPECTRA 200-2kHz MAX AMPLI RUN 2

Q-MHF-WIDE: Long tubeQ-MHF-WIDE: Short tube

Q-MHF standard


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Accelerometer reponse:MOUNT:ENGINE:-Z

The long tube at lowfrequencies a significantgain is obtained instructural response.

13000100 1000200 300 400 500600 800 2000 3000 4000 6000 8000

Hz

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(m/s2)2/Hz

F PSD LONG:0001:+Z NORMAL_burst100%_han_500avg_200-10kHz

F PSD LONG:0001:+Z WIDE_LONG_burst100%_han_500avg_150-3kHz

13000100 1000200 300 400 500 700 2000 3000 4000 6000

Hz

1.00

0.00

Amplitud

e


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Accelerometer reponse:MOUNT:ENGINE:-Z

With the short tube athigh frequencies asignificant gain isobtained in structuralresponse and an

improvement incoherence. 13000.003000.00 100004000 5000 6000 7000 8000 9000

Hz

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-120.00

-110

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-90

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-95

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dB

(m/s2)2/Hz

F PSD LONG:0001:+Z NORMAL_burst100%_han_500avg_200-10kHz

F PSD LONG:0001:+Z WIDE_SHORT_burst100%_han_500avg_400-10kHz

13000.003000.00 100004000 5000 6000 7000 8000 9000

Hz

1.00

0.00

Amplitude


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Challenges with high frequency acoustic excitationNoise level - Q-MHF vs Q-MHF-WIDE(hort tube)

Curve: Coherence FRF interior mic to microphone in engine compartment

Comparing the coherence of a microphone near engine compartment shows ansignificant improvement.

120003000 4000 5000 6000 7000 8000 9000 10000 110003500 4500 5500 6500 7500 8500 9500 10500

Hz

1.00

0.00

Amplitude

/

3000.00 10000.00

Curve Average Hz

0.38 /

0.58 /

F Coherence ENCO:frnt:S/Q_NORMAL:S

F Coherence ENCO:frnt:S/Q_WIDE:short:S


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Challenges with high frequency acoustic excitationNoise level - Q-MHF vs Q-MHF-WIDE (short tube)

FRF interior mic to microphone inengine compartment

To obtain a full bandwidth FRF, thetwo FRF sets can be easily mergedwithin LMS Test.Lab environment.

10000.001.00 Hz

0.10

100e-9

Lo

g

Pa/(m

3/s2)

180.00

-180.00

FRF ENCO:f rnt:S/Q_WIDE:long:S

10000.000.00 Hz

1.00

0.00

Amp

litude

F Coherence ENCO:frnt:S/Q_WIDE:long:S


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Overview


Noise level


Housing radiation


Accessibility

Mass loading


Housing radiation

Reproducability


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Challenges with high frequency structural excitationaccessibility

The LMS Qsources shakers are based on the inertiaprinciple making it possible to excite structures without anyexternal support.

Shakers are self aligning making the test efficient.

Internal force and acceleration sensors reduce space

constraints and alignment work.

The uncoupled mass is kept to a minimum.

Shakers allow testing from a safe location.

Frequency range: 20-2000Hz

50-5000Hz


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Challenges with high frequency structural excitationMass loading

Reference accelerometer with added weights

Four types of transfer functions withvolume source excitation wereperformed:

No added weight With added weight close to the

response point of the accelerometer :8.3g , 132.3g, 950 g


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Challenges with high frequency structural excitationMass loading

For every weight added you can see a peak in the FRF and the level drops justafter this peak.

Each peak corresponds to the local mode of the added mass on a spring with thestiffness of the metal sheet of the body.

For 800Hz we have a stiffness of around 5e7 N/m. If we calculate the resonance fora 132g mass, it will be around 1000 Hz.

5000.002.00 Hz

-20.00

-110.00

dB

(m/s

2)/(m3/s2)

180.00

-180.00

800.00 3067.95


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Housing radiationInfluence of airborne noise emitted by minishaker

_Normally attached

_Insulated all around with foam

_ Decoupled (on foam)

100002000 Hz

70.00

-30.00

dB

Pa/N

180.00

-180.00

FRF Mic:S/Q-MSH:+X Run 1

FRF Mic:S/Q-MSH:+X Run 3_w ith_insulationFRF Mic:S/Q-MSH:+X Run 6_decoupled

100002000 Hz

1.00

0.00

Amplitude

/

FRF

Coherence Functions


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Challenges with high frequency structural excitationfrequency response internal force sensor

The frequency response of the internalforce sensor is flat up to 5kHz.

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Hz

1000.00

1.00

Log/

F FRF accelerometer:-Z/Force cell:+Z


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Challenges with high frequency structural excitationBlocked force spectrum vs floornoise

Setup:

Shaker mounted to rigid base

max output voltage: 2.5V

Amplifier level:+16dB

Red curve:maximum force level in 1/3 octaves

Excitation frequency range: 50-5000Hz

Green curve: maximum force level in 1/3 octaves

Excitation frequency range: 50-600Hz

Black curve: Background noise during no excitation.

10.00 10000.00Hz

100e-6

1.00

Log

N


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Challenges with high frequency structural excitationForce level - example

100000 2000 4000 6000 8000Hz

1.00

0.00

Amplitude

/F Coherence Mic:S/shaker:+X

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Hz

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d

B

Pa/N

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-180.00

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2893.18 5262.82

FRF Mic:S/Force Cell:+X Shaker_on_force_cell Run 1


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27 copyright LMS International - 2005

LMS Qsources Integral ShakerVery high data accuracy and reliability

Typical operator variation:

Hammer worst repeataiblity

Q-ISH best repeatability resulting in high data accuracy and confidence in themeasurement results.

Integral shakerModal hammerConventional shaker

Exact excitation position andorientation is critical in highaccuracy measurements.

Following comparison has beenshows that the Integral Shakeris most robust in operatorreproducability and repeatability.


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28 copyright LMS International - 2005

Challenges with high frequency structural excitationReproducability

Typical operator variation:

Q-ISH: Positioning error

Misalignment errorRepeatability

A comparison between

Integral shaker

Conventional shaker Modal hammer

Q-ISH shows a minimumvariation in vibro-acoustic

FRF on a passenger car.

35.00

70.00

dB

Pa/N

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70.00

dB

Pa/N

200 600300 400 500220 240 260 280 320 340 360 380 420 440 460 480 520 540 560 580

Hz

35.00

70.00

dB

Pa/N

Misalignment of conventional shaker5degrees

Positioning error of ring on structure2mm

Reproducability of 10 persons with modal hammer


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Overview


Noise level


Housing radiation


Accessibility Mass loading


Housing radiation

Reproducability

Questions?

Date post:	14-Apr-2018
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LMS Webex - Supporting High Frequency Noise Analysis

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