High frequency FRF testingTom knechten – LMS Engineering Services 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 2 Focus on performance attributes: system dynamics. ASQ. 3 .… Innovative customized solutions Benefits: Increased efficiency of Transfer Function (FRF) measurements by enabling reciprocal measurements.LMS Qsources hardware completes LMS NVH test solution LMS ES provides unique structural and acoustic excitation hardware & services used in NVH by engineering. Extended measurement capability . EMA. comfort/sound quality.Excitation at difficult to reach locations. Offering: Standard set of structural and acoustic exciters covering most of the typical applications. TPA. body isolation testing. 00 0.89 rpm] 0.00 MG2 related MG1 related PSD related Engine related 4 dB g .00 s Time 14.07-949.00 -90.00 0.Identify all the orders on the same waterfall diagram 6000.00 Time Domain GEAR:-X (CH4) Hz AutoPow er GEAR:-X WF 163 [152. 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). 5 . Airborne Source Quantification(ASQ) In order to increase measurement efficiency reciprocal measurements are common. Direct FRF & Q1 F2 & &2 x F2 & Q 1 p1 6 . Reciprocal FRF & &2 x p1 = .Reciprocal acoustic excitation The reciprocity principle: Vibro-acoustic system transfer Acoustic system transfer p1 a2 =− & F2 Q 1 p2 p1 =− p1 p2 Volume acceleration enables measuring vibro-acoustic FRFs without post-processing as most common motion sensors are accelerometers which output an acceleration signal. LMS Qsources Mid Frequency Volume Source Working principle 1. 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 7 . An electrodynamic speaker to excite the structure 2. 5 meter distance. every 30° Mic frequency response chart Freefield-pressure @ 00 incidence 8 .Challenges with high frequency acoustic excitation Directivity of noise emission Directivity becomes more relevant as frequency increases as ratio between wavelength and hardware size decreases. Directivity plot shows 3 frequency ranges [dB] 1000Hz 4000Hz 10000Hz Pressure measurement at 0. 7000 6000 5000 4000 3000 2000 Log 1000 900 800 700 600 500 400 300 200 Mic frequency response chart Freefield-pressure @ 00 incidence 100 100 Hz 20000 9 .Challenges with high frequency acoustic excitation Directivity of sensors 10000 8000 The frequency response of the measurement equipment should be acceptable for high frequencies. Noise radiation from tubing or housing should be avoided Compact driver design 100 Reinforced tubing 90 Double sealed driver connection 85 80 70 65 Pa dB 60 55 50 45 40 35 30 20 89 Octave 1/3 Hz 22387 A L 20.00 Nonlinear tube acoustics make radiating noise uncorrelated and therefore not critical 75 10 .Challenges with high frequency acoustic excitation Housing radiation The monopole source should excite the acoustic environment with the noise that is emitted at the nozzle only.00 dB [0-20480 Hz] Pa 100. the time signal of the reference sensor is deteriorated. the emitted noise is symmetric. The pressure in the tube is in the range where nonlinear acoustics apply.Challenges with high frequency acoustic excitation Nonlinear tube acoustics At low source output level. At maximum output level. Symmetric waveform Asymmetric waveform 11 . 12 . & Q p Volume acceleration reference sensor in free-field and in an engine bay show consistent source strength quantification.Challenges with high frequency acoustic excitation Stable sound source strength measurement Volume acceleration as a quantity for sound source strength is more independent of acoustic environment. Pressure reference sensor in free-field and in an engine bay show variable sound source strength in function of acoustic environment. compared to sound power calculation based on pressure measurements. This triggered LMS to develop a special version of the current source which allows a higher noise level at frequencies above 3kHz. so do the noise levels of the source need to increase.Challenges with high frequency acoustic excitation High frequency reciprocal FRF measurement As isolation performance increases with frequency. 2 versions exist: A “normal” mid high frequency source with 4 meter tube A “wide frequency range” mid high frequency source with a 2 meter tube that can be extended to a 6 meter tube. 200-10000Hz 150-10000Hz Higher noise level 13 . Challenges with high frequency acoustic excitation Noise level .Q-MHF vs Q-MHF-WIDE(long&short tube) -10 -13 -15 -18 -20 -23 -25 -28 -30 (m3/s 2) 2/Hz -33 dB -35 -38 -40 -43 -45 -48 -50 -53 -55 -58 -60 50 60 70 80 90 100 200 300 400 500 600 700 800 1000 Hz 2000 3000 4000 5000 6000 7000 10000 15000 Q-MHF-WIDE: Long tube Q-MHF-WIDE: Short tube Q-MHF standard F F F PSD VOLACC:S SHORT TUBE MAX SPECTRA 500-10kHz MAX AMPLI RUN 2 PSD VOLACC:S LONG TUBE MAX SPECTRA 150-2kHz MAX AMPLI RUN 2 PSD VOLACC:S STANDARD TUBE MAX SPECTRA 200-2kHz MAX AMPLI RUN 2 14 . 00 0. (m/s 2) 2/Hz dB -60 -65 -70 -75 -80 -85 -90 -95 -100 -105 -110.Challenges with high frequency acoustic excitation Noise level .00 -55 F F PSD LONG:0001:+Z NORMAL_burst100%_han_500avg_200-10kHz PSD LONG:0001:+Z WIDE_LONG_burst100%_han_500avg_150-3kHz Accelerometer reponse: MOUNT:ENGINE:-Z The long tube at low frequencies a significant gain is obtained in structural response.Q-MHF vs Q-MHF-WIDE(long&short tube) -50.00 100 200 300 400 500 700 1000 Hz 2000 3000 4000 6000 13000 Amplitude 15 .00 100 200 300 400 500600 800 1000 Hz 2000 3000 4000 6000 8000 13000 1. 00 4000 5000 6000 Hz 7000 8000 9000 10000 13000.Challenges with high frequency acoustic excitation Noise level .00 3000. (m/s 2) 2/Hz dB -80 -85 -90 -95 -100 -105 -110 -115 -120.00 0.00 4000 5000 6000 Hz 7000 8000 9000 10000 13000.00 Amplitude 16 .00 -75 F F PSD LONG:0001:+Z NORMAL_burst100%_han_500avg_200-10kHz PSD LONG:0001:+Z WIDE_SHORT_burst100%_han_500avg_400-10kHz Accelerometer reponse: MOUNT:ENGINE:-Z With the short tube at high frequencies a significant gain is obtained in structural response and an improvement in coherence.00 3000.00 1.Q-MHF vs Q-MHF-WIDE(long&short tube) -70. 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 an significant improvement.Challenges with high frequency acoustic excitation Noise level .00 3000 3500 17 .58 / 10000.00 F F Coherence ENCO:frnt:S/Q_NORMAL:S Coherence ENCO:frnt:S/Q_WIDE:short:S / Amplitude Curve Average Hz 0. 1.38 / 0.00 3000.00 4000 4500 5000 5500 6000 6500 7000 7500 Hz 8000 8500 9000 9500 10000 10500 11000 12000 0. 00 Hz 10000.00 F Amplitude Coherence ENCO:frnt:S/Q_WIDE:long:S 0.00 0. FRF ENCO:frnt:S/Q_WIDE:long:S 100e-9 180.Q-MHF vs Q-MHF-WIDE (short tube) 0.00 Hz 10000.00 18 Log .Lab environment.00 1.00 ° -180.00 1.10 Pa/(m3/s 2) FRF interior mic to microphone in engine compartment To obtain a full bandwidth FRF.Challenges with high frequency acoustic excitation Noise level . the two FRF sets can be easily merged within LMS Test. 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 19 . The uncoupled mass is kept to a minimum. Shakers allow testing from a safe location. Internal force and acceleration sensors reduce space constraints and alignment work. Frequency range: 20-2000Hz 50-5000Hz 20 .Challenges with high frequency structural excitation accessibility The LMS Qsources shakers are based on the inertia principle making it possible to excite structures without any external support. Shakers are self aligning making the test efficient. 3g. 950 g Reference accelerometer with added weights 21 . 132.3g .Challenges with high frequency structural excitation Mass loading Four types of transfer functions with volume source excitation were performed: • No added weight • With added weight close to the response point of the accelerometer : 8. 00 2.00 (m/s 2)/(m3/s 2) -110. Each peak corresponds to the local mode of the added mass on a spring with the stiffness of the metal sheet of the body. -20. it will be around 1000 Hz.Challenges with high frequency structural excitation Mass loading For every weight added you can see a peak in the FRF and the level drops just after this peak. 22 .00 Hz 3067.00 dB For 800Hz we have a stiffness of around 5e7 N/m.00 800.95 5000.00 180.00 ° -180. If we calculate the resonance for a 132g mass. 00 180.00 _ Normally attached _ Insulated all around with foam _ Decoupled (on foam) -30.00 Pa/N dB -180.00 2000 Hz 10000 23 .00 2000 FRF Mic:S/Q-MSH:+X Run 1 FRF Mic:S/Q-MSH:+X Run 3_w ith_insulation FRF Mic:S/Q-MSH:+X Run 6_decoupled Hz 10000 ° Coherence Functions 1.00 / Amplitude 0.Housing radiation Influence of airborne noise emitted by minishaker FRF 70. 00 The frequency response of the internal force sensor is flat up to 5kHz.Challenges with high frequency structural excitation frequency response internal force sensor 1000. / Log F FRF accelerometer:-Z/Force cell:+Z 1.00 50 1000 2000 3000 4000 Hz 5000 6000 7000 8200 24 . Challenges with high frequency structural excitation Blocked force spectrum vs floornoise 1.00 N Log 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.5V Amplifier level:+16dB 100e-6 10.00 Hz 10000. 25 .00 Setup: Shaker mounted to rigid base max output voltage: 2. 82 8000 10000 -180.00 FRF Mic:S/Force Cell:+X Shaker_on_force_cell Run 1 0.00 180.example 90.Challenges with high frequency structural excitation Force level .18 0 2000 4000 6000 5262.00 0 2000 4000 Hz 6000 8000 10000 26 .00 4000 Hz 6000 8000 10000 1.00 ° Pa/N dB 2000 2893.00 / Amplitude F Coherence Mic:S/shaker:+X 0. Conventional shaker Modal hammer Integral shaker Typical operator variation: Hammer worst repeataiblity Q-ISH best repeatability resulting in high data accuracy and confidence in the measurement results. Following comparison has been shows that the Integral Shaker is most robust in operator reproducability and repeatability.LMS Qsources Integral Shaker Very high data accuracy and reliability Exact excitation position and orientation is critical in high accuracy measurements.2005 27 . 27 copyright LMS International . 00 Pa/N dB Misalignment of conventional shaker ±5degrees Pa/N dB 35.00 Reproducability of 10 persons with modal hammer Pa/N dB 28 copyright LMS International .2005 35.Challenges with high frequency structural excitation Reproducability 70.00 70.00 70.00 200 220 240 28 260 280 300 320 340 360 380 400 Hz 420 440 460 480 500 520 540 560 580 600 . 35.00 Positioning error of ring on structure ±2mm Typical operator variation: Q-ISH: Positioning error Misalignment error Repeatability A comparison between Integral shaker Conventional shaker Modal hammer Q-ISH shows a minimum variation in vibro-acoustic FRF on a passenger car. 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 Questions? 29 .
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