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Microflown Technologies The Netherlands www.microflown.com info@microflown.comMicroflown: a new category of sensors 1

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Agenda• Introduction to Microflown Technologies [3-29]• Sound Intensity [30-69]• Measurement techniques – traditional [70-88] systems• Advanced scanning techniques : Scan&Paint [89-126] 2

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Company introduction 3

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Company Introduction1994: Invention Microflown by Hans-Elias de Bree at University Twente1997: Ph.D. Hans-Elias de Bree1998: Founding Microflown Technologies B.V. (de Bree, Koers)2001: Industrializing product2003: Introduction broad banded sensor element2004: First applications scientifically proven / first arrays sold2005: Rapid growth in (automotive + aerospace) industry2005: De Bree appointed Professor ‘Vehicle Acoustics’ at the HAN University, Arnhem School of Automotive Engineering2008: Strategic decision to explore the defense & security market2010: 20 FTE company, 1,3 MEURO turnover2011: Microflown AVISA 4

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Working principleMicroflown SEM picture: two heated wires 5

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Working principleMicrophone measures sound pressure (result)Microflown measures Particle Velocity (cause)Acoustical <-> electrical <-> energySound pressure <-> voltage <-> potentialParticle velocity <-> amperes <-> kinetic 6

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Working principlePRESSURE WAVE 7

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Working principlePRESSURE WAVE ≠ PARTICLE VELOCITY 8

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Working principleT[k] velocity output 0 upstream downstream distance 9

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Working principle u uu u u u pp p p p p C u u sum sum p p A B D 10

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Working principleSurface velocity measurement:• No background noise and reflection problems Figure of eight Low surface velocity High surface velocity and high surface and low surface pressure pressure 11

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Working principle Mems based sensor Clean room technology is used to create the small elements on a waver University of Twente 12

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Working principleWirebonded elements 13

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Microflown probes 14

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Standard probes Scanning Probes • 1D Velocity • For small object • High temperatures • Non contact vibration 15

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Standard probes PU probes • Particle Velocity • Sound Pressure • 1D Sound Intensity • 1D Sound Energy • Impedance 16

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Standard probesPU Probes: Placement of p and u 17

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Standard probes Metal Mesh • Wind shield, DC flow up to 2 m/s • Protecion of the wires • Calibration including mesh 18

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Standard probes USP probes • 3D Particle Velocity • Sound Pressure • 3D Sound Intensity • 3D Sound Energy • Impedance • Acoustic Vector Sensor 19

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Standard probesHigh dB Scanning Probe• Above 135dB acoustics becomes non linear• Standard sensor overloads at 130dB• Measurement at 170dB is possible with packaged sensor 20

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Microflown applications 21

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Standard probesFrom product development till end of line control 22

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Automotive Scan & Paint Scan&ListenAcoustic camera PNCAR Insitu abosorption 23

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Space and Aerospace3D intensity stream lines In-situ impedance Reverberant room characterization PNCAR 24

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Environmental noise3D sound source location Virtual Arrays 25

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Manufacturing industries Scan & Paint In situ impedanceAcoustic Camera Point by point intensity 26

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End of Line Control Leak testingAcoustic EOL 27

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Room acousticsSound diffusion Impedance measurement 3D intensity visualization 28

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Military applications Air to ground applications Aircrafts location Hostile fire locationSurveillance 29

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What is intensity?

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Theoretical approach

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IntensitySound intensity is useful for measurement of sound power, identification and ranking ofsources, visualization of sound fields, measurement of transmission loss, identification oftransmission pathsSound intensity is defined as the sound power per unit areaIntensity: Time averaged rate per unit area at which work is done by one element of fluidon an adjacent elementIntensity and Particle velocity are vectors, therefor they have a direction related to theirmagnitude.Sound intensity units are W/m2 32

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IntensityIn far field pressure and velocity have equal phase so I is a real quantity. However, in the near field pressure and velocity are out of phase, leading to have an active and reactive part of the intensity Active intensity [ I ] Active intensity is the real part of the time Imaginary averaged product between pressure and velocity. This term is commonly called ‘acoustic intensity’ because is associated to the acoustic energy that propagates away J from the source Reactive intensity [ J ] I Real Active intensity is the imaginary part of the time averaged product between pressure and velocity. This term is associated with the evanescent energy carried by the particle velocity 33

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Reactivity index The reactivity index is the ratio between reactive (J) and active intensity (I) When reactivity takes high values lead to low active intensity. This can be seem as lack of radiation efficiency, i.e. there is a vibrating surface which moves the air but is not able to compress it. The size of the near field is related to the wavelength assessed, therefore the reactivity index also depends on frequency. 34

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Pressure-Intensity index 35

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Sound Intensity-PP probes

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PP intensityTraditionally the measurement of sound intensity is performed by P-P probes.The measurement procedure makes use of two microphones. The sound pressureis the average of the two corresponding pressure signals. The intensity is calculatedat the center of the space separating the two microphones.The P-P intensity is then obtained by the following relation: ˆ p1 t p2 t t p1 p2 I pp ˆˆ pu t d 2 r tWhere the first term is the promediated pressure value and the second term is theestimated particle velocity. 37

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PP intensity Average pressure between the two closely spaced microphonesEstimation of the particlevelocity from the pressuregradient valid for free fieldplane waves 38

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PP intensity - ERRORS Phase mistmatch error between pressure microphonesPressure-intensity index is directlyrelated with the measurement error 39

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PP intensity - ERRORSPhase mismatch error : 2 2 ˆ peprms prms / c pe I pp I pp I 1 k r c k r IA small error in the microphones phase matching can lead to an uncorrect intensityestimation.This is the reason why the manufacturers need to pair the microphones, to try tofind in the production, the more similar sensors to form the probe. 40

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PP intensity - ERRORS Finite difference error (depends on the microphone separation): ˆ sin k r I pp / I k r The estimation of the velocity term is the pressure gradient between the two pressure signals, this can lead to the following casesToo low frequency: the pressure gradient istoo small to determine the velocityToo high frequency: the wavelength istoo small compared to the microphonespacing 41

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PP intensity - LimitationsReverberant sound fields:The usable frequency region of thesesensors is drastically reduced when thepressure-intensity index is HIGH,because of the small ratio betweenphase measurements at microphonepositions. This effect appears inreverberant conditions where: – high pressure level – Intensity level tending to 0Free field conditions:The spacer needs to be changed foreach frequency range, in order toadapt it to the interest wave legth. 42

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PP intensity - LimitationsNear field measurements:The probe can be used but the usablefrequency range is reduced drasticallybecause of the appearance ofevanescent waves.The gradient of pressure to estimatethe particle velocity is on longer usableNOTE: Evanescent waves: Anevancescent wave is a near fieldstanding wave with an exponentialamplitude decay from the boundary atwhich the wave was formed 43

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Properties of PP probes Advantages • Not sensitive to DC flow • Flat frequency response Disadvantages • Only for plane waves • Distributed sensor • Exact microphone pairing needed • Microphone spacing depends on frequency • Accuracy is strongly dependent into the pressure-intensity index 44

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Sound intensity P-U probes

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PU intensity The working principle is based upon measuring the temperature difference in the cross sections of two extremely sensitive heated platinum wires that are placed in parallel. The incident sound flown produces a difference in temperature, leading into a voltage difference proportional to the flow. P-U intensity :Pressure and particle velocity are directly measured so no assumptionsabout the sound field are requiredIntensity is then described by the real part of the product of pressure andparticle velocity, both measured quantities. 46

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PU probes - ERRORSReactivity index:The reactivity is the ratio between active (Re) andreactive ( Im) intensity of the sound field.Reactive intensity : J pu 1 / 2 Im puIf the reactivity takes a HIGH value there is notintensity produced, the sound source is only pushingair back and forward. In this case a small phasemismatch between P and U sensor can produce anerror: ˆ J I pu I 1 e I1 e tan field IThis is due to happen fat the vicinity of the soundsource at low frequencies.This effect can be solved by the usage of the particlevelocity itself for sound localization purposes. 47

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Calibration errors of P-U probes Measurements show that a phase matching of 1 degree is possible with a calibration based on a short standing wave tube method or the piston on a sphere method. The enhanced calibration based on the sound power ratio technique a phase matching error of 0.15 degrees can be obtainedOne can state that if the measured phase of the sound field is less than 80 degrees(less than 7dB of reactivity index), a calibrated P-U probe has a measurement errorless than 0.5dB 48

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Properties of P-U probes Advantages • Small size. Point measurement • Usable for near field measurement • Broadband solution • Usable in reverberant conditions • Pressure and Velocity measured almost in same point ( non distributed sensor). Disadvantages • Response decreases with frequency • Sensitive to DC flow • Accuracy is dependent into the reactivity index 49

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PU and PP performance comparisson

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Experimental resultsSound intensity measurements of a broadband noise source using a P-Uprobe (red) and a P-P probe (blue) 51

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PP and PU intensity measurements 52

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PP and PU intensity measurements 53

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PP and PU intensity measurements Difference because of area assigned in each method 54

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Sound power measured at two surfacesExpected 10,6 dB difference because of Dipole sound sourcedimensions 1 dB deviation because of bad location of PP probe while measuring 55

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Sound power measured at two surfaces Very reactive worst scenario for PU probe After correction of phase mismatch of PU, intensity graphs coincide 56

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PU probes calibration method

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Piston on a sphereAs there is not a reference particle velocity sensor the principle is to insert thepressure and velocity sensors into a known sound field in which P and U arerelated by the known acoustic impedance ( Z). Problem: this is not possible for all frequencies, low frequencies: • Lower loudspeaker radiation • Spherical waves Solution: 3 step method • Step 1: High frequencies • Step 2: Low frequencies Step 3: Combination Applying the 3 steps the calibration is usable for 20-20KHz 58

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Step 1: High frequency calibration• A known sound field is generated• Known relation between U-P via Z ( Z= P/U)• Usable for 100-20.000 Hz. 59

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Step 2: Low frequencies• Loudspeaker cannot radiate as much energy as in high frequencies  Backgound noise too much influence• Different method: • Pref is inserted IN the sphere • U is located next to the membrane • Known noise field generated• From the relation of the difference in pressure inside the sphee and the movement of the membrane, is obtained the response.• Usable until first mode of sphere• The phase is obtaine dbut the results magnitud is not determined. Need of Step 3 60

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Step 3: Combination 1 and 2• Not known magnitude of calibration at low frequencies because of lack of: • Vo: exact sphere volume • Ao: piston area • R: exact distance to membrane Step 1 and 2 overlay 61

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ResultResult: non flat response of the sensor. Needs to be equalized via Signalconditioner + 62

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3D intensity

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3D sound intensity probes 64

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3D sound intensity probes 100HzSound intensity streamlines of loudspeakers vibrating in phase (left) and vibrating in anti-phase (right). 65

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3D sound intensity probes 500HzSound intensity streamlines of loudspeakers vibrating in phase (left) and vibrating in anti-phase (right). 66

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3D sound intensity probes Sound intensity streamlines of a loudspeaker driven close to a metal plate. 67

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3D sound intensity probesNoise mapping 68

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3D sound intensity probesEnergy characterization and difussion 69

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Measurement Techniques

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Theoretical approach

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Type of noises Noise Deterministic Non deterministic Periodic Non periodic Random Transient Complex Non-Sinusoidal Stationary periodic Stationary Ergodic Non-Ergodic 72

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Deterministic noise• Deterministic: a signal whos values can be predicted from current or past information – Numerical: denoted by a number or colletion of numbers – Analytic: denoted by an equation which defines the process. 73

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Non deterministic noise• Random / Stochastic process: a function usually of time, that takes on a definite wave form each time a chance experiment is performed that cannot be predicted in advance.• DEF 2: a family of time dependent signals for which the value at a specific time may be regarded as a random variable.– Stationarity: invariance of stadistical properties with respect to the time origin. • Narrow band process: stationary process in which significant samples are limited to a slim band of frequencies in relation with a central frequency of the band. – Color noises: narrow band processes which energetic content and statistical properties are distributed in a certain manner • Wide band process: stationary process which significant values appear in a range proportional of the magnitud of the central frequency of the band. 74

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Color noiseWhite noise is a signal/process with a flat spectrum. The signal has equal power in any band of a given bandwith.Grey noise: is random white noise subjected to a psychoacoustic equal loudness curve over a given range of frequencies, giving the listener the perception that it is equally loud at all frequenciesPink noise: the frequency spectrum is linear in logarithmic space, it has equal power in bands that are proportionally wide.Brown noise: stationary random signal whos power spectrum falls of at a constant rate of 6 dB per octaveViolet noise: Violet noises power density increases 6 dB per octave with increasing frequency(density proportional to f 2) over a finite frequency rangeBlue noise: Blue noises power density increases 3 dB per octave with increasing frequency (density proportional to f ) over a finite frequency range 75

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Transient noiseImpulse: unwanted, almost instantaneous (thus impulse-like) sharp soundsBurst noise : sudden step-like transitions between two or more discrete levelsSweep noise: a signal, commonly of constant amplitude, that locally resembles a sine wave but whose instantaneous frequency changes with timeChirp noise: rapid frequency sweep signal 76

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Measurement techniques

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Conventional measurement techniques

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Point by point measurementsSuitable for: – Stationary noiseMeasurement process: – Definition of an imaginary measurement plane. – Definition of a grid on the plane – In every grid position perform a measurement for every noise component to be characterizedResult: – Vector per grid point. 79

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Traditional scanning techniqueSuitable for: – Stationary noiseMeasurement process: – Definition of an imaginary measurement plane. – Scanning of the whole interest areaResult: – Single intensity value per area promediated value 80

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Simultaneous measurement• Suitable for: – Stationary noise – Transient noise• Measurement process: – Allocation of sensors/ array deployment – Audio capture of several channels – Direct measurement, no signal processing• Result: – Color maps of noise distribution in time 81

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Advanced measurement methods

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New scanning techniques: Scan&PaintSuitable for: – Stationary noiseMeasurement process: – Definition of an imaginary measurement plane. – Scanning of the whole interest area – Automatic post process assigning location of probe- audio measurementResult: – Color map of various indexes – Spectrograms of every located measurement point – Global index to characterize an area 83

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Intensity based sound source localizationSuitable for: – Any noiseMeasurement process: – Sensors allocation – Simple signal processingResult: – DOA: direction of arrival of noise – Spectrograms of each 3D directions – Global and narrow band levelsLimitations: ₋ Free field assumptions for simple algorithm ₋ Increase number of sensors to detect coherent noise sources 84

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Conventional beamformingSuitable for: – Stationary noise – Transient noiseMeasurement process: – Definition: Signal processing techniqued ised in arrays for directional signal transmission . This directional information is obtained by combining elements in the array – Allocation of sensors/ array deployment – Audio capture of several channels – Beam forming signal processingResult: – Color maps of noise distributionLimitations ― Frequency limitations by spacing and array size ― High cost 85

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HolographySuitable for: – Stationary noiseMeasurement process: – Definition: Method to estimate the sound field near a source by measuring acoustic parameters away from the source via an array of pressure and/or particle velocity transducers. – Processing after acquiring information from arrayResult: – Color map of the interest areaLimitations: — Frequency limitations because of spacing and array dimension — Assumes free field — Regular grid — Heavy calculations — High cost 86

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Airborne transfer path measurementsSuitable for: – Stationary noiseMeasurement process: – Combination of the characterization of a noise source with the propagation path to the listener in order to 𝑦 obtain information about the contribution of a specific noise in the whole perceive sound pressure level S 𝑥 – Measurements divided in two steps: source and transfer path characterizationResult: – Noise source listener rankingLimitations: — High cost — Typically measured in reverberant environments — Surface noise source detected not structural problems 87

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Virtual arrays beamformingSuitable for: – Stationary noiseMeasurement process: – Deffinition of an imaginary measurement plane. – Scanning of the whole interest area – Measurement of two reference positions – Automatic post process assigning location of probe- audio measurementResult: – Color map of various indexes – Spectrograms of every located measurement point – Global index to characterize an area and noise source locationLimitations: – Size and distance – Heavy calculations 88

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Advanced measurement techniques: Scan & Paint

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Theoretical approach

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Scan&Paint principleThe PU probe is moved along the virtual plane while the movement is recorded by the video camera.The location of each measured position is extracted from the video and synchronized with the 2 audio channels. 91

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Scan&Paint principlePressure Velocity 92

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Scan&Paint principle: post-processingTwo methods to cover the fullfrequency range: - Velocity method (for lowfrequencies) - Intensity method (for highfrequencies) 93

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Low frequenciesIn the near field of the surface the particle velocity is equal to the surfacevelocity. The influence to background noise is low.At higher frequencies the velocity method fails because: • The area of consistent velocity is too small. There are many modes in the material which require many measurement points • The sensor is not in the near field any more High frequencies At high frequencies the sensor is not in the (very) near field any more and the intensity is used. There are no P-I index problems like with P-P intensity probes At low frequencies the intensity method fails because the sound source is too reactive 94

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Measurement procedure

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Measurement procedure 96

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Measurement examples

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Scan & PaintExample 1: Large gas turbine enclosureThere are big stationary engines ( used for Heat & Power )The goal was to measure the performance of the special designed enclosures.Specially regarding acoustic leakages. With Scan & Paint we could perform themeasurement on a large surface in short time period in highly reverberantconditions. 98

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Scan & Paint Example 1: Large gas turbine enclosureSelection of measurement points on the backside of the housing 99

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Scan & PaintExample 1: Large gas turbine enclosureVelocity map at 65Hz 100

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Scan & PaintExample 1: Large gas turbine enclosure Low frequency High frequency 101

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Scan & PaintExample 2: Building acoustics 102

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Scan & PaintExample 3: Leak detection in buildings Studying the spectra of different areas allows to produce narrow band maps focused on detecting weaknesses This technique is suitable for localizing acoustic leakage with a very high spatial resolution in a clear and easy way 103

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Scan & PaintExample 4: Automotive | Comparison test of compo-nents in a windtunnel See the effect of the noise due to windflow related to the interior noise when using different type of components or make adjustment to the components used on the outside of a car like a mirror or window wiper. The test are performed with the car in a windtunnel when using Scan & Paint to map the effect to the noise in the interior inside the car. 104

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Scan & PaintExample 4: Automotive | Comparison test of compo-nents in a windtunnelLeft the velocity map of the standard wiper and right the velocity map ofthe wiper with adjustments made. 105

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Scan & PaintExample 4: Automotive | Comparison test of compo-nents in a windtunnelLeft the velocity map of the car without rearview-window and right thevelocity map of the car with the rearview-window. 106

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Scan & PaintExample 5: Automotive | Optimization material package To see where to place absorbing materials effectively and measure the effect after installing the materials. First the door was measured without materials and secondly with materials placed based on the first measurement. A sound source is positioned in the interior and with pink noise as excitation. 107

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Scan & PaintExample 5: Automotive | Optimization material package Door | no damping Door | with damping 108

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Scan & Paint Example 6: Automotive NVH| Component optimization The opening of the ventilation system of the dashboard show important acoustic leakagesThe shell radiation of an intake systemis measured on the test bench excitingthe plastic filter with white noise froma loudspeaker 109

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Scan & PaintExample 6: Automotive NVH| Component optimizationA volume super-charger show the crankfrequency emission from the aluminumcase. The intake system radiation at lower frequency in engine running condition can be optimized 110

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Scan & PaintExample 7: Automotive NVH| Sound source localization Exterior noise of a car. The colormap show the engine radiation through the weak areas.The front part of the engine withoutcover show high velocity emission. 111

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Scan & PaintExample 8: Electronic / white consumer goods Optimize the noise performance of a washing machine. Localize the hotspots suggest and adapt changes and compare result. Overall result of this case: 4dB lower Sound Power ( measured by the standard sound pressure method ) 112

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Scan & PaintExample 8: Electronic / white consumer goods Dominant source200Hz 113

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Scan & Paint 0.5Kg damping72dB PVL 68dB PVL 114

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Scan & PaintExample 9: Electronic goods | Commercial printer Optimize the noise performance of a printers developed for offices. Localize the origin of the noise problem. A mode is created in the backplate. This mode made the printer very noisy but the origin causing the mode was needed to be localized. 115

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Scan & PaintExample 9: Electronic goods | Commercial printer With Scan & Paint the gear wheel that was causing structure borne noise ( the created mode in the backplate). The amount of teeth, the material of the gear wheel or the connection with the backplate could be options to reduce this structure borne noise. 116

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Scan & PaintExample 9: Electronic goods | Commercial printer 117

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Scan & Paint Example 10: Electronic goods | Clima and Microwave With Scan & Paint low frequency noise from the cooling system (airflow)can be separated by the noise coming from the body frame of the clima.The button panel on the rightside show higher sound emissionabove 2000Hz. 118

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Scan & PaintExample 11: Ground Vehicles | High speed train | In situtransparency measurements In situ transparency measurements using Scan & Paint were performed as alternative to the traditional transmission loss measurements. Mainly to identify positions of leakages. 119

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Scan & PaintExample 11: Ground Vehicles | High speed train | In situtransparency measurements The pressure distribution (and the velocity distribution) is measured both out and inside the train, to correct the non- uniformity of the sound field as the excitation pattern (emitter side). The average velocity over the surface is calculated for both sides, and a simple formula is applied to estimate the transmission loss from the velocity or so called the transparency: 120

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Scan & Paint Example 11: Ground Vehicles | High speed train | In situ transparency measurementsOutside TGV – velocity distribution Inside TGV – velocity distribution 121

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Scan & PaintExample 11: Ground Vehicles | High speed train | In situtransparency measurements 122

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Scan & PaintExample 12: Industrial machinery | Sound source localization Industrial machinery can be tested in non-anechoic conditions. 123

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Scan & PaintExample 13: Airplane | Leakage detectionAcoustic leakage on a plane section. Thesound is generated out from the plane tosimulate the engine noise level. 124

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Scan & PaintExample 14: Airplane | In situ absorptionThe Scan&Paintabsorption showthe effect of theflame cover on aplane seat.The colormap ofabsorption canbe calculatedmeasuring withthe impedancegun. 125

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Scan & PaintExample 15: Absorption & Reflection coefficients 126