The Seismology Division of the Royal Netherlands Meteorological Institute (KNMI) has build up expertise in infrasound measurements by investigating low frequency events in order to distinguish between seismic and sonic events. KNMI operates, amongst others, a sixteen element microbarometer array with an aperture of 1.5 km, the Deelen Infrasound Array (DIA). Sonic booms and events of Comprehensive Test Ban Treaty (CTBT) interest are recorded within the frequency range of 100 seconds and 40 Hertz. Recently, KNMI and Microflown Technologies B.V. started a collaboration concerning infrasound measurements. This paper reports the use of a novel sensor. The so-called Microflown [1] is an acoustic sensor, sensitive for frequencies from 0Hz up to 1kHz. The Microflown is developed at the University of Twente and commercialised by Microflown Technologies B.V [3]

Bree, H-E. de

Evers, L.

Haak, H.W.

Koers, A.A.

English

The Seismology Division of the Royal Netherlands Meteorological Institute (KNMI) has built up expertise in infrasound measurements by investigating low frequency events in order to distinguish between seismic and sonic events. KNMI operates, amongst others, a sixteen element microbarometer array with an aperture of 1.5 km, the Deelen Infrasound Array (DIA). Sonic booms and events of Comprehensive Test Ban Treaty (CTBT) interest are detected and identified within the frequency range of 500 seconds up to 40 Hertz. Recently, KNMI and Microflown Technologies B.V. started a collaboration concerning infrasound measurements. This paper reports on the use of a novel sensor. The so-called Microflown[1] is a micro-machined particle velocity sensor, sensitive to frequencies from 0Hz up to at least 1kHz. Experiments with the Microflown for infrasound detection show promising results. 1

L. G. Evers

H-e De Bree

H. W. Haak

A. A. Koers

CiteSeerX

The Deelen Infrasound Array for Recording Sonic Booms and Events of CTBT Interest

The Seismology Division of the Royal Netherlands Meteorological Institute (KNMI) has built up expertise in infrasound measurements by investigating low frequency events in order to distinguish between seismic and sonic events. KNMI operates, amongst others, a sixteen element microbarometer array with an aperture of 1.5 km, the Deelen Infrasound Array (DIA). Sonic booms and events of Comprehensive Test Ban Treaty (CTBT) interest are detected and identified within the frequency range of 500 seconds up to 40 Hertz. Recently, KNMI and Microflown Technologies B.V. started a collaboration concerning infrasound measurements. This paper reports on the use of a novel sensor. The so-called Microflown[1] is a micro-machined particle velocity sensor, sensitive to frequencies from 0Hz up to at least 1kHz. Experiments with the Microflown for infrasound detection show promising results

Evers, L.G.

de Bree, H.E.

NARCIS 

The Deelen infrasound array for recording sonic booms and events of CTBT interest

University of Twente Research Information

The Deelen Infrasound Array for Recording Sonic Booms and Events of CTBT InterestL. G. Evers1, H-E de Bree2, H. W. Haak1 and A. A. Koers31 Royal Netherlands Meteorological Institute, Seismology Division, PO Box201, 3730 AE De Bilt, The Netherlands evers@knmi.nl2 MESA Research Institute, University of Twente, PO Box 217, 7500 AEEnschede, The Netherlands, H.E.deBree@el.utwente.nl3 Microflown Technologies B.V., PO Box 300, 6900 AH Zevenaar, TheNetherlands, www.Microflown.com, Koers@Microflown.comReceived 25 November 2000ABSTRACTThe Seismology Division of the Royal Netherlands Meteorological Institute(KNMI) has built up expertise in infrasound measurements by investigating lowfrequency events in order to distinguish between seismic and sonic events.KNMI operates, amongst others, a sixteen element microbarometer array withan aperture of 1.5 km, the Deelen Infrasound Array (DIA). Sonic booms andevents of Comprehensive Test Ban Treaty (CTBT) interest are detected andidentified within the frequency range of 500 seconds up to 40 Hertz. Recently,KNMI and Microflown Technologies B.V. started a collaboration concerninginfrasound measurements. This paper reports on the use of a novel sensor. Theso-called Microflown[1] is a micro-machined particle velocity sensor, sensitiveto frequencies from 0Hz up to at least 1kHz. Experiments with the Microflownfor infrasound detection show promising results.1. INTRODUCTIONRecently, the significance of infrasound measurements has been established inthe Comprehensive Test Ban Treaty (CTBT) as a technique to detect andidentify possible nuclear explosions. For this purpose a world-wide network of60 infrasound arrays is presently being constructed. KNMI has, since l999,operated an experimental array in the Netherlands. The Deelen InfrasoundArray (DIA) consists of in-house developed micro-barometers, based on adifferential pressure sensor. Detailed array response calculations have resultedin an omni-directional sensitive array configuration. Efficient discriminationbetween infrasound events and noise, is done by a detector based on Fisherstatistics. Characteristic values like apparent sound velocity and azimuth, canbe derived. Including the data of two other small arrays (aperture of 100 meters)results in an accurate event location, through cross-bearing, and origin time.Pressure variations consist of compression and dilatation with a certainfrequency. The frequency is resolved by the Microflown through the frequencyof the induced temperature differences. The Microflown has advantages, whichare tested together with KNMI for infrasound applications, compared to micro-barometers and pressure microphones. The Microflown has no moving parts,which make it very reliable. Resonances do not occur. Given its underlyingJournal of Low Frequency Noise, 123Vibration and Active ControlVol. 19 No. 3 2000thermal principle, the self drying Microflown is moisture resistant and it ismade from inert materials (platinum, silicon) so no corrosion problems can beexpected. All materials used tolerate high temperatures. The Microflown wasdeveloped at the University of Twente and commercialised by MicroflownTechnologies B.V.[3]2. THE DEELEN INFRASOUND ARRAYIn general, an array consists of a number of instruments which is, through itslayout, able to detect signals and localise the incoming direction of energy. Thearray configuration controls the resolution of the array. An optimal array isequally sensitive to all infrasonic signals, independent of incoming angle anddirection. Array design and calculations are based on signal coherency. Theoptimal array is capable of homogeneously  sampling the surroundingatmosphere.[4] Figure 1 displays the layout of the 16 micro-barometers andcorresponding response. To each micro-barometer six porous hoses areconnected in star configuration to reduce noise. The circular response impliesan optimal array.THE DEELEN INFRASOUND ARRAY FOR RECORDING SONIC BOOMS124Figure 1. Deelen Infrasound Array (DIA), instrument lay-out on the left andarray response on the rightFigure 2a. Examples of infrasonic signals. Sonic boom3. INFRASONIC SIGNALSSources of infrasound emit pressure fluctuations between 500s and 40Hz.Examples of infrasonic sources are: planes flying through the sound barrier,meteors entering the earth’s atmosphere, volcanic explosions, nuclearexplosions etc. Wind causes noise within the same frequency band, between 1and 10 Hz. Figure 2 shows two examples of infrasound recorded by the 16micro-barometers, a high frequency sonic boom (Figure 2a) and a lowerfrequency meteor detection (Figure 2b). Two packages of incoming energy canbe distinguished in the meteor recording. The delay time of the secondaryarrival of 50 seconds and its lower frequency contents (with respect to the firstarrival) indicate a probable thermospheric reflection.[6]4. DATA PROCESSING: FREQUENCY SLOWNESS ANALYSISAs proposed by[5], data processing at KNMI is done on the basis of a frequencyslowness, being the inverse of apparent sound velocity, analysis combined withFisher statistics. Frequency domain processing, instead of conventional timedomain, enables the development of detection tools by making use of phase,amplitude, frequency and coherency characteristics. Fisher values are ameasure for the signal coherency. An infrasonic wave which travels over thearray coherently will add up to high Fisher values. Uncorrelated signals, likewind, will result in low Fisher values, typically between 0 and 4.Mathematically the Fisher value represents a signal to noise ratio scaled withthe number of instruments minus one.Figure 3 shows the result of the frequency domain processing. All frameshave the same time axis. The top frame displays the best beam or the sum of allsignals.The lower three frames represent three parameters each. Frequency andtime axes are the same for all frames, but in each frame a third parameter isplotted (as a grey coloured contour). Coherency is plotted in the middle frameas Fisher value. Resolved event characteristic: azimuth and velocity are plottedTHE DEELEN INFRASOUND ARRAY FOR RECORDING SONIC BOOMS125Figure 2b. Examples of infrasonic signals. Meteor recordingin the lower two frames. Azimuth is defined as the angle from which the energycomes with respect to the North. The resolved velocity is the apparent soundvelocity or the projection of the sound velocity on the horizontal.As follows from the Fisher values, coherent energy is present throughout therecording between 0 and 3 Hz. This can be seen as the presence of light to darkgrey colours between 0 and 3 Hz. The major arrival after 85 seconds results inthe highest Fisher values of 160, due to its high amplitude and preservedwaveform while travelling over the array.The coherency of the signal is also reflected when evaluating the incomingangle of the energy. The band of light grey colours between 0 and 3 Hz roughlyindicate that the source is located to the North- west of the array, i.e. an azimuthof 330 degrees.The apparent velocity in the lower frame seems less well resolved. A mix ofall grey colours appears between 0 and 3 Hz. The lobe of light grey between 2and 3 Hz around 85 seconds correspond to reasonable apparent sound velocitiesTHE DEELEN INFRASOUND ARRAY FOR RECORDING SONIC BOOMS126Figure 3. Best beam (top frame), coherency or Fisher value (middle frame)and resolved angle or azimuth and apparent sound velocity in thelower two frames for the sonic boom (signals displayed left inFigure 2)of 350 m/s. To obtain exact values for the apparent sound velocity and azimuth,a detailed analysis is done.Results from detailed frequency slowness analyses of the sonic boom, aredisplayed in Figure 4. Frequency slowness power can be interpreted as a shiftedarray response due to the phase differences of the signal travelling over thearray. The highest values of the fp-power correspond to the value for slownessfor which the signal characteristics are best resolved. In Figure 4 the maximumcoherent energy around 0.5 Hz at 85 seconds is exactly located. The whiteslowness vector resolves an apparent sound velocity of 350 m/s, being itslength. The source is located to the North-west of the array by the angle withrespect to the North of the slowness vector, being 324 degrees.5. INSTRUMENTS5.1 The KNMI Micro-barometerWhen developing an infrasound sensor one can either choose to make a lowfrequency microphone or a barograph high frequency. KNMI choose the laterapproach for amongst others robustness with respect to field applications.Figure 5 shows the in-house developed micro-barometer based on a differentialpressure sensor. The pressure fluctuations are measured with respect to thebacking volume. Doing so, one would also measure very low frequencymeteorological pressure variations. Therefore, a thin capillary is includedwithin the backing volume, as a leak. Through its acoustical resistance, thecapillary controls the low frequency cut-off of the micro-barometer, this being500 seconds.5.2 The MicroflownThe Microflown is a silicon-based sensor that is fabricated in a cleanroom. It isin fact a highly sensitive mass flow sensor (a sensor that is designed to detectDC flow) made in such a way that it has a very fast response time. The result isTHE DEELEN INFRASOUND ARRAY FOR RECORDING SONIC BOOMS127Figure 4. Frequency-slowness power plots. The white slowness vectordenotes the resolved apparent sound velocity of 350 m/s by itslength and resolved azimuth of 324 degrees by its angle withrespect to the Northan acoustic sensor that is capable of measuring the particle velocity from DC(0Hz) up to 1kHz with a flat frequency response and with a high signal to noiseratio. The Microflown has a decay in response for frequencies higher than1kHz. The polar pattern or directivity is a figure of eight at all frequencies.Since its invention in 1994 it has mostly been used for measurementpurposes (1D and 3D-sound intensity measurement, or acoustic impedance).The Microflown is also used for measuring DC flows. DC flow is in factparticle velocity with a frequency of 0Hz. Nowadays sound-energydetermination and three-dimensional impulse response are underinvestigation. [2],[3] The Microflown is capable of measuring particle velocityinstead of sound pressure, closely related to the pressure gradient. So in anaudio perspective the Microflown can be seen as a pressure gradientmicrophone (with a figure-of-eight directivity pattern) having a high signal tonoise ratio from 0Hz up to 1kHz. The Microflown itself consists of two veryclosely spaced thin wires (spacing 350 m m) of silicon nitride with an electricallyconducting platinum pattern on top of them. A SEM photograph of aMicroflown is depicted in Figure 6. The size of the two wires is 1000x10x0.5micrometer (lxwxh). The metal pattern is used as temperature sensor andheater. The silicon nitride layer is used as a mechanical carrier for the platinumresistor patterns. The sensors are powered by an electrical current, causing thesensors to heat up. The temperature difference of the two cantilevers is linearlydependent on the particle velocity. The two squares S1 and S2 in Figure 7represent the two temperature sensors of the Microflown. The temperaturesensors are implemented as platinum resistors and powered by an electricalcurrent dissipating an electrical power Pel, causing it to heat-up, leading to atypical operational temperature of about 200°C to 400°C. When thetemperature of the sensors increases the resistance will also increase. WhenTHE DEELEN INFRASOUND ARRAY FOR RECORDING SONIC BOOMSFigure 5. Schematic drawing of the KNMI micro-barometer based on adifferential pressure sensor128particle velocity is present, it alters the temperature distribution around theresistors. The temperature difference of the two sensors quantifies the particlevelocity. When no particle velocity is present all the heat is transferred in thesurrounding air (qstat.). When particle velocity is present a convective heattransfer of both sensors (qconv1&2) will cause a temperature drop of both sensors.The upstream sensor however, will drop more in temperature than thedownstream sensor since the downstream sensor is heated by the upstreamconvective heat loss (qconv.), see Figure 7. A temperature difference will be theresult. The temperature difference is proportional to the particle velocity. Notall the convective heat loss of S1 will be transferred to S2, a certain percentage( j ) will be lost. This percentage will rise if the sensors are positioned furtherapart from each other. If, on the other hand, the sensors are brought closertogether another phenomenon will become dominant. The particle velocityinduced temperature difference will cause a conductive heat flow in theopposite direction. This feedback heat flow will temper the sensitivity. Thecloser the sensors are placed, the more conductive heat flow will take its effect.Several temperatures of the two sensors of the Microflown are shown in Figure8. Due to the thermal mass of the sensors and diffusion effects, after 1kHz thesensors cannot follow the thermal signal. The Microflown exhibits a highfrequency roll-off because of this.THE DEELEN INFRASOUND ARRAY FOR RECORDING SONIC BOOMS129Figure 6. Photo of a part of a MicroflownFigure 7. Schematic overview of the heat flows around a Microflown6. MEASUREMENTS6.1 The Microflown without a (wind) noise reducing mountingWith the use of a 12” loudspeaker a 20Hz sine tone was generated. Signals andnoise levels of the micro-barometer and the Microflown were compared. Thedistance between the loudspeaker and the transducers was 50cm. Furthermorethe on- and off axis output of the Microflown was measured. The output signalsof the Microflown and the micro-barometer were respectively 85mV and20mV, their noise levels about 0.2mV and 2mV. A rough estimation would bethat the signal to noise ratio of the Microflown is about 30dB higher than themicro-barometer. The lateral reduction at 20Hz was measured at 40dB. Themeasurement was performed by rotating the Microflown in a normal room.6.2 The Microflown with a (wind) noise reducing mountingAt one input of the Microflown a closed rigid tube was mounted and at the othera flexible tube was mounted. A sound pressure wave would deform the flexibletube and not the rigid one, inducing a particle velocity inside the mounting. Thisway of reducing noise will make the set up sensitive to sound pressure insteadof particle velocity; the directivity properties of the Microflown will thereforebe lost. The rigid tube will act as backing volume. We used a volume of 0.61.We expect the system to measure lower frequencies if the backing volume isenlarged. Opening the door of the lab resulted in signals shown in Figure 9.Despite the difference in transducing, the general trend of the signal is similar.The “noise” seen on the Microflown is due to electronic interference, it is notselfnoise. The flexible tube is used as a pressure to particle displacementconverter. We expect a higher sensitivity and a better noise reduction when thelength of the tube is increased.THE DEELEN INFRASOUND ARRAY FOR RECORDING SONIC BOOMS130Figure 8. The (measured) temperatures of the Microflown as the result of aparticle velocity wave. A particle velocity wave will cool downboth sensors in a different manner. The difference signal of bothsensors represents the particle velocity and the sum (the commonsignal) the cooling down6.3 Microflown and microphone experiments in the fieldThe Microflown has been tested in the field. For this purpose, the Microflownwas incorporated in an existing array of six microphones (figure 10). Windreduction for the Microflown was achieved with foam.Figure 11 shows the performance of the different instruments. A sonic boomcoherently travelled over the array. Although exactly locating the energy is notthe purpose of this experiment, clearly the energy is coming from the North-west since the Microflown and microphone number 1 were the first to detect thesignal. The highly sensitive Microflown, measuring particle velocity, shows agood performance with respect to the microphones. The Microflown traceseems more noisy, this is mainly due to the primitive wind reduction. Currently,research concerns the construction of a robust instrument and noise reductionfor durable operation in the field.THE DEELEN INFRASOUND ARRAY FOR RECORDING SONIC BOOMS131Figure 9. The measured signals of a micro-barometer (A) and a Microflown(B) as result of an atmospheric distortion in a room with a volumeof 144m3Figure 10. Microflown within a six element microphone array7. CONCLUSIONSThe Deelen Infrasound Array is capable of detecting and localising infrasound.Identification of sources of infrasound is achieved by using frequency-slownessanalysis combined with Fisher statistics. Experiments with the Microflownhave shown promising results, as a low noise, directional and particle velocitysensor. Proven durability in the field, developments to obtain a proper lowfrequency cut-off and wind noise reduction will allow the Microflown to be ahighly competitive infrasound sensor.REFERENCES1 H-E. de Bree et al., The Microflown: a novel device measuring acousticalflows, Sensors and Actuators A 54, 1996, pp. 552–557.2 W. F. Druyvesteyn et al., A new acoustic measurement probe; theMicroflown IOA London, volume 21, pt3 1999, pp. 139–149.3 www.microflown.com4 H. W. Haak, An acoustical array for subsonic signals, Tech. Rep. WR96-03, KNMI, De Bilt, the Netherlands, 1996, 22 pp.THE DEELEN INFRASOUND ARRAY FOR RECORDING SONIC BOOMS132Figure 11. Recording at approximately 200 km distance of a plane flyingthrough the sound barrier above the North Sea5 E. Smart and E. A. Flinn, Fast frequency-wavenumber analysis andFisher signal detection in real-time infrasonic array data processing,Geoph. J. R. astr. Soc., volume 26, 1971, pp. 279–284.6 Garcés et al., Traveltimes for infrasonic waves propagating in a stratifiedatmosphere. Geoph. J. Int. Volume 135, 1998, pp. 255–263.THE DEELEN INFRASOUND ARRAY FOR RECORDING SONIC BOOMS133

THE DEELEN INFRASOUND ARRAY FOR RECORDING SONIC BOOMSAND EVENTS OF CTBT INTERESTL.G. Evers*, H-E de Bree**, H.W. Haak*, A.A Koers****Royal Netherlands Meteorological Institute, Seismology Division, PO Box 201, 3730 AE De Bilt, The Netherlands evers@knmi.nl**MESA Research Institute, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands, H.E.deBree@el.utwente.nl***Microflown Technologies B.V., PO Box 300, 6900 DH Zevenaar, The Netherlands, www.Microflown.com, Koers@Microflown.com1. ABSTRACTThe Seismology Division of the Royal Netherlands Meteorological Institute (KNMI) has build upexpertise in infrasound measurements by investigating low frequency events in order to distinguishbetween seismic and sonic events. KNMI operates, amongst others, a sixteen element microbarometerarray with an aperture of 1.5 km, the Deelen Infrasound Array (DIA). Sonic booms and events ofComprehensive Test Ban Treaty (CTBT) interest are recorded within the frequency range of 100 secondsand 40 Hertz. Recently, KNMI and Microflown Technologies B.V. started a collaboration concerninginfrasound measurements. This paper reports the use of a novel sensor. The so-called Microflown [1] isan acoustic sensor, sensitive for frequencies from 0Hz up to 1kHz. The Microflown is developed at theUniversity of Twente and commercialised by Microflown Technologies B.V [3].2. INTRODUCTIONRecently, the significance of infrasound measurements has been established in the ComprehensiveTest Ban Treaty (CTBT) as a technique to detect and identify possible nuclear explosions. For thispurpose a world-wide network of 60 infrasound arrays is presently being constructed. KNMI operatessince 1999 an experimental array in the Netherlands. The Deelen Infrasound Array (DIA) consist of inhouse developed microbarometers, based on a differential pressure sensor. Detailed array responsecalculations have resulted in a omni-directional sensitive array configuration. Efficiently discriminatingbetween infrasound events and noise, is done with a detector based on Fisher statistics. Characteristicvalues like apparent sound velocity and azimuth, can be derived. Including the data of two other smallarrays (aperture of 100 meters) results in an accurate event location, through cross-bearing, and origintime.Pressure variations consist of compression and dilatation with a certain frequency. The frequency isresolved by the Microflown through the frequency of the induced temperature differences. TheMicroflown has advantages, which are tested together with KNMI for infrasound applications, comparedto microbarometers and pressure microphones. The Microflown has no moving parts, which make it veryreliable. Resonances do not occur. Given its underlying thermal principle, the self drying Microflown ismoisture resistant and it is made from inert materials (platinum, silicon) so no corrosion problems canoccur. All materials used are corrosion resistant and sustain high temperatures.3. THE DEELEN INFRASOUND ARRAYIn general, an array is a number of instruments which is, through its layout, able to detect signals andlocalise the incoming direction of energy. The array configuration controls the resolution of the array. Anoptimal array is equally sensitive to all infrasonic signals, independent of incoming angle and direction.Array design and calculations are based on signal coherency. The optimal array is capable ofhomogeneously sampling the surrounding atmosphere [4]. Figure 1 displays the layout of the 16microbarometers and corresponding response. To each microbarometer six porous hoses are connected instar configuration to reduce noise. The circular response implies an optimal array.Figure 1: DIA array layout (left) and corresponding response (right)4. INFRASONIC SIGNALSSources of infrasound emit pressure fluctuations between 500s and 40Hz. Examples of infrasonicsources are: planes flying through the sound barrier, meteors entering the earth’s atmosphere, volcanoexplosions, nuclear explosions etc. Wind causes noise within the same frequency band, between 1 up to10 Hz. Figure 2 shows two examples of infrasound record by the 16 microbarometers. A high frequentsonic boom on the left and a low frequent meteor detection ant the right.Figure 2: Examples of infrasound, sonic boom (left) and a meteor(right)5. DATA PROCESSING: FREQUENCY SLOWNESS ANALYSISAs proposed by [5], data processing at KNMI is done on the basis of a frequency slowness, being theinverse of apparent sound velocity, analysis combined with Fisher statistics. Frequency domainprocessing, in stead of conventional time domain, enables the development of detection tools by makinguse phase, amplitude, frequency and coherency characteristics. Figure 3 shows the result of the frequencydomain processing. The top frame displays the best beam or the summed signal with azimuth andvelocity for which the maximum coherency is found. Coherency is plotted in the middle frame as Fishervalue (which is a scaled signal to noise ratio). Clearly, coherence increases around the meteor detection.The secondary arrival is more low frequent, combined with its delay time with respect to the first arrival,a probable thermospheric reflection is seen [6]. Resolved angles are plotted in the lower frame. Themeteor energy is coming from the North-east while other coherent signal seems to come form moreNorth-western angles.Figure 3: Best beam (top frame), coherency or Fisher value (middle frame) and resolved angle (bottom frame) for the meteorsignal (right in Figure 2)Results from detailed frequency slowness analyses of the two sources of infrasound shown in Figure3, are displayed Figure 4. Frequency slowness power can be interpreted as a shifted array response due tothe phase differences of the signal travelling over the array. The highest values of the fp-powercorrespond to the value for slowness for which the signal is best resolved. The meteor energy in the rightframe is coming from 82 deg East. A storm depression on the Atlantic ocean generated standing waves inthe ocean. These standing waves create low frequent signal, so-called microbaroms, through itsatmospheric coupling. This energy comes from the North-west with respect to the array and is shown inthe left frame of Figure 4.Figure 4: Frequency-slowness power plots. The vector denotes the resolved apparent sound velocity by its length andresolved azimuth by its angle with respect to the North6. INSTRUMENTS6.1 The KNMI microbarometerWhen developing an infrasound sensor one can either choose to make a microphone low frequent or abarograph high frequent. KNMI choose the later approach for amongst others robustness with respect tothe field applications. Figure 5 shows the in house developed microbarometer based on a differentialpressure sensor. The pressure fluctuations are measured with respect to the backing volume. Doing so,one would also measure very low frequent meteorological pressure variations. Therefore, a thin capillaryis included within the backing volume, as a leak. Through its acoustical resistance, the capillary controlsthe low frequency cut-off of the microbaromter, being 500 seconds.Figure 5: Schematic drawing of the KNMI microbarometer based on a differential pressure sensor6.2 The MicroflownThe Microflown is a silicon-based sensor that is fabricated in a cleanroom. It is in fact a highlysensitive mass flow sensor (a sensor that is designed to detect DC flow) made in such way that it has avery fast response time. The result is an acoustic sensor that is capable of measuring the particle velocityfrom DC (0Hz) up to 1kHz with a flat frequency response and with a high signal to noise ratio. The polarpattern or directivity is a figure of eight at all frequencies.Since its invention in 1994 it is mostly used for measurement purposes (1D and 3D-sound intensitymeasurement, or acoustic impedance. The Microflown is also used for measuring DC flows. DC flow isin fact particle velocity with a frequency of 0Hz. Nowadays sound-energy determination and three-dimensional impulse response are under investigation [2], [3]. The Microflown is capable of measuringparticle velocity in stead of sound pressure, closely related to the pressure gradient. So in an audioperspective the Microflown can be seen as a pressure gradient microphone (with a figure-of-eightdirectivity pattern) having a high signal to noise ratio from 0Hz up to 1kHz. For frequencies higher than1kHz the frequency response has a decay of –6dB/oct. The Microflown itself consists of two very closelyspaced thin wires (spacing 350µm) of silicon nitride with an electrically conducting platinum pattern ontop of them. A SEM photograph of a Microflown is depicted in Figure 6. The size of the two wires is1000×10×0.5 micrometer (l×w×h). The metal pattern is used as temperature sensor and heater. Thesilicon nitride layer is used as a mechanical carrier for the platinum resistor patterns. The sensors arepowered by an electrical current, causing the sensors to heat up. The temperature difference of the twocantilevers is linear dependent on the particle velocity. The two squares S1 and S2 in Figure 7 representthe two temperature sensors of the Microflown. The temperature sensors are implemented as platinumresistors and powered by an electrical current dissipating an electrical power Pel, causing it to heat-up,leading to a typical operational temperature of about 200ºC to 400ºC. When the temperature of thesensors increases the resistance will also increase. When particle velocity is present, it alters thetemperature distribution around the resistors. The temperature difference of the two sensors quantifies theparticle velocity. When no particle velocity is present all the heat is transferred in the surrounding air(qstat.). When particle velocity is present a convective heat transfer of both sensors (qconv1&2) will cause atemperature drop of both sensors. The upstream sensor however, will drop more in temperature than thedownstream sensor since the downstream sensor is heated by the upstream convective heat loss (qconv.),see Figure 7. A temperature difference will be the result. The temperature difference is proportional to theparticle velocity. Not all the convective heat loss of S1 will be transferred to S2, a certain percentage (ξ)will be lost. This percentage will rise if the sensors are positioned further apart from each other. If, on theother hand, the sensors are brought closer together another phenomenon will become dominant. Theparticle velocity induced temperature difference will cause a conductive heat flow in the oppositedirection. This feedback heat flow will temper the sensitivity. The closer the sensors are placed, the moreconductive heat flow will take its effect. Several temperatures of the two sensors of the Microflown areshown in Figure 8. Due to the thermal mass of the sensors, after 1kHz the sensors cannot follow thethermal signal. The Microflown exhibits a –6dB/oct high frequency roll of because of this.Figure 6: A Photo of a part of a Microflown..Figure 7: Schematic overview of the heat flows around a Microflown.Figure 8: The (measured) temperatures of the Microflown as result of a particle velocity wave. A particle velocity wave willcool down both sensors in a different manner. The difference signal of both sensors represents the particle velocityand the sum (the common signal) the cooling down.7. MEASUREMENTS7.1 The Microflown without a (wind) noise reducing mountingWith the use of a 12” loudspeaker a 20Hz sine tone was generated. Signals and noise levels of themicrobarometer and the Microflown were compared. The distance between the loudspeaker and  thetransducers was 50cm. Furthermore the on- and off axis output of the Microflown was measured. Theoutput signals of the Microflown and the microbarometer were respectively 85mV and 20mV, their noiselevels about 0.2mV and 2mV. A rough estimation would be that that the signal to noise ratio of theMicroflown is about 30dB higher than the microbarometer. The lateral reduction at 20Hz was measured40dB. The measurement was performed by rotating the Microflown and in a normal room.7.2 The Microflown with a (wind) noise reducing mountingThe Microflown is a particle velocity sensitive microphone. It is therefore very sensitive for wind(even more than pressure sensors). Two ways of reducing wind have been tested. First we mounted foamin front of both inputs of the Microflown. This resulted in a reasonable reduction of the wind noise for anopen structure foam. A more dense foam reduced the wind noise more but also damped the sensitivity.The Microflown was tested outdoors overnight under rather windy and rainy conditions. Although theMicroflown itself showed no physical damage, the noise reduction appeared insufficient.Another attempt showed more promising results. At one input of the Microflown a closed rigid tubewas mounted and at the other a flexible tube was mounted. A sound pressure wave would deform theflexible tube and not the rigid one, inducing a particle velocity inside the mounting. This way of noisereducing will make the set up sensitive to sound pressure in stead of particle velocity; the directivityproperties of the Microflown will therefore be lost. The rigid tube will act as backing volume. We used avolume of 0.6l. We expect the system to measure lower frequencies if the backing volume is enlarged.Opening the door of the lab resulted in signals shown in Figure 9. Despite the difference in transducing,the general trend of the signal is similar. The “noise” seen on the Microflown is due to electronicinterference, it is not selfnoise. The flexible tube is used as a pressure to particle displacement converter.We expect a higher sensitivity and a better noise reduction when the length of the tube is increased. Figure 9: The measured signals of a microbarometer (A) and a Microflown (B) as result of a atmospheric distortion in aroom with a volume of 144m3.8. CONCLUSIONSThe Deelen Infrasound Array is capable of detecting and localising infrasound. Identification ofdifferent sources of infrasound occurring in the same frequency band is possible with the high resolutionarray (p.e. microbaroms, sonic booms and meteors). Experiments with the Microflown have shownpromising results, as low noise, directional and particle velocity sensor. Proven durability in the field anddevelopments to obtain a proper low frequency cut-off, will allow the Microflown to be a highlycompetitive infrasound sensor.9. REFERENCES[1] H-E. de Bree et al., The Microflown: a novel device measuring acoustical flows, Sensors and Actuators A 54, 1996,pp. 552-557[2] W.F. Druyvesteyn et al., A new acoustic measurement probe; the Microflown IOA London, volume 21, pt31999, pp. 139-149[3] www.microflown.com[4] H.W. Haak, An acoustical array for subsonic signals, Tech. Rep. WR96-03, KNMI, De Bilt, theNetherlands, 1996, 22 pp[5] E. Smart and E.A.Flinn, Fast frequency-wavenumber analysis and Fisher signal detection in real-timeinfrasonic array data processing, Geoph. J. R. astr. Soc., volume 26, 1971, pp. 279-284[6] Garcés et al., Traveltimes for infrasonic waves propagating in a stratified atmosphere. Geoph. J. Int.Volume 135, 1998, pp. 255-263

A new acoustic measurement probe;

An acoustical array for subsonic signals,

frequency-wavenumber analysis and Fisher signal detection in real-time infrasonic array data processing,

The Microflown: a novel device measuring acoustical flows,

Traveltimes for infrasonic waves propagating in a stratified atmosphere.

https://ris.utwente.nl/ws/files/6160340/Evers00deelen.pdf

The Deelen infrasound array for recording sonic booms and events of CTBT interest

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