This paper aims to clarify the principal advantages and disadvantages of using sound intensity probes which implement different measurement principles: p-p probes versus p-u probes or Microflowns. A novel measurement technique based on scanning principles called “Scan &amp; Paint” had been chosen to evaluate their performanc

Fernandez Comesana, Daniel

Grosso, Andrea

Holland, Keith R.

Wind, Jelmer

English

Southampton (e-Prints Soton)

PERFORMANCE OF P-P AND P-U INTENSITY PROBESUSING SCAN & PAINTDaniel Fernandez Comesana, Jelmer Wind, Andrea GrossoMicroﬂown Technologies, PO BOX 2205, 6802 CE Arnhem, The Netherlands, e-mail: fernan-dez@microﬂown.comKeith HollandInstitute of Sound and Vibration, University of Southampton, UKThis paper aims to clarify the principal advantages and disadvantages of using sound intensityprobes which implement different measurement principles: p-p probes versus p-u probes orMicroﬂowns. A novel measurement technique based on scanning principles called “Scan &Paint” had been chosen to evaluate their performance.1. IntroductionScan & Paint is a novel measurement technique under development for mapping stationarysound ﬁelds [1–3]. It minimises the measurement time whilst maximising the ﬂexibility. Basically, atransducer is swept around a virtual 2D plane close to a noise source while a video is recorded. Soundmaps can be created by mixing the tracking information with the signal acquired. Sound intensitycan be characterised either using a p-u probe or Microﬂown intensity probe to measure pressure andparticle velocity directly; or using a p-p probe to approximate the velocity from the gradient betweentwo pressure microphones close to each other. Advantages and disadvantages of each method havebeen discussed.2. Theory2.1 Particle velocityThe local velocity of a ﬂuid moving backwards and forwards due to a moving surface whichdisplaces a volume is known as particle velocity. Consequently, this magnitude is proportional to theexcitation source displacement. Depending on the direction of the ﬂow this quantity can be positiveor negative. Then, this magnitude can be deﬁned as~ u(t) =@~ @t(1)where ~  is the particle displacement. If only the normal velocity is considered, Equation (1) canrelated to the pressure byun(t) =  (1=0)Z t 1(@p()=@~ n)d (2)ICSV18, 10–14 July 2011, Rio de Janeiro, Brazil 118th International Congress on Sound and Vibration, 10–14 July 2011, Rio de Janeiro, Brazilwhere 0 is the density of air and p is sound pressure. There are two different approaches to measureun(t): directly with a Microﬂown, or indirectly from pressure measurements.Direct methodNowadays, the Microﬂown particle velocity sensor is the only way to measure acoustic particlevelocity directly. The response of this transducer has to be corrected in order to reach a ﬂat frequencyresponse with also linear phase. Sensitivity (Sc) and phase response (c) of the sensor have beenmeasured and combined into a complex correction function Cf(!)Cf(!) = Sc ejc (3)Then, any measured signal will be divided by this correction function in the frequency domain.Indirect methodIndirect methods of measuring particle velocity are based on an approximation of the pressuregradient shown in Equation (2) by taking the pressure difference between signals produced by twomicrophones situated close to each other, separated by a distance d [4],un(t)   (1=0 d)Z t 1[p2()   p1()]d (4)Next, theFouriertransformofthelastexpressioncanbeundertakeninordertoreachadeﬁnitionin the frequency domain. Hence,Un(!)   1j! 0 d[P2(!)   P1(!)]d (5)Now, the square magnitude of the normal particle velocity can be deﬁned asjUn(!)j2      1j! 0 d   2(jP1(!)j2 + jP2(!)j2   P1(!)P2(!)   P2(!)P1(!)) (6)Furthermore, the above expression can be simpliﬁed by calculating the power spectral den-sity and the cross-spectral products between the two pressure signals in order to reach a simpliﬁedexpression.jUn(!)j2     1j! 0 d  2(Sp1p1(!) + Sp2p2(!)   Sp1p2(!)   Sp2p1(!)) (7)Finally, the sound particle velocity level can be calculated by taking the logarithmic form ofEquation (7), i.e.SUL(!)  10log10jUn(!)j2Uref2(8)where Uref is the particle velocity reference (in air 5x10 8).2.2 Sound intensityThe instantaneous intensity I(t) of a sound wave is the instantaneous rate per unit area at whichwork is done by one element of ﬂuid on an adjacent element [5]. It is obtained as the product ofpressure and particle velocity (I(t) = pu). It is a measure in Watts per square meter (W/m2). The218th International Congress on Sound and Vibration, 10–14 July 2011, Rio de Janeiro, Brazilintensity I is usually deﬁned as the time average of I(t), so the time-averaged rate of energy trans-mission through a unit area normal to the direction of propagation,I = < I(t) >T = < pu >T =1TZ T0pudt: (9)Because of its intrinsic dependency on particle velocity, there are two methods to estimatesound intensity: direclty using a p-u intensity probe; or indirectly by using a p-p intensity probe (twopressure microphones). An interesting discussion of the limitations of direct and indirect principlesis given in [6].Direct methodIn order to obtain the time-averaged intensity using a p-u probe, the general expression (Equa-tion (9)) can be implemented as,I(!) = Real(Spu(!)) (10)where Spu is the cross spectrum between the pressure and particle velocity signals acquired.Indirect methodUsing a p-p probe, an approximate method for estimating sound intensity has to be used. Start-ing with Equation (9), particle velocity can be approximated as the gradient between two pressuremicrophones close to each other (see Section 2.1)In(t)  (1=20 d)[p1(t) + p2(t)]Z t 1[p1()   p2()]d (11)Now, taking the Fourier Transform of Equation (11) leads to an expression for calculating soundintensity in the frequency domain,I(!)   1!0dImag(Sp1p2(!)) (12)where Sp1p2 is the cross spectrum of the two pressure measured.3. Measurement procedureThe measurements undertaken aim to characterise intensity variations in a virtual 2D plane.Moving the transducer perfectly on this virtual plane is impracticable, a robot arm would be requiredanditwouldincreasethecostoftheexperimentsand, atthesametime, itwoulddecreasetheﬂexibilityof the measurement technique. Consequently, alternative ways of deﬁning, at least, a reference havebeen established.A cross-laser can be used to create a visual reference to move the microphone across a plane.Even without any measure of the positioning error, the lack of technology may be replaced by care-fulness. A visual mark can be made in the microphone to move it with the laser light always close tothe mark. The accuracy of the measurements then depends on the person who undertakes the mea-surements. However, due to the intrinsic averaging of several sweeps, human error can be neglected,as can be seen in the next section.318th International Congress on Sound and Vibration, 10–14 July 2011, Rio de Janeiro, Brazil4. Results and discussionIntensity probes have to be tested under critical conditions in order to see their limitationsclearly. Therefore, a fairly soft broadband noise source would be a proper device to be measured.Following this criterion, the noise from a laptop computer has been measured with p-p and p-u inten-sity probes. Three different measurement environments were evaluated: the ISVR anechoic chamber(Southampton, UK), a semi-anechoic chamber in the University of Vigo (Spain) and a small isolatedroom at Microﬂown (Holland). According to [7], it has been proven that the measurement scenariodoes not bias the results signiﬁcantly as far as the noise ﬂoor remains constant. Due to the proximityof the measurement plane, only high reverberation would spread the energy smoothly around the mainnoise source. Nonetheless, the sound maps shown are not much affected by the environment due tothe low reverberation time of all rooms.Furthermore, the physical characteristics of p-p and p-u probes are another important pointto take into account. The technique relies on tracking the position of an LED light attached to thetransducer. The height difference between the light and transducer has to be corrected. The projectionerror corrections are explained in detail in [7], but it is important to consider that if the LED is faraway from the transducer, systematic errors will increase. For instance, the commercial p-p probeused for taking the measurements presented (B & K 3595) had the LED attached 0.12 m away fromthe transducer centre. In contrast, using the Microﬂown p-u probe, the LED was coupled only 0.02 maway from the sensor, leading to signiﬁcantly reduced errors on the projection correction.The efﬁciency of p-p and p-u probes have been compared in the following sections assessingseveral frequency bands. Moreover, results have been presented in multiple ﬁgures which follow acommon pattern. Two different situations have been distinguished. First row shows intensity mapswith low fan speed in an anechoic chamber using p-p probes using different techniques: step-by-step(left) and Scan& Paint (right). On the other hand, the second row presents Scan& Paint results fromhigh fan speed measurements using a p-u (left) and a p-p probe (right) at Microﬂown Technologiesand a semi-anechoic chamber, respectively. Therefore, the top left pictures should be considered asthe reference, since each point of the mesh has been measured individually, taking more than 3 hoursto acquire all data; whereas, the other three intensity maps were measured using Scan& Paint withdifferent probes which only took 3 minutes to record the data.4.1 Lower frequenciesManipulation noise has been proven to be the most critical issue using p-p probes [7]. Distur-bances are produced due to dragging the cable over the ﬂoor when the probe is swept. The lack ofisolation of commercial p-p probes to structural vibrations is the weakest feature of its performance.Manipulation noise mainly affects the lower frequencies, leading to random peaks in the sound in-tensity maps up to 1 kHz. In contrast, Microﬂown transducers are highly isolated against structuralnoise, leading to better results in the lower bands.Figure 1 shows intensity maps in the 125 Hz octave band. As can be seen from these ﬁgures, p-uprobes perfectly works from 125 Hz, achieving an outstanding signal to noise ratio against manipula-tion noise. There are no perceivable peaks at any point over the map even with a very low minimumlevel (32 dB). In contrast, p-p probes cannot visualize any clear pattern at all for 125 Hz. Results areeven more similar to the step-by-step measurements in the next octave band, at 250 Hz, but the p-pperformance is still far better than the p-u probe.4.2 Middle frequenciesThere is not much inﬂuence of manipulation noise at frequencies over 500 Hz. At middlefrequency bands, measurements undertaken with a p-p probe are not biased by any huge peak which418th International Congress on Sound and Vibration, 10–14 July 2011, Rio de Janeiro, BrazilFigure 1: 125 Hz intensity maps: step-by-step measurements with p-p probe and low fan speed (top-left); Scan&Paint with p-p probe and low fan speed (top-right); Scan&Paint with p-u probe and highfan speed (bottom-left); Scan&Paint with p-p probe and high fan speed (bottom-right).masks the useful information, as can be seen from Figure 2. Now, the intensity maps match perfectlywith each other1.Evaluating in detail the contour shapes, measurements undertaken using p-u probes have moreregular maps giving clearer pictures charactering intensity variations. Consequently, even withoutmuch inﬂuence of manipulation noise, again in the middle frequencies, p-u presents better perfor-mance.4.3 Higher frequenciesHigh frequency octave bands are completely isolated against manipulation noise. Consequently,the sound maps generated, either with the p-p probes or with p-u probe, perfectly measure the soundpressure and intensity variations around a virtual surface as far as there is no errors in the projectioncorrection.Figure 3 present results obtained from different tests performed with high and low fan noise.As can be seen from these ﬁgures, all results perfectly match independently of the environment ortransducer used.5. ConclusionsEitherp-porp-uintensityprobescanbeusedtoundertakescanningmeasurementssuchasusingthe Scan&Paint method. However, it has been proven that p-u probes are able to measure at lower1Note that intensity levels of graphs excited with high fan speed (ﬁrst and second row) and low fan speed (third andfourth row) have to be compared separately518th International Congress on Sound and Vibration, 10–14 July 2011, Rio de Janeiro, BrazilFigure 2: 1 kHz intensity maps: step-by-step measurements with p-p probe and low fan speed (top-left); Scan&Paint with p-p probe and low fan speed (top-right); Scan&Paint with p-u probe and highfan speed (bottom-left); Scan&Paint with p-p probe and high fan speed (bottom-right).frequency bands very clearly, with a high signal to noise ratio; while p-p probes cannot do it due to itspoor isolation from handling noise. Furthermore, p-u probes also present a signiﬁcant performanceimprovement in middle frequency bands. Moreover, systematic errors are minimised using a p-uprobe, due to its geometric features: it is possible to couple the LED light closer to the transducer,reducing errors on the projection correction. Therefore, it can be concluded that p-u intensity probeshave much better performance than p-p probes for measuring with scanning techniques.618th International Congress on Sound and Vibration, 10–14 July 2011, Rio de Janeiro, BrazilFigure 3: 8 kHz intensity maps: step-by-step measurements with p-p probe and low fan speed (top-left); Scan&Paint with p-p probe and low fan speed (top-right); Scan&Paint with p-u probe and highfan speed (bottom-left); Scan&Paint with p-p probe and high fan speed (bottom-right).References1 E. Tijs, H.-E. de Bree, and S. Steltenpool. Scan & paint: a novel sound visualization technique. InInternoise, 2010.2 H.-E. de Bree, J. Wind E. Tijs, and A. Grosso. Scan&paint, a new fast tool for sound sourcelocalization and quantiﬁcation of machinery in reverberant conditions. In VDI Maschinenakustik,2010.3 D. Fernandez-Comesana, J. Wind, and H-E. de Bree. A scanning method for source visualizationand transfer path analysis using a single probe. In SAE International, 2011.4 F. J. Fahy. Sound Intensity. Elsevier, 1989.5 L. E. Kinsler, A. R. Frey, A. B. Coppens, and J. V. Sanders. Fundamentals of Acoustics. SponPress, 3rd edition, 1982.6 Finn Jacobsen and Hans-Elias de Bree. A comparison of two different sound intensity measurementprinciples. The Journal of the Acoustical Society of America, 118(3):1510–1517, 2005. doi: 10.1121/1.1984860. URL http://link.aip.org/link/?JAS/118/1510/1.7 D. Fernandez-Comesana. Mapping stationary sound ﬁelds. Master’s thesis, Institute of Sound andVibration Research, December 2010.7

A comparison of two different sound intensity measurement principles.

A scanning method for source visualization and transfer path analysis using a single probe.

Fundamentals of Acoustics.

Mapping stationary sound ﬁelds.

Scan & paint: a novel sound visualization technique. In Internoise,

Scan&paint, a new fast tool for sound source localization and quantiﬁcation of machinery in reverberant conditions.

Sound Intensity.

Performance of P-P and P-U intensity probes using Scan &amp; Paint

Performance of P-P and P-U intensity probes using Scan & Paint

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Performance of P-P and P-U intensity probes using Scan &amp; Paint

Abstract

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Performance of P-P and P-U intensity probes using Scan & Paint