Acoustic particle velocity enabled methods to assess room acoustics Emiel Tijs

a

Microflown Technologies, Arnhem, the Netherlands Jonathan Botts Rensselaer Polytechnic Institute, New York, US / Microflown Technologies, Arnhem, the Netherlands Hans-Elias de Bree

MicroflownTechnologies / HAN University dept. Vehicle Acoustics, Arnhem, the Netherlands Eva Arato

b

Arato Acoustics Ltd, Budapest, Hungary

ABSTRACT Traditionally, room acoustic measurements are based upon sound pressure microphones only. With the invention of the Microflown sensor in 19941, acoustic particle velocity has become a directly measurable quantity. Since then, many new applications have been developed for various markets. The applications are based on the fact that particle velocity based approaches may provide additional information. Together with a regular sound pressure sensor, the whole acoustic vector can now be measured. Sound probes combining both sound pressure and particle velocity sensors can be used for a variety of room acoustics applications as well. Five applications will be discussed here:

1. In-situ measurements of acoustic impedance and absorption of separate structures 2. Localization of the direction of the direct sound source and early reflections in a hall 3. Energy density measurements 4. Quantification of diffusion 5. 3D intensity visualization around objects

Some recent results obtained at the Budapest Music Academy and the Musis Sacrum theater in Arnhem will be presented.

a Email address. tijs@microflown.com

b Email address. arato.eva@aratokft.hu

1. INTRODUCTION With a Microflown sensor it is possible to measure the acoustic particle velocity directly in one spot. Coupled with a pressure microphone, these probes can measure both kinetic and potential energy. With such a sensor several new applications can be developed for room acoustics. 1) The amount and distribution of absorption and materials in a room greatly affects the acoustic quality of a space. Often these materials are not removable, so to determine their properties, in-situ measurements are necessary. The free field PU impedance technique was tested

successfully for the first time in 20042. Only a small sample area is required for this method, and the susceptibility to background noise or reflections is low. Many other free field absorption methods require large samples and cannot be used in reverberant environments. The theory of the PU method will be discussed and results of measurement in a concert hall in Arnhem (the Netherlands) will be shown, together with new measurements taken in Budapest (Hungary). 2) Another application allows the mapping of the direction of incoming early reflections versus time. Early specular reflections influence the perception of a signal in a room, whether it is the sound of a symphony in a large concert hall, a singer in an opera house, or a speaker in a classroom. With a sound pressure microphone a scalar quantity is obtained, which provides no directional information. When the particle velocity is also measured in three directions the 3D intensity can be calculated. Intensity is a vector indicating the direction of energy flow, thus the location of the direct source and early reflections can be determined. Such measurements have been studied as early as 2002, by Meyer sound laboratories. Here the method is studied further and results are shown of a measurement in the Music Academy in Budapest. 3) Energy density can be used in the calculation of most room acoustics parameters like decay time, clarity, definition, and center time. Using only p^2 as an estimation parameter often leads to errors and variation between measurement positions, as sound pressure is a place dependent quantity. Acoustic energy is a place independent quantity, thus more uniformity can be achieved by including the kinetic energy term of velocity as well. The pressure-velocity probes can be used in diffuse fields, as they are not affected by a high pressure-intensity index. This is a significant advantage over PP intensity probes. 4) Another line of research is the quantification of the degree to which a sound field is diffuse. A metric can be developed relating the degree of diffuseness to the amount that the time-averaged intensity, pressure and energy are related to each other. Because PU probes can be used in conditions with a high pressure-intensity index, these quantities can reliably be measured in a diffuse field. 5) The last application is the visualization of the sound vector field. Usually an extremely complex sound field is present because of the many different scattering and absorbing surfaces. To gain further insight into the true behavior around objects a theatre seat has been analyzed. In one plane a many measurements were taken with a three dimensional intensity probe and the intensity vector field is visualized.

2. PU SURFACE IMPEDANCE

A. PU surface impedance method The PU free field surface impedance technique makes use of a particle velocity sensor and a microphone. Both sensors are mounted in one probe that is positioned close to the material and a sound source is positioned at a certain distance. The impedance can be derived from the ratio of pressure and velocity2-6. From this, material reflection and absorption can be calculated. The impedance measurement set up that is used (figure 1, left) consists of a spherically shaped loudspeaker. The radiation impedance in front of the loudspeaker is quite similar to a monopole7. The loudspeaker is mounted to a grip and mechanically decoupled from the structure that holds the probe. Measurements in the free field can be difficult because plane waves are practically impossible to create in a broad frequency range. Therefore an image source model is used that takes into account the spherical waves, which are present because of the point source that is used. The plane wave reflection coefficient can be derived from the spherical impedance via equation (1).

2

1

( ) 11

( ) 1

measure

ff ik hs

smeasure s s

ff s s

Z

Z h hR e

h hZ h h ik h h

Z h h ik h h

−+

=− − + +

+ + − + (1)

where k is the wave number and R is the planer reflection coefficient. When the measured impedance at the material (Zmeasure) and measured impedance in the free field (Zff) are measured close after each other, all amplifier settings, AD settings, calibration of the microphone and microphone etc. are likely to be unchanged. Also the temperature, and thus the characteristic impedance of air, is assumed to be the same during both measurements. As long as these conditions are met, the values do not have to be known as they will vanish in the ratio Zmeasure/Zff. The distance between the probe and source hs is kept at a constant 23cm. A probe-sample distance h of 5mm to 20mm is normally used.

The pressure and velocity can be measured in the whole audible range but the lower frequency limit of the impedance method is many times 100~300Hz. This is due to the low sound pressure emission from the loudspeaker at low frequencies, and the limited dimensions of the samples. Also close to a fully reflecting plane the particle velocity is practically zero, which will increase the error of the measurement.

B. Small sample size Because the sound pressure and particle velocity are measured in one spot, the probe itself can be positioned close to the material, and the required sample size is therefore small compared to other free field methods. Other laboratory methods require the cutout of a piece of material or of an entire object. Such procedures are time consuming and destructive. Also it is not possible to measure all materials with standard laboratory setups, because the absorption of some samples depends on the way they are mounted. For instance (perforated) wall panels absorb little sound themselves, but there can be absorption from the interaction with the cavity behind the panel.

C. Resistance to background noise and reflections Because the distance from the speaker to the material is small, the method is almost not affected by background noise or by reflections in most cases. The distance between the probe and the source is only 26cm, so reflections at some distance are less dominant than the signal from the direct source. The method has already been applied several times inside a cars6. To filter out any reflections, a moving average in the frequency domain is used, which gives many times gives a result similar to an anechoic measurement. A time windowing technique could also be used but the moving average is more robust [9]. When there are many random reflections the smoothed result should follow the actual impedance. However when the actual impedance has a sharp change this averaging should not be applied, so some care is required. Also when there is one dominant reflection (e.g. from only one wall) the moving average should not be used.

D. In-situ measurements in concert halls Here the result will be shown of measurement in the Musis Sacrum concert hall in Arnhem and in a concert hall of the Music Academy in Budapest. It was possible to measure several wall panels, seats and some floor sections. The probe should be placed near to the sample with the spherical sound source at a close distance. The setup can be held by hand or supported by a stand as is shown in figure 1, right. During some of the measurement the setup is scanned by hand along the surface to measure the average impedance of a larger area.

Figure 1: Left: PU free field impedance setup. Right: the setup positioned near the back part of a seat in

the Music Academy in Budapest.

The reflection coefficient is studied here instead of the absorption, because there is no fully reflective plate behind many objects under test. The sound can go partially through the material as well, and it is therefore difficult to determine the actual absorption by the material itself. The results of some wall panels and floors in Arnhem and some seat measurements in Budapest are shown, see figure 2. In one particular case the absorption of a podium was to be measured. It is expected that this acts as a membrane absorber. First comparisons with accelerometer based data show encouraging results at low frequencies that need further analysis (60-200 Hz).

0

0.2

0.4

0.6

0.8

1

1.2

100 1000 10000

Frequency [Hz]

Refl

ecti

on

co

eff

icie

nt

[-]

Balcony, floor

Podium, panel

Podium, panel w ith cavities

Podium, panel behind curtain

0

0.2

0.4

0.6

0.8

1

1.2

100 1000 10000

Frequency [Hz]

Refl

ecti

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[-]

Seat 1

Seat 2

Seat 3

Figure 2: Results from measurement in the concert hall in Arnhem (left) and in the in Budapest (right).

3. THREE DIMENSIONAL LOCALISATION OF REFLECTIONS The ability to visualize the direction of incoming reflections can be a powerful design and analysis tool. It allows the mapping of the direction of incoming early reflections versus time. Perhaps it could also help to determine the optimal location to place absorption in order to minimize unwanted reflections.

A. Principle With a 3D PU sound intensity probe is the time signal of an impulse source can be measured. A similar result can be obtained by calculating the impulse response from a known reference. Such a reference could be a sensor close to a source or the input signal from a speaker itself, which could be powered with a sine sweep or a MLS type of signal.

The time-averaged intensity is calculated in three Cartesian coordinate directions from the pressure and velocity signals. The three-dimensional intensity vector can then be calculated and used to show the direction of the windowed reflection.

B. Measurements description A pilot study is done to localize early reflections in a small concert hall of the Music Academy in Budapest. The measurements in the hall were taken in three positions. A reference sensor was placed as near to the source as possible. The first position in the stalls was approximately 6.9 m from the source in the center aisle of the hall (figure 3, left). The second measurement position in the stalls was approximately 6 m from the source in the center of the left half of the audience (figure 3, right).

Figure 3: Position 1 of the 3D intensity probe in the aisle (left), and position 2 in the seat area (right).

The source signal was a swept-sine (50 Hz - 6000 Hz) produced by a monitor on stage. The pressure channel from the reference measurement, as opposed to the signal fed to the loudspeaker, was used as an attempt to eliminate the response of the loudspeaker and microphone. is not the ideal reference, as the loudspeaker and microphone are near to the stage floor, and the measurement is not anechoic. However, it works well enough for this analysis.

C. Results form measurements at the Music Academy in Budapest An example of an impulse response of the probe at position 2 is given in figure 4. The arrows in the right picture resemble the direction of intensity of time sections. The length of the arrow resembles not the intensity strength, but the elevation.

Figure 4: Time-averaged intensity vector calculated (right) from the indicated time windows (left).

The direct sound (number 1) and the first side wall reflection (number 6) are predicted and verified by measurement. There are several other reflections occurring in between as well, which likely come from other scattering objects like the seats.

Figure 5 below shows the first predicted image source reflections as well as the calculated intensity vectors. The agreement between the predicted image source path and the measured intensity vector is quite good, within a few degrees.

Figure 5: Position of the receivers and the expected path from the source and the 1

st wall reflection (left).

Middle and right figure: the expected path (transparent) together with the direct path (thin arrows).

A more sophisticated numerical model could be useful to predict the other contaminating reflections. If these experiments can be examined more carefully, and the extra reflections accounted for, the method can be validated more completely.

4. ENERGY BASED MEASUREMENTS The calculation of energy density is straight forward. It is the sum of the potential energy, measured with pressure (p), and kinetic energy, measured with velocity (v).

( )( ) ( )

22

2

0

2

0

2 tv

c

tptE

ρ

ρ+= (2)

where ρ0 is the air density and c the speed of sound. Energy density has the advantage of being more spatially uniform8. Using only p^2 often leads to errors and increased variation between measurement positions. More uniformity can be achieved by including the kinetic energy term. This has been shown to lead to more practical reverberation chamber measurements8. Energy density can be used in the calculation of most room acoustics parameters like decay time, clarity, definition, and center time. The results should be more accurate and reliable with less variation. When using energy density, the results have less variation in low frequencies, extending the valid frequency range of the measurement8. A Microflown USP probe allows for this to be measured with a single probe, as opposed to six pressure microphones. The USP probe is also more valid to use in a diffuse, reverberant field, as it is affected less by a high pressure-intensity index. This is a significant advantage over the PP intensity probes9.

6. QUANTIFICATION OF DIFFUSION The Microflown probe can also be used in several ways to quantify the diffusion in a room. The underlying principle is that in a diffuse room the time-averaged intensity goes to zero because the energy flow is randomly oriented. The amount of diffusion in a room can be correlated to the degree that this occurs. First experiments look promising and indeed there is a decay of the measured intensity due to the fact that multiple reflections from different directions coincidence. The pressure-intensity index or energy-intensity index will be very high in a diffuse field. The ratio of potential energy density to total energy density could also be used in a similar way. As the field becomes more diffuse, this ratio should approach unity. The use of Microflown probes for these measurements should be more accurate, as they are not susceptible to phase error in a field with a high pressure-intensity index. They are subject to significant phase error in highly reactive fields, but in architectural acoustics the measurement positions are usually in the far field, so these errors are negligible. Using the pressure-intensity index as a metric to quantify the diffuseness of a room could offer the simplicity of a single impulse response measurement as well as a simple calculation afterward to arrive at a metric. This could be a benefit compared to several other methods that are used to quantify diffusion, which require a (high) number of microphones.

7. THREE DIMENSIONAL INTENSITY FIELD AROUND SEATS

A. Principle of 3D intensity vector visualization In room acoustics, many measurement techniques are hampered by the fact that the cavity usually is governed by a high number of (in)coherent sources and reflections. In an attempt to create further insight in the behavior of sound fields the 3D sound intensity vector field can be visualized. Instead of sound pressure (which is a scalar value), sound intensity (a vector) can be used to visualize the flow of sound around objects. PP intensity probes that are based on three pairs of two closely spaced pressure microphones are difficult to use because of their size, and more important it is difficult to measure in highly reverberant fields, as they are susceptible to a high pressure-intensity index. PU probes are not affected by the p-I index and because the intensity is measured in one spot, the spatial resolution can be much higher. Numerous 3D sound intensity measurements have been made by the Maritime Institute of Szczecin in Poland demonstrating an erratic pattern of sound intensity streamlines10-11 around simple objects. Later a similar approach was used inside the reverberant cavity of a car12.

B. Measurement description Because such approaches require a high density of measurement points it is not practical to measure large areas. Here the 3D sound intensity vector field is visualized around the top area of a theatre seat both with the seat up and down. To mimic the situation of a theatre or concert hall with a lot of reflections, the theatre seat was positioned in a regular office without furniture or floor carpet to make the room more reverberant. In one plane a high number of measurement points with a 2cm spacing were taken with a 3D intensity probe. The measurement point arrangement is shown in figure 6 and 7. Two speakers at different positions powered with broad banded white noise were turned on.

Figure 6: Left: Arrangement of the theatre seat and speakers inside a reverberant office.

Right: A three dimensional intensity probe measuring one point in a plane around the seat.

Figure 7: Measurement locations in one plane of the theatre seats. Left: 502 measurement points on an open theatre seat. Right: 245 points are measured on a closed theatre seat (point [5,8] is missing here).

C. Measurement results Here the 3D intensity vectors are visualized via three techniques:

1. Normalized 3D intensity vectors. 2. Color maps of the intensity level (High levels are colored red here, lower values blue). 3. Sound intensity streamlines (Curves that are fitted tangent to the intensity vectors).

Figure 8: Intensity vector visualization of an open seat. Left: 250Hz, speaker 2. Right: 500Hz, speaker 1.

Theatre seat

Speaker 2

Speaker 1

Measurement

plane

These plots show that the intensity path is not bending very strongly at very low frequencies with the seat open or closed (figure 8 and 9 left). At higher frequencies the sound field is much more complex (figure 8 and 9 right).

Figure 9: Intensity vector visualization of a closed seat. Left: 250Hz, speaker 2. Right: 500Hz, speaker 1.

It should be noted that the test engineer, who was present in the room during the measurements, may have influenced the sound field. The spatial resolution of the measurement points is important for the streamline algorithm. At higher frequencies many more local influences can be expected, which would require the sensor spacing to be smaller. Here the results are plotted at 250Hz and 500Hz, so no resolution problems are expected here.

The measurements show that the direction of the 3D sound intensity vector is highly dependent of the position and that the energy propagation paths are very complex. Such three dimensional intensity measurements could be used as input data for numerical models or to verify them, but also as an alternative method. It provides further insight to the complexity of sound field.

7. CONCLUSIONS Acoustic particle velocity based sound probes clearly have many important, novel applications in room acoustics. They enable the calculation of several quantities to be measured with a single integrated probe where it would have required at least six separate microphones or an array of microphones, if possible at all. Several applications have been demonstrated that are supported by both laboratory tests and on site measurements in two concert halls. Acoustic impedance measurements of separate surfaces can provide much information about the influence of individual materials on parameters like the decay time. Up to now the absorption of separate materials had to be measured in a laboratory or were not measured at all. Unlike other methods to measure absorption, the free field PU impedance method is almost not susceptible to background noise and unwanted reflections. The sample size can also be very small, which allows detailed studies. The time-averaged intensity vector can be used to localize reflections and image sources in a room. This application could be useful for acousticians either during design or the renovation of existing spaces. Knowing the physical location of reflections adds a great deal of unique data to inform design decisions, and it can all be done with simple measurements and very little post-processing.

The energy density could be used to provide more reliable estimation of standard energetic acoustic parameters such as decay time, clarity, definition, and center time. More uniformity can be achieved by including the kinetic energy term instead of using only p^2. Velocity based probes are not susceptible to p-I index problems and finite distance limitations of the spacer which is an advantage compared to the PP intensity method. The Microflown can be used to measure quantities that can be used to quantify diffusion, which is an important topic in architectural acoustics. In a diffuse field the pressure or energy density will be very high compared to the time averaged intensity since the energy flow is randomly oriented and averages to roughly zero. As the sound field is more diffuse the ratio of potential energy density to total energy should approach unity. With models the energy flow around objects can be simulations, but this is hardly supported by measurements. To create further insight, the sound intensity is measured at a large number of positions on a theatre seat. A visualization of the sound intensity field is made on a plane around the object. Several examples show that the sound intensity field is heavily dependant on for example the position of the source, the geometry of the structures, and on the frequency. Such 3D intensity measurements can give further insight to the complexity of sound field and can be used as input data for numerical models or to verify them. The flexibility of the Microflown enables new research opportunities in room acoustics. The unique features of such intensity probes make them important tools for the next generation of acousticians.

REFERENCES 1. H.E. de Bree, P. Leussink, M. Korthorst, H. Jansen, T. Lammerink, and M. Elwenspoek, The

Microflown: a novel device measuring acoustical flows, Sensors and Actuators A-Physical, 54, (1996), pp.552-557.

2. R. Lanoye, H.E. de Bree, W. Lauriks and G. Vermeir, a practical device to determine the reflection coefficient of acoustic materials in-situ based on a Microflown and microphone, ISMA, 2004

3. R. Lanoye, G. Vermeir, W. Lauriks, R. Kruse, V. Mellert, Measuring the free field acoustic impedance and absorption coefficient of sound absorbing materials with a combined particle velocity-pressure sensor, JASA, May 2006

4. HE de Bree, M. Nosko, E. Tijs, A handheld device to measure the acoustic absorption in situ, SNVH, GRAZ, 2008

5. J. D. Alvarez and F. Jacobsen, An Iterative Method for Determining the Surface Impedance of Acoustic Materials In Situ, Internoise, Shanghai, 2008

6. E. Tijs, E. Brandão, H.E. de Bree, In situ tubeless impedance measurements in a car interior, SIA, Le Mans

7. F. Jacobsen, V. Jaud, A note on the calibration of pressure velocity sound intensity probes, Journal of the Acoustical Society of America, 120(2), p 830-837, 2006

8. D. Nutter, T. Leishman, S. Sommerfeldt, and J. Blotter, Measurement of sound power and absorption in reverberation chambers using energy density, JASA 121(5), 2700-2710 (2007).

9. F. Jacobsen and H.E. de Bree, A comparison of two different sound intensity measurement principles, JASA 118(3), 1510 - 1517 (2005).

10. S. Weyna, Experimental 3D visualization of power flow around obstacles in real acoustic fields, ICSV11, St. Petersburg 2004

11. S. Weyna, Acoustic energy distribution in space around the pipe outlet, Noise Control, 2007

12. E. Tijs, H.E. de Bree, Mapping 3D sound intensity streamlines in a car interior, SAE, 2009