NL2007334C2

NL2007334C2 - Device and method for measuring sound levels and direction or localisation of sound sources.

Info

Publication number: NL2007334C2
Application number: NL2007334A
Authority: NL
Inventors: Gregorius Andreas Joseph Boersma; Marinus Marias Boone; Marcel Korver; Johannes Martinus Mulder
Original assignee: Munisense B V
Priority date: 2011-09-02
Filing date: 2011-09-02
Publication date: 2013-03-05
Also published as: WO2013032332A1; DE112012003657T5

Abstract

The invention relates to a directional sound measurement device, comprising at least two pressure gradient microphones having the same directional characteristics, being co-located in a different orientation and a processor adapted to divide the time in time windows, to determine the mean squared value of the output signals of the microphones in each time window, to determine the root of the mean squared values in each time window, execute goniometric combinations to the mean squared values of root mean squared values in each time window for obtaining the sound level and the direction of a dominant sound source in each time window. The invention also relates to a combination of at least two of such devices and a central unit to receive the signals transmitted by the transmitters and to calculate the location of at least one dominant sound source based on cross bearing and a corresponding method.

Description

Device and method for measuring sound levels and direction or localisation of sound sources.

The invention relates to a device and a method to acquire the direction and noise level 5 of a sound field at an observation point as a function of time. It also relates to a set of devices and a method to combine the results of several measurement points with crossbearing techniques to locate dominant sound sources with their geographical coordinates and to correlate the results with source-related audio-visual information.

10 More in particular the present invention relates to a directional sound measurement device, comprising at least two pressure gradient microphones having substantially the same directional characteristics, co-located in a mutually different orientation, and a processor adapted to combine the output signals of the microphones for obtaining the sound level and the direction of a dominant sound source in each time window. Herein 15 the expression ‘co-located’ is understood to express that they are virtually at the same position in relation to the angles with respect to the sound source.

A well-known method to acquire the direction of a sound wave from its wave front direction is to combine the outputs of several closely-spaced microphones to form a so-20 called intensity probe. A disadvantage of this method is that the microphones must be very identical in amplitude and phase characteristics, which will make the system expensive. Besides that, there remains the problem that the finite difference approximations lead to errors for high frequencies as is well known from p-p intensity probes as disclosed in F.J. Fay, Sound Intensity, St Edmundsbury Press, Suffolk, Great 25 Britain, 1989. Another approach is to make use of four pressure-gradient microphones that are placed on the surfaces of a regular tetrahedron as is known from the so-called Soundfield microphone, which is disclosed in GB-A-1 512 514. The proposed method makes use of such a configuration. In the normal application of the Soundfield microphone, a mix of the audio signals of the four microphones is taken to make 30 outputs with variable directivity in different directions. Use could be made of this principle, but the result depends on the similarity of the microphone characteristics, while further phase errors at high frequencies may occur.

2

To avoid these disadvantages the invention proposes a directional sound-measurement device, comprising of at least two pressure-gradient microphones having substantially the same directional characteristics, located in their vicinity (see note above) and in mutually different orientations, and a processor adapted to divide the time in time 5 windows, determine the mean squared value or the root mean squared value of the output signals of the microphones in each time window, and execute goniometrie combinations to the squared signals in each time window with the purpose of obtaining the sound level and the direction of a dominant sound source for each time window.

10 Consequently there is no need for phase sensitive signal mixing and no use of finite difference approaches, which makes the method a preferred for practical applications, as the signals are combined after eliminating the phase dependency of the signals.

The invention also provides a corresponding method.

15

Subsequently the present invention will be elucidated with the help of the following drawings, wherein depict:

Figure 1 : a spherical coordinate system with azimuth a and elevation 0;

Figure 2 : a diagram explaining the positions of the microphones in a special 20 embodiment; and

Figure 3 : a flow chart of the post processing of signals in the embodiment shown in figure 2.

A sound field is characterized by its spatial and temporal pressure and particle velocity 25 field. If the sound field is due to a single source and the propagation of the sound is in a stationary, homogeneous and isotropic medium without reflections taking place, the direction of the source is the same as the direction of the particle velocity in the point of observation. The same is valid for the intensity vector of the sound field. If we measure the sound field in the far field of the source, a local plane wave approximation is valid. 30 The particle velocity is then related to the sound pressure and the source direction by: v(r,f) = yP(r,0ns (1) with Zs the specific acoustic impedance of the medium (usually air), equal to p0C, the product of the mass density p0 of the medium and the sound velocity c, r being the 3 vector of the spatial coordinates of the observation point and nsthe normal vector of the wave front of the sound field that equals the direction from the source to the observation point.

The intensity of the sound field is given by: 5 I (r) = p(r,f)v(r,f) = p2rJZs, (2) where the overbar denotes time averaging.

From the measurement of the sound pressure, the immission level is normally found as 10 L = 201Og(prms/p0) [dB] (3) with prms being the root mean squared value of the sound pressure and p0 = 20 pPa. The source direction is related to the three vector-components of the particle velocity and to the vector components of the intensity. For the characterization of the direction of the sound source we will use the azimuth and elevation angles as defined in figure 1. 15

One component of the particle velocity can directly be obtained from a microphone with a so-called figure of eight directivity. Such microphones are sensitive to the first order gradient of the sound pressure, which is related to the particle velocity by Euler’s equation: 20 Pol?=“Vp' <4)

This means that in a plane propagating wave field the sensitivity of such a microphone equals G((p) = G0cos(p (5) with (p being the angle between the source direction and the main direction of the 25 microphone. G0 is the microphone sensitivity in the main direction. The gradient can be obtained from a microphone that makes use of the pressure difference at the two sides of the microphone membrane. It can also be obtained from measurements with two closely spaced pressure sensitive microphones and taking a finite difference approximation of Eq. (4). The particle velocity can also be measured with a very fast 30 flow sensor, such as the Microflown, as described in patent application WO-A-1996000488.

4

If we place three gradient microphones in the x-, y- and z-direction of the coordinate system of figure 1 at a coincident place in the centre, the outputs of the microphones are respectively: xx(t) = (cosa sin0)s(f) x2(t) = (sinasin0)s(f) . (6) x3(f) = (cos0)s(f) 5 It is assumed here that the microphones have been calibrated such that S(t) equals the sound pressure of the sound source at the position of the microphones. In the following discussions we will use the mean squared and the root mean squared values of the measured signals.

The mean squared value of signal x(£) is defined as: 10 x„ = l(7) T—>oc J 0 and the root mean squared value as = (8) In practice the time integral is taken over a sufficiently long time to get a statistically stable output. From the measurements with the gradient microphones according to Eq.

15 (6), the sound pressure level can easily be found from L = 101og((xlms + x2ms + x3ms)/p02). (9)

However, it is not so easy to calculate a and 0 because this has to be done based on the time dependent signals and not on the ms or rms values. That is because quadratic averaging would destroy the correct signs of the goniometrie factors. Therefore direct 20 application of the output of gradient microphones is not preferred.

The components of the intensity vector can be obtained with microphone probes that apply Eq. (2) by multiplying the time dependent pressure and particle velocity and averaging the result. Such probes can be combined to form a three-dimensional intensity 25 probe. The outputs of such a three-dimensional probe give the following components: I x = (cosa sin0 )pms / Zs

Iy = (sina sin0 )pms / Zs . (10) /z = (cos0)pms/Zs 5

The sound pressure level equals l=ioiog(^//J+7^+7J / /0) (id with l0 = pi / Zs.

The azimuth and elevation angles can be calculated as follows: 5 a = arctan( ly/1 x) (12) 0 =arccos(/z/>//^+ /^ + /^) . (13)

To obtain a over the full angular range between 0 and 27i, Eq. (13) can be computed with the well-known atan2 function.

10 It must be noted that intensity probes are very sensitive to deviations from the ideal characteristics due to errors causes by phase differences and finite difference approaches. This is mainly caused by the fact that outputs of the individual microphones of these probes must be combined as the time dependent signals and cannot be applied to the ms or rms values. This makes the application of intensity probes expensive.

15

The principle of our invention is the use of pressure-gradient microphones that are placed close together in such a way that the sound pressure level and the azimuth and elevation angles can be found in an easy way by post processing of ms or rms output values. In a preferred implementation this can be done with only four microphones.

20 The directivity characteristic of a pressure-gradient microphone is given by; G(cp) = G0(l + 6coscp)/(l + b). (14)

The variable b is normally taken > 0 and depends on the directivity characteristic of the microphone. For an omnidirectional microphone b = 0, for a cardioid b = 1 and for a velocity-type (or figure of eight) microphone, b » 1.

25

The basis of our invention can easily be understood by first looking at a onedimensional situation where we want to find the angle (p of the incident plane sound wave. Therefore we will make use of two pressure-gradient microphones with the same characteristic (same b) and place these microphones in opposite directions. With the 30 sound pressure of the sound field at the microphone position given by S(t) (we omit the sensitivity of the microphone as an extra multiplication factor here), we have: 6 xi^) = 77T^^1 + ÖC0S(P^ ’ 05a) (l + b) X2(f) = 77T^(1_öcos(p)s(f) • 05b) (1 + b)

We can easily find the sound pressure signal S(t) as s(() = t±*(x,(() + x2(0) . (16) 5 and ^ xTt)-xJt) } ....

cp = arccos , v'—. (17)

^b(x,(f)+X2(f))J

Due to symmetry, (p can only be obtained over an angular range between 0 and n.

Note that the processing of Eq. 16 and 17 is done on the time dependent signals and is therefore sensitive to phase differences caused by the microphone characteristics and 10 due to the fact that the microphones have a slightly different position in the sound field. In our invention we realize that equivalent relations can be found for the root mean squared output signals of these microphones.

*l,™s = 777T7(1 + ÖCOSCP)Srms , (18a) (1 + 6) *2 ,rms = 7777: (1 - fccostp )srms. (18b) (1 + 6) 15 A restriction is that Eqs. (18) are only valid as long as (l + 6cos(p)> 0, so in general, taking all angles of incidence into account, for b < 1. Pressure-gradient microphones with 0 < 6 < 1 are called sub-cardioid microphones. The post-processing equations are now: ^ = + (19) 20 and (p = arccos *Lrms . (20) 6(xlrms + x2mB)

These relations do not depend on the already mentioned phase differences and take much less computing power, because they are applied on the averaged signals only.

7

The principle as outlined here can easily be extended to three dimensions by using additional microphone pairs in orthogonal directions. If we label the microphone pairs in the indirections with 1 and 3, in the ±y-direction with 2 and 4 and in the indirection with 5 and 6, the microphone signals for a sound field from the direction (oc ,0 ) are 5 given by:

Xl (f) = —(1 + 6 cos oc sin0) s(f) 1 + 6 X2(t) = J-(l + bsma sin0)s(f) x3(f) = —(l — 6cosot sin0) s(f) ' + * . (21) x4 (f) = (1 - 6sina sin0) s(t) x5 (f) = —(l + 6cos0) s(f) 1 + 6 1 + 6

In a first preferred method, we first measure the rms signals of the outputs of the sub-cardioid microphones, giving \rms = 777:(1 + öcosa sin0) srms 1 + 6 xi,rms = ~77- (1 + 6sina sin0) x3,r™ = 777(1 - öcosa sin0) srms 10 l\b . (22) = Y^b (1 _ 0Sin0C Sin9 ^

X5’rTO = T+6^1+öcos0)SmB

X6,rms ~ ^ ^®*n0 ) ^rms

Here the restriction is that b < 1. We find as a result: ^x,rms ~ Xl,rms ~ X3,rms ^^rms ^OSOC SU10 ^y.rms ~ X2,rms ~ X4,rms ~ CSrmssin(X sin0 (23) ^z.rrns ~ X5,rrm ~ X6,rms ~ ^^rms COS0 15 with Crms = 26 / (1 + 6).

8

From Eq. (23) we can find: ^ = + (24) ^ms a = arctan (vy,Jvx rms )> (25) f \ 0 = arccos z,rrm . (26)

.f\/+ v1 W

y \ x>rms y,rms x,rms J

5

With this configuration it is also possible to calculate the results after obtaining the ms-outputs of the microphones. With this method b is not restricted to values < 1 so any type of pressure-gradient microphone is applicable. We first compute the ms values of the outputs. These values can be calculated from Eq. (21) to give: 10 x, ,= —-—^ (1 + 26cosa sin0 + b2 cos2 a sin2 0)

Urrs (1 + jb)2V ) ms x, = —-—- (1 + 20sina sin0 + b2 sin2 a sin2 0) s' 2,ms (1 + jb)2V ) ms x, = —-—t (1 - 2 6 cos a sin0 + b2 cos2 a sin20) s__

Xms ,1 + by\ } ms j • (27) x, m =-r (1 - 26sina sin0 + b2 sin2 a sin2 0) s__ 4,ms (! -(- b)2 1 m X5,ms = + 2ÖC0SÖ + C°S2 9 ) X6,ms = 7^(1 “ 20COS0 + bl C0S2 9 )

We can now compute the mean squared particle velocity components as: \ms = Xl,ms - = Csms cos« sin0 ^y,ms = X2,ms ~~ X4,rm = CS^sintt SU10 , (28)

Vz,ms ~ XS,ms ~ X6,rm ~ CS^COsQ

15 with CTO = 40/(1 + 0)2.

From Eq. (28) we can find: S =—— + v2 + \r (29) °/tb y x,ms y,ms x,ms ’ v 7

Ums 9 cc = arctan (^/^), (30) f \ 0=arccos Vz,ms . (31)

To obtain a over the full angular range between 0 and 2n, Eq. (30) can be computed with the well-known atan2 function. A benefit of this preferred method is that the value 5 of b is not restricted to values < 1.

The use of a tetrahedron as the basis for three-dimensional microphone configurations is well known. For instance the Soundfield microphone [4] makes use of four pressure-gradient microphones placed on the surfaces of a regular tetrahedron. The outputs of the 10 microphones are called the A-format and are usually remixed to obtain the B-format, consisting of an equivalent omni-directional microphone and three figure of eight microphones in the x-, y- and z-direction.

Another approach is known from R. Hickling and A.W. Brown, “Determining the 15 direction to a sound source in air using vector sound-intensity probes”, J. Acoust. Soc. Am. 129(1), January 2011, pages 219 - 224.1. who describe a method of constructing a three dimensional intensity probe by using a finite difference approach on four omnidirectional microphones, placed at the comers of a regular tetrahedron. We mention that the principle of the Soundfield Microphone can directly be used in the 20 preferred solutions of section 2.5, by first mixing the output to six virtual pressure- gradient microphones in the ±x~, ±y- and ±z-direction. A disadvantage of this method is that the mixing of these signals relies on a very precise phase behaviour of the microphones and the microphones must also be placed very close together. Therefore we propose a preferred method that is not dependent on such phase relations because 25 only the rms output values of the four microphones are used. The DSLM has tetrahedron geometry as shown in figure 2. The tetrahedron has edges that are surface diagonals of a cube with edges in the main directions of the carthesian coordinate system xyz. The microphones are placed on the faces of the tetrahedron, shown as circles in the figure.

30 10

The orientation of the microphones is according to table 1.

Table 1 - Position and orientation of the microphones in the tetrahedron planes Microphone Orentation Plane Direction (X-y/3) A back, left, down PQT -1+1-1 B front, left, up QRT +1+1+1 C front, right, down PQR +1-1-1 D back, right, up PRT -1-1+1 5 The output signals of the four microphones due to a plane wave from the direction (a, 6) are: A(t) = — (l + {>/3jb(-cosasin0 + since sin0 - cos0))s(f) 8(0 = -—(l +3>/3ib(+cosasin0 +sinasin0 +cos 0))s(f) l + b . (32) C(f) =-(l +^V36(+cosccsin0 - sina sin0 - cos0)) s(f) 1 + bx ' D(t) = (1 + 3 V36(- cosa sin0 - sina sin0 + cos0)) S(t)

The signal s(t) is the output of each of the microphones to the plane wave signal when the main axis of that microphone is pointed towards the source direction of the plane 10 wave source. The post processing of the microphone signals follows the procedure as shown in the flow chart of figure 3. The signals are first filtered with an appropriate frequency weighting function. This can be for instance A or C weighting as indicated in the figure, but other filters such as octave or 1/3-octave filters can be used here as well. These filters can also include a frequency dependent calibration. Next the rms-outputs 15 of the four microphone channels are computed. In this procedure it will be necessary that the pressure-gradient microphones have a value of b < 1, leading to:

Ams= -— (1 + 3 V3£>(- cosa sin0 + sina sin0 - cos0)) srms 1 + bx '

Brms = —— (l + i V3ó(+ cosa sin0 + sina sin0 + cos0)) srrm 1 + b . (33) C“ = I7h(1 + ^+COSastae -sinasine -cos6))^

Drms = (l+ ^ V3b(-cosa sin0 -sinasin0 +cos0)jsrms 11

These rms-signals are next matrixed into the following signals: 4b K = ~Arms + Brms + Crms - Drms = ^ srrm cosa sin0 4b

Vy = +Arms + Brms - Crms - Drms = ^ ^Sina sin0 . (34) 4b vz = -Arms + Brms - Crms + Drms = ^ Srms cos0

Notice that these signals look like figure of eight microphone signals, but with the procedure of our invention the goniometrie functions keep the correct signs, which is 5 not the case with rms-values obtained from figure of eight microphones.

The final post processing gives the required values for the immission level and the source angles: + + (35) a = arctan (Vy/Vx) (36) 10 and ( \ 0 = arccos z . (37) JV2 + V2 + V2

vx ^ vy ^ vz J

Here again, to obtain a over the full angular range between 0 and 2n, Eq. (36) can be computed with the well known a tan 2 function.

15 So far we discussed several preferred methods to acquire the SPL and azimuth and elevation angles (AE) for a single DSLM. It was assumed that there is only one dominant source that is stable in position and time. In real-life applications this will not be true and instead measurements will need to be time windowed and mapped to the corresponding AE-values.

20

In a first preferred method this will be done by energetic summation of sound-level contributions to a grid of azimuth and elevation values. This method can be refined by retaining the statistics of the source levels in the different grid cells. This gives the possibility to characterize and identify the sources from different directions according to 25 these statistic properties.

12

In a second preferred method, knowledge of possible source locations can be used to localize the source positions with a cross bearing between the AE-values and the possible source locations. If for instance the DSLM is placed at a high position, the elevation of a source direction can be used to calculate its distance when we know the 5 height of the sources above the ground.

In another preferred method a cross bearing is performed between DSMLs that are situated on different known positions.

10 For a practical implementation of these methods each DSML should be equipped with GPS and compass sensors to find its position and orientation.

For the characterization of the measured sound sources the results may be combined with audio-visual recordings of the surrounding of the DSMLs.

Claims

A direction-sensitive sound-measuring device, comprising at least two microphones sensitive to sound gradients with substantially the same characteristics, which are placed in the same position and with a mutually different orientation and a processor which is arranged for: - dividing the time into time windows; - determining the average value of the square of the output signal of the microphones in each time window; 10. determining the root of the average value in each time window; - providing trigonometric combinations of the average values of the squares of the root of the average of the squares of each time window to obtain the sound level and the direction of a dominant sound source in each time window while taking into account the angular position of the microphones. 15

A directional sensitive sound measuring device according to claim 1, characterized in that the microphones have a sub-cardioid characteristic and that the processor is adapted to determine the root of the average of the square of the signal from the microphones in each time window before their average is determined. 20

3. Directional sensitive sound measuring device according to claim 1 or 2, characterized in that the processor is adapted to subject the output signal of the microphones to frequency.

4. The direction-sensitive sound-measuring device as claimed in claim 1, 2 or 3, characterized in that the device comprises two microphones placed on a single axis in the opposite direction and that the processor is adapted to process the output signal of each of the microphones and for determining trigonometric combinations on the average of the square of the signals in each time window.

5. A directional sensitive sound measuring device as claimed in claim 4, characterized in that the device comprises three pairs of microphones, each pair being placed on an mutually orthognonal axis in the opposite direction and the processor being adapted to process the output signal of each of the microphones and for determining trigonometric combinations on the average of the square of the signals in each time window, taking into account the angular position of the microphones.

A directional sensitive sound measuring device according to claim 1, 2 or 3, characterized in that the device comprises four microphones with substantially the same characteristic, wherein each microphone is placed on a face of a regular quad face and the processor is arranged for processing of the output signal of each of the microphones and for determining trigonometric combinations of the mean of the square of the signals in each time window, taking into account the angular position of the microphones.

7. Directional sensitive sound measuring device according to claim 1, 2 or 3, characterized in that the processor is arranged for combining the output signals of the four microphones for simulating a configuration of three pairs of microphones and for simulating three pairs microphones for simulating a configuration of three pairs of microphones, each pair being positioned on an mutually orthogonal axis in opposite directions.

A direction-sensitive sound-measuring device according to any one of the preceding claims, characterized by a direction sensor, the processor being adapted to record the direction signal output in the trigonometric combination of the microphones' output signals.

A directional sensitive sound measuring device according to any one of the preceding claims, characterized by a position sensor and a transmitter, the transmitter being adapted to send the output signal from the processor and the output signal from the position sensor.

Combination of at least two direction-sensitive sound measuring devices according to one of claims 1-9, characterized by a central unit, comprising a receiver adapted to receive the signals transmitted by the transmitters and to calculate the location of at least one dominant sound source based on cross-polling.

11. Combination as claimed in claim 10, characterized in that the central unit is arranged for storing the transmitted signals and is arranged for calculating the location of the at least one dominant sound source in elapsed time windows on the basis of cross-polling. 5

12. Combination as claimed in claim 10 or 11, characterized in that the central unit shows a visual display device for visually displaying the position of the dominant sound sources.

A method of measuring for obtaining the sound level and direction of a dominant sound source within a time window, comprising the steps of providing at least a set of at least two pressure gradient microphones having substantially the same directional characteristics, which are at the same position and with a mutually different orientation, determining the average squares values of the square root of the mean squares of each time window and determining trigonometric combinations of the quadratic signals in the same time window.

A method according to claim 13, characterized in that the microphones have subcardioid characteristics and the square root of the average of the output signal of the microphones is determined within the time window for performing trigonometric combinations.

15. Method as claimed in claim 13 or 14, characterized by subjecting the output signal of the microphones to frequency paths.

A method according to claim 13, 14 or 15, characterized by including a signal representing the orientation of the microphones in the trigonometric combination of the output signal of the microphones. 30

Method according to one of claims 13 to 16, characterized by including a signal representing the position of the microphones in the trigonometric combination of the output signal of the microphones.

A method according to any of claims 13-17, characterized by receiving the signals transmitted by the transmitters of the multiple sets of microphones and calculating the location of at least one dominant sound source based on cross-polling. 5