Method and device for determining acoustic coefficients and acoustic power

1. A method for determining at least one of: the acoustic absorption coefficient of a material, the acoustic transmission coefficient of a material at a chosen position on a relevant surface in a space in which a certain sound field prevails as a result of the operation of a sound-emitting source, and the acoustic power emitted by the sound-emitting source, which method comprises the following steps of:

(a) generating a sound with the sound-emitting source and measuring the sound pressure p(t) and the particle velocity v(t) at the chosen position in space;

(b) calculating the Fourier transforms P, V of p(t), v(t) using a processor configured to calculate the Fourier transforms;

(c) calculating on the basis of P and V using a processor the time-averaged active intensity Iac(f) in the direction indicated with the vector n, being a normal vector on the relevant surface, and the time-averaged total intensity Itot(f), in the direction indicated with the vector n;

(d) determining using a processor the time-averaged incident sound intensity in the direction n:

I in( f )=½( I ac( f )+ I tot( f ));

wherein:

when the method comprises determining the acoustic absorption coefficient of a material the method further comprises: (e) determining the time-averaged absorption coefficient in the direction n:

α= I ac( f )/ I in( f );

when the method comprises determining the acoustic transmission coefficient of a material the method further comprises: (f) determining the transmission coefficient θ=Iac(f)/Iin(f) in the direction n for materials wherein the mechanical absorption can be disregarded; and

when the method comprises determining the acoustic power emitted by the sound-emitting source the method further comprises: (q) defining an enveloping surface around the sound-emitting source; and (r) determining the emitted acoustic power by integrating Iin over the enveloping surface.

2. The method as claimed in claim 1 , comprising the step of:

(g) performing step (c) on the basis of the relations:

I ac( f )=¼( P V · n + P V·n )

and

I tot( f )=¼( ΔcV·n V ·n+P P ( Δc ));

wherein Δ is the density of the air and c is the phase velocity or speed of sound.

3. The method as claimed in claim 1 , comprising the step of:

(h) calculating on the basis of P and V the time-averaged reactive intensity in the direction indicated with the vector n, i.e. Ire(f)=¼ i( P V·n−P V ·n); and

(i) performing step (c) on the basis of the relations:

I ac( f )=¼( P V ·n+ P V·n )

and

I tot( f )=(2/π)arcsin( I ac( f )/√( I ac( f ) 2 +I re( f ) 2 )) I ac( f )+ . . . +(2/π)| I re( f )|

wherein Ire(f)=¼i( P V·n−P V ·n) is the reactive part of the intensity.

4. The method as claimed in claim 3 , comprising the steps of:

(j) measuring the sound pressure p(t) close to the surface in the space and the particle velocity v(t) at a number of chosen positions in the space;

(k) calculating the Fourier transforms P, V of p(t), v(t);

(l) calculating on the basis of P and V the time-averaged active power Pac(f) obtained by integration of Iac(f), as determined under step (i), over the relevant surface, wherein n is the normal vector on the relevant surface, and calculating the time-averaged incident power Pin(f), obtained by integration of Iin(f), as determined under (d), over the relevant surface, wherein n is the normal vector on the relevant surface,

(n) determining the spatially averaged absorption coefficient α=Pac(f)/Pin(f); and/or

o) determining the spatially averaged transmission coefficient θ=Pac(f)/Pin(f) for materials wherein the mechanical absorption can be disregarded,

(s) determining the emitted acoustic power by equating the emitted acoustic power with the incident power.

5. The method as claimed in claim 4 , comprising the step of:

(p) performing step (a) or step (j) by measuring the particle velocity v(t) using two pressure sensors which are disposed such that together they form a particle velocity sensor, and measuring the sound pressure p(t) with one or two of these two pressure sensors, in which latter case the average of the two sound pressures is determined.

6. A device for determining the acoustic absorption coefficient, the acoustic transmission coefficient or the acoustic power emitted by a source with a method as claimed in claim 1 , the device comprising:

a sound pressure measurer for measuring the sound pressure p(t);

a particle velocity measurer for measuring the particle velocity v(t), which particle velocity measurer is disposed in the immediate vicinity of the sound pressure measurer;

a Fourier transformer for calculating the Fourier transforms P, V of p(t), v(t);

first calculator for calculating on the basis of p(t) and v(t) the time-averaged active intensity Iac(f) and the time-averaged total intensity Itot(f); and

a second calculator for determining the time-averaged active intensity Iac(f) and for determining the incident intensity Iin(f); and

a third calculator for determining the absorption coefficient α=Iac(f)/Iin(f) and/or transmission coefficient θ=Iac(f)/Iin(f).

7. The device as claimed in claim 6 , wherein the particle velocity measurer comprises two pressure sensors and the sound pressure measurer comprises at least one of these two pressure sensors.

8. The method as claimed in claim 1 , further comprising generating a signal representative of the absorption coefficient, transmission coefficient or emitted acoustic power with a signal output.

9. The method as claimed in claim 8 , further comprising displaying the signal on a display unit.

10. The method as claimed in claim 1 , further comprising arranging a reflecting wall.

11. The method as claimed in claim 1 , wherein the method comprises arranging the material at a distance from the sound-emitting source.