Building Acoustics

114 Building acoustics

The result is shown by the dashed line in Figure 4.3. We see that there is a good fit
between this result and the frequency averaged data in the frequency range above 200
Hz. However, it must be noted that we have performed a calculation just for one receiver
position. Determining the emitted power from a source in a standard reverberation room
test (see ISO 3741) the squared sound pressure is space averaged by using a number of
microphone positions. It is interesting to note that this standard requires a minimum room
volume of 70 m^3 (the volume in our example is approximately 64 m^3 ) permitting
measurements upwards from 200 Hz.

4.4.3 Impulse responses and transfer functions

The common measurement procedure today is to determine pertinent impulse responses,
hereby using these to calculate reverberation time, other room acoustic measures and
transfer functions if required. In the preceding section, we calculated the transfer
function between the sound pressure at a given position in a room and the volume
acceleration of a source at another position. Vice versa, by an inverse Fourier transform
of the transfer function we shall arrive at the impulse response, from which we may
calculate the reverberation time and check that it is correct. The latter means that it is 1.0
second independent of frequency, as presupposed when calculating the transfer function
shown in Figure 4.3.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Time (s)

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Relative sound pressure

Figure 4.4 Impulse response calculated from the transfer function shown in Figure 4.3.

The unfiltered impulse response (for the frequency range up to 1000 Hz)
corresponding to the transfer function in Figure 4.3 is shown in Figure 4.4. It should be
noted that when calculating the inverse transform one must ensure that the result, the
impulse response, turns out to be a purely real quantity, which implies a meticulous
treatment of the real and imaginary part of the transfer function.