Building Acoustics

Room acoustics 115

The unfiltered impulse response may now be filtered in either octave or one-third-
octave bands to arrive at the reverberation time in these bands. This is carried out using
octave bands with centre frequencies 125, 250 and 500 Hz and the decay curves are
shown in Figure 4.5. Fitting straight lines to these curves, one will find that the time for
the sound pressure level to decrease 60 dB is 1 second, which was input to the
calculations using Equation (4.15). For simplicity, the decay curves are not calculated
using the integration procedure given by Equation (4.2) but by a running short-time (50
milliseconds) integration of the squared response. In fact, such a procedure simulates the
working of the old level recorders.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Time (s)

-80

-70

-60

-50

-40

-30

-20

-10

Relative sound pr

essure level (dB)

500 Hz

250 Hz

125 Hz

Figure 4.5 Decay curves in octave bands with centre frequencies 125, 250 and 500 Hz, calculated from the
impulse response shown in Figure 4.4.

To conclude on this topic, we shall present examples of transfer functions based on
impulse responses obtained in a real room like the one shown in Figure 4.1. The purpose
is, for one thing, to show that transfer functions obtained in real rooms have the character
as calculated and depicted in Figure 4.3. We shall use transfer functions based on
impulse responses measured in the same auditorium as the one used for measuring the
impulse response in Figure 4.1. The result is shown in Figure 4.6 where the sound
pressure level (arbitrary reference) is given for the frequency range 100–200 Hz. One of
these curves corresponds to the impulse response shown in Figure 4.1, for the other two
curves the axis of the loudspeaker source is rotated 30° and 60°, respectively, from the
horizontal plane. It goes without saying that the results exhibit the expected deterministic
behaviour depending, among other factors, on the physical dimensions of the room.