108 Building acoustics
noise from the later part of the impulse response, thereafter compensating for the noise
by assuming that the energy decays exponentially with the same decay rate as the actual
one at a level 10–15 dB above the background level. Such a technique (see Lundeby et
al. (1995)) is used calculating the decay curves shown in Figure 4.2. The impulse
response shown in Figure 4.1 is filtered by a one-third-octave band of centre frequency
1000 Hz and the decay curves are calculated with and without being compensated for
background noise. In one set of curves, the level of the background is equal to the one
present at the time of measurement. In the second set, the background noise is artificially
increased to show that also in this case one will obtain a decay curve having an
acceptable dynamic range. Ideally, all the solid curves should be coincident but this will
only be the case if the decay rate is everywhere the same.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Time (s)-50-40-30-20-100Relative sound pressure level (dB)Figure 4.2 Decay curves based on filtering, one-third-octave band 1000 Hz and reverse time integration of the
impulse response shown in Figure 4.1. Solid curves – integration with background noise compensation. Dashed
curves – integration of the total impulse response. One set of curves is using an artificially added noise. After
Vigran et al. (1995).
4.3.2 Other parameters based on the impulse response
A large number of parameters suggested in the literature and applied over the years are
listed and commented on in ISO 3382. These are all derived from measured impulse
responses, and we shall present a selection of these measures.
The balance between the early and late arriving sound energy, which concerns the
balance between the clarity (or distinctness) and the feeling of reverberation, is important
for music as well as for speech. Several parameters are suggested to cover this matter in
room acoustics. The simplest ones deal with the ratio of the total sound energy received