144 Building acoustics
Most are developed for applications in performance rooms, i.e. for predicting the
acoustics in rooms for speech and music. The trend is not only to give visual descriptions
of the results but also to present the results by auralization. This implies that one may
listen to music or speech “played” in a room at the design stage. This is accomplished by
a process called convolution; the music or speech signal is convolved by the predicted
impulse response belonging to a given source–receiver configuration.
R 1
R 2
R 3
R 4
R 5 R 6
Figure 4.21 Sketch illustrating the concept of mean free path.It is outside the scope of this book to give an overview or a closer description of
this software based on the principles outlined in section 4.6. We shall, however, give
examples on some special models primarily developed to predict sound propagation in
large factory halls etc. The computer models mentioned above may certainly also be
applied to such rooms but the ones we shall present cover the most important quantities
to be predicted for such rooms: the attenuation of sound as a function of distance and the
reverberation time. These are the analytical image-source models of Jovicic (1979) and
Lindqvist (1982) together with the ray-tracing model of Ondet and Barbry (1989), the
last including scattering in a very ingenious manner.
4.8.1 The model of Jovicic
The aim is to find an expression for the sound pressure level as a function of distance
from a source of a given sound power level, which implies finding how the level
decreases analogous to the results shown in Figure 4.9, however, without the constraint
that the dimensions of the room should be fairly equal. Jovicic’s models are confined to
rooms of rectangular shapes, either “long” rooms, where one dimension is much larger
than others (corridors etc.), or “flat” rooms, where two dimensions are much larger than
the third one. We shall confine ourselves to the latter type, where the following
assumptions are made:
- The influence of the sidewalls are neglected.
- The ceiling is treated as a plane surface like the floor. A serrated ceiling or
ceilings with baffles etc. are treated as scattering objects. - The absorption factor used is the mean value for the floor and ceiling.
- The sound source is placed midway between floor and ceiling.
- The scattering objects, which may also be assigned an absorption factor, are
randomly distributed in the room.