106 Building acoustics
combining the principles outlined above. A number of them have the possibility of
simulating simple types of scattering effect.
4.3 Room acoustic parameters. Quality criteria
The parameters used for assessing the acoustic quality of a room obviously depend on its
intended use. Whereas the reverberation time and/or the sound level reduction by
distance from the source may be sufficient in an industrial hall, a more comprehensive
set of parameters must be used in e.g. concert halls. It is acknowledged that the
reverberation time has an important role and there is sufficient background experience on
how long or short it should be depending on the size of the room and related to the type
of the performance room; theatre, room for music performance etc. As for music
performance, the type of music will be a vital factor; see e.g. Kuttruff (1999).
A number of other parameters that correlates well with the subjective impression
are based on data calculated from measured impulse responses in the room; see ISO
- An example is shown in Figure 4.1, a measured impulse response using an MLS
technique (see section 1.5.2).
Figure 4.1 A measured impulse response in an 1800 m^3 auditorium using a MLS signal (sequence length of
order 16 and sampling frequency 25 kHz of which only every second point is shown). After Lundeby et al.
(1995).
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Time (s)-2-10123Relativ sound pressureIrrespective of the intended use of the room, whether for speech or music, it is
important to design the room in such a way as to give a balanced set (in time) of the early
reflections onto the audience area. Reflections following the direct sound within a time
span of approximately 50 milliseconds will contribute to the strength of the direct sound.
A listener will not perceive these reflections as a separate part or as an echo, but will if a
strong reflection has a longer delay. This phenomenon is called the precedence effect or