Civil Engineering Reference
In-Depth Information
CHAPTER 4
Room acoustics
4.1
INTRODUCTION
In talking about the concept of room acoustics we shall include all aspects of the
behaviour of sound in a room, covering both the physical aspects as well as the
subjective effects. In other words, room acoustics deals with measurement and prediction
of the sound field resulting from a given distribution of sources as well as how a listener
experiences this sound field, i.e. will the listener characterize the room as having “good
acoustics”? When designing for a good acoustic environment, which could be everything
from introducing some absorbers into an office space to the complete design of a concert
hall, one must bear in mind both the physical and the psychological aspects. This implies
having knowledge on how the shape of the room, the dimensions and the material
properties of the construction influences the sound field. Just as important, however, is a
knowledge of the relationship between the physical measurable parameters of this field
and the subjective impression for a listener. Finding such objective parameters, either
measurable or predictable, which correlate well with the subjective impression of the
acoustic quality, is still a subject of research. It goes without saying that the number of
suggested parameters is quite large. The reverberation time in a room has been, and still
is, an important parameter in any judgement of quality. Another large group of
parameters are also based on the impulse responses of the room but here the emphasis is
on the relative energy content in given time intervals.
In this chapter, the primary emphasis will be on the physical properties, partly to
give a background for the most common measurement methods in room acoustics.
Suggested requirements for parameters, other than the reverberation time, will to some
extent also be touched on.
4.2
MODELLING OF SOUND FIELDS IN ROOMS. OVERVIEW
In principle, we should be able to calculate the sound field in a room, generated by one
or more sources, applying a wave equation of the same type as used earlier in the one-
dimensional case (see section 3.6). There we introduced a sound source as a mass flux
q
,
having the dimensions of kg⋅m
-3
·s
-1
, in the equation of continuity. In the three-
dimensional case, we obtain
2
1
∂
pq
∂
2
∇− ⋅
p
+ =
∂
0
(4.1)
.
2
2
t
c
∂
t
0
Solving this equation analytically will normally become very difficult except for simple
room shapes and simple boundary conditions, e.g. an empty rectangular-shaped room
