64 Building acoustics
These two main types of process are denoted mechanical and aerodynamic/
hydrodynamic generated sound, respectively. The latter type is normally connected with
the building service equipment such as the air-conditioning system, pumps, compressors
etc.
There are also other types of sound generating mechanism such as explosions, just
to mention another common type, and there are instances where thermal energy may
transform directly into acoustic energy. The so-called Rijke tube is an example of the
latter form of sound generation. Conversely, acoustic energy may transform into other
types of energy, e.g. by the phenomenon of sonoluminence. We shall, however, not delve
further into this phenomenon but concentrate on the mechanisms coupled to sound
transmission in buildings.
The most important aspects concerning sound transmission are the mechanisms by
which building elements as beams, plates and shells generate sound when set in motion.
Questions to be asked could be: Why does a thin panel radiate less sound than a thicker
one even when the vibration amplitude is the same? Why does an additional thin panel
mounted on to a thicker wall being called an acoustical lining? Why is the amount of
sound energy produced by a building element dependent not only on dimensions and
material parameters but also on the way it is excited, by point forces, moments or sound
pressure?
Before moving on to the themes concerning the dynamics of buildings elements,
i.e. excitation, response and sound radiation, we shall use some elementary or idealized
types of sound sources to illustrate the basic properties of sources in general. One will
find the terms monopole, dipole, quadrupole, octopole and so on. Normally, however,
one uses the term multipole when the number of elementary sources exceeds four.
Multipoles of different order are useful in modelling sound radiation from plate-like
structures. However, through the so-called Rayleigh integral we have another efficient
basis for calculating the sound radiation from plane surfaces. A classic illustration of the
use of this integral is in calculating the radiated sound from a plane surface of circular
shape set into an infinite large wall. This type of source is called a baffled piston, which
is used as a first approximation for a loudspeaker in a closed box. Actually, this type of
source has much wider application.
3.4.1 Elementary sound sources
Vibrating surfaces in contact with the surrounding medium, which is air for most
practical applications in building acoustics, give a volume displacement and thereby a
wave motion is generated. A sensible way of calculating the sound radiated from a large
vibrating plate or panel could then be to divide it into small elements, calculate the sound
field from each element and thereafter sum these contributions, i.e. we make use of the
principle of superposition. Normally, this process is not so simple due to the fact that the
sound field from each element not only depends on the geometry of the surface, of which
the element is a part, but also on other neighbouring surfaces. This does not imply,
however, that combinations of elementary sources are not useful for modelling. In
addition, some simple expressions arrived at in this way are useful in practice.
3.4.1.1 Simple volume source. Monopole source
The simplest type of source may be envisaged as a pulsating sphere, an elastic ball
having radius a and where the volume fluctuates harmonically with a given angular