Building Acoustics

266 Building acoustics

sound or vibration levels vary with frequency, i.e. in octave or one-third-octave bands,
are normally sufficient.
Sound and/or vibration energy are the primary variables in SEA and it may
therefore be classed as an energy flow method. In that respect it is analogous to methods
calculating heat flow in a system. We shall therefore use this analogy to illustrate the
method.
Finally, we shall refer to some additional features important in practical
applications of the method:

• The most important choice is specifying the model, which is solely dependent
  on experience. The calculations themselves are relatively trivial.


• SEA normally works at its best on systems having many cross couplings
  between the elements (subsystems). The accuracy will normally be less when
  elements have a series connection only.


• SEA is most suitable for investigation of the type “what happens if...?”, i.e. in
  situations searching for the effects of modifications.


• Experiments are just as important as the analysis.


• Tools for estimating the accuracy of the results may be difficult to obtain.

7.2 System description

As pointed out in the introduction, the primary variable is the energy in the system. The
other dynamic variables are deduced from the energy. Specifically, the energy is the
modal energy, the energy per mode in the separate elements or subsystems. The modal
density of these subsystems is therefore an important parameter. Furthermore, the energy
loss mechanisms of each single subsystem are characterized by a loss factor and,
analogously, we shall use coupling loss factors to characterize the power flow between
subsystems. Fahy (1998) has pointed out that the latter may not be the best alternative to
describe the energy transport between subsystems, and he suggested using a power
transfer coefficient. We shall not, however, treat this development here.

7.2.1 Thermal–acoustic analogy

To illustrate the terms, we shall as a starting point use a simple model of a thermal
system depicted in Figure 7.1. Two identical components (subsystems) are coupled
together, one of them connected to a heat supply. We shall assume that the thermal
conductivity for both subsystems is high enough making the temperature the same inside
each subsystem.

System 1 System 2

Heat input

Radiation to surroundings

System 1 System 2

Heat input

Radiation to surroundings

Figure 7.1 Energy transport in a thermal system having two components (subsystems).