32 Building acoustics
assume that our physical system is linear within a certain magnitude range of excitation.
This applies to the sound pressure levels we normally encounter within a building. This
may not be the case when exciting structures using a mechanical source. Testing the
transmission of impact sound, one uses a hammer blow or several hammers (the
standardized tapping machine). In such cases one should be aware that the result might
depend on the applied force. The only way to control it is to perform measurements
varying the force level.
In the following we shall refer to examples using simple mechanical systems. As an
introduction, however, it could be useful to address a practical case containing the
aforementioned couplings between acoustical and mechanical components.
2.2 A practical example
Figure 2.1 shows a machine or generator of some kind mounted on a base plate, placed
on the floor in a building. Additionally, due to noise emission it is placed within an
enclosure. The machine needs cooling and therefore openings in the enclosure are
required, the openings are equipped with silencers. The problem to be addressed could be
of two kinds; there could either be an existing problem or there is a question of designing
a system, with the sketched components, to reduce the noise. In the latter case prediction
tools must be available, which again presuppose knowledge of all sound sources.
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Figure 2.1 A machine unit with enclosure and supplied with silencers. Transmission paths for sound energy are
indicated: a) direct transmission through enclosure walls. b) by way of structural connections. c) by way of
openings (silencers and leakages).
Some pertinent questions in this connection will be: how large is the force
amplitude input to the foundation and floor due to the vibration of the machine? How
large is the sound power radiated from the floor into the room below due to these forces?
What will the sound pressure level inside the enclosure be and furthermore, what is the
relation between this sound pressure level and the sound pressure level outside the
enclosure, partly due to transmission through the walls, partly through the silencers?
What is the possibility of “short-circuiting” any of the noise measures by unwanted
mechanical couplings, air leaks etc.?
All these questions require knowledge of transfer functions of some kind, the
relationship between physical variables that will normally be a function of frequency and