Air_US_rev_111.qxd
BACKGROUND INFO - ACOUSTICS
Standing Waves
Standing waves exist in all kind of rooms. The shape of the room, the dimensions of the room, and the relationship between the dimensions of the room, are important parameters that will determine the frequencies around which the phenomenon exists as well as the distribution of these standing waves. But how do they occur?
Imagine a sound source. When the sound is emitted the sound wave will propagate in all directions if no obstacles in sight. This will of course happen with the speed of sound. Now, if the sound source is placed inside a room the sound wave will hit the boundaries of the room. If the boundaries consist of acoustically hard (reflective) surfaces, the sound is reflected. If the angle of incidence is
The sound field is initially radiated having a radial wave front but within a few reflections the sound field has obtained a plane wave front.
90º the sound will be reflected right back where it came from. Under certain circumstances the sound wave will meet itself again. For instance if the sound is reflected between to parallel walls. This becomes a problem, when the sound wave not only meets itself, but when it meets itself in phase. And this will happen when the distance between the walls is half a wavelength of the radiated sound wave. Or one whole wavelength - or 1½, 2, 2½ and so on.
This phenomenon is called standing waves. Actually the sound wave is not standing. But it is experienced like that because the sound pressure maxima and minima are positioned in fixed places in the room.
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