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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
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.
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.