A loudspeaker can be
modelled as a piston-type source. This is a flat solid disk set in an
infinite plane baffle that moves to and fro along its axis of symmetry. It
is possible to calculate exactly the sound pressure fluctuations produced
at locations well away from the piston (the acoustic far-field).
From the animations below,
it can be seen that the radiation pattern of a loudspeaker is different
at low, medium and high frequencies.
At low frequencies, the
sound field radiated by a loudspeaker spreads out evenly in all
directions. For this animation, the acoustic wavelength
is about four times larger than the disk radius
frequencies, when λ
1.5R, the sound pressure
produced by a loudspeaker is mostly
contained within a cone around the piston axis. So, at
these frequencies, you will hear louder sound levels in front of the
speaker. If you move to either side it will get quieter.
At even higher frequencies,
0.7R, the sound field radiated by a loudspeaker is constricted within a
narrower cone around the piston axis. Now, the sound
pressure levels fall off very rapidly as a listener steps away from in
front of the loudspeaker. You can test this by listening to your hi-fi
loudspeakers from the side or the back. The sound will be 'muddier'
there because the high frequencies will only reach you by reflection from
the walls or objects in the room.
The degree of 'beaming' of the sound is governed by the
ratio of the size of the piston to the wavelength of the sound.
Since real loudspeakers are not infinite baffles the edges of the box can
also cause diffraction of the sound waves resulting in a much more complex
radiation pattern. All these effects have to be taken into account
when designing loudspeakers.