8. Positioning
When a bass loudspeaker is used in an environment with boundary surfaces, its placement
affects its frequency response. When such effects are properly understood, they can be used
to great effect in producing the desired sound quality without the aid of additional
amplification.
Consider fig (a) in the diagram below, here we see a loudspeaker in free field or anechoic
conditions. We measure its sound pressure level at a distance D, and refer to this as our
reference level, or 0 dB SPL.
If we now place a large reflective surface (i.e., a wall, ceiling or floor) next to the
loudspeaker, see fig (b), the sound that is radiated towards the boundary is reflected. As a
result, the sound pressure level can increase by as much as 3 dB (effectively doubling the
available amplifier power). The loudspeaker is radiating its power into half as much space,
this is known as half space loading. For each additional boundary the SPL can increase by
3dB. Corner placement or eighth space loading can increase a bass speaker’s efficiency by
up to 9dB.
This effect is not the same at all frequencies. Loudspeakers are only essentially
omnidirectional at low frequencies (where the wavelength is large in comparison to the
loudspeaker). At high frequencies sound radiates in a more directional manner. We can
position full range loudspeakers next to a boundary in order to boost the lower frequencies
while the highs remain unchanged.
Coupling, or placing bass cabinets together will also increase bass output.