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8
9. 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 Figure 8a 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, Figure 8b, the sound that is radiated towards the boundary is reflected.
As a result, the sound pressure level can increase by as much as 3dB (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 speakers’ efficiency by up to 9dB.
This effect is not the same at all frequencies. Loudspeakers are only essentially omni
directional at low frequencies (where the wavelength is large compared 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.
Figure 8a Figure 8b
Figure 8c Figure 8d