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 9a 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 Figure 9b, 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 (Figure 9d) 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 omni directional 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.
Figure 9a. | Figure 9b. Half Space |
Figure 9c. Quarter Space | Figure 9d. Eighth Space |
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