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8.4 Bass reinforcement
By bass reinforcement, we mean the effect of the room boundaries on the propagation of
sound. It is widely known that speaker placement relative to the floor and walls can affect the
relative amount of bass that the system produces.
To make this interaction more clear, let us refer to the optical analogy of the candle. Similarly
to the way that the mirror reflected the light of the candle, so can the surfaces near the
loudspeaker reflect the sound waves back into the listening room. However, when the path-
length-difference of the reflected wave is short relative to the wavelength of the sound, the
reflected wave is substantially in-phase with the original wave. When this condition is met,
the coupling coefficient between the speaker diaphragm and the air increases, and the
speaker efficiency increases. This changes the actual frequency response of the speaker,
and is not attributable to standing waves or other room resonances.
By selecting the distance from the speaker to the reflective surface, we can determine the
frequency at which the bass reinforcement takes effect. Please see Figures 8.3 and 8.4.
Furthermore, there are typically three reflective surfaces near each speaker, the floor, the rear
wall and the side wall. Each of these surfaces produces its own reflection, and hence
additional bass reinforcement.
Figure 8.3 - Change in frequency response resulting from placement of speaker 3.3 feet from a reflective
surface (relative to an anechoic environment).
Figure 8.4 - Same conditions as above, except speaker is 6.6 feet from the reflecting surface. Note how the
reinforcement now occurs at a lower frequency.