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-
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.
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