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Speaker boundary interference response (SBIR for short) is a little known and poorly understood issue that is responsible for deep dips or “suckouts” in bass response below the transition frequency. SBIR is caused by the destructive interaction of the direct sound wave from the speaker and the reflected, indirect sound wave from a nearby boundary.
If the difference between the direct and indirect path lengths (where path length is the distance the sound has to travel) is equal to half a wavelength then the two sound waves will combine destructively and a notch in the frequency response will occur. The frequency that interference occurs at can be calculated through application of the wavelength formula as follows:
cancellation frequency = speed of sound / (2 * path length difference)
All of the boundaries in a small room can cause interference and audible suckouts – the ceiling, front wall, back wall, side walls and floor. A couple of illustrative examples follow:
| Direct Path Length, ft | Indirect Path Length, ft | Difference, ft | Cancellation frequency, Hz |
| 11 | 18 | 7 | 81 |
| 9 | 15 | 6 | 94 |
Note the cancellation or suckout around 80Hz is very common in two channel systems, since many people place their speakers around 3-4 ft from the rear wall, which creates a path length difference of 7-8ft.
The following frequency response measurement reveals evidence of SBIR at 79Hz. A good measurement would not show any deep nulls. Note that measurements of frequency response in the bass region need to be taken with high resolution (here 1/24th octave smoothing is used).
For more information read the article from Wes Lachot which explains the phenomenon in more depth and watch the video from RealTraps to see some real life demonstrations.
Nyal Mellor, Founder, Acoustic Frontiers
Nyal Mellor
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