Presented at the 79th AES Convention
1985, October 12-16, New York

(...cont'd)

If the reflected wave sound pressure is less than 1/4 of the sound pressure of the direct wave measured at the listener's position, a simplification can be written accurately to 3%.

Here both the amplitude and phase distortion in the resultant wave are shown to have a B/A dependence, (the pressure ratio of reflected to direct wave), and also vary with frequency. We can plot these two distortion terms to watch the effects with frequency. For a typical situation, where the speaker is 1 1/4 meters from the corner, we see an amplitude distortion of 25% or +/- 1 dB and a phase distortion of 16* or 9%.

Leading edge of the tone burst will be usually under 20 ms. Of time. The second harmonic of a complex tone transient might be 90 Hz and whose first reflection is delayed some 9 ms. The composite wave misinforms the listener that the 90 Hz tone source quickly stepped forward 15cm, in the middle of the tone burst attack transient. By reducing the strength of the lst reflection, the transient phase distortion and resulting image position jitter is eliminated.

This problem, fortunately, is also easily controlled. The B/A coefficient of the oscillatory parts or both the amplitude and phase components of the resultant wave needs to be reduced. This is accomplished by reducing the strength of the reflected (B) wave rebounding out of the corner.

We've now covered problems and remedies involved in accurately tracking both tone burst transients. We can combine the remedies to control both the attack and decay transients of the tone burst simply by locating a good bass trap in the tri corner behind the speaker. We can simultaneously damp the strength of the first reflected wave, thereby correcting for attack phase distortion, and also damp the resonant modes of the room, thus correcting for decay distortions. We see that both transients of the musical tone burst can be cleaned up through bass trapping in the corner behind the speaker.