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

(...cont'd)

Rooms sound better when bass trapping is added. Prolonged resonant frequency decay times are reduced; non-resonant frequency rapid decay time is increased, and frequency shifted resonant boom is eliminated. Clearly, bass trapping in the listening room does equalize the tone burst decay constants, in that both the mean and the deviation of decay constants are reduced frequency to frequency.

Pink noise tests are typically used to EQ a room. Curiously, only a minimal 1-2 dB readjustment towards equalization in the mid bass is noticed after the transient features of the burst have been suitably controlled by trapping. The slow sine sweeps tests of a trapped room will show a slight 1-2 dB reduction in peaks and similar increase in levels of the valleys of the response curve. The curve's fine structure however, is obviously cleaned up and sharpness of the variations is softened. This change means the 'q' of the room has been reduced, and typically measured to be a factor of 4.

We've been discussing the decay transient of the tone burst. Now we move onto the second significant feature of the tone burst, it's leading edge, the attack. The critical element in the tone burst attack is phase alignment. It's been long established that the phase shifting of components of a complex musical tone is not discernable for the steady state condition. But phase alignment is easily noticed in the attack transient.

If we analyze the case of a speaker near a corner, we see that two wave trains are simultaneously heard at the listener's position. The direct signal from the speaker is laced with the weaker signal reflected off the nearby corner. If we compare the phase of the reflected wave train with that of the direct wave train, we see that the reflected wave runs through a series of relative phase shifts with frequency due to its turn-around path distance and subsequent time delay.

Now, at low frequency this first reflected wave is not heard as an ambiance effect, but rather as a simple sum effect. When we add two same-frequency wave trains together we get a resultant amplitude and phase shifted wave train that has frequency dependant features, as this formal calculation shows.

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