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Big errors in your thinking

Posted by David Aiken (A) on May 23, 2011 at 16:18:50

In Reply to: RE: Second thickness of gypsum posted by Richard P on May 23, 2011 at 10:42:21:

"how is it that, for example, a 60Hz fundamental with a wavelength of nearly 19 feet can be "reflected" by anything (rigid or not) until the fundamental's period is complete?"

All frequencies are reflected, whether the walls are rigid or not. As far as bass performance in a room goes, 3 factors are important:

1-when a room dimension is exactly the wavelength of the tone or an exact multiple of that, you get modal behaviour and that means fixed zones of reinforcement and cancellation along that dimension of the room.

2-stiffness of the wall effects how effectively the wall reflects the sound. A very stiff wall reflects bass frequencies more effectively than a flexible wall. More effective reflection also strengthens both the reinforcement and cancellation that occurs at modal frequencies so the behaviour noted in 1 above is more apparent in a room with stiff walls than in a room with flexible walls. Flexible walls reflect less at bass frequencies than do rigid walls, and that reduces the severity of modal behaviour.

3-you've got 3 axial dimensions in a rectangular room, more in some other shaped rooms. If 2 room axes have modes at the same frequency, the reinforcement and cancellation occurring when a mode is excited can be reinforced even more.

At non-modal bass frequencies, the reflections don't "overlap" precisely and therefore don't result in the reinforcement and cancellation that occurs at modal frequencies.

Modes have a bandwidth and it tends to be narrow in frequency. At low frequencies there are few modes per octave so each mode tends to "stand out", to be quite obvious. As the frequencies of the modes increase there are increasingly more modes per octave and while their bandwidths remain narrow, the greater number of modes per octave starts to cause the bandwidths to overlap and that starts to smooth out the modal behaviour which is why modal problems occur only at bass frequencies. Looked at over the whole audible range, the problem at bass frequencies is not that you find modal frequencies in that range but rather that you don't have anywhere near as many of them occurring as you have at higher frequencies. Modal behaviour can't be avoided in an enclosed space but the more modal frequencies you have per octave and the more they overlap, the less of a problem they are. The problem at bass frequencies is that there aren't enough of them to smooth the room response.

Reflection also has nothing to do with directionality in this regard. It's the reinforcement and cancellation which occur at fixed points in the room which are the issue. Strong reflections can influence our perception of the direction from which a sound is arriving but that's less related to frequency and more related to strength of the reflection.

If your basement listening room was open to the larger cellar area, Ie if it had a permanently open entryway or you listened with the door open, you would have the 3 axial dimensions of the room generating modal frequencies plus you'd also get some more modal frequencies generated by the dimension of the axis running between the walls opposing each other through the opening, plus the coupled space of the cellar outside the listening room would also influence low frequency behaviour in the room.



David Aiken

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