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RE: Here's my challenge to you.

Posted by KlausR. (A) on January 23, 2008 at 04:21:27

In Reply to: Here's my challenge to you. posted by David Aiken on January 21, 2008 at 14:16:08:

I jumped into the thread after you had recommended, amongst other things, to treat first reflection points and the questioner had asked a specific question regarding this issue.

Re: Standing waves
You have standing waves at ALL frequencies by superposition of a wave with its reflection from a room boundary which creates an interference pattern:

Waterhouse, “Interference patterns in reverberant sound fields”, J. of the Acoustical Society of America 1955, vol.27, no.2, p.247

When you speak of reflection treatment in general terms you don't think of standing waves, do you?

Be that as it may, standing waves come in two forms, the common standing wave and, when particular requirements are fulfilled, the resonant standing wave or mode. When the sound source is stopped, only the modes will continue to ring, their decay time is longer than for the non-modes.

Standing wave is not equivalent to mode so you are using the term incorrectly.

When I play pure sine tones I detect the interference pattern very easily when walking around the room. When I play synthesizer music where sustained notes are common such pattern is somewhere between very weak and non-existing. When I play pop, rock, jazz or classical, I don’t detect any pattern at all. So I wonder how big a problem interference patterns are in real listening situations.

> that [modal] behaviour will also be problematic over a wider range than it would in most normal, rectangular rooms.<

The frequency range where modes are widely spaced as to possibly cause problems is determined by the room’s size, not by its shape. In the room in question (12x12x12) the transition to closely spaced modes will, according to Schroeder’s formula fc = 2000 sqrt RT60/V occur at about 240 Hz (assuming 0.7 s for RT60).

> Tell me, do you think Toole would say that there are no reflection problems in a cube given standing wave behaviour in a cube? I somehow doubt it.<

Toole says : “The inevitable conclusion is that, in natural listening, room reflections are not problems.” Of course, he did not mean those reflections which generate room modes. Of course, he did not mean late reflections in the case when there are too many.

> What advice would you give to a person with a cube shaped room who wants to use that space as a listening room? There tends to be general agreement that a cube is the worst shape for a listening room and we have a cube here. What would you do with it?<

There is also general agreement about optimum room dimensions. That these people are fundamentally wrong has been shown.

Advice: First, I won’t gove the advice to treat first reflection points becausev there is no evidence that first reflections are a problem.

Second, I would give the advice to first try to find out whether or not the cube shaped room creates mode problems at all. Everyone seems to be sure that a cube shaped room is a problem, whatever the circumstances are. In order to have a problem, in any room, the modes have first to be excited and then they have to be perceived at the listening position. So my advice would be to place speakers and listening chair where it suits best and listen to normal music. If you play music instead of test signals only those modes that correspond to frequencies in the music will be driven, and only those modes will ring on when the signal stops. However, music is a continuous and the passage following the one which is able to excite a room mode may mask the ringing. So maybe the speakers are in a position where they don’t excite the modes and/or the listening chair is in a position where the excited modes are not perceived in full glory.

Further, when you measure SPL at listening position you always must keep in mind that a mike operates linearly, unlike human hearing which operates non-linearly. If you look at curves of equal loudness you see that for the same perceived loudness the absolute SPL may differ by up to 20 dB in the sub 100 Hz range. Which means that you have to apply A-weighting or similar to the absolute SPL figures.

Should there be a mode problem at listening position when playing music, the obvious solution is to change positions of loudspeakers and listening chair. In my room I have detected so far one mode, when I move my head 20 cm forward, I’m out of the high pressure zone.

Should nothing of the above work, one could apply treatments as proposed already. Another solution could be to use narrow-band equalisation or four subwoofers placed on the walls midway between the corners:

Welti, “How many subwoofers are enough”, Audio Eng. Society preprint 5602 (2002)
Welti, “In-room low frequency optimization”, Audio Eng. Society preprint 5942 (2003)
Welti, “Low-frequency optimization using multiple subwoofers”, J. of Audio Eng. Society. 2006, p.347


>In this particular case the original poster also said "I really like detail, imaging, and really enjoy hearing the soundstage extend outside the speakers.<

Since I could not find any research at all on this issue I would not know what actions have which effect.

> (incidentally I hear the soundstage often extending on the right side where the boundary is bare drywall, almost never on the left where the boundary is a large blinds-covered window)<

This looks like a matter of higher SPL from the right side which generates an interaural level difference such that the apparent source is shifted to that side. That’s perfectly in line with psychoacoustic knowledge. The measure to take would be as a simple as using the balance button on the amp! If there’s no such button, a change in toe-in might work or an increase in distance of right speaker to listening chair. Placing the chair off-center might work as well. You obviously could envisage to create a situation where right and left reflection points (read: wall surfaces) are of similar (or identical) acoustic nature.



Klaus

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