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RE: Echo threshold

Posted by KlausR. (A) on July 23, 2010 at 05:22:32

In Reply to: RE: Echo threshold posted by josh358 on July 20, 2010 at 16:37:43:

>Despite what Toole implies in his paper, early first reflections have deleterious effects:
"The requirement to place speakers at least 1m away from reflecting surfaces (measured from the tweeter) had been found empirically for box and panel type loudspeakers. All reflections are then delayed at least 6 ms, except for the floor reflection." <

I had a look at all publications Toole refers to w.r.t. early reflections, and some more, and apart from some indications there is no evidence for the deleterious effects of first reflections. These indications are based on

a. experiments with speakers with variable directivity index (Klein+Hummel O85a)

b. experiments where 4 (identical) loudspeakers were arranged in pairs of 2 such that in one pair the speakers were arranged vertically, in the other pair horizontally

c. Linkwitz’ monopole and dipole (AES paper 7162)

The requirement of placing speakers at least 1m from reflecting surfaces looks as if it was based on the Allison effect, ensuring that boundary reinforcement occurs for frequencies below 34 Hz, which is sufficient to avoid bass boom for the great majority of loudspeakers.


>Linkwitz has made some attempts to codify what engineers, reviewers, and listeners who have played with dipole radiators have known for many years, that side wall first reflections should be suppressed or delayed (in my experience, the more the better) and that rear wall reflections should also be delayed as far beyond 5 msecs as practical and are then instrumental in the reproduction of depth.<

Toole doesn’t exclude the possibility that early reflections may have nasty effects, e.g. in the case of loudspeakers with bad off-axis behaviour (http://www.aes.org/e-lib/browse.cfm?elib=5430). In the case of dipoles, side wall reflection may indeed be beneficial.


>But as you point out, it seems that little systematic research seems to have been done on the issue. It remains for the most part a matter of practical art rather than scientific understanding.<

Naqvi’s active listening room looks like very promising approach, but so far no results for 2-channel stereo: http://www.aes.org/e-lib/browse.cfm?elib=13418

Klaus

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