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RE: Echo threshold

Posted by KlausR. (A) on July 28, 2010 at 06:35:26

In Reply to: RE: Echo threshold posted by josh358 on July 23, 2010 at 19:50:35:

>>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 reflection.<<

>If you're fond of 40 dB nulls in the midrange. There really is a science here, whether Toole has discovered it or not.<

There is science, and there are thresholds:

http://repository.tudelft.nl/assets/uuid:7f0331e3-bc1a-4d7f-8d2a-eb5d6cc04fbf/as_salomons_19951220.PDF

So yes, reflections may generate audible coloration, but not always. In this respect Brüggen, “Coloration and binaural decoloration in natural environments”, Acustica/acta acustica 2001, vol. 87, p.400 presents interesting results.


>Early arrival first reflections impair midrange clarity and imaging.<

The simple experiments I made to test this didn’t show any such effects. There is no scientific evidence to support that statement.

>Why does Toole say that side wall reflections are beneficial in the case of dipoles?<

My fault, I intended to say that reflection treatment may be beneficial.


>One of the main advantages of dipoles is that they minimize side wall reflections without the need for diffusion and absorption at the first reflection points.<

Minimize yes, but to levels below perception thresholds? I was perhaps a bit hasty when saying that dipoles may benefit from reflction treatment. Loudspeakers with odd-looking off-axis response, and that doesn’t exclude dipoles, may benefit.

>>Naqvi’s active listening room looks like very promising approach, but so far no results for 2-channel stereo<<

>Their description of the results are similar enough to what one observes with two channel stereo. It will be nice to have more information about audibility thresholds, though.<

These pilot experiments merely confirm audibility of early reflections in Naqvi’s setup. The cues used for detection are similar or identical to those used by Olive&Toole, Bech, Schubert, Seraphim etc. Obviously, in order to detect the presence of a reflection, there must be a change in timbre, imaging or loudness, this is known since long so the Naqvi study doesn’t present anything really new. If that is interpreted as the reflection having deleterious effects, so be it. You will have noted that there is no agreement among listeners as to what cue is being used for detection (85% of the subjects, most listeners, some listeners).

Naqvi’s paper: how were reflection levels determined, do they represent real-life levels? The panels don’t work below 500 Hz, has this an effect? What about the radiation behaviour of the NXT? Did Naqvi take the absorption characteristics of real walls, floor and ceiling into account for the spectrum of the generated reflections? Are these possible flaws of Naqvi’s study? The paper has passed peer-review, were these (and other) issues discussed?

Klaus

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