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Old 6th March 2013, 06:06 PM   #1391
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I think that some of use understand how easy things become with true inwall enclosures taking the front wall somewhat out of the enclosure as close to an infinite baffle as we can create but what of on wall speakers? Why couldn't a shallow on wall enclosure with angled sides or smoothly radius-ed sides blending into the walls come very close to approximating the inwalls? With very little delay in reflections wouldn't this be nearly ideal rather than a box out in the room and trying to match all the room reflections?

Last edited by Kindhornman; 6th March 2013 at 06:08 PM.
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Old 6th March 2013, 06:21 PM   #1392
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Hi Rudolf
The difference between the with and without cases, show how much difference that made to the MTF measurements.

The drooped curve or arch shape shows how far from “measurement ideal” the system is, “perfect” is a flat line straight across.

But the re-occurring theme seems to be like with optical resolution, the MTF’s usually degrade at the highest rates first.

In Ra7’s measurements of two different loudspeakers, you can see they are very different at the higher modulation rates.

An easy way to change or see the overall effect of the direct to reflected ratio would be to take two sets of those measurements, one at the listening position as it is experienced and another say 1, 2 or 3 feet in front of the loudspeaker (which will present a higher direct to reflected level while everything else in the system is unchanged).

A third measurement could be taken at the same distance outdoors where there are no room reflections to see what part / magnitude may be the room vs possibly everything else.

While the ARTA MTF’s only go out to 20 Hz modulation (15hz was the upper limit for the STIpa measurements), at the seminar the fellow presenting said they measured out to 30 Hz and while not important for speech, was still detectable.

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Tom
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Old 6th March 2013, 08:02 PM   #1393
Elias is offline Elias  Finland
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Originally Posted by Tom Danley View Post
While the ARTA MTF’s only go out to 20 Hz modulation (15hz was the upper limit for the STIpa measurements), at the seminar the fellow presenting said they measured out to 30 Hz and while not important for speech, was still detectable.

Best,
Tom

It is way too limited as an analysis tool. Perception of modulation frequencies goes all the way up to 1kHz.

See for example:
Neural Processing of Amplitude-Modulated Sounds, P. X. JORIS, C. E. SCHREINER, AND A. REES, Physiol Rev 84: 541-577, 2004; 10.1152/physrev.00029.2003.
http://physrev.physiology.org/content/84/2/541.full.pdf

Click the image to open in full size.


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Old 6th March 2013, 08:03 PM   #1394
Rudolf is offline Rudolf  Germany
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Originally Posted by gedlee View Post
How is it that the impulse responses are not zero before the impulse?
Sorry Earl,
that is a problem of perception. My fault - it's not the usual way of presenting an IR . The start of the diagram is already 2 ms into the IR. The dark red line at the right end of the diagram is the end of a 12 ms gate. Look at the picture as a magnifying glass into the response. It is meant to show the effectiveness (or whatever) of the reflection suppression.

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Old 6th March 2013, 09:24 PM   #1395
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Originally Posted by gedlee View Post
I don't think that building speakers onto a wall, toed-in at 45 degrees, and in a way that has minimum diffraction is as easy as you suggest.
Could be eminently doable (depends on how handy someone is), and on the plus side you get a sensitivity boost at the bottom end, if not also a flattening of the power response.
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Old 6th March 2013, 11:28 PM   #1396
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Hi Elias
I am not sure your following this, I didn’t suggest it stopped at 15 or 30, I asked the fellow who did the research at the seminar I went to how high they measured because I suspected this was (the ability to convey or preserve specific auditory information) was also related to stereo imaging etc.

My thought was that since the STIpa measurement is now the main legal standard in Europe for measuring language independent intelligibility in emergency warning systems (coming here too) and this dealt with preserving or destroying auditory information that had a powerful effect on what one hears, that it might be of interest.

Personally I have always been interested in the ways loudspeakers don’t do what they are told by the signal and while your link deals with perception , what I am talking about is if a loudspeaker is faithful to the input or not, using that measurement domain. I was at the seminar because primary sound systems are increasingly doing double duty as the emergency warning system for those spaces and we sell those to installers and contractors at work.

I am not surprised modulation rates are detectable to 1KHz (limiting it to much higher frequency bands ) but the issue is that if you look at the measurements the two guys have posted, you can see they are far from the ideal even by 20Hz modulation.

For speech intelligibility, that work showed that 12 to 15Hz was sufficient to encompass Voice intelligibility. If you didn’t read the earlier posts, a set of 7 MTF’s in bands from 125Hz to 8Khz, speech spectrum weighted are used to arrive at the STIpa figure.

Here in RA’s earlier post we have the individual MTF’s for two different loudspeakers, one sounds more dynamic than the other. The MTF’s show (for the same frequency band) the more dynamic one generally has a higher MTF’s at higher rates and at that limited to rates 20Hz and below. Take a scan at his two speakers, keep in mind what is signal perfect would be straight across.

Understand too, I am in no way suggesting this is related to the pleasure-ability or niceness, only that in a way similar to the voice, some kinds of important spatial information are also carried in amplitude modulation.
While examination of the MTF’s don’t tell you anything about what to fix, it could at least be a compass in an area where it is hard to define true North.
Best,
Tom
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Old 7th March 2013, 01:31 AM   #1397
soongsc is offline soongsc  Taiwan
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Originally Posted by markus76 View Post
My conclusion would be a) make the front wall absorptive as much as possible or b) use in-/on-wall speakers.
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Originally Posted by Pano View Post
I know that's what most people do, but I disagree. Good diffusion is usually better. The problem is that it's expensive.
It seems reasonable to combine the two with good design considerations, which is what I always had in the back of my mind.
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Old 7th March 2013, 02:03 AM   #1398
soongsc is offline soongsc  Taiwan
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Originally Posted by markus76 View Post
Why not simply build the speakers into the wall or at least move them as close as possible to the wall?
I have not built into the walls, but with the baffle 15cm in front of the wall, some absorption is still desirable. A good way of doing it is having the absorption material up to or slightly in front of the baffle. Of course this will not work for dipoles.
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Old 7th March 2013, 07:04 AM   #1399
lolo is offline lolo  France
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Originally Posted by gedlee View Post
Where does one find one of those? Except on paper.
Oh please, come on! A little bit of honesty.. there are at least two in existence, and please note I wrote "fairly CD" as I knew you would chime in.. as always.
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Old 7th March 2013, 08:21 AM   #1400
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Could be eminently doable (depends on how handy someone is), and on the plus side you get a sensitivity boost at the bottom end, if not also a flattening of the power response.
You'll get high(er) directivity at low frequencies which is very hard to achieve. You'll also get more SPL for free.

I'd think linear distortion due to edge diffraction is minimized if the baffle is large. Edge diffraction is more severe with smaller baffles.

Destructive interference from the front wall reflection can be avoided if the enclosure is very shallow. This shifts front wall interference higher up in frequency. If the woofer is large enough and has therefore already started to radiate into 2pi space, that front wall interference simply won't emerge.
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