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Psychoacoustic effects of front and back reflections?

oivavoi

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I have read quite a bit psychoacoustic research about room acoustics, reflections etc and how it affects our perception. But one thing is striking: There seems to be quite a bit of research about lateral reflections, and also some research on vertical reflections. But I struggle to find any good studies on the effect of front and back reflections - that is, reflections which either come from the front, the same direction as the direct sound, and reflections which come from the back, behind the listener. There seems to be some informal consensus among acousticians and researchers that the reflections that are the least beneficial are reflections from the front, but I haven't been able to find out what studies this view is based on.

This is especially relevant for those of us who have a liking for dipole- and omni-speakers, which by necessity leads to more reflections from the front.
 

amirm

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There is a good bit of research here and when I have time, I will post it. As you can imagine, it is performed by likes of Dr. Toole and Sean Olive.

Briefly, the beneficial effects of side reflections is not there. So if the room is not too dead already, absorption or diffusion that bounces the sound to side walls can be used.
 
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oivavoi

oivavoi

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Thanks Amir! Looking forward to it. Recently read the last edition of Toole's book, and if there were any references to studies on front wall or back wall reflections there, I must have overlooked them. Also noticed that Toole chose the Mirage M1 as his personal loudspeaker of choice when living in Canada. Apparently they received higher ratings than any other loudspeakers in the NRC blind tests, I think. They were bipole and quasi-omni, and thus sent lots of sound back to the front wall. I struggle to reconcile the high ratings of the Mirage M1 with the dominant view that front wall reflections are so detrimental.

There was also an experiment from Bech in 1994 where different loudspeaker directivities were compared (reported here), in which dipole and omni speakers received higher ratings than other speakers, when optimally placed.

So I'm a bit confused as to what the systematic listening data actually implies at this point - what the data shows, and how far any results can be generalized beyond the experimental conditions.
 

Blumlein 88

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I have read quite a bit psychoacoustic research about room acoustics, reflections etc and how it affects our perception. But one thing is striking: There seems to be quite a bit of research about lateral reflections, and also some research on vertical reflections. But I struggle to find any good studies on the effect of front and back reflections - that is, reflections which either come from the front, the same direction as the direct sound, and reflections which come from the back, behind the listener. There seems to be some informal consensus among acousticians and researchers that the reflections that are the least beneficial are reflections from the front, but I haven't been able to find out what studies this view is based on.

This is especially relevant for those of us who have a liking for dipole- and omni-speakers, which by necessity leads to more reflections from the front.

The Linkwitz article cosmik linked earlier talks a little about it. Diffusion of the wall behind the speakers is something of the conventional wisdom. Don't have much experience with omni speakers though lots with dipoles. The rear wave of an omni or bipole (Mirage M3 for instance) will be out of phase when it bounces back. While a dipole starts off out of phase and bounces back in phase so to speak. Really weak anecdotal testimony having heard the Mirage M3, back to back Thiel CS 2, and back to back Snells it seems the dipoles have a more airy spacious sound than the bipoles (of course that might be for any number of reasons beyond just bipole vs dipole). That well could be as much from the bipoles have deeper higher level bass as anything however.
 

RayDunzl

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The rear wave of an omni or bipole (Mirage M3 for instance) will be out of phase when it bounces back... While a dipole starts off out of phase and bounces back in phase so to speak.

Isn't the phase of the reflection relative to the direct wave frequency dependent?
 
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fas42

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IME room acoustics and reflections are all handled inside the brain, when the actual output from the speaker drivers is of adequate quality. If the latter is not true, then our inner hearing, processing systems need a helping hand - by using speakers with differing dispersion characteristics, or adjusting room reflections. Since the maths of getting the room to do all the "right" things gets messy, very fast, it's a lot easier to use the brain for this sorting out stuff - feed it good quality 'food'.
 

Blumlein 88

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Isn't the phase of the reflection relative to the direct wave frequency dependent?

Yes of course. I simply was pointing out that the rear of a dipole is 180 degree out of phase and then reflects back the other direction where as a bipole is in phase with the front initially and reflects back. So the reflected wave if both are in the same position in the same room is in opposite phase for the two types. Now how the reflected wave cancels or enhances the front wave at any given position or frequency is dependent upon all those other things. Not to mention bipoles are almost always smaller radiating widths than dipoles.
 

Cosmik

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The rear wave of an omni or bipole (Mirage M3 for instance) will be out of phase when it bounces back. While a dipole starts off out of phase and bounces back in phase so to speak. Really weak anecdotal testimony having heard the Mirage M3, back to back Thiel CS 2, and back to back Snells it seems the dipoles have a more airy spacious sound than the bipoles (of course that might be for any number of reasons beyond just bipole vs dipole). That well could be as much from the bipoles have deeper higher level bass as anything however.
But in terms of transients and the time domain - where much audio information used by our hearing comes from - what bounces back from the rear of a dipole is inverted relative to what came out of the front - surely something very unlikely in nature. If our hearing system is relying on correlating the reflections of transients with the direct sound in order to separate them from the source (however it does it), it cannot do so with an inverted reflection.
 

Blumlein 88

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I can tell you a crossed pair of figure 8 microphones (dipoles) played back over a properly angled pair of electrostatic panels (dipoles) sounds about as real as I have heard any two channel stereo recording sound. Much due to the accuracy of space and imaging of the reproduction vs the original recording. Things sound the right size and in the right position.

Maybe that is because the back of a dipole mic is also out of phase with the front and it gets flipped with a dipole speaker upon playback. Maybe it is more than that.

Maybe it is related to the difference between a pressure transducer and a velocity transducer.
 

Thomas savage

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IME room acoustics and reflections are all handled inside the brain, when the actual output from the speaker drivers is of adequate quality. If the latter is not true, then our inner hearing, processing systems need a helping hand - by using speakers with differing dispersion characteristics, or adjusting room reflections. Since the maths of getting the room to do all the "right" things gets messy, very fast, it's a lot easier to use the brain for this sorting out stuff - feed it good quality 'food'.
That’s not true though.
 

RayDunzl

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But in terms of transients and the time domain - where much audio information used by our hearing comes from - what bounces back from the rear of a dipole is inverted relative to what came out of the front - surely something very unlikely in nature.

Do drum heads (or a natural equivalent) exist in nature?
 

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Brings to mind:

 

Cosmik

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fas42

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That’s not true though.
True for some, most probably not for all. All the systems I have evolved or come across that have this quality share the attribute that there is a great ease to the listening, that it projects a sense of specialness irrespective of where one is in the locality; and there is no disagreement amongst those listening about such being the case ... unfortunately, those positive qualities can evaporate very easily, if there happens to be some slight issue - the sound degenerates back to ordinary audio, with all normal, well known behaviours.
 

Cosmik

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Cosmik

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@fas42

Off topic, but thanks for the term 'Bulverism'. I hadn't heard that one before - it's a good one!
 

Wombat

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@fas42

Off topic, but thanks for the term 'Bulverism'. I hadn't heard that one before - it's a good one!


Bul(v)emia: An audiophile disorder that prevents consumption of unpalatable facts. :cool:
 
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oivavoi

oivavoi

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But in terms of transients and the time domain - where much audio information used by our hearing comes from - what bounces back from the rear of a dipole is inverted relative to what came out of the front - surely something very unlikely in nature. If our hearing system is relying on correlating the reflections of transients with the direct sound in order to separate them from the source (however it does it), it cannot do so with an inverted reflection.

I tend to agree with Cosmik here. Point source dispersion which is not out of phase in any direction seems more "logical" to me, given that reflections will be more naturally related to the direct sound that way. In theory, I would expect it to work better. Having said that, I've heard several dipole setups that I've really liked, and which didn't sound "unnatural" at all. But it's difficult to assess unless one does A/B comparisons, because our ear/brain seems to adapt to almost anything (nobody would use turntables otherwise).
 
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