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Old 26th August 2011, 02:08 PM   #151
graaf is offline graaf  Poland
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Quote:
Originally Posted by LineArray View Post

Just for visualization:

Sound Wave Interference Applet
nice! we can see the destructive interference of stereo crosstalk

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Originally Posted by LineArray View Post
- possible colorations, artefacts etc. arising from "inter speaker"
interaction solely
those problems are related to speaker directivity problem because (Toole, p. 139):
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File Type: jpg crosstalk.jpg (70.5 KB, 194 views)
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Old 26th August 2011, 02:18 PM   #152
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nice! we can see the destructive interference of stereo crosstalk
As perceived by an omnidirectional mono microphone which is simply summing the two wave-fronts at a point in space without consideration for their angle of arrival.

Quite different to what our binaural ears mounted on either side of our head do, at least at higher frequencies above middle midrange frequencies or so.
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Old 26th August 2011, 02:27 PM   #153
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Now primarily talking about the frequency range of say >2Khz,
where ILD is assumed being the dominant cue for localization
i'd like to drop the - surely - provoking question

how important is phase coherent radiation in stereo speakers ?

Given fine enough granularity of phase randomness over frequency
and angle ... that "naked ugliness" can be cured to some
extend at least above midrange.

Surely group delay has to stay well controled within sane
limits.

Even diffusivity of boundary reflections in the room will
increase without additional room treatment.
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Last edited by LineArray; 26th August 2011 at 02:37 PM.
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Old 26th August 2011, 02:32 PM   #154
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Originally Posted by DBMandrake View Post
As perceived by an omnidirectional mono microphone which is simply summing the two wave-fronts at a point in space without consideration for their angle of arrival.

Quite different to what our binaural ears mounted on either side of our head do, at least at higher frequencies above middle midrange frequencies or so.
yes, indeed, in case our sense of hearing it is more complicated but in principle this is it
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Old 26th August 2011, 02:55 PM   #155
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yes, indeed, in case our sense of hearing it is more complicated but in principle this is it
The effects of reduced crosstalk at high frequencies could be simulated by taking the incident angle of each source and feeding it through a HRTF before summing, so in principle it shouldn't be too hard to model, even in a java app like that.

Fortunately for us, cross-talk at high frequencies where there would otherwise be severe lobing/comb filtering effects between left and right speakers (as shown in that example) is not a big problem, provided that there is enough angular separation between the speakers to provide enough occlusion of the opposite speaker from each ear. Less than about 20 degrees separation and it starts to become a real problem. Greater than about 25 degrees and its a noticeable but small problem.
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Old 26th August 2011, 02:56 PM   #156
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Quote:
Originally Posted by LineArray View Post
Now primarily talking about the frequency range of say >2Khz,
where ILD is assumed being the dominant cue for localization
i'd like to drop the - surely - provoking question

how important is phase coherent radiation in stereo speakers ?

Given fine enough granularity of phase randomness over frequency
and angle ...
big problem is that we do not know how exactly our sense of hearing weighs all these different cues in such specific case as stereo speakers playing stereo program in acoustically small room

it is probable that in case of stereo in such a room neither (ongoing - phase) time (ITD - IPD) nor intensity (IID-ILD) cues play decisive role in the process because they are per se contradictory, ambigous and generally unreliable under such conditions

perhaps onset (wavefronts arrival times) time cues are more important? perhaps ISD (interaural spectral differences) cues?

it seems that stereo in a room is a puzzle to our sense of hearing

now - perhaps "phase coherent radiation" is not important from perspective of ongoing ITD because such cues are unreliable under discussed conditions
OTOH in may be important from perspective of onset ITD because of wavefronts deformations due to time smear
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Old 26th August 2011, 02:59 PM   #157
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Originally Posted by DBMandrake View Post
noticeable but small problem.
I don't know but Toole writes about naked ugliness of this problem, perhaps it is not that small
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Old 26th August 2011, 04:03 PM   #158
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Originally Posted by DBMandrake View Post
Fortunately for us, cross-talk at high frequencies where there would otherwise be severe lobing/comb filtering effects between left and right speakers (as shown in that example) is not a big problem, provided that there is enough angular separation between the speakers to provide enough occlusion of the opposite speaker from each ear. Less than about 20 degrees separation and it starts to become a real problem. Greater than about 25 degrees and its a noticeable but small problem.
I think the notion is correct but not for normal stereo angles.

I find if you feed mono pink noise to a stereo pair of speakers and set the triangle between you and the speakers to something conventional (usually 45 to 50 degrees seperation of the speakers when viewed from above) that the comb filtering is quite strong. Hearing directivity or head diffraction is not nearly enough to shade the opposite speaker.

Under those conditions you can shift your head left to right and hear the in and out of phase cancelation quite readily. The effect is of treble coming and going in a "swish swish". You can even find the location of exact time allignment for one ear or the other. (Not both at the same time)

The effect is very different if you move up to the midpoint of the speakers such that the angle between is 160 to 180 degrees. Then the phasiness disappears and you get a smooth shift from one side to the other as you shift across the geometric mid point.

In the first case you are hearing the comb filtering as it exists in space due to the different path lengths. In the second case you have enough hearing seperation that the sum is all in your head.

By the way you can listen to binaural recordings with the wide seperation approach. Unfortunately it images inside your hear rather than in front, much as with headphones.

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Old 26th August 2011, 05:24 PM   #159
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Originally Posted by speaker dave View Post
I think the notion is correct but not for normal stereo angles.

I find if you feed mono pink noise to a stereo pair of speakers and set the triangle between you and the speakers to something conventional (usually 45 to 50 degrees seperation of the speakers when viewed from above) that the comb filtering is quite strong. Hearing directivity or head diffraction is not nearly enough to shade the opposite speaker.

Under those conditions you can shift your head left to right and hear the in and out of phase cancelation quite readily. The effect is of treble coming and going in a "swish swish". You can even find the location of exact time allignment for one ear or the other. (Not both at the same time)
I'll agree there is still certainly quite a noticeable effect, but compare shifting your head from left to right when facing the speakers, to turning your head at right angles and then moving the same direction and amount in the room - the phasiness in the treble is MUCH worse with your head at right angles because the left and right speaker can sum in the nearer ear more or less like an omni microphone.

If it wasn't for the amount of cross-talk rejection that we do get I think conventional stereo reproduction would be something of a disaster, especially for centre mono signals.

50 degrees really isn't enough to minimize phasiness in the treble with movement, in my last post I was referring to the angle from centre to each speaker, so double those figures. My own speakers are only a little bit under 50 degrees at the moment simply because of small room constraints, and I would consider 50 degrees a bare minimum, certainly not optimal angle at least from the point of view of L-R channel crosstalk and comb filtering.
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The effect is very different if you move up to the midpoint of the speakers such that the angle between is 160 to 180 degrees. Then the phasiness disappears and you get a smooth shift from one side to the other as you shift across the geometric mid point.

In the first case you are hearing the comb filtering as it exists in space due to the different path lengths. In the second case you have enough hearing seperation that the sum is all in your head.
I'm not sure you need to go quite that far, I find when you get out to about 60-70 degrees total separation treble phasiness with movement on pink noise is mostly gone, and also that the angular change between where you get phasiness and where you don't is quite small.

I'm guessing this could be the angle where the outer ear is fully occluded by the face for the opposite speaker, and if so, this critical angle is very likely different for different people depending on the shape of their face and ears.

Someone with ears that stick out to the sides a lot may need wider speaker separation to get good stereo reproduction than someone with flatter ears (just a hypothesis)

If the angle is too narrow I find it tends to make the imaging unstable (with head movement) so that you only get a good focused image at one particular spot, and moving a few inches either way messes it up. A significantly wider angle and this hyper sensitivity to sweet spot seems to go away. On pink noise to me it sounds like less comb filtering with sideways head movement when the speakers are more widely separated, which directly corresponds to a more stable centre image.
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By the way you can listen to binaural recordings with the wide seperation approach. Unfortunately it images inside your hear rather than in front, much as with headphones.
I remember trying that years ago with binaural recordings on a couple of full range drivers - each about half a metre away and directly to the sides 180 degrees from each other. From what I remember it sounded very good, but I don't recall whether the sound localized in my head or externally. Immersion was very good though.
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Old 26th August 2011, 06:54 PM   #160
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I'll agree there is still certainly quite a noticeable effect, but compare shifting your head from left to right when facing the speakers, to turning your head at right angles and then moving the same direction and amount in the room - the phasiness in the treble is MUCH worse with your head at right angles because the left and right speaker can sum in the nearer ear more or less like an omni microphone.
I typically use the sideways technique to find the exact center between two speakers. It works especially good if you connect one out of phase. At least part of the improvement is because facing straight forward you get a different "answer" for each ear.
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If it wasn't for the amount of cross-talk rejection that we do get I think conventional stereo reproduction would be something of a disaster, especially for centre mono signals.
In a lot of stereo there is more left/right going on than center and we tend to ignore poor center postion or accuracy. Meridian did a balance control using time shift to let you sit down, use the remote for balance, and hopefully get perfect centering. I remember it took a little practice to hear the effect on typical program.

Real three channel would be a nice improvment. You could have a real center image and you could open the left and right speakers to a wider spread as you like, without fear of a "hole in the middle".

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