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Old 31st August 2011, 11:19 AM   #221
6.283 is offline 6.283  Germany
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Quote:
Originally Posted by Humdinger View Post
I don't know of a situation where comb filtering is desirable in hi-fi reproduction. It's a fact of life (law of physics) unless you're floating in space more than 75 feet from anything, or in an anechoic chamber, that you would be wise to fully understand and deal with somehow. Maybe you could explain yourself in much more detail. I'm confident that you misunderstood what Toole was saying, or I just don't get what you are saying.
Cause and effect have to be separated !
Reflections in a small room vitually always cause "comb filtering". But there are wanted reflections, e.g. lateral >10ms. As a result you will have comb filtering. But where is the damage then ? All the combs are, is something that can be measured with a microphone.
"Damage" only comes from unwanted reflections, which also causes comb filtering.
So it is not the comb filtering that is bad in general but the specific reflections causing it, e.g. floor reflections for some people here.
Ref. Toole, chapter 9, The Audibility of Acoustical Interfrence - Comb filtering, page 151. The four bullets on the page together with the paragraph above and below.
The reason I write all this is: In forums there are certain buzz words that are being used loosely, uncontrolled and even incorrectly. The rookie readers will likely catch these words and more confusion and BS will emerge from that such as "comb filtering must always be bad. It looks ugly so it must sound ugly". And it takes long discussions to sort it out again. Sorry for making you a victim
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Old 31st August 2011, 11:26 AM   #222
graaf is offline graaf  Poland
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Quote:
Originally Posted by 6.283 View Post
Cause and effect have to be separated !
Reflections in a small room vitually always cause "comb filtering". But there are wanted reflections, e.g. lateral >10ms. As a result you will have comb filtering. But where is the damage then ? All the combs are, is something that can be measured with a microphone.
"Damage" only comes from unwanted reflections, which also causes comb filtering.
So it is not the comb filtering that is bad in general but the specific reflections causing it, e.g. floor reflections for some people here.
Ref. Toole, chapter 9, The Audibility of Acoustical Interfrence - Comb filtering, page 151. The four bullets on the page together with the paragraph above and below.
The reason I write all this is: In forums there are certain buzz words that are being used loosely, uncontrolled and even incorrectly. The rookie readers will likely catch these words and more confusion and BS will emerge from that such as "comb filtering must always be bad. It looks ugly so it must sound ugly". And it takes long discussions to sort it out again. Sorry for making you a victim
exactly!
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Old 31st August 2011, 12:38 PM   #223
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I happened to listen e.g. in/near dipole "nulls" with different arrangements
and also experienced some systems based on high reverberant ratio in the
Khz range.

A friend of mine had an experimental (fun) setup using a small fullranger
aiming into the two upper front corners of his room.

Some of those arrangements sounded "better than expected" to me, if
you get tonal balance right as often the power response of the system
gets more dominant, which is usually falling with frequency.

Such arrangements may have phantom sources sound (very)
wide (ASW), also deep, maybe even somewhat "involving" in a
wide range of the room, but resolution and neutrality is always missing
to an extent, that makes it unacceptable to me personally even if
appropriate EQ is applied.

For easy listening, say in a bar or restaurant, why not.
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Old 31st August 2011, 01:14 PM   #224
graaf is offline graaf  Poland
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Quote:
Originally Posted by LineArray View Post
I happened to listen e.g. ... some systems ... even somewhat "involving" ... For easy listening, say in a bar or restaurant ...
ok! so You have listened to "something", found it even "involving" and so... suitable for "easy listening"... ???

and You think that You can criticize things You haven't heard but which You think are somewhat similar, yes?
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Old 31st August 2011, 02:24 PM   #225
keyser is offline keyser  Netherlands
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Guys! You're moving so fast, I have trouble keeping up just reading! But don't let me hold you back .
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Old 31st August 2011, 03:47 PM   #226
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Great topic guys. This is core-business.

I think that in discussing this topic there are some effects that have to be considered:

- Stereo is a compromise in a spatial sense. You can only hope to listen into the recording venue.

- I think most people agree that early reflections are unwanted. However, when listening to very directional speakers, some people almost experience in-head or very near localisation of the auditory scene. Lateral reflections, which in a natural setting aid localisation, are unfortunately absent with stereo (or arrive only very early from the side walls).

- People differ greatly in their sensitivity and preference for early reflections, as described by Toole in 'Sound Reproduction'. There is no doubt that at least some preferences are learned or differ depending on the situation. Toole writes for example that most recording engineers prefer an absence of early reflections during mixing, but many prefer them for casual listening. Some find stereo to be spatially unsatisfying otherwise, which is not really surprising (see previous point). It may be considered desirable by some to 'enhance' the spatial impression of a stereo system with early reflections.

--

- On the requirement of a flat on-axis response: I'm not convinced that this requirement is correct because of the large difference in HRTFs between 0 and 30 degrees. Most sources (or the most important ones) in stereo reproduction are in the center. Personally I find a slight on-axis HF down-shelving to be desirable with virtually every speaker.

- I'm also not sure about the requirement for either rising directivity. It might be 'necessary' to preserve the balance intended by the recording engineer because all recordings are mixed on speakers with such directivity. It might also be 'required' to compensate for the HRTF coloration of the phantom center. There are proponents for both flat and rising directivity; I'm not sure about either however.

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Old 31st August 2011, 10:17 PM   #227
boris81 is offline boris81  United States
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Quote:
Originally Posted by keyser View Post
Guys! You're moving so fast, I have trouble keeping up just reading! But don't let me hold you back .
You are actually reading all of this?


On this note let me respond to something from over 70 posts a go. I mean *yesterday*

Quote:
Originally Posted by DBMandrake View Post
Direct to reflected ratio does seem to be a critical factor, especially for side-wall reflections, and as you say, the optimal directivity pattern may depend on how dead the room is, along with the listening orientation in the room. (Speakers firing across or along a rectangular room)

I have speakers that are moderately directional above 1-2Khz and I've recently realised that for the room I currently have (fairly small 4.8m x 3.45m and quite reflective/reverberant, with not a lot I can do to change it) that the optimal toe in of the speakers is different depending on whether the listening axis is along or across the room.

For a long time the speakers were on the long wall firing across the short axis of the room. This arrangement worked quite well and put the speakers some 1.5 metres from the side-walls. Imaging was excellent, and I found I got the best results with the speakers toed to converge about 2 feet behind the listening position.

If I toed them to converge in front of the listening position by an equal amount the result was a strong centre phantom image, but very little apparent source width, and a very dry overall sound. I definitely preferred the more toed out configuration.

Some time later the room was reconfigured to put the speakers on one of the short walls firing along the long axis of the room. (For other room use reasons, not for sound...)

For quite a while I was unhappy with the results of this. Although this orientation can work in a large room I've found it generally doesn't work well in a reverberant room this small. It put the speakers less than a metre from the side walls and increased the listening distance considerably, while reducing the speaker to speaker spacing, making the angular separation quite small.

With the speakers in their normal toe in to converge behind the listener configuration sweet spot was narrow and vague, imaging in general was vague and disappointing, and the reverberent nature of the room was all too apparent, with a very strong room characteristic.

Some time later on a "what do I have to lose" hunch I decided to try some strong toe in to cross them over well in front of the listener. (~2 feet) I'm not a fan of crossing speakers in front of the listener but to my chagrin and delight the difference was quite staggering.

It went from sounding like I was sitting well back in the reverberant field with very diffuse imaging to sounding like I was once again in the direct field with pin point imaging, wide sweet spot, and a lot less contribution from the room. In fact overall the result with toeing in front of the listener was very similar to toeing behind the listener in the other room configuration, and overall is very satisfactory, considering that it's the "non-optimal" room orientation.

Thinking about it since then it seems obvious that adjusting the toe in of somewhat directional speakers is directly manipulating the direct to reflected ratio of the side-wall reflections to get a "pleasing" balance.

If there isn't enough side wall reflection, as in the wide room orientation, toe the speakers less, illuminating the side wall more. If there is too much side wall reflection, as in the narrow room configuration, toe them in more.

The implications are interesting, if perhaps a little obvious - if you like precise imaging rather than feeling buried in the reverberant field, more directional speakers are probably better suited to use on the short wall of a rectangular room than speakers with very wide dispersion, and with the directional speakers you can gain more control of the direct/reflected ratio of the first reflection with toe in adjustment. This makes toe in far more critical, but at the same time gives you more control to get the balance you want without needing to change the speaker design or room damping.

On the other hand for speakers mounted along the long wall of a room with side-walls fairly distant, wider dispersion may be more appropriate, and in the case of directional speakers, a fairly minimal toe in. (Crossed behind the listener)

If the toe in is largely about controlling the side wall reflection that also suggests that the optimal toe in angle will change depending on how dead the room is, especially the side-walls.

I have a similar experience with controlling the imaging width and clarity by varying the toe in-out. I'm convinced the ratio of the direct to reflected signal from the sidewalls plays a great role in this. I also suspect that longer time gaps between the direct and reflected sound allow for different tolerances of the intensity of the reflection. When I have the spare time I'd like to try different toe in angles and take some listening tests. I think plots of the Initial Pulse and the first side wall reflection accompanied with the subjective image perception of each speaker position will be enough to make some initial assumptions.

Maybe if more of us are interested in something like this we could organize an official format by which people can conduct such listening tests at home and upload the data to a central repository. If indeed a relation between the sidewall reflections and imaging exists, patterns in the data should be easy to discover.
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Old 31st August 2011, 10:26 PM   #228
Elias is offline Elias  Finland
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Transients, those bästards !

I used to be in that camp also. I like dipole line arrays too, remember! I'm not sure if I still am with that since gained new knowledege, but it certainly feels wierd to think otherwise. Old habits are too deep in me.


Quote:
Originally Posted by LineArray View Post
I was never statisfied with systems that mess up transients too
much. Acceptable reproduction of transients is essential to me.

A (loudspeaker/room) system that gets the transients well, makes me
"lock in" to the music.

See again the document I posted earlier:

Quote:
Originally Posted by Elias View Post
A good one to start on human sound perception in modulation domain:
http://physrev.physiology.org/content/84/2/541.full.pdf

- Elias

Transient is an AM modulation. How fast can it be perceived? How fast is the human auditory system?

From the document we can read -10dB cut off points of the modulation transfer function of the auditory nerve fibers, and they are something like this:
at 500Hz: 100Hz
at 1kHz: 200Hz
at 2kHz: 400Hz
etc

To convert this into the time response in the modulation domain it is something like:
at 500Hz: 10ms
at 1kHz: 5ms
at 2kHz: 2.5ms
etc

It seems to follow constant Q characteristics.

Is this fast? Or rather slow?

At 1kHz the 'speed' is 5ms. How many reflections can fit in this time frame already


What is your "transient" ?

- Elias
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Last edited by Elias; 31st August 2011 at 10:30 PM.
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Old 31st August 2011, 11:07 PM   #229
ra7 is offline ra7  United States
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Scott, how often have you come across a listening room that meets all your criteria? Even a dedicated one may have trouble meeting your criteria.
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Old 31st August 2011, 11:22 PM   #230
ScottG is offline ScottG  United States
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Quote:
Originally Posted by Elias View Post
What's wrong with "tough nut" soffa pillow?

First you test, by any means necessary to get a result, then only try to make it practical. That's the way to do research.

- Elias

..piffle on your sofa pillow.

-FAR more advanced than a sofa pillow. Multi-miniature curtains! This way you can vary the pattern.


I'm thinking to do it with a radial will require 2 identical drivers (side to side) with delay and possible phase rotation. Not exactly my area of expertise.
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