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post #1 of 15 Old 03-24-2013, 03:29 PM - Thread Starter
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Hi all,

After getting some great help and eduction in my other thread on covering bass traps, I decided I'd pop in for a second bit of education:

There seems to be a couple ideas for treating the walls of your theater. One is to put duct liner around the room up to ear level, the other is to place panels at the early reflection points. I know Ethan (Winer) is an opponent of the former method.. but..

I moved my seats in temporarily to check out where my reflection points are. On my right wall, my 2nd reflections (using the mirror trick) land almost dead center on my equipment rack.

My question is: Does this change which method I should follow? Obviously I can't hang a panel over my poorly placed rack, and I can't move the equipment as the racks are embedded in the wall.


Advice and information always greatly appreciated. Thank you!

-Dave

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post #2 of 15 Old 03-25-2013, 10:09 AM
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There is no one right or wrong answer regarding side wall treatment. In the context of this post I am assuming you are talking about treatment for the screen channels, not surrounds.

In my book the right way to do it is to measure the speaker's off axis response. You do this using an acoustic measurement package and moving the speaker in say 22.5 or 30 degree increments until you get to 90 degrees. You need to keep the speaker to baffle distance consistent.

The results should look something like this...the response below is for a very nicely designed speaker using a constant directivity waveguide. Let's call it speaker A. The main characteristic here is that the off axis response at angles past 45 degrees is noticeably lower than the on-axis response yet it does resemble is spectrally until 3k or so before a smooth taper down in the highest frequencies.



Speaker B, a normal cone/dome type speaker, is very different. The off and on axis response are very similar meaning the speaker 'broadcasts' much more sound out off axis than the speaker above. There is also an obvious discontinuity between 2-3k which is a typical suckout that appears off axis in cone/dome speakers. What happens is that at the crossover point the directivity of the cone and dome do not match. The cone is beaming and the dome has very wide dispersion. On axis the responses might look the same but off axis they do not.



Why is the off axis response important? Well what you hear in most normal sized domestic listening rooms is a combination of the direct and reflected sounds. Our ears are particularly sensitive to lateral reflections so what you do with them can make a big difference in the sound quality. Suffice to say there is no one right or wrong answer and acoustic designers or companies that always specify the same treatment for side walls are not looking at things in enough sophistication.

The factors that should go into a decision about side wall treatment are numerous and include:
- speaker off axis response
- angle of incidence for the major side wall reflection points, which tells you what the spectral balance of the sound striking the wall looks like
- path length difference for the reflections, which tells you what level they will be at relative to the direct sound, since the further sound has to travel the lower in amplitude it will be
- room use - is it for movie watching or music. For movies the ventriloquism effect is very powerful and so imaging specificity is for me less important than for music
- personal preference - does the client like a very spacious soundstage, pinpoint imaging or perhaps a balance of the two. This is in turn typically related to their content preferences; for example rooms I have designed for people who only listen to symphonic classical you can really design to make the soundstage huge and not worry about pinpoint imaging

Hope that gives you a starting point!
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post #3 of 15 Old 03-25-2013, 10:48 AM
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Nyal, good simple explain to complex subject.

For measuring on/off axis speaker response, for a hobbyist one would need to do that outside if possible, correct?
(To eliminate room reflections from the measurement)


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post #4 of 15 Old 03-25-2013, 12:51 PM
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Originally Posted by mtbdudex View Post


For measuring on/off axis speaker response, for a hobbyist one would need to do that outside if possible, correct?
(To eliminate room reflections from the measurement)

You can do it inside, close to the speaker and use impulse response gating to exclude reflections. It gives you enough to characterize the speaker in the frequency region of interest without having to take it outside.

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post #5 of 15 Old 03-29-2013, 11:09 AM - Thread Starter
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I'm still trying to absorb (no pun intended) this, but I wanted to take a moment and say thanks Nyal.

-Dave

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post #6 of 15 Old 03-29-2013, 07:21 PM
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Originally Posted by Spall View Post

I'm still trying to absorb (no pun intended) this, but I wanted to take a moment and say thanks Nyal.

NP!

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post #7 of 15 Old 03-30-2013, 08:59 AM
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Originally Posted by Nyal Mellor View Post

There is no one right or wrong answer regarding side wall treatment. In the context of this post I am assuming you are talking about treatment for the screen channels, not surrounds.

In my book the right way to do it is to measure the speaker's off axis response. You do this using an acoustic measurement package and moving the speaker in say 22.5 or 30 degree increments until you get to 90 degrees. You need to keep the speaker to baffle distance consistent.

The results should look something like this...the response below is for a very nicely designed speaker using a constant directivity waveguide. Let's call it speaker A. The main characteristic here is that the off axis response at angles past 45 degrees is noticeably lower than the on-axis response yet it does resemble is spectrally until 3k or so before a smooth taper down in the highest frequencies.



Speaker B, a normal cone/dome type speaker, is very different. The off and on axis response are very similar meaning the speaker 'broadcasts' much more sound out off axis than the speaker above. There is also an obvious discontinuity between 2-3k which is a typical suckout that appears off axis in cone/dome speakers. What happens is that at the crossover point the directivity of the cone and dome do not match. The cone is beaming and the dome has very wide dispersion. On axis the responses might look the same but off axis they do not.



Why is the off axis response important? Well what you hear in most normal sized domestic listening rooms is a combination of the direct and reflected sounds. Our ears are particularly sensitive to lateral reflections so what you do with them can make a big difference in the sound quality. Suffice to say there is no one right or wrong answer and acoustic designers or companies that always specify the same treatment for side walls are not looking at things in enough sophistication.

The factors that should go into a decision about side wall treatment are numerous and include:
- speaker off axis response
- angle of incidence for the major side wall reflection points, which tells you what the spectral balance of the sound striking the wall looks like
- path length difference for the reflections, which tells you what level they will be at relative to the direct sound, since the further sound has to travel the lower in amplitude it will be
- room use - is it for movie watching or music. For movies the ventriloquism effect is very powerful and so imaging specificity is for me less important than for music
- personal preference - does the client like a very spacious soundstage, pinpoint imaging or perhaps a balance of the two. This is in turn typically related to their content preferences; for example rooms I have designed for people who only listen to symphonic classical you can really design to make the soundstage huge and not worry about pinpoint imaging

Hope that gives you a starting point!

Nice post.

The off axis response is important in relation to the amount that you actually hear it in terms of reflections. In a untreated room, you hear the reflections much more than you do in a treated one where the early reflections are suppressed or redirected. So the significance of the power response of the speaker is proportional to magnitude and timing of the reflections as a result of the power response. I would argue that if all your early reflections (up to 20ms) are -20db or better down, then a uneven power response isnt that significant.

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post #8 of 15 Old 03-30-2013, 01:58 PM
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Originally Posted by jim19611961 View Post

The off axis response is important in relation to the amount that you actually hear it in terms of reflections. In a untreated room, you hear the reflections much more than you do in a treated one where the early reflections are suppressed or redirected.

I'll argue this from a completely different perspective. You don't actually hear side-wall reflections unless the room is so wide that the reflections arrive more than ~20 milliseconds after the direct sound. In more typical size rooms the reflections cause comb filtering. That's what causes the real damage to the sound. So with this in mind, how does off-axis response affect the comb filtering response? The more the highs roll off to the sides, the less sound is reflected and in turn the less comb filtering you get. So while I'd never argue for speakers that lose highs off-axis, there is a benefit from the perspective of reflections. I believe some people argue that side-wall reflections are less damaging when speakers are more flat off-axis, though I think it's actually the opposite!

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post #9 of 15 Old 03-30-2013, 02:35 PM
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Originally Posted by Ethan Winer View Post

I'll argue this from a completely different perspective. You don't actually hear side-wall reflections unless the room is so wide that the reflections arrive more than ~20 milliseconds after the direct sound. In more typical size rooms the reflections cause comb filtering. That's what causes the real damage to the sound. So with this in mind, how does off-axis response affect the comb filtering response? The more the highs roll off to the sides, the less sound is reflected and in turn the less comb filtering you get. So while I'd never argue for speakers that lose highs off-axis, there is a benefit from the perspective of reflections. I believe some people argue that side-wall reflections are less damaging when speakers are more flat off-axis, though I think it's actually the opposite!

--Ethan

OK, shifting to this perspective, id still say the more the side walls redirect or absorb the energy there, the less comb filtering your going to get (at least from that source). Now , if all you have is one or two 2' x 4' panels on each side wall to deal with the first reflection point(s), then although you are lessening those reflections that cause comb filtering, there is still a lot of wall untreated and consequently may still have comb filtering issues.

When I speak of hearing early reflections, what I really mean is hearing the effects they cause. That can be comb filtering or a altered FR.

A flatter off axis response I can see as beneficial if your redirecting that energy for a later (>20ms) return. My point is, if your absorbing this energy, what difference does it make what its spectrum is?

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post #10 of 15 Old 04-01-2013, 11:59 AM
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Originally Posted by jim19611961 View Post

My point is, if your absorbing this energy, what difference does it make what its spectrum is?

Agreed, there's no difference as long as the absorption is broadband. My comment was meant only for rooms that do not have absorption at all on the side walls. That's the argument I've seen many times, that if your speakers have a good off-axis response, then absorption isn't needed. So I showed why it is in fact needed even more. biggrin.gif

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post #11 of 15 Old 04-01-2013, 07:42 PM
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Originally Posted by Ethan Winer View Post

Agreed, there's no difference as long as the absorption is broadband. My comment was meant only for rooms that do not have absorption at all on the side walls. That's the argument I've seen many times, that if your speakers have a good off-axis response, then absorption isn't needed. So I showed why it is in fact needed even more. biggrin.gif

--Ethan

Don't equate 'good off axis response' to flat off axis response. The speakers with falling off axis response in the diagram above are, in my book, speakers with good off axis performance. That is the off axis frequency response is spectrally similar to the on axis sound. Those particular speakers I've used more than a few times and believe it or not to my ears they sound better without any acoustic treatment at the side wall reflection points even when the speakers are 2ft from the side walls: anything that disturbs the nicely balanced off axis response makes the sound worse.

I honestly don't know where you get this comb filtering thing from? If the off axis sound is spectrally similar our ear does not hear what a measurement microphone shows. Sure the superposition of these reflections causes some soundstage shifts but they are quite benign, especially when most acoustic treatment used at side wall locations is not spectrally consistent down to the low modal region when looking at their absorption or diffusion characteristics from the perspective of specular energy or even considering diffraction effects rather than based on the absorption coefficients they show when measured in a test chamber.

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post #12 of 15 Old 04-01-2013, 09:28 PM
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Originally Posted by Nyal Mellor View Post

If the off axis sound is spectrally similar our ear does not hear what a measurement microphone shows.
If it had, 2-speaker stereo wouldn't have worked all this time.

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post #13 of 15 Old 04-02-2013, 11:09 AM
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Originally Posted by Nyal Mellor View Post

Don't equate 'good off axis response' to flat off axis response. The speakers with falling off axis response in the diagram above are, in my book, speakers with good off axis performance. That is the off axis frequency response is spectrally similar to the on axis sound.

But it's not a smooth roll-off. Speakers "beam" and "lobe" off-axis, which is itself a form of comb filtering. biggrin.gif
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I honestly don't know where you get this comb filtering thing from? If the off axis sound is spectrally similar our ear does not hear what a measurement microphone shows.

Well, one ear sure does! The difference between two ears and one microphone is each ear receives a different response. So nulls in one ear are filled in at the other. But...
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Sure the superposition of these reflections causes some soundstage shifts but they are quite benign

These shifts are not benign to me! To me it's the difference between vague imaging and pinpoint imaging.
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most acoustic treatment used at side wall locations is not spectrally consistent down to the low modal region

Of course, but imaging and comb filtering are mainly an issue at mid and high frequencies.

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post #14 of 15 Old 04-02-2013, 11:48 AM
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The higher the frequencies, the lower our auditory resolution. See my article here on perception of room reflections: http://www.madronadigital.com/Library/RoomReflections.html. Here is a useful graph from it:

ERB.png

As noted, this combined with the differential signal heard by each ear results in audibility of comb filtering to be nothing like the measurements show. The proper analysis then is in perceptual domain guided by listening tests.

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post #15 of 15 Old 04-03-2013, 11:06 AM
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this combined with the differential signal heard by each ear results in audibility of comb filtering to be nothing like the measurements show.

Yes, our ears are like a 1/x octave FFT. biggrin.gif But the imaging still shifts as you move around, even if the notches themselves are less audible. There has to be some reason imaging is universally considered to improve when side-wall reflections are absorbed. Even if one subscribes to the belief that clarity improves when early reflections are present, imaging and image stability are not better. Further, "clarity" can seem to improve when comb filter nulls are in the low-mid range around 200-300 Hz. I'm convinced this is the real reason some research shows some people claim that dialog seems clearer with reflections allowed.

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