Speaker placement vs treatment - AVS Forum
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Old 01-05-2012, 11:17 PM - Thread Starter
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Hi all

I am running aperion verus grands in my 18x33 room . It's not a dedicated theater but the so called great room. It's powered by Denon 3311 & EMo XPA-3

I have been researching some speaker placement topics & what I read is that the distance between the back of the speaker & the front wall needs to be at least 2.5' to get the minimum 5 ms delay between direct & reflected sound.

Currently that distance is around 21" & it won't be possible to move the speakers 9" more. My question is - can we get the same effect of moving the speakers forward by putting some absorption materials ( like this one http://www.amazon.com/Auralex-Studio...5830370&sr=8-4) on the front walls? Really appreciate your input

Thx

Jay

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Old 01-06-2012, 01:50 AM
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In terms of adding materials behind the speakers, you'll be reducing or canceling or diffusing the backwave, not increasing the delay to the listening position. At least that's my understanding - I may be mistaken. Regardless, the Auralex product you've linked seems to be a refractor/diffusor though (just based on the shape in the picture). An absorber will just be a big panel of fibreglass or some other insulation - you can build them yourself fairly easily (and for less than $100).
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Old 01-06-2012, 05:41 AM
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As far as speaker placement you just move them and listen and hopefully you will get the desired effect.
If that doesn't do it a little room treatment could help!!
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Old 01-06-2012, 06:53 AM - Thread Starter
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Thanks. If we can diffuse /absorb the back wave, then the delay won't matter ( at least the amplitude will be much less) - is that understanding correct?

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Old 01-06-2012, 07:47 AM
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Old 01-07-2012, 10:47 AM
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Quote:
Originally Posted by jaymalya View Post

Hi all

I am running aperion verus grands in my 18x33 room . It's not a dedicated theater but the so called great room. It's powered by Denon 3311 & EMo XPA-3

I have been researching some speaker placement topics & what I read is that the distance between the back of the speaker & the front wall needs to be at least 2.5' to get the minimum 5 ms delay between direct & reflected sound.

Currently that distance is around 21" & it won't be possible to move the speakers 9" more. My question is - can we get the same effect of moving the speakers forward by putting some absorption materials ( like this one http://www.amazon.com/Auralex-Studio...5830370&sr=8-4) on the front walls? Really appreciate your input

Thx

Jay

Hi Jay, where did you get that 5ms figure? I have not heard of that reasoning before...in any event I would not use foam, anything used needs to have a constant absorption profile down to around 250hz, where the frequency response becomes dominated by the effect of room modes. That means 3" fiberglass or 6" of foam....

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Old 01-07-2012, 11:11 PM - Thread Starter
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Old 01-07-2012, 11:53 PM
 
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From the first paper:

"When the brain cannot localize reflections it ignores them..."

And "There are two reasons for sitting close to the rear wall. First, at the room boundaries (walls) the sound pressure is high and the velocity is low. Sitting in the maximum pressure area gives the best perception of deep bass. Secondly, the reflections are shorter than the circumference of the head, so the brain cannot measure the time delay between the ears, and therefore cannot localize the source of sound. When the brain cannot localize reflections it ignores them.

Here is a simple example of the brain ignoring unwanted or unessential information. Imagine being in a noisy public place and conversing with the person next to you. Even though a recording made where you are standing would sound like random noise, you can isolate the conversation. If you hear your name spoken several feet away, you can change your focus, and listen in on the other conversation. Our brains do this automatically all the time, for example, to filter out a distracting natural resonance of a
room to facilitate speech, or to identify potential dangers. So, in this listening position your brain will listen in to the primary source and ignore the reflections."


That is incorrect.

What he incorrectly characterizes as being "ignored" is part of what is called the Henry Precedence Effect.

The fact is that when early signals arrive within what is the called the Haas (time) interval, the ear-brain cannot resolve the two or more arriving signals into distinct separate arriving signals. They are not ignored. Instead, they are merged into one less distinct signal, with the cues from the first arriving signal being used to localize the signal.

The result is a reduction in intelligibility... What Dick Heyser referred to as "time smear distortion".

The same phenomena results in what is characteristically experienced in reflective environments like high school gymnasiums or Airports, where the PA has plenty of gain, but it remains totally unintelligible due to the arrival of high gain indirect signals (reflections) within the Haas interval - that period whee the ear-mind cannot resolve the various signal arrivals into separate signals, and instead fuses them into a single fuzzy signal. The other signals are not "ignored" at all, despite the first arrival being used to localize the group of signals.

The paper is full of inaccuracies regarding psycho-acoustics - how the ear-brain interprets acoustical signals. It has been evaluated before on other forums, and I would suggest finding another source of information.

Oh, and please don't sit immediately adjacent to a boundary!

Oh, and you might enjoy observing the fact that the second paper contradicts the first with:

"With regards to soundstaging, you'll find that depth is dramatically influenced by rear wall proximity. Increasing the distance from the speaker to the wall behind will increase soundstage depth. However, pulling the speaker too far out may degrade focus. In most cases, room layout dictates the maximum distance the speakers will be allowed to intrude into the space, but experiment to as a great degree as possible.

Most speakers need to be a minimum of a foot or two away from the side and back walls to reduce early reflections (early reflections reach the listener out of step with the direct sounds, causing image degradation)."
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Old 01-08-2012, 12:39 PM
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Quote:
Originally Posted by dragonfyr View Post

From the first paper:

"When the brain cannot localize reflections it ignores them..."

And "There are two reasons for sitting close to the rear wall. First, at the room boundaries (walls) the sound pressure is high and the velocity is low. Sitting in the maximum pressure area gives the best perception of deep bass. Secondly, the reflections are shorter than the circumference of the head, so the brain cannot measure the time delay between the ears, and therefore cannot localize the source of sound. When the brain cannot localize reflections it ignores them.

Here is a simple example of the brain ignoring unwanted or unessential information. Imagine being in a noisy public place and conversing with the person next to you. Even though a recording made where you are standing would sound like random noise, you can isolate the conversation. If you hear your name spoken several feet away, you can change your focus, and “listen in” on the other conversation. Our brains do this automatically all the time, for example, to filter out a distracting natural resonance of a
room to facilitate speech, or to identify potential dangers. So, in this listening position your brain will “listen in” to the primary source and ignore the reflections."


That is incorrect.

What he incorrectly characterizes as being "ignored" is part of what is called the Henry Precedence Effect.

The fact is that when early signals arrive within what is the called the Haas (time) interval, the ear-brain cannot resolve the two or more arriving signals into distinct separate arriving signals. They are not ignored. Instead, they are merged into one less distinct signal, with the cues from the first arriving signal being used to localize the signal.

The result is a reduction in intelligibility... What Dick Heyser referred to as "time smear distortion".

The same phenomena results in what is characteristically experienced in reflective environments like high school gymnasiums or Airports, where the PA has plenty of gain, but it remains totally unintelligible due to the arrival of high gain indirect signals (reflections) within the Haas interval - that period whee the ear-mind cannot resolve the various signal arrivals into separate signals, and instead fuses them into a single fuzzy signal. The other signals are not "ignored" at all, despite the first arrival being used to localize the group of signals.

The paper is full of inaccuracies regarding psycho-acoustics - how the ear-brain interprets acoustical signals. It has been evaluated before on other forums, and I would suggest finding another source of information.

Oh, and please don't sit immediately adjacent to a boundary!

Oh, and you might enjoy observing the fact that the second paper contradicts the first with:

"With regards to soundstaging, you'll find that depth is dramatically influenced by rear wall proximity. Increasing the distance from the speaker to the wall behind will increase soundstage depth. However, pulling the speaker too far out may degrade focus. In most cases, room layout dictates the maximum distance the speakers will be allowed to intrude into the space, but experiment to as a great degree as possible.

Most speakers need to be a minimum of a foot or two away from the side and back walls to reduce early reflections (early reflections reach the listener out of step with the direct sounds, causing image degradation)."

A lot of misinformation being spread in these two papers as dragonflyr points out. The key point I would restate again is the precedence effect. Everyone interested in understanding the effect of reflections should read a little about it. It it so well accepted it is termed the 'law of the first wavefront'. Further reading: http://www.hifizine.com/2011/12/list...gy-time-curve/

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Old 01-11-2012, 07:28 AM - Thread Starter
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Thanks Nyal & Dragonfyr. That was a lot of info. So sounds like I should shelve the idea of putting those foams behind speakers for now. One more question, if these panels/foams can't absorb below 1000 hz & potentially alters the sound, why should anybody use it ? Taking the same argument further along, any type of room treatment would alter the sound ( given the absorption level would be different for different frequency ranges) - so why room treatments remain one of the key parameters for audio performance?

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Old 01-11-2012, 08:19 AM
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Quote:
Originally Posted by Nyal Mellor View Post

Further reading: http://www.hifizine.com/2011/12/list...gy-time-curve/

the very first statement from that article:

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Originally Posted by article View Post

In our listening rooms reflected sounds – sounds that bounce off walls and other objects – are more often louder than the direct sound from our loudspeakers

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Originally Posted by article View Post

When we listen to music on our stereo system in rooms that are the size of those found in most typical homes we are more often than not listening at a distance where the sound pressure level of the reflected sounds is greater than that of the direct sound from the loudspeakers

what? how is this even possible?

of all of the ETC's presented within that article, NONE display indirect specular reflections arriving HIGHER in gain than the direct signal...

this goes right back to the very simple but all-too-common misinterpretation of how sound behaves in large acoustical spaces vs that of small acoustical spaces...
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Old 01-11-2012, 12:37 PM
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Quote:
Originally Posted by jaymalya View Post

One more question, if these panels/foams can't absorb below 1000 hz & potentially alters the sound, why should anybody use it ? Taking the same argument further along, any type of room treatment would alter the sound ( given the absorption level would be different for different frequency ranges) - so why room treatments remain one of the key parameters for audio performance?

this is why treatments such as broadband absorbers (panels) of which are deployed to attenuate (absorb) early specular reflections need to be effective enough to sufficiently to attenuate the entire broadband reflection with respect to gain. applying thin absorbers (as many here blindly do), merely attenuate the HF band and allow the mid-low specular band to persist - which filters, EQ's, colors the reflection.

this is why recommendation of use of the ETC is used to identify whether such high-gain early specular reflections impede the listening position exist (identify if you have a problem before procuring 'treatment'), and to also verify the attenuation of the reflection once the treatment has been placed.

the material chosen (the porous insulation's gas-flow-resistivity), the thickness of the absorber (+spacing from boundary) are the two primary (for simplicity) characteristics which can determine the effective range of a broadband (velocity-based) porous absorber. if properly constructed, the panels are certainly effective below 1000hz, and ideally constructed to be effective to the lower specular region (~250-300hz depending on room dimensions)
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Old 01-11-2012, 03:18 PM
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Quote:
Originally Posted by Nyal Mellor View Post

A lot of misinformation being spread in these two papers as dragonflyr points out. The key point I would restate again is the precedence effect. Everyone interested in understanding the effect of reflections should read a little about it. It it so well accepted it is termed the 'law of the first wavefront'. Further reading: http://www.hifizine.com/2011/12/list...gy-time-curve/

Nyal, did I miss the above link in the ETC thread here??
I'll read at home.

I'm specifically intrested in "What should the targets be for reflections?", from your link:
Quote:


What should the targets be for reflections?
The most popular approach is simply to analyze the level of reflections on an ETC and compare these to the direct sound, setting a target for the reflections to be 10dB/15dB/30dB etc lower than the direct sound after a certain number of milliseconds (ms). These targets appear to have been arrived at by people reviewing the results of studies that examined the threshold at which reflections caused perceptual effects and then setting the allowable level of reflections to be below these values. This approach is overly simplistic, since it does not examine whether these perceptual effects are good, bad or neutral. Listening tests summarized in Toole's book reveal that most listeners actually show preference for high levels of reflections that are spectrally similar to the direct sound. The evidence is that they add to timbral richness and give body and fullness to images within the soundstage. We also know that ETCs are spectrally blind (i.e. they contain no information as to the spectral content of the reflected sound) and from Benade's summary that the auditory system is discerning in its requirements for spectral balance between the direct and reflected sounds in a room.

In the absence of properly smoothed waterfall type measurements we must therefore rely upon the flawed ETC as a way of measuring our rooms and determining the changes we should make. In my recent white paper [Ref. 2] co-authored with Jeff Hedback we proposed that the ETC of the left and right speakers (measured separately) should:

Be visually identical (with only minor deviations) from 0-40 ms
Show that the peaks are down to at least -10 dB by 40 ms to prevent breakdown of the precedence effect
Clearly show a decrease in the amplitude of energy over 0-40 ms. The decay pattern may or may not be continuous (**).
Show the consecutive peaks of the highest amplitude reflections viewed across the time axis to be relatively smooth in pattern and density.
An important point is that examination of the ETC by itself is not enough to tell you if a room is good or bad. These criteria should be considered in conjunction with the other stated targets in the white paper for consistency of midrange decay time and left / right frequency response. Figure 1 shows an example of an ETC that meets these standards.

not to hijack this thread, if posted in the ETC thread I'll find it later tonight...if not I feel its a good re-post for that thread, including locals comments.
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Old 01-12-2012, 11:34 AM
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The second sentence, upon re-reading, is factually incorrect and badly written I agree with you localhost.

What I was trying to say in general in that introductory section was that reflections are a very important contributor to the perceived loudness of sounds in our listening rooms because of the way the ear integrates all sound within a certain time window and therefore it is critical for us to consider their effect on what we hear.

I am on the same page with you localhost regarding your final point. I understand that critical distance and the concept of a diffuse reverberant field do not apply in the study of rooms that are the size of our listening rooms and home theaters.

MTBDudex, I did not post a link to the article in the other thread, that Hi-Fi Zine article was only published a few weeks ago.

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