Acoustic treatment for my living room - Page 2 - AVS Forum
Forum Jump: 
Reply
 
Thread Tools
post #31 of 594 Old 04-02-2012, 08:48 AM
AVS Addicted Member
 
amirm's Avatar
 
Join Date: Jan 2002
Location: Washington State
Posts: 17,633
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 359 Post(s)
Liked: 288
Quote:
Originally Posted by localhost127 View Post

a thin panel does not filter out high frequencies from the speakers - as the panel on a sidewall has no impact on the direct signal from the source.

however, a thin porous absorber that is only effective in the high band of the specular region will EQ/filter/color the indirect reflection, as the lower-mid specular band will not be fully attenuated and will still superpose at the listening position with the direct signal.

Last I checked, the wall doesn't play music by itself Since the indirect sound comes from the speaker just the same, then effect is as I said. What you hear is the sum total of direct sound from the speaker and all the reflections in the room. If you filter out the high frequencies of the indirect sound from the speaker, you are indeed changing the overall sound from the speaker.

Quote:


if you choose to attenuate a high-gain, sparse, early arriving indirect specular reflection - then the absorber needs to be effective throughout the entire specular region, which extends down to 250-300hz in typical residential room sizes. bear in mind also that the lower frequencies have inherently more energy content along with longer wavelengths of which require thicker absorption.



a porous absorber's performance is based on a few things, such as the gas-flow-resistivity of the porous material, the thickness of the absorber, the spacing from a rigid boundary, and angle of incidence of the indirect specular reflection.

That's right. "But wait, there is more." Importantly, the graphs published for absorption panels is for large (diffused) rooms which our typical listening rooms are not. I looked up the source of your graph above. It came from this forum post: http://www.gearslutz.com/board/7237454-post133.html

The post has no mention of how that graph was generated. The AFMG Soundflow marking though shows it to be a simulation. Without data as to what was simulated, I would not at all look at such curves. Most of such data is for 0 degree angle of incident which is useless for as you are not going to put the absorber directly in front of your speaker. At least I hope you are not From: http://books.google.com/books?id=sGm...rglass&f=false

Quote:
Originally Posted by Dr. Toole's View Post

In the end, it really
does not matter because for our purposes in small
rooms, these data are of limited usefulness
. They
are measurements optimized for the calculation of
reverberation times in large rooms with highly
diffuse sound fields. Small listening rooms do not
have diffuse sound fields; reverberation time is a
factor of much diminished importance and achieving
satisfactory values is straightforward. For our
purposes, we need to have other data, and in a form
that allows us to anticipate what the absorbing
material might do to the sound from a loudspeaker.

Honestly guys, you are not going to get to the truth here by glancing at stuff post on forums. You either need to bite the bullet and spend good bit of time reading expert's writings such as Dr. Toole above, hire an expert, or do nothing. Doing something *can* make things worse. Do not let folks guilt you into putting stuff on your wall and don't use your intuition of something sounding like it would work (like all the talk of absorbing reflections). They are both liable to get you in the ditch more than helping you . And cost you money to boot. The real data is much more complex and not understanding it at least to a proper level can be problematic.

Amir
Founder, Madrona Digital
"Insist on Quality Engineering"

amirm is online now  
Sponsored Links
Advertisement
 
post #32 of 594 Old 04-02-2012, 10:06 AM
AVS Special Member
 
Ethan Winer's Avatar
 
Join Date: Apr 2003
Location: New Milford, CT, USA
Posts: 5,744
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Liked: 131
Quote:
Originally Posted by localhost127 View Post

to the OP,
additionally, if you want to maintain symmetry, you would not apply treatment to one side of the room and not the other

Agreed 100 percent.

--Ethan

RealTraps - The acoustic treatment experts
Ethan's Audio Expert book

Ethan Winer is offline  
post #33 of 594 Old 04-02-2012, 10:24 AM
AVS Special Member
 
localhost127's Avatar
 
Join Date: May 2009
Posts: 2,277
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 24
Quote:
Originally Posted by amirm View Post

Not a good idea. A thin "panel" like that will be like an EQ which will filter out the high frequencies of your speakers.

a sidewall panel in no way obstructs the direct signal (straight vector) from speaker to listening position. the speaker itself is NOT EQ'd - it is the indirect reflection that is EQ'd with thin absorption of which will still superpose with the direct signal at the listening position. exactly as i stated.

a sidewall panel IN NO WAY changes the response of what is emitted from the source/speaker. the speaker is most certainly NOT "EQ'd" by placement of an absorber. perspective is key.


Quote:
Originally Posted by amirm View Post

Since the indirect sound comes from the speaker just the same, then effect is as I said.

there is a difference between the speaker being "EQd" and a reflection being "EQd" - as the direct signal itself is NOT being modified when a sidewall absorber is placed.

Quote:
Originally Posted by amirm View Post

What you hear is the sum total of direct sound from the speaker and all the reflections in the room. If you filter out the high frequencies of the indirect sound from the speaker, you are indeed changing the overall sound from the speaker.

im sorry, but modifying the room in no way changes what "sound" comes from the speaker (eg, the direct signal). the direct source is NOT changed at all. the superposed response at the listening position changes, yes - but that is not what you are saying. the direct signal ("speaker") is NOT modified in any way with the placement of a sidewall absorber panel or not.


Quote:
Originally Posted by amirm View Post

That's right. "But wait, there is more." Importantly, the graphs published for absorption panels is for large (diffused) rooms which our typical listening rooms are not. I looked up the source of your graph above. It came from this forum post: http://www.gearslutz.com/board/7237454-post133.html

The post has no mention of how that graph was generated. The AFMG Soundflow marking though shows it to be a simulation. Without data as to what was simulated, I would not at all look at such curves.

there are many different standards for absorption coefficient measurement - including impedance tube of which is NOT diffuse field/reverberant chamber spec. why don't you read the marketing brochures for AFMG Soundflow? we all know how much people like brochures here. it is available for a free 30-day trial. may want to have a look at AFMG Reflex as well. wonderful tools.

and your commentary does NOTHING to invalidate my statements that a thin porous absorber is NOT sufficient to fully attenuate a broadband indirect specular reflection of which the lower cut-off frequency for his room will be ~250-300hz. the thin absorber will merely EQ/filter/color the reflection, exactly as i stated many times within this thread. where do you disagree? are you disagreeing with toole regarding this statement?

Quote:
Originally Posted by localhost127 View Post

...and the 4" absorber with a 4" air-gap is based on updated gas-flow-resistivity figures calculated using the Delaney, Bazely, and Miki models (and subsequent modifications, as they satisfy all of the various mods to it) - based on the ACTUAL values for the recommended fiberglass and rockwool porous materials.


Quote:
Originally Posted by amirm View Post

Most of such data is for 0 degree angle of incident which is useless for as you are not going to put the absorber directly in front of your speaker. At least I hope you are not From: http://books.google.com/books?id=sGm...rglass&f=false

yep - and as angle of incidence increases, then you effectively have a thicker absorber of which is more effective. this is true up to grazing angles where absorption coefficient will be significantly reduced.

and you do realize one is able to mount a sidewall panel at an angle (with respect to the boundary) such that the ingress signal is actually hitting the absorber at 0*/normal incidence, yes?
localhost127 is offline  
post #34 of 594 Old 04-02-2012, 10:54 AM
 
diomania's Avatar
 
Join Date: Dec 2008
Posts: 1,389
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 49
Quote:
Originally Posted by localhost127 View Post

im sorry, but modifying the room in no way changes what "sound" comes from the speaker (eg, the direct signal). the direct source is NOT changed at all. the superposed response at the listening position changes, yes - but that is not what you are saying. the direct signal ("speaker") is NOT modified in any way with the placement of a sidewall absorber panel or not.


where do you disagree? are you disagreeing with toole regarding this statement?

What amirm should have said was, "EQ which will filter out the high frequencies of what you hear", but as you may have already noticed, he doesn't like to admit his mistakes.
diomania is offline  
post #35 of 594 Old 04-02-2012, 11:10 AM
AVS Special Member
 
localhost127's Avatar
 
Join Date: May 2009
Posts: 2,277
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 24
Quote:
Originally Posted by diomania View Post

What amirm should have said was, "EQ which will filter out the high frequencies of what you hear", but as you may have already noticed, he doesn't like to admit his mistakes.

"say what you mean, mean what you say"

i really don't know what his problem is or why he is disagreeing with toole. if the user makes the decision to attenuate a high-gain early indirect reflection, then the 'treatment' needs to be broadband such that the ENTIRE reflection is fully attenuated and not EQ'd. toole states this explicitly as i quoted in an earlier post. it's not me telling the user to attenuate reflections - im just saying if the user makes the decision to do so, then the treatment needs to be sufficient! say what i mean, mean what i say

the reflection can even be attenuated with a flat reflector panel (a large sheet of wood) - but the constraints to be broadband and effective to lower cut-off frequency STILL APPLY ... eg, in this 'treatment' scenario, the sheet of wood needs to be LARGE in physical size with respect to wavelength of the lower (250-300hz) cut-off frequencies. the same goes for applying diffusion at such a reflection point - the diffuser needs to be broadband to be effective to the lower frequencies, like this:



not this:



regardless of what you do, the spectral content should not be modified (EQ'd)
localhost127 is offline  
post #36 of 594 Old 04-02-2012, 11:18 AM
AVS Addicted Member
 
amirm's Avatar
 
Join Date: Jan 2002
Location: Washington State
Posts: 17,633
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 359 Post(s)
Liked: 288
Quote:
Originally Posted by diomania View Post

What amirm should have said was, "EQ which will filter out the high frequencies of what you hear", but as you may have already noticed, he doesn't like to admit his mistakes.

You said it better than both of us did . Thanks.

Amir
Founder, Madrona Digital
"Insist on Quality Engineering"

amirm is online now  
post #37 of 594 Old 04-02-2012, 12:01 PM
AVS Special Member
 
localhost127's Avatar
 
Join Date: May 2009
Posts: 2,277
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 24
Quote:
Originally Posted by diomania View Post

What amirm should have said was, "EQ which will filter out the high frequencies of what you hear",

"color", not "filter out" - as nothing from the direct signal is "lost" in such a scenario - what is "lost" is in the sense that the spectrally modified reflected signal (featuring a lack of highs) will cause unevenly distributed comb filtering, notching out an inordinate amount of the specular mids and low-mids frequencies.

Quote:
Originally Posted by localhost127 View Post

if you are using porous absorption to attenuate a high-gain specular reflection, then the absorber needs to be effective down to the Schroeder cut-off frequency/transition region (typically 250-300hz in smaller rooms; based on room dimensions). thin absorption will merely attenuate the HF band and allow the lower-mid band to persist - which will filter/EQ/color the reflection which will then superpose with the direct signal at the listening position. the lower frequencies are going to be more off-axis with typical speaker designs, and they also contain longer wavelengths with inherently more energy content. so the absorber's design must be with respect to effectively attenuating the lower-mid specular band.

localhost127 is offline  
post #38 of 594 Old 04-02-2012, 02:54 PM
AVS Addicted Member
 
amirm's Avatar
 
Join Date: Jan 2002
Location: Washington State
Posts: 17,633
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 359 Post(s)
Liked: 288
Quote:
Originally Posted by localhost127 View Post

where do you disagree? are you disagreeing with toole regarding this statement?

I wouldn't bring up Dr. Toole on the issue of absorbing first reflections. He is not very kind on the subject. From just a page or two after the link I provided above from his book:

Quote:
Originally Posted by Dr. Toole View Post

21.5 SUMMARY
It is clear that the traditional method of specifying absorption coefficient, the random-incidence reverberation room method, provides incomplete information so far as using these devices in small listening rooms. When we look at examples of how sounds arriving from specific angles are modified, a very different picture emerges, and it is one that strongly suggests that a naked wall may be a better option than thin absorbing panels. I may exaggerate, but it is difficult to be restrained when attempting to counter a practice that has gone on for many years and actually is encouraged by some international standards (see Section 18.5.1). In the days of mediocre loudspeakers, the effects were perhaps not obvious, but now there are increasing numbers of excellent loudspeakers that have quite uniform and wide dispersion. These products have no opportunity to exhibit their inherent excellence in rooms with areas of absorbing panels of the kind shown in Figure 20.9, or thinner, especially if those panels are placed at the points of fi rst reflection.

Translation: if you have a good speaker, you may be screwing it up with these panels. Check out these curves for thicknesses of 1, 2 and 4 inches:



We see that increasing panel depth does not just amplify the effect of the thinner one but gives you a different transfer function. And regardless, none of these have flat response people imagine them to have of filtering out the highs and then nicely dropping down to zero. The curves have peaks and valleys in them. With zero knowledge of the off-axis (off-angle) response of your speaker (i.e. the sound hitting the side wall), you are taking an unknown waveform and modifying it with another unknown non-linear transform, hoping for the best.

Net, net, you better have a good reason to resort to such solutions and accept their trade offs. At the risk of repeating myself, please don't let folks talk you into such things. If you have a good speaker in a typical living room, you may not having anything that is broken to be fixed this way.

Quote:


and you do realize one is able to mount a sidewall panel at an angle (with respect to the boundary) such that the ingress signal is actually hitting the absorber at 0*/normal incidence, yes?

Thanks. I am well aware of possible solutions like this:




Amir
Founder, Madrona Digital
"Insist on Quality Engineering"

amirm is online now  
post #39 of 594 Old 04-02-2012, 03:38 PM
AVS Special Member
 
localhost127's Avatar
 
Join Date: May 2009
Posts: 2,277
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 24
Quote:
Originally Posted by amirm View Post

I wouldn't bring up Dr. Toole on the issue of absorbing first reflections. He is not very kind on the subject. From just a page or two after the link I provided above from his book:

Quote:
Originally Posted by toole View Post

. If reflected sounds are absorbed, the listener is placed in a predominantly direct sound field, making the experience more intimate, and the imaging tighter and more precise. If the reflections are allowed to add their complexity, the overall illusion is altogether more spacious and open, to many listeners, more realistic. In part, this is a matter of taste.

my first post in this thread stated:

Quote:
Originally Posted by localhost127 View Post

treatment any/all first reflection points is a blind approach and can quickly lead to a highly damped/dead room. is this a design goal?

Quote:
Originally Posted by localhost127 View Post

if you are using porous absorption to attenuate a high-gain specular reflection, (note - there is NO recommendation here, just a statement that if the user chooses to do something, then ....)then the absorber needs to be effective down to the Schroeder cut-off frequency/transition region (typically 250-300hz in smaller rooms; based on room dimensions). thin absorption will merely attenuate the HF band and allow the lower-mid band to persist - which will filter/EQ/color the reflection which will then superpose with the direct signal at the listening position. the lower frequencies are going to be more off-axis with typical speaker designs, and they also contain longer wavelengths with inherently more energy content. so the absorber's design must be with respect to effectively attenuating the lower-mid specular band.

where is this so-called commentary of mine telling people to just "absorb all first reflections"?

and speaking of toole,

Quote:
Originally Posted by toole View Post

. If reflected sounds are absorbed, the listener is placed in a predominantly direct sound field, making the experience more intimate, and the imaging tighter and more precise. If the reflections are allowed to add their complexity, the overall illusion is altogether more spacious and open, to many listeners, more realistic. In part, this is a matter of taste.

seems to go hand-in-hand with my commentary that the user decides if they want an accurate listening space or a subjectively pleasing one. eg, the user decides what they want in their room, NOT I.



Quote:
Originally Posted by amirm View Post

Translation: if you have a good speaker, you may be screwing it up with these panels. Check out these curves for thicknesses of 1, 2 and 4 inches:



We see that increasing panel depth does not just amplify the effect of the thinner one but gives you a different transfer function. And regardless, none of these have flat response people imagine them to have of filtering out the highs and then nicely dropping down to zero. The curves have peaks and valleys in them. With zero knowledge of the off-axis (off-angle) response of your speaker (i.e. the sound hitting the side wall), you are taking an unknown waveform and modifying it with another unknown non-linear transform, hoping for the best.

hmm, it's too bad we don't have a measuring tool that could be useful in this scenario...

and the strawman arguement on those panels are a nice touch. where did i recommend those?
http://www.megasorber.com/soundproof...ster-wool.html
localhost127 is offline  
post #40 of 594 Old 04-02-2012, 05:01 PM
AVS Addicted Member
 
amirm's Avatar
 
Join Date: Jan 2002
Location: Washington State
Posts: 17,633
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 359 Post(s)
Liked: 288
Quote:
Originally Posted by localhost127 View Post

"color", not "filter out" -

???

Quote:
Originally Posted by localhost127 View Post

thin absorption will merely attenuate the HF band and allow the lower-mid band to persist - which will filter/EQ/color the reflection which will then superpose with the direct signal at the listening position.

Seems like Dio had it right .

In that post you also said this:
Quote:
Originally Posted by localhost127 View Post

the frequency response details you absolutely nothing other than the fact that you are seated in an interference pattern due to superposition of direct and indirect signal(s).

Wouldn't frequency response also tell you that you have colored the sound with that treatment? Or without for that matter?

Also, if you have a thin absorber, would you not get rid of fair bit of comb filtering?

Amir
Founder, Madrona Digital
"Insist on Quality Engineering"

amirm is online now  
post #41 of 594 Old 04-02-2012, 05:30 PM
AVS Special Member
 
kiwi2's Avatar
 
Join Date: Nov 2010
Location: New Zealand
Posts: 1,628
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 5 Post(s)
Liked: 144
Quote:
Originally Posted by amirm View Post

Last I checked, the wall doesn't play music by itself Since the indirect sound comes from the speaker just the same, then effect is as I said. What you hear is the sum total of direct sound from the speaker and all the reflections in the room.

But then each and every room is going to sound different. And each possible speaker and listening position placement with in a given room is going to sound different.


Quote:


If you filter out the high frequencies of the indirect sound from the speaker, you are indeed changing the overall sound from the speaker.

You don't change the sound of the speaker. You change the sound of the speaker plus room mix. You get to hear less of the room and more of the speaker. How much someone wants to reduce the room's contribution is a matter of personal taste and experimentation with the particular room they have.
kiwi2 is offline  
post #42 of 594 Old 04-02-2012, 05:33 PM
AVS Special Member
 
localhost127's Avatar
 
Join Date: May 2009
Posts: 2,277
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 24
Quote:
Originally Posted by amirm View Post

???


Seems like Dio had it right .

"filter" vs "filter-out". the highs are NOT "filtered-out" as the content of the direct signal is still present. but, the colored reflection due to thin absorption is indeed a filtered reflection. say what i mean, and mean what i say.


Quote:
Originally Posted by amirm View Post

Wouldn't frequency response also tell you that you have colored the sound with that treatment? Or without for that matter?

Also, if you have a thin absorber, would you not get rid of fair bit of comb filtering?

doesn't matter if you get rid of a "fair bit" - toole states NOT to change the spectral content of the reflection with insufficient absorption:


Quote:
Originally Posted by Toole View Post

Although reflections appear not to be great problems, it
is reasonable to think that there must be a level above
which the good attributes are diminished and negative attributes
grow. Obviously an empty room is not a comfortable
listening environment, even for conversation. The
furnishings and paraphernalia of life tend to bring normal
living spaces into familiar acoustical territory. Custom listening
spaces need to be treated. In all rooms absorption,
scattering or diffusion, and reflection occur, and devices to
encourage each are commonly used by acousticians.


It appears that much of what we perceive in terms of
sound quality can be predicted by the anechoic characterization
of loudspeakers. Because most of these data pertain
to sounds that reach listeners by indirect paths, it is proper
to suggest that nothing in those indirect sound paths
should alter the spectral balance.
For example, a 1-inch
(25.4-mm) layer of fiberglass board at the point of a strong
first reflection is effective at removing sound energy
above about 1 kHz.
From the perspective of the loudspeaker,
the off-axis response of the tweeter has just been
greatly attenuated—it will sound duller and less good.
Obviously if the purpose of the absorbing material is to
attenuate the reflection, the material should be equally effective at all frequencies.


Given the duplex nature of
sound fields in small rooms, it seems reasonable to expect
similar performance at all frequencies above the transition
region.
In their examination of the audibility of reflections, Olive
and Toole looked at detection thresholds as high frequencies
were progressively eliminated from the reflected
sounds, as they might be by frequency-selective absorbers.
They found that only small to moderate threshold elevations
occurred for low-pass filter cutoff frequencies down
to about 500 Hz, where the investigation ended. Removing
the high frequencies alone is not sufficient to prevent audible
effects [32].


Finally there are the indications that the precedence effect
is maximally effective when the spectra of the direct
and reflected sounds are similar [4], [18], [20]. If the spectrum
of a reflection is different from that of the direct
sound, the probability that it will be heard as a separate
spatial event is increased—not a good thing.


Quote:
Originally Posted by Toole View Post

Thus from the perspectives of maintaining the excellence
in sound quality of good loudspeakers, rendering an
unwanted reflection inaudible, and preserving the effectiveness
of the precedence effect, there are reasons not to
alter the spectrum of reflected sounds. One is free to redirect
them with reflectors or diffusers, or to absorb them
with lossy acoustical devices, but in each case, the process
should not alter the spectrum of the sound above some
frequency toward the lower side of the transition region in
a small room.
It seems reasonable to propose, therefore,
that all acoustical devices used in listening rooms—
reflectors, diffusers, and absorbers—should be uniformly
effective above about 200 Hz. For resistive absorbers this
means thicknesses of 3 inches (76 mm) or more.

applying a thin absorber that does not sufficiently attenuate the full broadband indirect signal is altering the spectral content of the reflection. this goes directly against toole's commentary.
im afraid i really do not understand why you disagree with toole in this regards...i was under the impression you were strictly in favor of his research.
localhost127 is offline  
post #43 of 594 Old 04-02-2012, 05:44 PM
AVS Special Member
 
fotto's Avatar
 
Join Date: Nov 2003
Location: N.E. OH
Posts: 1,859
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 5 Post(s)
Liked: 18
You know Amir, even as an audio "dolt" I (and probably many others) get the direct vs. reflected vs. combined aspect of this argument. Whether you label (or argue) the reflection as colored, or EQ'd, or filtered is just semantics.

This petty bickering BS over topics that are very clear to those following, is what got the other thread locked. These threads typically start out as educational, then to moderately entertaining, and inevitably dip down to JUST....PLAIN....BORING.
fotto is online now  
post #44 of 594 Old 04-02-2012, 06:09 PM
AVS Special Member
 
localhost127's Avatar
 
Join Date: May 2009
Posts: 2,277
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 24
Quote:
Originally Posted by amirm View Post

Wouldn't frequency response also tell you that you have colored the sound with that treatment? Or without for that matter?

only if you can somehow isolate that boundary (indirect reflection) ONLY from the acoustical space - otherwise, how would you know what interference in the frequency response is a result of WHICH indirect reflection (from WHICH boundary)?

it's like trying to do multi-variable calculus: A*B*C = 5 (solve for B).
localhost127 is offline  
post #45 of 594 Old 04-02-2012, 06:37 PM
 
dragonfyr's Avatar
 
Join Date: May 2005
Posts: 809
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 13
Facinating.

So we get to listen to the one trick pony interpretations of Toole now adamantly opposed to early reflection treatment stating that "When we look at examples of how sounds arriving from specific angles are modified, a very different picture emerges, and it is one that strongly suggests that a naked wall may be a better option than thin absorbing panels."

Yeah, as he repeatedly states that ANY treatment that is NOT sufficiently broadband ADVERSELY MODIFIES THE SPECTRAL CONTENT OF THE REFLECTED ENERGY!!!!

Meaning ANY treatment MUST be sufficiently broadband!

This is further reinforced by admonitions from Toole consistent with what ALL of the best minds have advocated about treatment for what seems like forever:

"7.1.2 Obviously if the purpose of the absorbing material is to attenuate the reflection, the material should be equally effective at all frequencies. Given the duplex nature of sound fields in small rooms, it seems reasonable to expect similar performance at all frequencies above the transition region.

9.2.1 Any device inserted into a reflected sound path— reflector, absorber, or diffuser—should perform uniformly well at all frequencies above the transition frequency region, say, 200–300 Hz. This is in order to preserve the spectral balance of the loudspeakers, to uniformly attenuate the full spectrum of reflections, and to ensure that the precedence effect is maximally effective.

Thus from the perspectives of maintaining the excellence in sound quality of good loudspeakers, rendering an unwanted reflection inaudible, and preserving the effectiveness of the precedence effect, there are reasons not to alter the spectrum of reflected sounds. One is free to redirect them with reflectors or diffusers, or to absorb them with lossy acoustical devices, but in each case, the process should not alter the spectrum of the sound above some frequency toward the lower side of the transition region in a small room. It seems reasonable to propose, therefore, that all acoustical devices used in listening rooms— reflectors, diffusers, and absorbers—should be uniformly effective above about 200 Hz. For resistive absorbers this means thicknesses of 3 inches (76 mm) or more.
"

A fly in the ointment being that what many persist in believing is adequate performance is predicated on data based on flawed measurement procedures.

The irony being that measurements utilizing the heretofore accepted absorptive test standards ASTM-C423 and ISO-354, as a result of Ron Sauro's recent research, have been rendered politely "inaccurate at best" (as are the methods for calculating scattering coefficients as specified in ISO-17497-1).

This is further complicated by a testing methodology assuming a diffuse energy field that is not applicable to situations common to our usage where the incident sound is limited to more directionally restricted oblique angles of incidence and where edge diffraction of ALL edges, including non-incident edges, are not at play; in addition to he old measurements ignoring the important factoring of the anisotropic nature of the material - meaning that absorbent material functions differently depending upon its physical orientation, as the material structure is not random, but distributed in patterns where incident energy directed in one direction is effected differently and less well than energy directed from other directions.

(A simple example that may make this much more easy to envision is a 1D QRD diffusor. If sound is indecent perpendicular to the 'grooves/wells', the diffusor functions optimally. If the incident sound is parallel to the wells, the wells are almost totally ineffective. Such is the case with typical oriented semi-rigid Fiberglass as well.)

Now I am not expecting most to simply go, "oh, of course" as many of you express frustration with any reference to technical issues. So the larger point here is that much of the heretofore test results commonly cited are NOT what they seem to be. They are overstated not only by virtue of their ability to have absorption coefficients greater than 1, but they have been rendered flawed and are currently in the process of revision to render them more accurate as their accuracy has been successfully challenged!

But results using the new methodology are available on a more limited basis. And a few object to the use of Soundflow and the more advanced models, techniques and considerations being used as it contradicts and or modifies those old vested assumptions. (Yup, and just listen to someone here tell us what Soundflow is based upon; someone who was not even aware of it until we pointed it out, and who remains ignorant of the use of gas flow resistivity values being used in the calculations, let alone as to the myriad models that are cross correlated in the results... And where does this nonsense about calculations assume a perpendicular incidence when it is able to be specified and explored for any angle? Especially when they cannot even get Toole's own admonition that such purely resistive absorptive material be at least 3" thick based upon out of date data!)

And if you accept Toole's premise which also agrees with just about every other major school of thought that has preceded him, that means that 'almost good enough' treatment is NOT a sufficient solution!!!! It means that 'almost good enough' treatment is NOT a partial solution, but that it instead merely moves the problem around and makes it worse!

But let's take all of this one step further.

So what does Toole offer the average person here???

He states over and over again that merely treating a room is not satisfactory!

Unless one has 'ideal' speakers with a uniform power response, they are basically hosed.

So, folks, let's see a raise of hands who has ideal uniform power response speakers?

Hmmmmmm....A bit underwhelming, wouldn't you say? So it looks like we are left trying to figure out what the vast majority of you can do.

And who has an ideal room that will not adversely affect the symmetry or spectral balance of the reflected energy? Why do I get a feeling that the results are about the same as for those who lack ideal power response speakers?

And I suspect that this group also includes all of you who have cried out so loudly that adequate broadband treatment is simply not in the cards...as how there are so many real world mitigating conditions sufficient to cause one to compromise one's treatment....

So, per Toole, if you do NOT have ideal uniform power response speakers, and you cannot apply spectrally neutral treatments effective over the full specular bandwidth beginning at ~200 Hz, you are exaggerating the problems.


The reality need not be so draconian nor bleak.

One CAN largely address the problems caused by non-uniform power response speakers - the ones the vast majority have.

And for some, there will be a tradeoff due to the need to use adequate treatment.

But before they tune out and give up or simply complain some more, for those who so loudly lament the requirements for the thickness of purely porous absorption, that you face your constraints and pursue low Q hybrid porous/'membrane' absorbers that can achieve similar results using a thinner hybrid construction, that will in most cases cost more for the increased sophistication of the design, unless you are willing to spend a bit of time to learn the physics of the behavior and construction of such tools. And the most practical of these may be a 4" layer of porous material faced with a binary amplitude grating providing a combination of low frequency absorption with moderate degree of mid high frequency diffusion, thus resulting in greater lower frequency control coupled with the retention of the mid-high energy. Just realize the practical constraints of this topology, and it can offer significant benefit. Likewise one might also pursue a broadband VPR topology sandwiching a membrane between two porous material layers.

So let's review the basic behavior we are dealing with if you have non-ideal power response speakers...

A typical speaker with a non-uniform power response features what is commonly known as 'collapsing polars'. this means that the low frequencies are low Q, meaning that the lower in frequency and the longer in wavelength, the greater they spread out quickly in an almost cardioid pattern.

The mid range tends to beam in a medium Q pattern, somewhere in between a wide floodlight like distribution pattern, and the high frquencies tend to beam in a high Q pattern, rather like a spotlight. In other words, as you increase in frequency, the more narrow and focused the spatial power radiation pattern.

That means that ONLY in the narrow Q region of the tweeter are all of the frequencies present covering the same region evenly. And with decreasing frequency, you are becoming more and more weighted to the mids and finally the low frequencies in the off-axis regions.

As a result, assuming uniformly reflective walls over the specular bandwidth from ~200 Hz up, since the polar distribution of energy from the speakers is not even, the resulting reflections will not be uniform and will result in the coloration (effective EQing) of the frequency content of the direct sound.

Can we address this? Yes.

But it requires a few things. And it does have a few tradeoffs.

First, what does it require?

It requires that all treatments for reflections be effectively broadband. Meaning, as said earlier above, that all treatment must be effectively broadband, and able to control all energy over the FULL broadband bandpass, especially the long wavelength energy down to ~200 Hz.

You see, if it is able to handle the low frequency long wavelengths, it is generally pretty easy for it to handle the high frequency short wavelengths as well.

So by using the large thick absorbers s necessary to control the low specular frequencies, the mids and highs are 'automatically addressed'.

So, by identifying the incident patterns on the boundaries, one can surgically treat the points where such incidence is a real problem, and help control the uneven power response of a non-ideal speaker, as it is the low and low-mids that create the 'non-uniform' spectral portion of the spatial dispersion.

By making the treatment broadband, it will control the worst that may occur, while easily dealing with the mids and higher frequencies.

But if you only use thin limited treatment, you will only control the high frequencies and leave the even more critical low-mids and lows to wreak havoc with the response and color the sound, as well as to adversely affect the imaging, localization and intelligibility of the direct signal.

And while you never want early arriving sparse (non-diffuse) high gain reflections in any case, you also do not want significant early arriving reflections whose spectral balance has been modified by the wall surface or by inadequate treatment spectral balance will them also color the sound of the direct signal.

So, the way you deal with this situation is to use BROADBAND absorption (or diffusion in limited cases which are beyond he scope of this already too long post) that effectively can be used to even out the typical speaker's (and the room's) non-uniform power response, treats it adequately, and results in a direct signal whose integrity is preserved.

And without a plethora of well-controlled diffuse early reflections (of not too high gain or sparse a nature), you may not have a wide fuzzy less well defined but BIG image, but instead you will be 'stuck' with a more accurate (as in identical to what was recorded), more precisely defined image. And heaven knows that there are worse things!

But this tighter more defined imaging is precisely what Toole complains about (and which most recording engineers prefer - further supported by the fact that the users of Blackbird Studio C have covered the MEGA side diffusors with packing blankets, rendering them absorbers of early arriving energy, to thus tighter up the image in accordance with their professional PREFERENCE...) as it is not found as pleasing by some who like larger fuzzier images in multi-channel sources.

So, if you do not have ideal power response speakers, and you do not have perfect room, you are basically stick making selective surgical adjustments. And Toole's all or nothing admonitions that are essentially akin to the admonitions of communists advocating the perfect ideal classless society. Given the pre-requisite assumptions of a diffuse well behaved early arriving soundfield are achieved by both the room AND by uniform power response speakers, you are good to go.

But if you lack EITHER, you have big trouble in River City.

You are stuck with a non-uniform power response distribution, either because of the speakers, AND/OR as a result of the inadequacy of the room boundaries AND due to insufficiently broadband treatment.

So, you are faced with a rather obvious choice:

Follow Toole's advice and buy new speakers and completely redo your room that either preforms optimally without treatment or utilizes approriate adequately broadband treatment (an no more complaining about 'thick' porous panels if you choose to stick with porous absorption) (which if you could do that, I doubt you would be here...),

...or take measurements, determine the actual dispersion pattern of your speakers and the actual points of incidence of specular energy on the boundaries, and treat them APPROPRIATELY with adequate broadband treatment in order to correct for the real world uneven power response of your speakers and for the geometry of the room while being prepared to listen to the Toole camp repeatedly tell you that your speakers suck and that your room and treatment are inadequate - something you already know is you are here looking for solutions.

And given the later choice where an early arriving diffuse soundfield is not an option due to room and/or speakers, an achievable well behaved tighter more defined and accurate imagine is not such a bad alternative, coupled with a later well-behaved laterally arriving diffuse exponentially decaying soundfield that provides a complimentary sense of spaciousness and envelopment. All in all, such an achievable result may not be such a bad thing after all compared to a design basically telling you cannot get there unless you buy new speakers and heavily modify your room such that you do not need fancy treatment - although that which you do use, MUST be broadband.

It IS your choice.

Just ponder those professionals who have access to Blackbird, opting to place blankets over those absolutely amazing diffusors... Someone desperately needs to tell them about how large amorphously defined fuzzy imaging is preferred by some...
dragonfyr is offline  
post #46 of 594 Old 04-02-2012, 07:14 PM
AVS Special Member
 
jproy13's Avatar
 
Join Date: Jan 2011
Location: East of West, Canada
Posts: 1,178
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 2 Post(s)
Liked: 22
^^^^
thank you for this post. I will reread it again and again.
jproy13 is offline  
post #47 of 594 Old 04-02-2012, 07:26 PM
Member
 
Perry R's Avatar
 
Join Date: Jun 2008
Location: Lindbrook Alberta Canada
Posts: 159
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 24
Between this one and the recently closed thread....even with the bickering.... I have learned more in the last few days than I ever thought I could.

Thank you gentlemen, its been very educational.
Perry R is offline  
post #48 of 594 Old 04-02-2012, 07:35 PM
AVS Addicted Member
 
amirm's Avatar
 
Join Date: Jan 2002
Location: Washington State
Posts: 17,633
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 359 Post(s)
Liked: 288
Thanks for the thoughtful post. A few questions if I may:
Quote:
Originally Posted by kiwi2 View Post

But then each and every room is going to sound different. And each possible speaker and listening position placement with in a given room is going to sound different.

It is a given that once a speaker is placed in a room, it sounds different. Whoever disagrees can leave the room now! Question then: if you were to characterize the effect of the room on the speaker, which area do you think has the most impact?

1. Below transition frequency.

2. In transition frequency.

3. Above transition frequency.

Quote:


You don't change the sound of the speaker. You change the sound of the speaker plus room mix.

Agreed. I think we have beat this point to death.

Quote:


You get to hear less of the room and more of the speaker.

So what if I don't hear the room at all? Would that be best? Is that what the talent heard/approved?


Quote:


How much someone wants to reduce the room's contribution is a matter of personal taste and experimentation with the particular room they have.

I am all for folks experimenting but man, that is a tall order. How would folks be in a position to try all of these acoustic treatments and perform reliable AB tests? It would be quite expensieve and onerous to say nothing of the marital complains as you started to adorne your living room that way . Would you say these tests should be doulbe blind?

Would it be wrong to draw on research on what people might like? Or should it be completley self-service?

Amir
Founder, Madrona Digital
"Insist on Quality Engineering"

amirm is online now  
post #49 of 594 Old 04-02-2012, 07:43 PM
AVS Addicted Member
 
amirm's Avatar
 
Join Date: Jan 2002
Location: Washington State
Posts: 17,633
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 359 Post(s)
Liked: 288
Quote:
Originally Posted by localhost127 View Post

doesn't matter if you get rid of a "fair bit" - toole states NOT to change the spectral content of the reflection with insufficient absorption:

So between having comb filtering and proper spectral response, the latter wins. Did I understand you correctly? If not, I apologize and appreciate understanding why getting rid of so much comb filtering was not beneficial with thin absorbers.

Quote:


im afraid i really do not understand why you disagree with toole in this regards...i was under the impression you were strictly in favor of his research.

I have no disagreement whatsoever with Dr. Toole. He however, does not take the same position as you do regarding impact of comb filtering and hence my question above. In that regard, quoting him is not the answer .

Amir
Founder, Madrona Digital
"Insist on Quality Engineering"

amirm is online now  
post #50 of 594 Old 04-02-2012, 08:17 PM
 
dragonfyr's Avatar
 
Join Date: May 2005
Posts: 809
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 13
Quote:
Originally Posted by amirm View Post

So between having comb filtering and proper spectral response, the latter wins. Did I understand you correctly? If not, I apologize and appreciate understanding why getting rid of so much comb filtering was not beneficial with thin absorbers.



It is absolutely amazing that anyone would make such a fundamentally erroneous assertion that there is a disconnect between spectral content of the component signals and the comb filtering that results from the superposition of the direct and indirect energy!


It doesn't matter what you understand of local's post, as it is apparent that you have no idea as to the behavior of superposition and the fact that the it ultimately plays in creating the coloration by way of the combination of the direct with the spectrally modified indirect signal.


The fact is that thin absorbers no NOT remove "so much comb filtering" as you assert, as the remaining energy which the bandwidth limited treatments are insufficient to mitigate results in substantial comb filtering that is the very source of the perceived coloration!
dragonfyr is offline  
post #51 of 594 Old 04-02-2012, 08:32 PM
AVS Special Member
 
kiwi2's Avatar
 
Join Date: Nov 2010
Location: New Zealand
Posts: 1,628
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 5 Post(s)
Liked: 144
Quote:
Originally Posted by amirm View Post

Question then: if you were to characterize the effect of the room on the speaker, which area do you think has the most impact?

1. Below transition frequency.

2. In transition frequency.

3. Above transition frequency.


I think the first most noticeable difference would be the effects of frequency nulls and peaks below transition frequency.

That's not to say that other effects aren`t going on as well.



Quote:
I am all for folks experimenting but man, that is a tall order. How would folks be in a position to try all of these acoustic treatments and perform reliable AB tests? It would be quite expensieve and onerous to say nothing of the marital complains as you started to adorne your living room that way . Would you say these tests should be doulbe blind?

No, just introduce treatments a little at a time into a room and see what happens.
kiwi2 is offline  
post #52 of 594 Old 04-02-2012, 09:23 PM
 
dragonfyr's Avatar
 
Join Date: May 2005
Posts: 809
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 13
Quote:
Question then: if you were to characterize the effect of the room on the speaker, which area do you think has the most impact?

1. Below transition frequency.

2. In transition frequency.

3. Above transition frequency.




I think the first most noticeable difference would be the effects of frequency nulls and peaks below transition frequency.

That's not to say that other effects aren`t going on as well.




Quote:
I am all for folks experimenting but man, that is a tall order. How would folks be in a position to try all of these acoustic treatments and perform reliable AB tests? It would be quite expensieve and onerous to say nothing of the marital complains as you started to adorne your living room that way . Would you say these tests should be doulbe blind?



No, just introduce treatments a little at a time into a room and see what happens.





So now frequency domain anomalies are postulated as being perceived in sequence in the time domain?
I don't think even Abbott could have anticipated this development!
dragonfyr is offline  
post #53 of 594 Old 04-02-2012, 09:40 PM
AVS Special Member
 
kiwi2's Avatar
 
Join Date: Nov 2010
Location: New Zealand
Posts: 1,628
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 5 Post(s)
Liked: 144
I think some of you "professional" types are overcomplicating this.
kiwi2 is offline  
post #54 of 594 Old 04-03-2012, 02:39 AM - Thread Starter
AVS Special Member
 
Skylinestar's Avatar
 
Join Date: Jun 2008
Posts: 1,304
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 18 Post(s)
Liked: 54
Quote:
Originally Posted by localhost127 View Post

to the OP,
additionally, if you want to maintain symmetry, you would not apply treatment to one side of the room and not the other

Does that mean sidewall absorbers on left & right walls are not required as it's now in symmetry?

According to the audioholics DSX education page here...
Quote:
2) Width is more important than height!
Width is crucially important in placing instruments and recreating acoustical space. Concert halls with side walls perpendicular to the stage are considered by experts on the subject to be acoustically superior spaces to fan shaped halls. One of the reasons here is the importance of the early reflections from the right and left side walls perpendicular to the stage. In the "fan shaped" concert halls, the splayed side walls did not support the same kind of early reflections and are one of the main reasons these halls are not judged as good by the experts. To quote Mr Holman, "it is known in concert hall acoustics that the first side wall reflection is the single most important reflection direction, it sets the auditory source width... Channels constrained to plus/minus 30 degrees are too narrow for that". For this reason, the DSX standards (7.1, 9.1 and 10.2) support a left wide and right wide channel at plus/minus 60 degrees to reproduce the kind of side wall reflection you would hear if you were seated in a great concert hall. (Note: Those who recall Audioholics' original article on 10.2 may note that the front widths were at 55 degrees, not 60 degrees. According to professor Chris, this small adjustment was made because the difference between the two angles was minimally audible, and "60 degrees is easier to eyeball").

... the early reflections seems to be a good thing if applied properly.

Since I am having a DSX Wide sound stage setup for movies, is there any need to reduce this early reflections in my room using absorber? IMO, it will be a good thing because it's best to kill off this nasty haphazard early reflections using absorbers & replace it with the desirable sound that is created by the DSX Wide channels. Am I correct?


From the photo above (Audyssey's Surround Sound Room), looks like there are absorbers next to the speakers.


Skyline diffusers on front wall?
Skylinestar is offline  
post #55 of 594 Old 04-03-2012, 03:43 AM
AVS Addicted Member
 
arnyk's Avatar
 
Join Date: Oct 2002
Location: Grosse Pointe Woods, MI
Posts: 13,648
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 286 Post(s)
Liked: 997
Quote:
Originally Posted by Skylinestar View Post

Does that mean sidewall absorbers on left & right walls are not required as it's now in symmetry?

No.

Quote:
According to the audioholics DSX education page here...

... the early reflections seems to be a good thing if applied properly.

You seem to be confusing building a concert hall with building a listening room.

The two tasks are very different. I hope you have noticed the obvious difference in size! ;-)

Do you want to localize where your speakers are, or do you want to localize where the instruments were in the concert hall?
arnyk is online now  
post #56 of 594 Old 04-03-2012, 04:46 AM
Senior Member
 
audiophilesavant's Avatar
 
Join Date: Sep 2009
Posts: 453
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 13
Can someone recommend a good commercially available 2'x4' BROADBAND absorption panel?
audiophilesavant is offline  
post #57 of 594 Old 04-03-2012, 04:51 AM
AVS Addicted Member
 
arnyk's Avatar
 
Join Date: Oct 2002
Location: Grosse Pointe Woods, MI
Posts: 13,648
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 286 Post(s)
Liked: 997
Quote:
Originally Posted by audiophilesavant View Post

Can someone recommend a good commercially available 2'x4' BROADBAND absorption panel?

Anything with 2" of good sound high density absorbing material (e.g. 705), spaced 2 or more inches off the wall would be a good start. 4" thick is even a bit better. I think I've seen these on several sites.
arnyk is online now  
post #58 of 594 Old 04-03-2012, 05:44 AM
AVS Special Member
 
localhost127's Avatar
 
Join Date: May 2009
Posts: 2,277
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 24
Quote:
Originally Posted by audiophilesavant View Post

Can someone recommend a good commercially available 2'x4' BROADBAND absorption panel?

gas-flow-resistivity (not density) is the key value in determining a porous materials absorption characteristics (+ other variables). density and GFR generally have a linear correlation for the materials we commonly use, but two materials with identical densities can have differing flow-resistivity values (and thus ,different performance).

as is relevant to the context of the conversation, if you are utilizing a broadband porous absorber to attenuate (absorb) an indirect specular reflection, then the absorber needs to be sufficient to fully attenuate the entire broadband reflection down to the lower cut-off frequency. porous absorbers are velocity-based absorbers, meaning they convert the kinetic energy of the waveform as it passes through the porous material into heat. thus, to be effective, the absorber needs to be placed into areas of high particle velocity (note that particle velocity is NOT the same as the speed of sound of the waveform (which is a constant within your room's medium (air) - although speed of sound will change as the soundwave is passing through the absorber). http://en.wikipedia.org/wiki/Particle_velocity

pressure and velocity components of the soundwave are inversely proportional. right at the rigid boundary, pressure is maximized and velocity goes to zero. thus, there is little flow directly at the boundary and thus the absorber will not be effective. at 1/4, 3/4 wavelengths away from the boundary will be velocity-max. so, for very short wavelengths (high frequencies), a thin absorber (2") will be within this high particle velocity range - and thus, be an effective absorber for those higher frequencies. but for lower frequencies (longer wavelengths), the absorber will need to be thicker and/or spaced further away from the rigid boundary such that the porous material is in areas of high particle velocity for a given frequency in order to maximize effectiveness. at 300hz, the wavelength is ~45inches and 1/4wavelength corresponds to 11.25". this is why thin absorbers are not effective especially when placed directly on the rigid boundary (where pressure is high but velocity is not). this is also why spacing the porous material away from the rigid boundary (with an air-gap) is akin to getting a "free lunch" - as you're increasing the effectiveness of the absorber to lower frequencies without using any additional material. this is also why pressure-based resonator absorbers are recommended for LF absorbers (bass traps), simply because porous absorbers need to be quite thick in order to be placed into areas of high particle velocity for the LF wavelengths that are much much longer. pressure-based resonator traps need to be placed at areas of high pressure to be effective - which just so happens to be right against the rigid boundary (and thus, NOT protruding into the user's room like a porous, velocity-based absorber).

4" absorber with a 4" air-gap is based on updated gas-flow-resistivity figures calculated using the Delaney, Bazely, and Miki models (and subsequent modifications, as they satisfy all of the various mods to it) - based on the ACTUAL values for the recommended fiberglass and rockwool porous materials. and this is recommended for the commonly/easily sourced materials such as OC703 (3pcf) and 4pcf Rockwool MineralWool.


Quote:
Originally Posted by dragonfyr View Post

As Andre states, you are dealing with the acoustical Impedance of the boundary material.

Please note this and be careful as so many are want to reduce absorption to resistance - which is only one part of the complex impedance.

And this issue becomes very important if one stops for a moment and examines that the the reactance has both capacitive and inductive components which function not only as frequency selective resistance with a time aspect, but which also impact the energy aspects of the system behavior, as the real domain which corresponds to resistance also correlates to the potential energy in the system, while the imaginary realm corresponds to the reactance component and the kinetic energy of the system, which relate directly with pressure and velocity aspects (and more) of the overall system.

One makes a significant mistake to reduce the absorption model to simply a function of 'resistance'.

localhost127 is offline  
post #59 of 594 Old 04-03-2012, 05:51 AM
AVS Special Member
 
localhost127's Avatar
 
Join Date: May 2009
Posts: 2,277
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 24
Quote:
Originally Posted by arnyk View Post

You seem to be confusing building a concert hall with building a listening room.

The two tasks are very different. I hope you have noticed the obvious difference in size! ;-)

Do you want to localize where your speakers are, or do you want to localize where the instruments were in the concert hall?

and the fundamental behavior of the acoustical energy is different - as in a Small Acoustical Space (SAS) where what reverberant sound-field exists is above our hearing range and below the ambient noise floor (thus, no appreciable reverberation exists - but we do have specular 'room decay'), we deal with sparse reflections bouncing around the room like laser beams of which the reflection's magnitude, time-arrival, and vector components can be determined (eg, virtual images) --- and thus, destructive interference with respect to intelligibility, localization, and imaging with respect to the direct signal. you can take measurements from differing points within the room and notice the great discrepancies in response from one point to another as the energy is not "well-mixed".

in a Large Acoustical Space (LAS) such as a concert hall - with the development of a statistically homogenous reverberant sound-field (at frequencies based on volume calculation from Dr. Schroeder's FsubL equation), the energy is equal and probable in any and all directions. you cannot determine magnitude and vector components of the indirect energy as it is "well-mixed". you also have the constraint of being past the critical-distance, of which the reverberation is higher in gain than the direct signal. but you will not find a critical-distance within a Small Acoustical Space like our residential rooms.

this is why we must continue to encourage users NOT to emulate the 'treatments' they see in a LAS or movie theater to that of a SAS like their residential home theater.

dragonfyr's/mas' commentary here is a worthy read if you are interested further: http://www.hometheatershack.com/foru...rt60s-sas.html
and: http://www.avsforum.com/avs-vb/showp...postcount=8563

Quote:
Originally Posted by dragonfyr View Post

To reduce the distinction to its most basic functional charactrisitc, a large acoustical space features a developed statistically random reverberant sound field where reflections at any location are equally probable to radiate from any direction. Conversely, a small acoustical space LACKS a statistically random reverberant sound field where reflections at any location are equally probable to radiate from any direction – and instead is DOMINATED by focused specular reflections which are definitely identifiable as discreet phenomena as energy vectors with both direction, intensity and a discreet time of arrival, while their intensity decay and their distribution in regards to their arrival time can be measured and all of these characteristics easily identified and isolated via such measurements as the envelope time curve (ETC).

localhost127 is offline  
post #60 of 594 Old 04-03-2012, 05:55 AM
Senior Member
 
audiophilesavant's Avatar
 
Join Date: Sep 2009
Posts: 453
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 13
So you are recommending a 4" absorber with a 4" air gap rather than a 2" absorber with a 2" air gap for BROADBAND absorption?

Am I to understand than BROADBAND in this context means uniform absorption down to 300Hz?
audiophilesavant is offline  
Reply Audio theory, Setup and Chat

User Tag List

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off