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Can small rooms have real reverb?
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#241
19th March 2013
Old 19th March 2013
  #241
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Quote:
Originally Posted by Rod Gervais View Post
Listen, you can (obviously) measure any space to determine whether or not there is a reverberant field, in cases where there is none (as explained previously) this will be revealed just as it was in that particular case.
Wonderful. Please outline the steps.

Quote:
I believe a description of (at least one of) the techniques involved was posted.
I am not asking for "technique" nor am I asking for cut and paste of stuff online. Did kasmira really just give us a patent extract as his proof point? Application # 2004/0213415. Determining reverberation time - Patents.com Let's please not waste each other's time with stuff like this.

How can you guys have such a strong belief in something in audio and be comfortable that you have never, ever tried to verify it with your own measurements? You guys are technical. You record things for a living. I see tons of threads on acoustics here. If you build a bass trap surely you run before and after frequency response measurements to see if it did any good. How can you trust this disputed notion and not once, try to verify it either way? Why not spend a fraction of time spent arguing about it, and go gather some data?

I ask the above not even knowing the outcome. For all I know you prove your point and not mine. What I do know is that I will learn something if you shared some data instead of just arguing with stuff I have read, and know to not be making your case.

Quote:
Actually - the optimal RT60 value for speech (or anything else for that matter) is room dependent....
So now RT60 is a valid measurement for that?

Quote:
I do not see him saying anything is "problematic" - I simply see him reporting some data, is there part of what he said about this space that you aren't sharing with us?
No, all I am asking you is a simple question: can RT60 be used with rooms have not developed reverberant field as Davis did? I provided all the proof for that. He reported the RT60 at 0.5 and said the room did not have a reverberant field. Both were said in black and white.

Quote:
I would also point out that he is (apparently) not talking about how anything amounts to anything else - this appears to be physical data gathered within a particular space where he was relaying the information gathered in that space relating to the decay of sound.
I will give you the footnote that was in Local's larger version of page copy:

"Figure 8-1 Proof that there is a fundamental difference between small and larger reverberant spaces."

So now we hear that the small room was also reverberant! See what you have done? Ethan's room at decay of ~2.0 fits his example of large reverberant space. Yet we are told Ethan's room is too small to ever act like a large room. Sound waves traveled the equiv. of some 1,800 feet before they died down 60 dB. Do you know how many reflections that represents in his room?
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Last edited by amirm; 20th March 2013 at 01:56 PM.. Reason: Mistakenly said "Yannick" when I meant to say kasmira.
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#242
19th March 2013
Old 19th March 2013
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Quote:
Originally Posted by amirm View Post
How can you trust this disputed notion and not once, try to verify it either way? Why not spend a fraction of time spent arguing about it, and go gather some data?
Either you are reading this thread selectively or not paying attention. I've given you the very simple process earlier today. You can accomplish it in 5 minutes. I personally have also done the old school dB meter process and found that it closely matched the simple process I posted earlier today. So in my mind, there is no disputed notion. I've witnessed a reverberant soundfield and have measured to confirm. If you sincerely want to learn, you should be willing to invest 5 minutes. If not, please don't pretend to care.
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#243
19th March 2013
Old 19th March 2013
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Quote:
can RT60 be used with rooms have not developed reverberant field as Davis did?
Even Ethan agrees that you would not use it to calculate absorption needed. Maybe there are people doing that for smaller rooms where music does not matter, I don't know - perhaps to just "ball park" it?, but we are talking in terms of rooms where music does matter.

My test room is 17'x13'x8', in what way would I use RT60 to treat it for music? I don't have a reverberant sound field so what am I measuring? flutter?
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#244
19th March 2013
Old 19th March 2013
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Quote:
Originally Posted by amirm View Post
Wonderful. Please outline the steps.
YOU provide the proof and data to support YOUR claims:

surely someone in your position should already know how to do this?

why would you write an article claiming the existence of such if you are unaware of the steps to verify and thus resort to us to provide them for you.

we have asked for proof on the verification of a reverberant sound-field and this has been met with silence. and providing an RT60 measurement does NOT automatically imply a reverberant sound-field exists. it merely demonstrates operator error.

the silence is deafening!!

long specular decay times do not automatically imply statistical reverberant sound-field!!!


Quote:
Originally Posted by amirm View Post
I have his .mdat. Walk us through what you like me to do please. I will post whatever you say whether it helps your case or mine .
just upload it for us.

and can you please re-verify whether this measurement was indeed taken "well past Dc"? so that we know the procedure was followed correctly.

edit: well past the ACTUAL/MEASURED Dc - not some fictitious calculated Dc.
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#245
19th March 2013
Old 19th March 2013
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Quote:
Originally Posted by amirm View Post
Ethan's room at decay of ~2.0 fits his example of large reverberant space. Yet we are told Ethan's room is too small to ever act like a large room. Sound waves traveled the equiv. of some 1,800 feet before they died down 60 dB. Do you know how many reflections that represents in his room?
YOU wrote the article claiming 3.1ft Dc in the garage (and also claim 3.2ft Dc in your home theater). what we are in contention with is your lack of data to support your claims.

a number spit from an erroneously used calc does NOT automatically imply true representation of what is actually happening in the real world.

Quote:
Originally Posted by Article
So even 3.1 feet away from the sound source in my garage is enough to be beyond the critical distance. So much for claims that critical distance prevents true reverb from existing in a small room!
so much for science!



Quote:
Originally Posted by amirm View Post
Do you know how many reflections that represents in his room?
no, i do not - what process do you use to determine this?
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#246
19th March 2013
Old 19th March 2013
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Quote:
Originally Posted by amirm View Post
Wonderful. Please outline the steps.


I am not asking for "technique" nor am I asking for cut and paste of stuff online. Did Yannick really just give us a patent extract as his proof point? Application # 2004/0213415. Determining reverberation time - Patents.com Let's please not waste each other's time with stuff like this.
Come on now - this is actually funny....... what do mean when you say you are not asking for a technique? In order for measurements to be valid they have to follow some sort of standard in the industry...... but you are not interested in the requirements of a standard?

If you do not have a very exacting set of requirements to follow when testing (for this or any other data) then how do you assure repeatability?

So you would prefer

"hey I just kinda walk into a space and set my stuff up wherever there isn't someone passed out on the floor - then I make a crap load of noise with whatever happens to be convenient at the time and capture it with a mic of some sort (that's connected to my computer of course - would not want to miss a step here ya know) and once I finish that I run it through my computer program to analyze it all, and it spits out a bunch of data and stuff and I'm done."

Quote:
How can you guys have such a strong belief in something in audio and be comfortable that you have never, ever tried to verify it with your own measurements? You guys are technical. You record things for a living. I see tons of threads on acoustics here. If you build a bass trap surely you run before and after frequency response measurements to see if it did any good. How can you trust this disputed notion and not once, try to verify it either way? Why not spend a fraction of time spent arguing about it, and go gather some data?
I don't (as a rule) "record things" for a living - and I seriously doubt that I ever will.


Quote:
I ask the above not even knowing the outcome. For all I know you prove your point and not mine. What I do know is that I will learn something if you shared some data instead of just arguing with stuff I have read, and know to not be making your case.


So now RT60 is a valid measurement for that?


No, all I am asking you is a simple question: can RT60 be used with rooms have not developed reverberant field as Davis did? I provided all the proof for that. He reported the RT60 at 0.5 and said the room did not have a reverberant field. Both were said in black and white.


I will give you the footnote that was in Local's larger version of page copy:

"Figure 8-1 Proof that there is a fundamental difference between small and larger reverberant spaces."

So now we hear that the small room was also reverberant! See what you have done? Ethan's room at decay of ~2.0 fits his example of large reverberant space. Yet we are told Ethan's room is too small to ever act like a large room. Sound waves traveled the equiv. of some 1,800 feet before they died down 60 dB. Do you know how many reflections that represents in his room?
Listen up - out of everything you said in this last clip I am gong to address one particular thing - perhaps if you get focused you can make it from A to B.......

Quote:
No, all I am asking you is a simple question: can RT60 be used with rooms have not developed reverberant field as Davis did? I provided all the proof for that. He reported the RT60 at 0.5 and said the room did not have a reverberant field. Both were said in black and white.
Davis did not "do" anything with those rooms - he did not "do" anything with the data other than record/report it.... he did not "use" the data as part of a design analysis.......

He simply measured a couple of spaces - and reported his findings.......

The fact that he reported his findings, in no way suggests that were he (afterwards) to be hired by whatever particular companies any of those properties were owned/managed by, to improve the rooms acoustically - that that particular piece of his analysis would have played any part in his design other than letting him know that he did not have to concern himself with any part of the space (in at least one of the cases) exhibiting a reverberant sound field to any degree, and only to a small degree in the other.

Now, I am quite certain that when he decided to determine whether or not their was a reverberant sound field in the space he did not simply walk into it, whistle, clap his hands - make bird noises, etc., or do anything other than gather data in a manner that was consistent with scientific means to ascertain whether or not the rooms supported the formation of a reverberant sound field.

Now a question for you...... let's take a look at the room he determined did not have a reverberant sound field......

Once you know (for a fact) that the room does not have a reverberant sound field - what then becomes any further value in the use of RT60/30/etc. , as relates to the room's analysis & acoustic design?

Rod
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#247
19th March 2013
Old 19th March 2013
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Would it be too much to have Ethan, Glenn, et al post ETCs for their rooms?

Logically,
Andre
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#248
19th March 2013
Old 19th March 2013
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Quote:
Originally Posted by avare View Post
Would it be too much to have Ethan, Glenn, et al post ETCs for their rooms?

Logically,
Andre
Andre,

would any data gathered in such a manner that it did not meet testing procedures either prove or disprove the existence of reverberant fields in either case?

Rod
#249
19th March 2013
Old 19th March 2013
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Quote:
Originally Posted by Rod Gervais View Post
would any data gathered in such a manner that it did not meet testing procedures either prove or disprove the existence of reverberant fields in either case?

Rod
+1

and the scale of those diagrams from SSE were intentionally OMITTED - as it is the characteristic PATTERN of the sound-field which is critical; the NATURE of each sound-field.

and did ethan use an omin-source for that measurement?



Quote:
Originally Posted by Sound Reproduction: Floyd Toole

Reverberation time is a property of the room alone, and a correct measurement
of it should employ an omnidirectional sound source
capable of “illuminating”
all of the room boundaries.
The reason for this is that it is assumed
that the boundaries consist of areas of reflection and absorption and that the
central volume of the room is empty. The several formulae by which we estimate
RT confirm this, and the values of absorption coefficient for the materials are
“random incidence” values, meaning that there is an assumption of some considerable
diffusivity in the sound field. Some practitioners incorrectly use conventional
sound-reproduction loudspeakers as sources
. The directivity of these
is such that the resulting reflection patterns and decays are not properties of
the room but of the room and loudspeaker combination—a very different situation.

Also, as we will see in Chapter 20, absorption at specific angles is quite
different from random-incidence absorption. Figure 4.15 illustrates the fundamental
difference between a proper RT measurement and what it is that we
listen to.


FIGURE 4.15 (a) How RT should be
measured, using an omnidirectional source

aiming its sound at all of the room surfaces,
and an omnidirectional microphone.

earlier, armirm stated:

Quote:
Originally Posted by amirm
Therefore we have met the conditions of reverberant space even though our speaker was not omni-directional.
Quote:
Originally Posted by amirm
To be fair, there was a condition of "omni-directional" source put forward. In your garage you were making noises. I would say those noise sources are fairly omni-directional. You were in the middle of a large space making sounds in mid-air. So we now essentially satisfy both conditions as stated by them for you to have a reverberant space above transition frequencies!
!!
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#250
19th March 2013
Old 19th March 2013
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Quote:
Originally Posted by Rod Gervais View Post
Andre,

would any data gathered in such a manner that it did not meet testing procedures either prove or disprove the existence of reverberant fields in either case?
I am not clear on what you asking. Spikes with nothing in between is no reverberant field. Random fluctuations decaying are an indication of a reverberant field. HOW diffuse the reverberant field is, is a different question. A conbination of the 2 (spikes and decaying fluctuations) is a conbination of the 2.

One could posit that Schroeder integration decay curve would of value as non contigous decay would show up the monotonic curve as jumps. Opps, I guess I just did posit that.

Positly, or positively,
Andre
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#251
19th March 2013
Old 19th March 2013
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Quote:
Originally Posted by avare View Post
One could posit that Schroeder integration decay curve would of value as non contigous decay would show up the monotonic curve as jumps. Opps, I guess I just did posit that.

Positly, or positively,
Andre
And you posit correctly my friend . Very correctly.... It is a point that I showed in glory detail in the AVS thread. And answers the question of "how far off is RT60 if it is wrong?"

Folks like you can easily put me out of business in this thread .
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#252
19th March 2013
Old 19th March 2013
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Quote:
Originally Posted by avare View Post
Would it be too much to have Ethan, Glenn, et al post ETCs for their rooms?

Logically,
Andre

I did this test yesterday. Test room is 17'x13'x8'. No absorption.
The mic is around 13 feet or so away from the monitor. Put it up in the air and as far back as I could without it right up against the wall.




As far as I can tell I do not see a reverberant sound field.

The mic is not in the same spot but here is a recording with music playing. Abbey Roads ain't got nothing on me!!!!!!!! ha ha Honestly the recording tells you nothing but does let you know how ALIVE the room is. The recording was done for another test I did and it is around 5 feet from the monitors.
http://gikacoustics.com/wp-content/u.../untreated.wav
The recording of the treated room is located here
http://gikacoustics.com/audio-exampl...ntreated-room/
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#253
19th March 2013
Old 19th March 2013
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Quote:
And you posit correctly my friend . Very correctly.... It is a point that I showed in glory detail in the AVS thread. And answers the question of "how far off is RT60 if it is wrong?"
I honestly think that is a fair question. But still would not use it to treat a small room. Fun to talk about though.
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19th March 2013
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Quote:
Originally Posted by avare View Post
I am not clear on what you asking. Spikes with nothing in between is no reverberant field. Random fluctuations decaying are an indication of a reverberant field. HOW diffuse the reverberant field is, is a different question. A conbination of the 2 (spikes and decaying fluctuations) is a conbination of the 2.

One could posit that Schroeder integration decay curve would of value as non contigous decay would show up the monotonic curve as jumps. Opps, I guess I just did posit that.

Positly, or positively,
Andre
LOL....... cute dat......

OK - I'll 'bite", I suppose there may well be evidence of some nature regardless of exactly how the sample was taken.........

well at least in the direction of proving for if evidence existed - proving the existence of a reverberant sound field in a room (in either direction) would be difficult if the samples were taken in the direct field.... and that was my point in the question I posed.......

Rod
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#255
19th March 2013
Old 19th March 2013
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Lightbulb

Quote:
Originally Posted by cborg View Post
To quote Sabine:
"... the duration of audibility of the residual sound, which measures that acoustical property of a room commonly called reverberation."
That works for me.

--Ethan
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#256
19th March 2013
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Quote:
Originally Posted by amirm View Post
Our aim for any measurement is to understand the acoustic space. Once we understand it, then we can plot a course of action that helps improve it.
This is exactly the point.

--Ethan
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#257
19th March 2013
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Quote:
Originally Posted by amirm View Post
Our aim for any measurement is to understand the acoustic space. Once we understand it, then we can plot a course of action that helps improve it.
Quote:
Originally Posted by Ethan Winer View Post
This is exactly the point.
But wait!

Back in the 30th post to this thread, you stated:

Quote:
Originally Posted by Ethan Winer View Post
My sole objective is to debunk the common myth that small rooms can't have real reverb. Nothing more, nothing less.
So Ethan, tell me about how you're going to improve the reverb in your garage?
#258
19th March 2013
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has the measurement file from the OP been shared as of yet?

was the measurement from the OP performed with an omni-source?

Quote:
Originally Posted by Ethan Winer
I removed my car so the garage was empty, measured the RT60 in third-octave bands and made a new recording, then wrote the following as an article to describe the test. Some people believe that small rooms can't have real reverb due to Critical Distance (Dc) limitations, so I also show the math used to calculate Dc and list the distance for my garage. Enjoy.
Quote:
Originally Posted by Ethan Winer
So even 3.1 feet away from the sound source in my garage is enough to be beyond the critical distance. So much for claims that critical distance prevents true reverb from existing in a small room!
have these statements from the OP been validated with proof or data to support the claims?
was the actual (measured) Dc = to the calc'd Dc of 3.1ft?
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#259
19th March 2013
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Quote:
Originally Posted by localhost127 View Post
has the measurement file from the OP been shared as of yet?

was the measurement from the OP performed with an omni-source?
Local,

I believe Andre's point was (and he knows he can free to correct me if I am wrong) that even if this test was not performed "by the book" - there should still be physical results evident........

So although the value of the data might be called into question as regards anything other than the "curiosity factor" (my term not his) that it should serve the purpose to help answer the general question.......

Rod
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Quote:
Originally Posted by Rod Gervais View Post
Local,

I believe Andre's point was (and he knows he can free to correct me if I am wrong) that even if this test was not performed "by the book" - there should still be physical results evident........

So although the value of the data might be called into question as regards anything other than the "curiosity factor" (my term not his) that it should serve the purpose to help answer the general question.......

Rod
hence the simple request to have the measurement file provided to the audience.

in the OP, the statement was made that: "I removed my car so the garage was empty, measured the RT60" --- the context of the OP is RT60 and that is why i was asking whether the measurement was taken with an omni-source or a traditional loud-speaker.

Quote:
Originally Posted by Sound System Reproduction - Floyd Toole
Reverberation time is a property of the room alone, and a correct measurement
of it should employ an omnidirectional sound source
capable of “illuminating”
all of the room boundaries.
The reason for this is that it is assumed
that the boundaries consist of areas of reflection and absorption and that the
central volume of the room is empty. The several formulae by which we estimate
RT confirm this, and the values of absorption coefficient for the materials are
“random incidence” values, meaning that there is an assumption of some considerable
diffusivity in the sound field. Some practitioners incorrectly use conventional
sound-reproduction loudspeakers as sources
. The directivity of these
is such that the resulting reflection patterns and decays are not properties of
the room but of the room and loudspeaker combination—a very different situation.

Also, as we will see in Chapter 20, absorption at specific angles is quite
different from random-incidence absorption. Figure 4.15 illustrates the fundamental
difference between a proper RT measurement and what it is that we
listen to.


FIGURE 4.15 (a) How RT should be
measured, using an omnidirectional source

aiming its sound at all of the room surfaces,
and an omnidirectional microphone.


edit: i missed it initially, but the speaker type is given in the OP: http://www.mackie.com/pdf/hr624_ss.pdf
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#261
19th March 2013
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Quote:
Originally Posted by Glenn Kuras View Post
I did this test yesterday. Test room is 17'x13'x8'. No absorption.
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19th March 2013
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In my studio room I took IR when it was (unluckily not completely) empty.
I also took an IR when it was 85% finished.

The speaker is not omnidirectional, but a directional speaker will increase Dc, will it not ?

The RMS levels at 2m95cm of the speaker, room volume 111 m3:

empty: direct sound -5.9 dB / room sound + 1.5 dB / room=direct+7.4 dB
treated: direct sound -7.3 dB / room sound -8.7 dB / room=direct-1.4 dB

This with a HP filter at 120 Hz
Now without filter:

empty: direct sound -5.8 dB / room sound + 2.3 dB / room=direct+8.1 dB
treated: direct sound -7.2 dB / room sound -8.3 dB / room=direct-1.1 dB

Based on these levels, I would conclude I was well beyond Dc when the space was untreated. I am just within Dc when it IS treated.
This with a directional speaker. The use of a omnidirectional speaker would only increase the "room" part of the signal - would it not ?

Amazingly, the inclusion of room modes below 120 Hz does not make that big a difference.
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http://www.fujitsu-ten.com/business/...l/pdf/5-6E.pdf

Again, look at p73.
They measured the directivity diffusion ratio. If this would be =1, then the reverberant sound field definition would be fulfilled.

Amazingly, for most halls this is between 0.8 and 0.9 - so there is no reverberant sound field, following the strict definition. So Rt60 measurements are useless ?

More amazingly, in the cathedral (which has the biggest volume of all, even when it is not specified, look at the length) the measurement was done at 30 meters (!) from the omni source. I think we can ALL agree this is well beyond Dc in a cathedral with Rt60 at 500Hz = 4.96 s
Here the directivity diffusion ratio is the LOWEST, only 0.753, far from "the flow of energy in all directions is equally probable"
#264
19th March 2013
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Quote:
Originally Posted by Rod Gervais View Post
Local,

I believe Andre's point was (and he knows he can free to correct me if I am wrong) that even if this test was not performed "by the book" - there should still be physical results evident........

So although the value of the data might be called into question as regards anything other than the "curiosity factor" (my term not his) that it should serve the purpose to help answer the general question.......
Thank you Rod. More like "it may not be a diffuse reverberant field but it porbably still has reverberance, as used quolocialy." But your summary is great also.

Ethan's garage would be great for this as soundclips for the space have been posted to use for aural comparision of what ambience is measured and what that ambience sounds like.

Reverberantly appreciative,
Andre
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#265
19th March 2013
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Quote:
Originally Posted by avare View Post
Thank you Rod. More like "it may not be a diffuse reverberant field but it porbably still has reverberance, as used quolocialy." But your summary is great also.

Ethan's garage would be great for this as soundclips for the space have been posted to use for aural comparision of what ambience is measured and what that ambience sounds like.

Reverberantly appreciative,
Andre
That is about as diplomatic as one can get!

Diplomatically Speaking,
Glenn
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#266
19th March 2013
Old 19th March 2013
  #266
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amirm is offline
Quote:
Originally Posted by Syncamorea View Post
Why do you need me to explain it? If you think it's such an awesome paper, you explain it.
It would be my pleasure .

The paper is about different ways of measuring Critical Distance. The researcher measured direct to reverberant sound field and reported the resulting critical distance here:



All three rooms fall in the category of acoustically "small rooms", yet they all have critical distances shorter than their lengths. The first two rooms value is just 3 to 4 feet which means you can easily “go way past critical distance.” As I noted, the second room is roughly the size of Ethan's garage. Both rooms have lots of absorption which is the enemy of late reflections and development of “reverberant sound field”. Yet critical distance was precisely measured. And again, it was 3 to 4 feet in rooms. So the notion that “Dc” does not exist in small rooms does not hold relative to this data. The author is from Bell Labs by the way so probably knows what he is doing especially since the paper was from the journal of ASA and hence, peer reviewed.

Alas, we like to see the comparison between theoretical values and real measured and the above paper does not show that. Fortunately we have a much more recent paper (2010) that performs that very analysis for us with even more sample points: Reverberation Radius in Real Rooms, by Miji? and Mašovi? from the journal of Telfor (peer reviewed). Unlike ASA and AES I believe Telfor puts its documents in public domain so you may be able to find them. I am just referencing the study in my library locally so can’t give you the link.

Anyway, as the name indicates they set out to measure the reverberation radius which is another name for critical distance in real room and by that they mean small rooms with non-diffused, potentially non-reverberant sound fields. Here are the stats and results for each room:

”Room 1
This room is rectangular, its dimensions are 8.5 x 8.3 x 4.3 m and its volume is 303 m3. It is a classroom, but with no acoustic treatments in the interior. All four walls and the ceiling are solid, and the floor is covered with parquet. The only significant absorption is on upholstered chairs (backs and seats).”


In imperial units, the room is 27x27x14. So somewhat larger than Ethan’s garage. Here are the results:

Reverberation times are similar to Ethan’s at around 1.8. Note that for 500 Hz, the measured critical distance was just under one meter or 3.1 feet. Again very similar to Ethan’s garage. Computed value based on theoretical Sabine formula was 3.4 feet. Hardly different! At other frequencies there are some differences and the article digs into that some.

” Room 2 is a laboratory room which is also frequently used as a classroom. It has a rectangular parallelepiped shape, with dimensions 5 x 4 x 2.5 m and volume of 50 m3. There is an intense acoustic treatment in it in the form of a suspended ceiling made of highly absorbing panels. The absorption coefficient of ceiling material is ? > 0.9 at middle and high frequencies. The side walls and the floor are hard.”

This one is smaller at 16x13x8.


At 500 Hz, measured value is 2.6 feet, theoretical at 3.4 feet. Still pretty small numbers and well within the full length of the room at 16 feet.

I let you do the unit conversions from here on.

”Room 3
This room is a control room of a music studio. Its base is slightly irregular in shape, but the room can be approximated by a parallelepiped of dimensions 6.5 x 8 x 3.2 m, and volume 166 m3. There is a usual acoustic treatment for such a function of the room, so the reverberation time is about 0.4 s at middle frequencies.”


Hey, finally a case you all may care about!!! A small studio. Here are its stats:


So critical distance was found to be 2 meters or at less than one third the length of the room. I will stop here and not quote the whole article but the other rooms follow suit. Here is a comparison shot of the calculated vs real measurements of critical distance:



We see that there are some deviations from classic formulas but nothing remotely the same as what you guys have been saying, namely that it is so long as to never be within the actual space of a “small” room.

Also note how there is not just one critical distance. Recall how I said there was some basics of acoustics that was needed to understand what is involved in measurement of critical distance. You have one example of it here.

I hope you agree that this is very strong and damaging evidence to the statement that critical distance does not exist within the space of a “small” acoustic space. Yes there are some deviations from theoretical values but nothing to get upset over.
Quote
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#267
19th March 2013
Old 19th March 2013
  #267
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Joined: Feb 2010
Posts: 1,310

localhost127 is offline
why is it amirm adamantly refuses to back his claims regarding Dc of 3.2ft in his home theater and Dc of 3.1ft in ethan's garage with valid data. surely the onus is on the user making the claim to satisfy said claim, and not on the audience.

why do we need all of the continued distractions as evident in his last post - the OP is with respect to one very-specific acoustical space that is of context, and the claimed existence of a reverberant sound-field within it. that is the claim, let's see the data to support the claim.

to date - there have been NO demonstrations of a reverberant sound-field with measurements within this thread.

Quote:
Originally Posted by amirm View Post
I have his .mdat. Walk us through what you like me to do please. I will post whatever you say whether it helps your case or mine .
just upload it for us.
Quote
2
#268
20th March 2013
Old 20th March 2013
  #268
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Joined: May 2009
Posts: 3,365

Syncamorea is offline
Quote:
Originally Posted by amirm View Post
While we are waiting on that, here is something to chew on from peer reviewed journal of Acoustic Society of America paper, "Critical distance measurement of rooms from the sound energy spectral response:
If you look at the proceedings, here's what the abstract says (bolded by me):

Critical distance measurement of rooms from the sound
energy spectral response. J.J. Jetzt (Bell Laboratories,
Holmdel, NJ 07733)

At any point in an ideal room, the standard deviation of the
acoustic energy spectral response from its mean is shown to
be a function of the direct-to-reverberant acoustic energy
ratio at that point. This function yields a method for measuring
critical distance, at which these energies are equal, which is
more powerful than the traditional total energy response
method. The new method uses measurements made within the
critical distance; it does not require that the reverberant
energy be uniform in the room.
Calculations made for computer-
simulated rooms show that the method is valid for rooms
having a wide range of average absorption coefficient. In some
cases, results can be obtained with the new method where the
total energy method fails.

---------------

Hmmm, I bet Localhost didn't peer review it...

So these guys don't believe it's important to work outside Dc. Do you see how your small room pseudo-RT-60 has parallels to this approach?

Quote:
Originally Posted by amirm View Post
Both rooms have lots of absorption which is the enemy of late reflections and development of “reverberant sound field”.
Absorption pulls Dc towards the room boundaries. So think about that, relative to Ethan's garage result. Can you get across the mental divide?

Quote:
Originally Posted by amirm View Post
Yet critical distance was precisely measured.
If I had to choose between precision and accuracy, I'd choose accuracy.

Quote:
Originally Posted by amirm View Post
So the notion that “Dc” does not exist in small rooms does not hold relative to this data.
I don't find that surprising in the least.

Quote:
Originally Posted by amirm View Post
I hope you agree that this is very strong and damaging evidence to the statement that critical distance does not exist within the space of a “small” acoustic space.
If you measure inside Dc, you get what you deserve. Erroneous conclusions.

Quote:
Originally Posted by amirm View Post
The author is from Bell Labs by the way so probably knows what he is doing especially since the paper was from the journal of ASA and hence, peer reviewed.
Probably knows what he is doing? I wonder if that was the content of the peer reviews?

You don't do much of this science stuff, do you?

I don't have free access to the whole paper and am not going to pop $30 for it. If it modeled large space down through small space, then maybe I'd be interested in giving it a through look but have plenty of my group's current research to get published and don't want to spend any more time on this estimation method.

I see you've been avoiding my recommendation to conduct a 5 minute speech test. At the least, you could go do the simple dB meter approach. Unless you are afraid of what you might measure when you do the real thing.
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1
#269
20th March 2013
Old 20th March 2013
  #269
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Joined: Jan 2009
Location: London, Europe
Posts: 478

microwave is offline
My small room has a coat hanger...
#270
20th March 2013
Old 20th March 2013
  #270
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Bobecca's Avatar
 
Joined: Aug 2012
Posts: 409

Bobecca is offline
Quote:
Originally Posted by amirm View Post
It would be my pleasure .

The paper is about different ways of measuring Critical Distance. The researcher measured direct to reverberant sound field and reported the resulting critical distance here:



All three rooms fall in the category of acoustically "small rooms", yet they all have critical distances shorter than their lengths. The first two rooms value is just 3 to 4 feet which means you can easily “go way past critical distance.” As I noted, the second room is roughly the size of Ethan's garage. Both rooms have lots of absorption which is the enemy of late reflections and development of “reverberant sound field”. Yet critical distance was precisely measured. And again, it was 3 to 4 feet in rooms. So the notion that “Dc” does not exist in small rooms does not hold relative to this data. The author is from Bell Labs by the way so probably knows what he is doing especially since the paper was from the journal of ASA and hence, peer reviewed.

Alas, we like to see the comparison between theoretical values and real measured and the above paper does not show that. Fortunately we have a much more recent paper (2010) that performs that very analysis for us with even more sample points: Reverberation Radius in Real Rooms, by Miji? and Mašovi? from the journal of Telfor (peer reviewed). Unlike ASA and AES I believe Telfor puts its documents in public domain so you may be able to find them. I am just referencing the study in my library locally so can’t give you the link.

Anyway, as the name indicates they set out to measure the reverberation radius which is another name for critical distance in real room and by that they mean small rooms with non-diffused, potentially non-reverberant sound fields. Here are the stats and results for each room:

”Room 1
This room is rectangular, its dimensions are 8.5 x 8.3 x 4.3 m and its volume is 303 m3. It is a classroom, but with no acoustic treatments in the interior. All four walls and the ceiling are solid, and the floor is covered with parquet. The only significant absorption is on upholstered chairs (backs and seats).”


In imperial units, the room is 27x27x14. So somewhat larger than Ethan’s garage. Here are the results:

Reverberation times are similar to Ethan’s at around 1.8. Note that for 500 Hz, the measured critical distance was just under one meter or 3.1 feet. Again very similar to Ethan’s garage. Computed value based on theoretical Sabine formula was 3.4 feet. Hardly different! At other frequencies there are some differences and the article digs into that some.

” Room 2 is a laboratory room which is also frequently used as a classroom. It has a rectangular parallelepiped shape, with dimensions 5 x 4 x 2.5 m and volume of 50 m3. There is an intense acoustic treatment in it in the form of a suspended ceiling made of highly absorbing panels. The absorption coefficient of ceiling material is ? > 0.9 at middle and high frequencies. The side walls and the floor are hard.”

This one is smaller at 16x13x8.


At 500 Hz, measured value is 2.6 feet, theoretical at 3.4 feet. Still pretty small numbers and well within the full length of the room at 16 feet.

I let you do the unit conversions from here on.

”Room 3
This room is a control room of a music studio. Its base is slightly irregular in shape, but the room can be approximated by a parallelepiped of dimensions 6.5 x 8 x 3.2 m, and volume 166 m3. There is a usual acoustic treatment for such a function of the room, so the reverberation time is about 0.4 s at middle frequencies.”


Hey, finally a case you all may care about!!! A small studio. Here are its stats:


So critical distance was found to be 2 meters or at less than one third the length of the room. I will stop here and not quote the whole article but the other rooms follow suit. Here is a comparison shot of the calculated vs real measurements of critical distance:



We see that there are some deviations from classic formulas but nothing remotely the same as what you guys have been saying, namely that it is so long as to never be within the actual space of a “small” room.

Also note how there is not just one critical distance. Recall how I said there was some basics of acoustics that was needed to understand what is involved in measurement of critical distance. You have one example of it here.

I hope you agree that this is very strong and damaging evidence to the statement that critical distance does not exist within the space of a “small” acoustic space. Yes there are some deviations from theoretical values but nothing to get upset over.
So, there is some people that is getting Dc afterall. And if there is a Dc that is at 1/3 of the lenght of a room then there must be a reverberant sound field aswell beyond that distance. as per defintion as stated in my link in post 9.
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