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post #91 of 191 Old 02-24-2013, 05:34 AM
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Quote:
Originally Posted by sdurani View Post

When you send a test signal through your speakers that sweeps the frequency scale, the signal itself is equal volume at all frequencies (looks like a straight line), but the in-room measurement has peaks & dips (looks like a roller coaster). The up & down response you see is a result of the room reflecting sound back, which interferes constructively (peaks) and destructively (dips) with the direct sound from your speakers.

Those peaks tend to dominate what you hear, making sounds at those frequencies really loud, which masks sounds at other frequencies that are quieter. If you listen to a recording of a bass guitar, you'll wonder why you only hear some of the notes. It's a sobering exercise to listen to a recording through a pair of headphones and then listen through your speakers to hear what details have been covered up.

If you can smoothen out the response, you'll hear all sounds more or less equally. Which means that sounds that used to be covered up will now be audible, letting you hear much more detail in the bass range. Listening to that bass guitar recording again, you'll hear all those notes that had been masked by the loud peaks and buried in the dips. Meanwhile, your friends will be wondering how you found such an articulate and detailed sounding subwoofer, not suspecting that you tuned the room and not your sub.

What a perfect and well written explanation; Clearly illustrating the need for room tuning in a way even the most non-technical person can understand.

Thanks Sanjay

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post #92 of 191 Old 02-24-2013, 11:05 PM
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post #93 of 191 Old 02-25-2013, 09:58 AM
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Quote:
Originally Posted by Ethan Winer View Post

Nice post Amir. I suspect it's falling on deaf ears. eek.gif
Thanks Ethan. Even though you are very much right, I think it is worthwhile to set the record straight with respect to the data/science being put forward. On that front, as you recall we had this same discussion about a year ago. I am sure you remember this poster who Local copies in both style and substance giving you the same as arguments as him but with this additional specificity (underline mine):
Quote:
Originally Posted by dragonfyr View Post

Manfred Schroeder defined the large room frequency, and derivatively, the volume of a room necessary to support a reverberant field at the large room frequency and above.

There is NO reverberant sound field below that frequency. Since we are concerned with volume, and using a generally accepted breakpoint value for RT of 1.6 seconds:

Volume = K^2 (RT60/(Lg Room freq^2)) K in SI (metric) is 2000 and 11,885 in imperial/US terms

Thus, for 300 Hz and above, the room must be a MINIMUM of : (11,885)^2 (1.6/(300^2)) = 2511.168 or 2511 ft^2

When someone has a room larger than 2511 ft^2 and is ONLY concerned with reverberant times ABOVE 300 Hz, determined properly - NOT with a directional home speaker but with a true omni-directional source stimulus - call me.

Let's forgive the typo there where he says the volume is feet squared rather than cubed and perform the same math for two rooms, the one that I mentioned in my last response and your garage:

My "small" listening room was 19 by 12 feet and had a height of 8 feet. That makes the volume 1,824 feet^3. Computing the transition frequency as the above number is called, we get 352 Hz.

For your garage, I am guessing that that it is roughly 30 by 20 with a 10 foot ceiling. That gives us a volume of 6,000 feet^3 resulting in a transition frequency of 194 Hz.

Recall how this argument started with Local stating, "again, to limit confusion to other novices on this forum, there is no appreciable reverberant sound-field at any frequency us humans are concerned with in small acoustical spaces (residential rooms, living rooms, etc)."

I think we all agree that humans are very much concerned with frequencies above 194 to 352 Hz in the above example spaces. We have the rest of the response all the way up to 20,000 limit of our normal hearing. So therefore Local’s statement is not supported by the evidence his mentor puts forward.

Local also said, "if you are challenging this notion then you are challenging the entire careers of Leo Berenek, Manfred Schroeder..." Seeing how the formula above came from Schroeder, he can rest in peace knowing we are not abusing his findings smile.gif.

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! smile.gif

How about my room then? Speakers as a rule are not omni-directional at higher frequencies and we usually don’t hang them in the middle of the room. So perhaps that is their get out of jail card? I will have a lot more to say later about this when I dig deep into the theory of how we compute Reverberation time and its history but for now, let's look at a frequency response graph of a sample home (and therefore 'small') listening space:

Room-Speaker-Effect.png

You can easily see that the room has two completely different characters. On the left not only do we have large swings in frequency response, but they also vary based on what seat we measure at. Our response therefore changes with “space” (measuring/hearing location) and hence violates the precondition of a reverberant room being space invariant in its response as I mentioned in my last post.

Above transition frequencies of a few hundred hertz however, that problem essentially goes away. Seat to seat variations almost entirely go away and the response becomes very smooth – one that can actually be computed using the anechoic chamber response of the speaker! If that can be done, then the room impact is essentially non-existent (in this context). Therefore we have met the conditions of reverberant space even though our speaker was not omni-directional.

Of course there is some accuracy error at the extreme. But such an error also exists in larger spaces. You simply need to know what you are doing and in this case, it means being above the transition frequencies. Once there, for the purposes that we want to use this measure, namely, how much overall absorption we need in the room, this measurement generates sound data, pun intended. biggrin.gif

So whether we stay with the evidence the other side puts forward or our own data, they all invalidate the strong claims made.

As I said, there is a lot more to come including bringing this theory down to real knowledge of how we (correctly) perform such measurements with tools like REW.

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post #94 of 191 Old 02-25-2013, 10:26 AM
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and now amir publically displays his lack of understanding of acoustics as he confuses the Schroeder/Davis frequency which dictates a transition from modal to specular behavior in a small acoustical space, and the Schroeder Large Room Frequency of which is the minimum volume to support a statistically random-incidence reverberant sound-field (but does not guarantee, just the minimum supported volume for doing so).

LOL !!

and i asked him continuously over the course of a week to provide the "conditions for reverberant sound-field" ... and the best he can do after all this time is quote dragonfyr?!? LOL.
and he still ignores the other remaining criteria. we'll wait as he furiously googles and searches through past threads as he attempts to find ANY relevant information (because he relies on copy-paste; he does not have the information committed to memory or understood).

he confuses so many terms and behaviors - just like how he implied an ECHO is REVERB. LOL !! the two definitions are logical contradictions of each other. and on top of this he STILL ignores the fundamental requirement for Reverberation Time (RT) of which the measurement must be taken Well Past Dc (critical-distance) in order for the calculations to be considered valid. FACT.

Quote:
Originally Posted by Sound Reproduction: Floyd Toole 

It is therefore necessary to conclude that the
large-room concept of critical distance is also irrelevant in
small rooms.


Quote:
Originally Posted by amirm 
I think we all agree that humans are very much concerned with frequencies above 194 to 352 Hz in the above example spaces.
Quote:
Originally Posted by amirm 
Therefore we have met the conditions of reverberant space even though our speaker was not omni-directional.

yes - by all means, show me a measurement well-past Dc at 194-352hz in your room of which the steady-state reverberant sound-field is higher in gain than the direct signal. I'LL KEEP WAITING.
you can't even list all of the "conditions for reverberant space", yet you are attempting to dictate you have "met" them? LOL!


ethan has ignored numerous times to dictate whether there is a critical-distance in his garage - which would then allow us to move forward with whether the calculations can then be considered valid. he also is concerned with "ignoring official definitions" of reverb, and instead merely wants to know if it sounds like reverb, not whether it actually is statistically random-incidence reverb.

mickey mouse acoustics for public display!

all these guys are concerned with is "ball-parking" - most certainly NOT interested in acoustics.
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post #95 of 191 Old 02-25-2013, 10:43 AM
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Quote:
Originally Posted by jdlynch View Post

Clearly illustrating the need for room tuning in a way even the most non-technical person can understand.
Thanx for the kind words David. Not knowing the OP's background, I deliberately tried to keep my reply as non-technical as possible.

Sanjay
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post #96 of 191 Old 02-25-2013, 10:56 AM
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Quote:
Originally Posted by amirm View Post

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! smile.gif

How about my room then? Speakers as a rule are not omni-directional at higher frequencies and we usually don’t hang them in the middle of the room. So perhaps that is their get out of jail card?

"fairly omni-directional". "essentially satisfying" LOL. "mickey mouse acoustics".

you really should spend a little more time reading Toole's book, he goes into great detail and provides many citations to that of Ted Schultz and Leo Beranek.


Russ Berger and Bolt Beranek & Newman must be clueless on reverb as well! and as a representative of BBN, Leo Beranek also concurred!!
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post #97 of 191 Old 02-25-2013, 11:45 AM
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Quote:
Originally Posted by amirm View Post

For your garage, I am guessing that that it is roughly 30 by 20 with a 10 foot ceiling. That gives us a volume of 6,000 feet^3 resulting in a transition frequency of 194 Hz.

My garage is a bit smaller at 24.5 feet long by 22 feet wide, by 8 feet high = ~4300 square feet. But it's still large enough to get reverb nearly as rich as that from the live reverb rooms in pro recording studios.
Quote:
therefore Local’s statement is not supported by the evidence his mentor puts forward.

Indeed. Also, I was hoping someone would post a link or quote to a formal definition by the ASTM or another standards agency. Is there even a formal definition? Not that standards don't change over time. But so far all I've seen from Local is the logical fallacy known as Argument From Authority. What I don't understand is why he's so rude and hostile. Can't civilized people carry on a technical discussion without resorting to immature nonsense like this:

LOL !!

I wonder how old Local is? He just undermines his own credibility with such boorish behavior.
Quote:
Seeing how the formula above came from Schroeder, he can rest in peace knowing we are not abusing his findings smile.gif.

Others here may not be aware, but according to this post at Local's other hangout, Gearslutz, he doesn't even have a proper listening room:

http://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/639773-fundamentals-between-prds-qrds.html#post6944750

You can see that Local later deleted all of his posts in that thread, but in the one linked above you can see this admission in the quote from someone else's post. biggrin.gif

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post #98 of 191 Old 02-25-2013, 11:46 AM
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Quote:
Originally Posted by Ethan Winer View Post

Quote:
Originally Posted by amirm View Post

For your garage, I am guessing that that it is roughly 30 by 20 with a 10 foot ceiling. That gives us a volume of 6,000 feet^3 resulting in a transition frequency of 194 Hz.

My garage is a bit smaller at 24.5 feet long by 22 feet wide, by 8 feet high = ~4300 square feet. But it's still large enough to get reverb nearly as rich as that from the live reverb rooms in pro recording studios.
Quote:
therefore Local’s statement is not supported by the evidence his mentor puts forward.

Indeed. Also, I was hoping someone would post a link or quote to a formal definition of reverb by the ASTM or another standards agency. Is there even a formal definition? Not that standards don't change over time. But so far all I've seen from Local is the logical fallacy known as Argument From Authority. What I don't understand is why he's so rude and hostile. Can't civilized people carry on a technical discussion without resorting to immature nonsense like this:

LOL !!

I wonder how old Local is? He just undermines his own credibility with such boorish behavior.
Quote:
Seeing how the formula above came from Schroeder, he can rest in peace knowing we are not abusing his findings smile.gif.

Others here may not be aware, but according to this post at Local's other hangout, Gearslutz, he doesn't even have a proper listening room:

http://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/639773-fundamentals-between-prds-qrds.html#post6944750

You can see that Local later deleted all of his posts in that thread, but in the one linked above you can see this admission in the quote from someone else's post. biggrin.gif

--Ethan

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post #99 of 191 Old 02-25-2013, 12:34 PM
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Originally Posted by Ethan Winer View Post

I wonder how old Local is? He just undermines his own credibility with such boorish behavior.

the root of your confusion!! no one cares about my "credibility", especially me. i'm not the one selling products and services here. i am not the one with my company in my signature file. i am not the one courting potential customers with my commentary or "articles". NOR am i describing myself (read: self-proclaiming) as an "acoustic specialist" - such as yourself.

physics is not modified based on what i personally do or say. why a "nobody" such as myself should have to debate such fundamental aspects of acoustics to you (the "acoustic specialist") is beyond me.

i have cited my sources. if you wish to go against Sabine, Schroeder, Beranek, Davis, Russ Berger, and Ted Schultz (and toole, as he cites Beranek and Schultz!), then by all means at least come out and directly say it! step up to the plate!

Quote:
Originally Posted by Ethan Winer View Post

Also, I was hoping someone would post a link or quote to a formal definition by the ASTM or another standards agency. Is there even a formal definition? Not that standards don't change over time.

formal definition? but i thought you weren't concerned with such:
Quote:
Originally Posted by Ethan Winer 
So - ignoring official definitions - does this sound like reverb to you?

and yes, the definition was given to you in this thread and the very first reply to you in the GS thread. of which you clearly did not read or process. yet you apparently had enough spare cycles to refer to me as a "know-it-all baffoon" and "troll". i even linked you to authoritative source on the subject (unless Ted Schultz, Manfred Schroeder, Leo Beranek, and Don Davis are suddenly not viable sources - even Toole cites Schultz repeatedly). did you read the sources? no - the first thing you asked is whether or not this Don Davis fellow's experience "trumps yours". !!!

but then again you're the one performing RT60 in a 37.75 by 22.5 by 14.5 inch box. eek.gif
http://www.realtraps.com/art_surfaces.htm

Quote:
Originally Posted by Ethan Winer 
And again, an empty room can definitely have enough individual echoes that they fuse together with sufficient density to sound like "reverb."

one last time: sounds like reverb or is reverb. which are you asking?
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post #100 of 191 Old 02-25-2013, 01:05 PM
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Quote:
Originally Posted by our buddy localhost 
These are not Sabine spaces, and it is not appropriate to employ
calculations and measurements that rely on assumptions of diffusivity.
Well, the bolding, underlineng and bright colors are gone, but this is another case of poor reading comprehension.

Calculation and measurement are simple words. One would think them conspicuous by their ABSENCE.

Conversations devoid of those words should then be free from your interference, as in this thread. Please make it so.
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post #101 of 191 Old 02-25-2013, 02:55 PM
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Quote:
Originally Posted by sdurani View Post

Thanx for the kind words David. Not knowing the OP's background, I deliberately tried to keep my reply as non-technical as possible.

Actually it was one of the more useful posts in this thread as it relates to the OP's question. I liked it. I think the discussion at this point probably isn't doing much to educate the vast majority of viewers. I think most are tuning in to watch a fight. Kind of like the school bus stop when you were a kid.

Regards,
John
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post #102 of 191 Old 02-25-2013, 10:57 PM
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Quote:
Originally Posted by Ethan Winer View Post

Others here may not be aware, but according to this post at Local's other hangout, Gearslutz, he doesn't even have a proper listening room:

http://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/639773-fundamentals-between-prds-qrds.html#post6944750

You can see that Local later deleted all of his posts in that thread, but in the one linked above you can see this admission in the quote from someone else's post. biggrin.gif

--Ethan

Sorry, but why is this relevant? He even clearly states in the post why he does not, and it's because his room is too small to adhere to the standards that he wishes it to be on par with. Plus, that was well over a year ago. Just because a man drives a Toyota doesn't mean he can or can't fix a Porsche.
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post #103 of 191 Old 02-26-2013, 10:35 AM
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Originally Posted by GIK Acoustics View Post

Sorry, but why is this relevant?

It's relevant because Local is clearly just an armchair theoretician who has zero practical knowledge. How many rooms has he measured and treated? Anyone can read a book and memorize a bunch of stuff to parrot later. Who is Local and why is he hiding behind a screen name? Why does he feel the need to challenge me at every opportunity? Is he a GIK customer? Does he work for GIK? Speaking of deleted posts Alex, I see that you deleted all of yours in this thread too. I honestly don't understand why you're involving yourself in this. If you have something to add to the discussion at hand about reverb, please do. Otherwise you're just lowering the s/n even further.

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post #104 of 191 Old 02-26-2013, 12:09 PM
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Quote:
Originally Posted by Ethan Winer View Post

Does he work for GIK?

too funny! another delusional distraction to the thread by the user who then complains about not being on topic:
Quote:
Originally Posted by Ethan Winer 
If you have something to add to the discussion at hand about reverb, please do.

"do as i say, not as i do", he says!

why is Ethan so clearly threatened to begin with? i'm not the one selling products and services here. i am not the one with my company in my signature file. i am not the one courting potential customers with my commentary or "articles". NOR am i describing myself (read: self-proclaiming) as an "acoustic specialist".

i have cited my sources. if you wish to go against Sabine, Schroeder, Beranek, Davis, Russ Berger, and Ted Schultz (and Toole, based on his citations of Beranek and Schultz!), then by all means at least come out and directly say it! step up to the plate!



getting back on topic to what has repeatedly been ignored:
Quote:
Originally Posted by Ethan Winer 
So - ignoring official definitions - does this sound like reverb to you?
Quote:
Originally Posted by Ethan Winer 
And again, an empty room can definitely have enough individual echoes that they fuse together with sufficient density to sound like "reverb."

one last time: are you concerned with what sounds like reverb, or what actually is reverb. which is it you are asking?
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post #105 of 191 Old 02-26-2013, 12:23 PM
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No, he does not work for GIK. No clue if he's a customer or not.

Edit: Not sure why my motives for deleting a post are being questioned, but in case it needs to be clarified for some reason, I deleted & edited my posts to keep my involvement in the thread on topic.
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post #106 of 191 Old 02-26-2013, 02:29 PM
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time to move on please

thanks
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post #107 of 191 Old 02-27-2013, 06:43 AM
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Quote:
Originally Posted by Ethan Winer View Post

It's relevant because Local is clearly just an armchair theoretician who has zero practical knowledge. How many rooms has he measured and treated? Anyone can read a book and memorize a bunch of stuff to parrot later. Who is Local and why is he hiding behind a screen name? Why does he feel the need to challenge me at every opportunity? Is he a GIK customer? Does he work for GIK? Speaking of deleted posts Alex, I see that you deleted all of yours in this thread too. I honestly don't understand why you're involving yourself in this. If you have something to add to the discussion at hand about reverb, please do. Otherwise you're just lowering the s/n even further.

--Ethan

Now now there Ethan I don't see any reason to start behaving in this manner. You know Alex has been nothing but respectful to you and you also know local does not work for GIK. cool.gif

Try to keep your cool there big guy. You know Alex is one of the good guys. Not a Ethan HUNTER. ha ha smile.gif

Now with that said I tried to read through all the posts so excuse me if this has been answered, but from your test what on a scientific level are you saying that is reverb? I just want to understand what you are trying to get at. Is it a new meaning that you think it should be? Or something different?


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post #108 of 191 Old 02-27-2013, 12:07 PM
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Glenn, it's all spelled out clearly in my first post to this thread concerning reverb:

http://www.avsforum.com/t/1453370/do-bass-traps-produce-noticeable-audible-difference/30#post_22969669

Comments relating to reverb and how its perceived etc are welcome. Insults and accusations will be reported henceforth. Same for other OT posts that question my motives. I'm here to educate, not elevate my status. If only others felt the same way.

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post #109 of 191 Old 02-27-2013, 01:39 PM
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Ethan,

Thanks. I guess I was looking at how you would define reverb in a more scientific fashion vs just making a statement
Quote:
I used to accept that too until I actually tested the theory. biggrin.gif Now, if you add "typically furnished" to "small acoustical spaces" then I'd agree. But an empty room can be highly reverberant, even if it's relatively small. My two-car garage is no larger than many living rooms, but when it's empty it produces a very nice sounding reverb that extends for at least several seconds! The reverb decay time is also fairly uniform versus frequency. If anyone doesn't believe me I'll make a recording and post an MP3 file here.

Needless to say the world is not going to end over it. smile.gif
Quote:
Comments relating to reverb and how its perceived etc are welcome. Insults and accusations will be reported henceforth. Same for other OT posts that question my motives. I'm here to educate, not elevate my status. If only others felt the same way.

Was my first post above insulting to you? confused.gif I don't think I have ever been insulting to you nor you to me. Email me if something is up buddy.

Relax/breath

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post #110 of 191 Old 02-27-2013, 02:31 PM
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Quote:
Originally Posted by myfipie View Post

Ethan,

Thanks. I guess I was looking at how you would define reverb in a more scientific fashion vs just making a statement
If I may make an analogy, on your web site you have a video on how Diffusion works. It starts with "Diffusers are used to treat sound imperfections in rooms such as echoes."

When you use the term "echo" which way do you mean per your question to Ethan? Is it proper? Is it "slang?" Is there echo in that small room in the video? Should you be criticized for the use of the term either way?

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post #111 of 191 Old 02-28-2013, 11:15 AM
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Quote:
Originally Posted by amirm View Post

If I may make an analogy, on your web site you have a video on how Diffusion works. It starts with "Diffusers are used to treat sound imperfections in rooms such as echoes."

When you use the term "echo" which way do you mean per your question to Ethan? Is it proper? Is it "slang?" Is there echo in that small room in the video? Should you be criticized for the use of the term either way?

Amirm, with all due respect, Glenn hasn't questioned whether Ethan's definition of reverb was 'proper' or 'slang' (neither have I). If one were to bring it up to question, as you have, I could simply suggest what I meant by it. Our articles are not on our website to argue about definitions - they are there to explain to someone the concepts of acoustics without too much jargon and to maximize understanding of the subjects so they can make informed decisions. If you explained to me that I used such a term incorrectly, I would explain the reasoning for the use or correct it if need be. None of us at GIK are here to argue that our definition of echo is more correct than your own, or that it is more correct than an acoustics textbook.

Glenn's questions were simply asking what the point Ethan was trying to get at with his sound clip. In other words:
Are you saying that local's definition is incorrect and yours is correct?
Or that there are too many technical definitions to consider one correct, so the term reverb should be always used loosely?
Or is it something else?

- I'm replying for Glenn in this case as he's out of town, and we've obviously discussed this together so I know why he posted.

I'm also confused as to what Ethan is arguing, as I was under the impression the original discussion was about a calculation, and not about the verbiage of the term reverb.

In case it all needs to be laid out, Local pointed out (and I agreed) that RT60 is an invalid calculation in small rooms because RT60 is the measurement of the decay time of "a well-mixed reverberant sound field well beyond critical distance". Note that in this argument about RT60, it doesn't matter whether you think something "sounds like" reverb or not. In fact, the verbiage has absolutely no use to argue in the original discussion about the validity of RT60 calculations in SAS. Regardless of whether you think a reverberant sound field is "x" or you think a reverberant sound field is "y" there is still another part of the criteria not met for this to be a valid calculation - the measurement needs to be taken "well beyond critical distance" which is impossible to do in a small room. Secondly, it is quite obvious that in the text, they are referring to some technical definition of a "well-mixed reverberant field" - not "a room that subjectively sounds like it has reverb". Ethan, I'm 100% with you that long decay times "sound like" reverb. Certainly they do. I might 'incorrectly' use the term from time to time to describe simple concepts to people who aren't in the field of course.

Again, this entire discussion about reverb is completely irrelevant to the matter at hand discussing RT60 as we're still missing part of the criteria - regardless if the room has reverb or not, or regardless if it has this reverb or that reverb, the measurement still needs to be taken beyond critical distance.

I don't own nearly enough acoustic texts to shout that I am right and others are wrong about the validity of RTXX - I just simply see that RT60 is invalid because the books I do own outline why (and I've quoted them) - and from personal experience testing, I simply get much more data from ETC, filtered ETC, Waterfall, and Burst Decay charts than I do from RT60 in a small room. So not only do I believe they are invalid calculations based on my readings, but I also believe they are practically useless as it doesn't give information to treat specular reflections, nulls, nor resonances, all of which are essentially the main problems that need to be dealt with when treating a small room IMO.

Now if anyone wants to argue that fact based on other things they've read, testing data they've gathered, please, feel free to do so. Amirm has provided good counterexamples IMO, but I honestly don't know enough about the subject to again say what is right or wrong - I only state my opinion based on what I have read and used. I find his examples intriguing and am further investigating it myself to see if I can draw the same conclusions. Ethan, I'm not attacking you. Just because I don't agree with you doesn't mean I'm insulting you. I haven't been rude, nor have I been even discourteous with you. I haven't threatened you nor questioned your motives on posting. Please, lend me the same respect. I've provided my reasoning for RT60 being invalid. If you agree or disagree, some data to back up your claims would be helpful to educate yourself, myself, and others. Which parts do you agree with or disagree with? Or do you even have an opinion about RT60 being valid/invalid? (I understand if not, your original post in this thread was not about RT60 specifically).
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This whole what is reverb thing seems unnecessarily confusing to me. When local talks about "random incidence reverb" that seems to me to be a subset of reverb. Sort of like somebody telling my my pickup isn't a truck because then THEY say truck they mean the tractor for an 18 wheel rig. The term "truck" encompasses both, IMO, and if you want to narrow it you need an adjective or somepin'.

I can't help thinking about the many stories of guys putting their vocal mics or guitar amps in the bathroom or a stairwell at a recording studio in order to capture "reverb" that they find particularly desireable (or Anderson and Howe from Yes putting up tile dividers to make the studio sound like a bathroom when recording Tales from Topographic Oceans). IIRC, Jeff Beck stuck his amp in a stairwell to get the sound he wanted on his Crazy Legs project. To my mind, those sounds, as well as the sound of the spring based reverb in my guitar amp, the sound of plate reverbs in studios, etc etc etc are "reverb." And often the fact that they are not some kind of "perfect" reverb is the very point of using them artistically. (of course it's gotta be pleasingly imperfect. The noise that made it to my big condenser from the floor joists 7.5 feet above the floor in my basement was "crap," not "reverb," at least in my book. Anyway, I'll keep on calling them reverb, because that's what everybody calls 'em in my little world.
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Quote:
Originally Posted by myfipie View Post

Was my first post above insulting to you?

Not to dwell on the "personality" problems running through this thread: My objection was Alex repeatedly defending and siding with someone who constantly takes pot shots at me here and at Gearslutz, rather than address the issue of what constitutes reverb. The behavior of forum trolls is bad enough without others egging them on and encouraging them. When such encouragement comes from a competitor, it's equivalent to the competitor taking the pot shots. Same for Local's claim that he's not selling anything. He hides behind an anonymous screen name, so how do we know he's not a competing vendor (or friend of yours)? I could easily create an account under a fake name and take pot shots at you and Alex in every forum I see you post in. I wouldn't have to list anything in my sig, yet I'd still be successful at tearing you guys down. Not that I'd ever do that.

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Originally Posted by JHAz View Post

This whole what is reverb thing seems unnecessarily confusing to me.

Yes, some people enjoy making things seem overly complicated. Me, I prefer to simplify things. It's a lot better for the people who genuinely want to learn!

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Whoops, double post. See below -

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Ethan, just because you felt threatened that I disagreed with you does not mean that I intended to harm your company in some explicitly arbitrary manner.

In fact, I never even disagreed with a single thing you wrote! I actually defended both of your viewpoints about reverb here:
http://www.avsforum.com/t/1453370/do-bass-traps-produce-noticeable-audible-difference/30#post_22985309

I never even "sided" with local "against" you. I simply agreed with him that RT60 is not valid in my original post in this thread - which had nothing to do with you:
http://www.avsforum.com/t/1453370/do-bass-traps-produce-noticeable-audible-difference/30#post_22985309

I wasn't trying to prove either of you correct/incorrect about reverb. As I stated here:
http://www.avsforum.com/t/1453370/do-bass-traps-produce-noticeable-audible-difference/60#post_22994592

As to "repeatedly defending local" - he isn't a criminal and I can defend whoever I want. First off, I was defending my own viewpoint - not local - he can defend himself. If a textbook points to me that something is correct, I'm allowed to voice that opinion! It doesn't mean I'm attacking you just because someone you don't get along with has the same viewpoint! The only time when I defended local was when you personally attacked him which I felt was completely uncalled for.
Quote:
Originally Posted by Ethan Winer View Post

He hides behind an anonymous screen name, so how do we know he's not a competing vendor (or friend of yours)? I could easily create an account under a fake name and take pot shots at you and Alex in every forum I see you post in. I wouldn't have to list anything in my sig, yet I'd still be successful at tearing you guys down. Not that I'd ever do that.

So why in the hell would you accuse us of doing that?! Seriously, Ethan. We're better than that. You're better than that. We know this, you know this.

Please treat me with respect as I've treated you. I haven't been rude, I haven't accused you of trying to damage GIK, I haven't done anything harmful to you at all in this thread. Re-read my posts if for some reason you need clarification.

This thread is completely ridiculous and I am done posting in it! Any commentary henceforth about GIK or myself will be simply ignored.
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As promised, here is the deep dive into Reverberation Time. It is long but I hope that it tells the story from start to finish and hopefully provides useful information beyond settling this argument. smile.gif

History Part I
The origin of Reverberation Time is from Wallace Clement Sabine who around 1890 performed a ton of experiments in his basement to see how long it would take for sound to decay 1,000,000 times once it is shut off. Expressed in dB, we get 60 dB which happens to also be the sound level of a human. Sabine discovered that the decay time was proportional to volume (V) and inversely so to the total amount of absorption (Sa): RT60 = K * V / Sa (K is just a constant)

That kind of makes sense, no? You have more reverberations in a larger space than small. And an empty room like Ethan’s garage has more of it than one stuffed with furniture.

The Sabine formula was a godsend for early acousticians. Remember, this is a century before we have personal computers and nearly as long before we had pocket calculators. Anyone could perform the above math by hand and immediately get a sense of how a room would “sound” even prior to building it. People then performed surveys of performance halls and realized there is good correlation between how they sounded for different applications and RT60. It is not often that complex physics of a room with sound waves bouncing and absorbing becomes this simple to analyze.

History Part II
Decades later, Schroeder advanced our understanding of room reflections by introducing the concept of transition frequency below which, the room resonances/modes are separate and hence not random. He came up with a formula to determine this frequency which I showed in my last post. In reality there is no single frequency delineation (nature hates sudden changes) but a range above which we have the randomness we desire. For our home listening spaces, that ranges from 200 to 300 Hz typically.

The second thing he did was to come up with a reliable way to measure reverberation time. This is called the Schroeder Integral and is simply the sum of the square of measured decaying sound. I will show how this works in a minute.

Measuring vs Predicting RT60
So far we have talked about computing RT60 using the Sabine formula. I call that the predicted value of RT60. Its simplicity means that one can find cases where the prediction does not work as well. Let’s take the simple case of having a fully reverberant room meaning it has diffused reflections from all the hard walls. Now put an absorber in it. By definition the absorber will not bounce back some of the sound wave energy hitting it. As soon as you did that, your room is no longer fully reverberant. Put another way, the idea of a reverberant one does not exist in a realistic situation in big or small rooms!

Now think of a practical situation. You have a room with some furnishings in it. You add an absorber to it. The furnishings in your room caused your space to not have fully diffused reflections. That means the absorber experiences a different situation than it did in the reverberation chamber above. Sabine’s formula will mispredict how much of an effect that product has since its absorption coefficient was determined in a more reflective situation.

Instead of using the simple formula we can actually measure the decay of reflections in the room. Once there, we are free of many of the limitations of Sabine RT60. This is one of the reasons I am pushing back on all the theoretical objections against RT60. Often the text cited is old and implied in there is that the issues raised relate to Sabine formula, not in situ version we use. Let’s see how this works.

Measuring RT60 with Room EQ Wizard (REW)
I am going to walk through how we can use REW to determine the proper value of RT60. If you have used REW, you know that it has a button for RT60. If you click on it, you may be puzzled that none of the graphs are named RT60. Instead, there are odd names such as EDT, T20, T30 and Topt. Let’s table that for a minute and learn a bit more about theory behind the measurement.

The concept for measuring reverberation is rather simple: excite the room with an impulse – a sharp spike of sound energy that has high/known amplitude and zero time – and then measure how long it takes for the reflections that are created as a result of it to die down by 60 dB. In practice we don’t use such impulses as they are hard to create in our rooms and can damage our gear. REW uses a modern technique called log swept sine which produces very good results without the need for loud test signals. But the end result is the same: we are measuring an “impulse” even though it sounds like a sine wave going from low to high frequency.

Let’s look at what happens when we hit the “Filtered IR” button. IR is short for Impulse Response (IR). This is what the output looks like:
i-cgRFQVS-L.png
The sharp peak at (roughly) zero time is our (computed) impulse. To the left we see our noise floor prior to our “impulse” playing. On the right we see us returning to the sane noise level after the reflections die off. Reverberations in a room drop off exponentially post the impulse going away. In the above graph the decay appears to be a straight line. The reason for that is that the vertical scale is in dB which is a log scale. Log is the inverse of an exponent so we wind up with a line.

Speaking of the line, there is one in black. That is the Schroeder integral. If this were an ideal reverberation chamber, we would have a perfectly straight line. We would then follow where it crosses the -60 dB point on the Y axis, and read our RT60 on the X axis which is in time. That is not the case here. Our line is not straight. We will fix this shortly.

Overlaid on the Schroder integral line is a blue line. If you look to the top right box, you see “Topt” selected. Other choices are T20 and T30. The latter two are standardized methods for finding the RT60 value from a less than perfect Schroeder integral. Instead of using the whole line to compute our RT60, we use a part of it and extrapolate from there. T20 uses the time it takes for reflections to die down between -5 and -25 (difference of 20 dB). Simply multiply that value by 3 and we get our RT60 time. T30 is similar and uses -5 to -35 and multiplies by 2. Both of these methods solve the problem of what to do if our noise floor is above -60 dB point. They also avoid looking at the start of the line which may be different. These are standardized computational methods per ISO 3382. As a result researchers use the same methods for consistency.

Topt is a much cleverer one in that it attempts to analyze the Integral and try to fit a line to it wherever it needs to start. Use Topt unless you have a reason not to (e.g. complying with ISO standard).

Looking at Topt line, we see that it pretty much tracks the Schroeder Integral except at the beginning of our impulse (between 0 and 50 milliseconds). There, we have a sharper drop than our Topt line represents and hence shorter reverberation time (the line would be steeper if we made it parallel to the early portion). That early part is called EDT (Early Decay Time). If you look at the box on the left, the RT time reflects that: Topt computation gives us an RT value of 0.659 seconds whereas EDT gives us 0.428.

What is the reason for that sharper early decline? The room that is being measured has carpeted floor. Once a sound wave hits that, it loses a lot of its power. Whatever bounces off the carpet is likely going to die soon. And once it does, it no longer contributes to the energy of the reflections that exist between the rest of the (reflective) surfaces such as drywall constructed walls. Another reason for the sharp drop is that in early stages we have strong reflections which push up the value of Schroeder Integral. Those go away after a while and we are left with our random/diffused reflections. As noted, all the standardized measurements of RT avoid the earlier part of the graph and hence, represent the “late” reflection timing of our room.

Back to our graph, REW provides a measure of how far off our RT measurement is from Schroeder integral with that “r” value (correlation coefficient). -1 is perfection meaning our single line matches the integral completely. Our Topt line is at -0.998 which is darn close with respect to our reflections past the initial 50 milliseconds or so.

So far we have looked the full spectrum RT. We know however that the low frequency modes likely are not following our rules. Let’s ask REW to show us RT60 for an octave of frequencies centered at 63 Hz:

i-QPGfH8P-L.png
We see a pretty chewed up line now since the reflection energy clearly is not random. We can try to fit a line to this but obviously there are a lot of deviations which REW notes with “r” values in orange. This means that the computation of RT for lower frequencies is probably not accurate.

Climbing up to 500 Hz cleans up the graph significantly:

i-WqT6qCr-L.png

With a nice line fit, you see why in research we used RT60 times for 500 Hz or higher. Let’s see what happens if we keep going up in frequency:

i-hqHP8M4.png

Even better. Now we are ready to go to the RT60 tab since you know what the different parameters mean. Here I have selected only Topt:

i-V5ZM2qb-L.png

This is really a duplicate of our previous display except that the REW is doing the work for us of changing the frequency bands and plotting them as single values. We can easily read the mid-frequency RT60 times of 0.8 seconds. As a way of reference, the predicted RT60 using the Sabine formula for the same room is 0.66 seconds. Measured RT60 is higher due to one of the factor I mentioned: the absorption of material in the room is less than the ideal used in Sabine computation.

What does it mean to you?
A lot of objections were raised saying RT60 measurement is useless/meaningless in small rooms. With the knowledge at hand of what it is and how it is measured in a real small room, let’s see if that is the case.

Imagine you are playing a movie and actor says the word “hat.” We have three distinct parts to that with different loudness levels: “h,” a” and “t.” Let’s say it takes one second to pronounce “hat.” That means each part of it roughly takes 1/3 of a second or ~0.3 seconds. In our sample room we had a reverberation time of 0.8 seconds. This means that the pronunciation of “h” lasts another 0.8 seconds after it stops. That 0.8 seconds then overlaps the rest of the word. That may make it harder to hear the softer (in level) parts of that word. We may think the person said “had” instead of "hat." If we shortened our reverberation time to say, 0.2 seconds, the chances of that happening becomes much smaller and intelligibility improves.

Note that you don’t want to go too far here. The reflections help make the direct sound louder and hence easier to understand. Think of trying to talk to someone far away outside. It takes more energy than being indoor, right? Now you see why we have a lower bound for RT 60 values. The early reflections can be useful and we want to preserve them. But have the later ones die fast.

So as you see, RT60 is not useless at all. If we know what we are doing and measuring, we can extract useful data from it in order to determine if our room is too naked/live or alternatively over treated and too dead. Yes, with experience you too can determine the same by eye. Ethan did in his garage. And I could too without him posting the recording. That doesn’t invalidate RT60. It just says that our ears and experience can show the same. Indeed, Sabine did all of his measurements using his ears just the same!

If someone wants to challenge all of this, I hope they post measurements and data demonstrating their case.
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why now discussing validity for RT60 and Schroeder?? on the first page, it was stated in contradiction:
Quote:
Originally Posted by amirm 
Likewise, references to Schroeder and conditions for reverberant field are inappropriate in this context.

and yet he has still been unable to expand on his statements and actually detail for us what these "conditions for reverberant sound-field" are.
can anyone detail this further ?

unfortunately, we once again see the confusion between the schroeder/davis small room transition frequency with the schroeder Large room frequency.
and really, much more could be said regarding schroeder than simply quoting dragonfyr eek.gif

on-topic and quoting Toole:
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.

Quote:
Originally Posted by Sound Reproduction: Floyd Toole 

Figure 4.2 shows a familiar portrayal of idealized behavior in one of these
halls. In this depiction, an omnidirectional sound source is located well away
from the room boundaries, such as the center front of the stage. As a function
of distance from the source, the level of the direct sound follows the inverse square-
law rate of decay (−6 dB per double distance, dB/dd) until it encounters
the underlying steady-state reverberant sound field
that is assumed to extend
uniformly throughout the space
(Beranek, 1986; Schultz, 1983). The distance
from the source at which the direct sound equals the level of the reverberation
is known as the critical distance (also known as reverberation distance or reverberation
radius).

notice how his measurements are unable to display a steady-state reverberant sound-field? notice how it doesnt extend uniformly throughout the space. is it because he is in a Small room and not a Large room?> eek.gifeek.gifeek.gif

Quote:
Originally Posted by Sound Reproduction: Floyd Toole 

In the abundance of reflections that we collectively
call reverberation, there are so many individual events
that it has been common to think of them as a statistical
entity distributed randomly in time and space
. As
a result, classic concert hall acoustical theory often
begins with the simplifying assumption that the sound
field throughout a large relatively reverberant space is
diffuse.


Quote:
Originally Posted by Sound Reproduction: Floyd Toole 

4.3.4 What Is a “Small” Room?
Diffuse-field theory may not apply perfectly to concert halls, but it applies even
less well to other kinds of rooms
. In the acoustical transition from a large performance
space to a “small” room, it seems that the significant factors are a
reduced ceiling height (relative to length and width), significant areas of absorption
on one or more of the boundary surfaces, and proportionally large absorbing
and scattering objects distributed throughout the floor area

These are not Sabine spaces, and it is not appropriate to employ
calculations and measurements that rely on assumptions of diffusivity
. Schultz
(1983) states, “The amount of sound-absorbing material in the room cannot be
accurately determined
by measurement, either with the decay-rate (reverberation
time) method or the steady-state (reference sound source) method. . . . One
cannot trust the predictions of the Diffuse Field Theory for a non-Sabine
room.”

Quote:
Originally Posted by Sound Reproduction: Floyd Toole 

It is therefore necessary to conclude that the
large-room concept of critical distance is also irrelevant in
small rooms.

applying Large Room calculations into Small Rooms that lack the pre-requisites = operator error and compromises the results. garbage in = garbage out.

Quote:
Originally Posted by amirm View Post

If someone wants to challenge all of this, I hope they post measurements and data demonstrating their case.

there's no need to challenge anything you say as you simply aren't interested in the fundamentals, just "ball-parking".

you attempted to discuss RT60 without-so-much-as-even-mentioning critical-distance. not once! - of which is required for RT60 to even be considered valid.

RT60 is a property of an acoustical space, just as Toole clearly outlined above. as such, we are not interested in the transfer function between your loudspeaker and your room - if we were, we have better tools at our disposal.
another case of operator error.


it's a shame as such concepts have been well known and understood for decades.

but thanks for the "history lesson" - i now know what not to do.

tongue.giftongue.giftongue.gif
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Quote:
Originally Posted by JHAz View Post

I can't help thinking about the many stories of guys putting their vocal mics or guitar amps in the bathroom or a stairwell at a recording studio in order to capture "reverb" that they find particularly desireable (or Anderson and Howe from Yes putting up tile dividers to make the studio sound like a bathroom when recording Tales from Topographic Oceans). IIRC, Jeff Beck stuck his amp in a stairwell to get the sound he wanted on his Crazy Legs project. To my mind, those sounds, as well as the sound of the spring based reverb in my guitar amp, the sound of plate reverbs in studios, etc etc etc are "reverb."

they're all a form of decay. a guitar amp's "reverb knob" is not acoustical reverb, it is an "FX". same goes for the plate/spring reverb units. it's merely slang for acoustical reverb of which likely took hold in popular vocabulary when the very guitar amps you mentioned started including the "reverb" knobs. and hey, "reverb" sounds cool so many people like to use it in a myriad of ways (thus the muddying of the definition). there's nothing wrong with using it as slang as reiterated many times here. the problem is when someone using the term reverb for any type of room decay then implies that by mere use of the word that such energy is indeed random-incidence. long decay times do not automatically imply statistically random-incidence reverberant sound-field (of which is required for RT60, for example, to be considered valid).

Ted Schultz put this to bed a long time ago.
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