Using energy time curve for acoustic analysis: developing a Home Theatre primer - Page 2 - AVS Forum | Home Theater Discussions And Reviews
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post #31 of 292 Old 08-17-2011, 08:45 PM
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Originally Posted by aackthpt View Post

One more note: while I think that having an ISD-gap and possibly even an ISD termination might be desirable for the LCR in a home theatre environment, I've never seen (and perhaps it might simply be through failure to competently drive a search engine) mentioned a best ETC response for surround speakers. I'd guess there might be a difference between movie use (where dipoles are recommended) versus multichannel music use (where monopoles are recommended). My guess is that for multichannel music the desirable response might be the same as that for the mains, but for movie use the desirable response would be as much of the decay in the "spaciousness" zone of the perceptual curves - and it would surprise me if it was desired to put their decay up into the "image effects" range. In other words, for movies, as decorrelated and psychoacoustically confusing as possible without being obvious or noticeable through echo.

Then again, I haven't really kept in great touch with things in a while so... does anyone really listen to multichannel music anyway?

the ambechoic model is precisely regarded for surround sound! (the next evolution)
http://www.cari-llc.com/pdfs/Mono2Surround.pdf


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Originally Posted by Andreas View Post

Perhaps I'm repeating this too often; the standard 5.1 surround setups have the rear speakers at about +-110' (relative to straight ahead) for good reasons. It's a great direction for arrival of later energy! If the delay is sufficiently short, the ears will integrate it into the sensation of the direct sound.

This gives a bunch of subtle effects.

The fusing of the direct and delayed sound can increase and enhance the sensation of the direct sound. The idea is exploited in Bob Katz's K-Stereo. To me, an example of how a patent can be granted to the first one to apply for a patent for an old idea. The trick is easy to do with any typical mixing setup. Add a 20-30 millisecond delay and mix it in with the direct sound. You can try polarity inversion in one channel and messing about with phase to change the effect a bit. I do use such tricks at the odd times when I need a bit of "ambience recovery" in mastering. It's very seldom to do so, about as infrequent as adding reverb. Though it can be very handy. Try it! It'll give you a coarse idea of what may happen with a similar acoustical response. The sales blurb for the K-stereo effect is describing it well enough; K-Stereo

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post #32 of 292 Old 08-18-2011, 07:24 AM
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Originally Posted by localhost127 View Post

the ambechoic model is precisely regarded for surround sound! (the next evolution)
http://www.cari-llc.com/pdfs/Mono2Surround.pdf

...OK, and your point with regards to building the best practical home theatre?

Quote:


fyi - that's Andreas' room, not Jens!

You posted too quickly - you will notice that I edited it when I quickly recalled it was Lupo not Jens.

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a dense return from the rear wall + rear side-walls should provide sufficient envelopment for the entire listening positions (rows) for the front speakers where the bulk of the energy content is in a 5.1 setup, no?

I somewhat disagree (not much), and I don't see how anything in my post implies that I might. I even said "In any case the one thing whose value I don't really question is a decent logarithmic decay of diffuse reflections." However it's not really practical to implement diffusion at all my sidewalls and rear wall as I have potential listening positions from a few inches to a few feet from all of those surfaces - as does Mike. He has more space than this so his biggest problem might be listening distance, but in my theatre nothing more than a couple inches thick can go at listener height. I'm sure D.E. would say I'm stuffing too much into the room... except he's not designing my theatre, I am. Luckily, I can think of solutions and I'm in the process of implementing them - but it's not going to look remotely like a typical LEDE/RFZ setup by any chance, and it simply cannot. Seven channels (that matter), nine potential listening positions... it's just not as simple as a control room in a home theatre.

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correct! hence why it is considered a new room model! (ambechoic) with very specific specular criteria!

in Blackbird Studio C! the difference is the peak of the termination (which is MUCH lower (obviously at -30dB...which should be anechoic)) but the DENSITY and the decay *rate* is the key factor there .... as George Massenberg comments here:

Again, obviously I understand the time response of the ambechoic model. My question is (a) whether that makes it a "better" response than RFZ and (b) how that relates to home theatre. As in, should we really be trying to achieve the strong ISD termination (and in which seat)? Or are we simply trying to create an ISD and then provide a dense/diffuse field of reflections?

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immediately attenuates any reflection to the noise floor

Not true. Immediate decay to the noise floor is called anechoic, and since that room has a decay evident on its ETC, clearly any reflection is not "immediately attenuated to the noise floor". That may be close to true in the short term, but it's clear from the ETC that that the energy was not so much attenuated as "managed".

You can quote SAC all you want, it doesn't change physics. He can say there is no reverberant field in a small room all he wants, but there's been plenty of discussion on GS saying "maybe there isn't, but looking at ETCs it sure _looks_ like there is". Meaning, call it whatever you want, there is later decaying energy. Meanwhile I specifically never used the term "RT60" or referred to it as "reverberation", rather calling it "decay" specifically because people love to pick nits on the Interwebs. You may of course continue if your object is to pick up one of SACs less-endearing traits.

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the decay and the LEDE model in itself is related to small acoustical spaces (and important factor regarding all of this conversation!!)

Indeed. But you are ignoring the point that not all small acoustical spaces are the same. There is a big difference between a live room, a vocal / iso booth, a control room, possibly a mixing room (I don't think everyone makes this distinction), and a home theatre (and I'm sure I have ignored plenty of mentionable small acoustical spaces). I suspect that professionals' thoughts on home theatre situations run much toward a "no bad seats" mentality than a "golden seat" mentality. That changes things. I believe it might even suggest that providing a strong ISD termination is one of the last things we should worry about. It'd be pretty far down the acoustical list (unless one wants to try to make a theatre with an excellent two channel listening spot, which is actually what I'm trying to do... twitch).

Use of ETC has been explained here fairly completely I'd say. I think the question now is, how to best apply it in a home theatre room (given that this is a home theatre forum)? Possibly a better question: where does Mike (OP) wish to take the thread now?
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post #33 of 292 Old 08-18-2011, 07:36 AM
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Since it seems to be a recipe for confusion, I won't edit this in, rather throw another post out to make this point. Since SAC was only writing about control rooms, I don't think that posting SAC's views (once you get beyond the basics of what ETC is and how to apply it) is really getting us there. There will have to be some research into use of the tools for home theatre, and actual thought [that is, one would hope] based on understanding of some physics and psychoacoustics.

SAC constantly crossed the fine line between attribution and name-dropping. Let's not do some twisted recursion of that by spouting his stuff until we're blue in the face!

We should do better than that, I suggest, by turning the discussion toward how the use of the tool (ETC) might relate to the goals of the OP.
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post #34 of 292 Old 08-18-2011, 09:03 AM
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"ISD termination" I actually mean a specific burst of energy intended to strongly terminate the ISD gap.

I probably missed it but I have not been able to find defintions of ISD, ISD termination, and ISD gap. Could someone please point me to a source for these definitions?
Thanks.
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post #35 of 292 Old 08-18-2011, 10:58 AM
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Originally Posted by GGA View Post

I probably missed it but I have not been able to find defintions of ISD, ISD termination, and ISD gap. Could someone please point me to a source for these definitions?
Thanks.

the Inter Signal Delay (ISD) gap is a length in time after the original signal reaches the listening position that is essentially anechoic (no reflections or specular energy is allowed to arrive within this window in time) - you are ONLY hearing the direct signal from the speakers and no other reflections from any boundaries within the room.

if you were in an anechoic room, your ISD gap would be infinity, as you'll never get a reflection off a boundary (as all reflections are fully attenuated - just as if you were outdoors or in an infinitely large room).

in small rooms where the boundaries are relatively close, reflections off the sidewalls and ceiling will take a slightly longer path than the direct signal, which means these reflections will arrive slightly longer in time and combine with the original signal at the listening position.

if a specular reflection arrives within the ISD gap, then it is considered to be an 'early' reflection. but it is important to note that not all first-order-reflections are early-reflections. in a large room, the reflection path is so great that the reflection arrives at the listening position outside of this gap - hence it is not necessary an 'early reflection'.

for example - a 20ms ISD means that once the original (direct) signal from the speaker reaches the listening position (straight line, hence the shortest path), there is a 20ms anechoic gap where no other specular energy impedes the listening position. this allows the brain to 'digest' the original signal in totality before also hearing the reflections within the room.

any early-reflections arriving at the listening position within 15ms or so of the original signal will cause time-smearing and issues related to clarity. any early reflections arriving in this window need to be identified and attenuated (either by redirection, absorption, etc)...if that is indeed the room model you are emulating and the specular response you are looking to achieve.

by attenuating these early reflections (or delaying the time in which they reach the listening position AFTER the original signal), you're also essentially tricking the brain into thinking you're in a larger room.

the termination of this gap is the first significant burst of energy (specular or diffused) that arrives after the anechoic ISD gap, hence "terminating" the gap.

there are psycho acoustic effects that relate to this termination (Haas effect). if you read some of SAC's commentary that i quoted on the previous page, this will be better explained.



it was born out of control room models (LEDE/RFZ) - which each have very specific criteria for how the specular energy arrives and decays at the listening position - but these room models are directly applicable to critical listening rooms (where the room needs to be made as neutral as possible, if that is the design requirement).

with regards to recording (which is not really relevant here, but it does add insight) ... the ITD (Initial Time Delay) is generally used for the length in time in the recording room, and the ISD (Initial Signal Delay) is generally referred to the gap within the control (mixing/listening) room.

so, if music was recorded in a large space where the first reflection arrived 30ms after the original signal ... and you were to play this back in your control room that is much smaller in space (eg first reflection arrives at 15ms) ... this means that your control/listening room is masking the recording., and instead of 'hearing' the room the music was recorded in, you're hearing your listening room mask its sound on top of the recording.

so, the general criteria is to have the ISD of the listening/control room be 2-5ms later than that of the recording room ... so you can hear the room in the recording in totality before your listening room has a chance to mask its sound on top. think about listening to a recording from a large concert hall, and playing it back (reproducing it) in your bathroom. you are going to 'hear' the small acoustical space via the early reflections, versus 'hearing' the concert hall ambiance. if you close your eyes, you'll be aware you're in a very small room vs being transported into 'being' in the room of the recording.


go into your bathroom and close your eyes and begin talking. when you talk, the reflections off the boundaries arrive back at your ear very quickly because of how close the boundaries are, and your brain processes this and you can 'hear' or 'feel' how small the room is. if you did this in a large room (movie theater, warehouse,etc) the reflections would take a much longer time to reach your ears (possibly turning the reflection into a distinct 'echo'), and you could 'feel' how large of a room you're in, even if you were blindfolded.
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post #36 of 292 Old 08-18-2011, 11:14 AM
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Thank you. That was superb.

I used to use ETF5 measurement software. It has, to me, an excellent impulse graph, which I found easy to read. I now use REW, which has an ETC graph in addtion to the impulse.

Is there any advantage to using the ETC graph over the impulse graph?
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post #37 of 292 Old 08-18-2011, 11:30 AM
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Quote:
Originally Posted by GGA View Post

Thank you. That was superb.

I used to use ETF5 measurement software. It has, to me, an excellent impulse graph, which I found easy to read. I now use REW, which has an ETC graph in addtion to the impulse.

Is there any advantage to using the ETC graph over the impulse graph?

ETC should be pink-noise impulse, containing all freq content.
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post #38 of 292 Old 08-18-2011, 03:17 PM
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Originally Posted by localhost127 View Post

ETC should be pink-noise impulse, containing all freq content.

LOL, not close.

ETC contains more data than the straight-up impulse response. From this page:
Quote:


To briefly summarize this idea, the energy-time curve is the envelope (as opposed to the magnitude) of the impulse response. To find that envelope, we combine the magnitude of the impulse response in an RMS fashion with the magnitude of its Hilbert transform. The complex (or quadrature) sum of the impulse response with its Hilbert transform is called the analytic impulse response (AIR) of the system, and the energy-time curve (ETC) is the magnitude of the AIR.

When I compare impulse response to ETC in REW, the only difference I see is the lower part of the curve, no difference on the upper part. SAC said one time this was typical when viewing the curves macroscopically but he claimed that if you moved in you would see shape differences to the curves. I've never examined further to figure that out myself.

Conceptually, a regular impulse response contains only kinetic energy in the system while ETC contains the kinetic energy PLUS the potential energy. Therefore it's claimed to be a more complete picture of the energy content in the acoustical system (the path from speakers to listener aka "the room").
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post #39 of 292 Old 08-18-2011, 03:30 PM
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Originally Posted by aackthpt View Post

LOL, not close.

ETC contains more data than the straight-up impulse response. From this page:


When I compare impulse response to ETC in REW, the only difference I see is the lower part of the curve, no difference on the upper part. SAC said one time this was typical when viewing the curves macroscopically but he claimed that if you moved in you would see shape differences to the curves. I've never examined further to figure that out myself.

Conceptually, a regular impulse response contains only kinetic energy in the system while ETC contains the kinetic energy PLUS the potential energy. Therefore it's claimed to be a more complete picture of the energy content in the acoustical system (the path from speakers to listener aka "the room").

thanks for that clarification !
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post #40 of 292 Old 08-18-2011, 04:01 PM
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Originally Posted by localhost127 View Post

it was born out of control room models (LEDE/RFZ) - which each have very specific criteria for how the specular energy arrives and decays at the listening position - but these room models are directly applicable to critical listening rooms (where the room needs to be made as neutral as possible, if that is the design requirement).

I presume by "it" you are referring to the ITDG/ISD. Actually I believe the ISD concept was co-opted for recording spaces and control rooms from the original place that ITDG came from - large-space acoustics (having been practiced for MUCH longer than small-space acoustics). According to this page:

ITDG (initial time-delay gap) Acoustics. The difference in time between the first arrival of direct sound and the first arrival of reflected sound at the listener. The sensation of intimacy is quantitatively measured by the ITDG. First defined in 1962 by Beranek in Music, Acoustics & Architecture.
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post #41 of 292 Old 08-18-2011, 05:01 PM
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if there is further history regarding the terminology, then i am unaware (and havent investigated it)

if you have a more elegant way of describing it to GGA then by all means, please provide. it's not entirely relevant (recording background re: this conversation) - but like i said above, i think it adds invaluable insight especially for those interested in reproduction.
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post #42 of 292 Old 08-18-2011, 06:38 PM
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Originally Posted by aackthpt View Post

First defined in 1962 by Beranek in Music, Acoustics & Architecture.

Your explanation was fine, I just wanted to correct your history. The idea of ITDG didn't begin with LEDE, though they (Davis & Davis) may have been the first to try to artificially create it in small acoustical spaces.

This reference is a book. First edition from 1962 but second edition was published fairly recently.

I'd be most interested in an overall timeline of historical acoustical thinking. In most fields that provides a great view of how the current state of the art developed, which tends to be very instructive .
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post #43 of 292 Old 08-18-2011, 06:51 PM
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agreed - thank you,
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post #44 of 292 Old 08-18-2011, 08:38 PM
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localhost, where do those last images you posted come from? Wherever it is looks like a nice source.
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post #45 of 292 Old 08-20-2011, 01:02 AM
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mtbdudex! have you by chance found any time to proceed with making progress on the ETC responses (measured on speaker at a time? eg, start with simply the L and R front speakers from a single, primary listening position?)? you are in a great position to present to the avs community the significance of the ETC and how simple it is to actually verify that one has attenuated a particular 'early reflection' ... if you follow through with a proper workshop, then ill donate a few dollars to REW on your behalf.
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post #46 of 292 Old 09-24-2011, 04:42 AM - Thread Starter
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Alright, summer distractions have ended, school has started, and my wife wants her computer back in the craft room!

So, I was up early this morning getting back into using REW.

visual of Set-up, calibrated mic on boom stand, I used 45 deg cal file.
200-20khz sweep.


Some ETC's:
LH main at default graph limits:


Graph limits changed to see more detail:


RH main at default graph limits:


changed graph limits for more detail



So, my first time looking at these.....where is my ISD gap hiding??
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post #47 of 292 Old 09-24-2011, 06:46 AM
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nice work!

zoom in further so you get a lot of resolution on the 0-150ms range. it looks like there's a few early specular reflections there you may wish to hunt down,
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post #48 of 292 Old 09-24-2011, 07:56 AM - Thread Starter
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kids soccer games 9-10am, came home, so in between the next game @ 11:30.

Here is 150ms resolution

LH


RH


RH has spike @ 16ms, possible repeats itself @ 32ms?
Looks like I'll do the string trick to find that.
(1ms = how far in feet again at normal air temp/pressure, about 1.1 feet? )

Ha, could be the 27" iMac...which is right next to the RH speaker at ear level about 8'-ish from the mic, I'll put that on the floor with a blanket for next readings....
I will remove the side wall panels to see hard drywall comparison, the ceiling ones will stay up though...
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post #49 of 292 Old 09-24-2011, 08:21 AM
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Originally Posted by mtbdudex View Post

kids soccer games 9-10am, came home, so in between the next game @ 11:30.

That's dedication to the task at hand

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post #50 of 292 Old 09-24-2011, 09:06 AM
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Originally Posted by mtbdudex View Post

RH has spike @ 16ms, possible repeats itself @ 32ms?
Looks like I'll do the string trick to find that.
(1ms = how far in feet again at normal air temp/pressure, about 1.1 feet? )
.

if you notice, there's a bunch of clutter right after the original signal as well --- 1-3ms after original signal. could be edge diffraction off an object very close to the speaker or very close to the mic. HUNT IT DOWN AND DESTROY IT.

string trick:
http://www.gearslutz.com/board/6133764-post8.html

Quote:
Originally Posted by SAC View Post

1.) - the most basic mechanical method is the string method.

Quite frankly, I don't expect anyone to do this more than a handful of times until you can more easily visualize the process, but it illustrates the concept very simply and effectively - if a bit awkwardly!

From the total time of travel, you calculate the distance of travel (TOF X 1.13'/ms or .344m/ms). You might want to leave a few inches on each end to hang on to, but mark the precise endpoints corresponding to the distance. Find a few friends who you will not mind being a bit worn out by the process and have then each hold one end of the string, one end with the point marked on the string precisely located where the measurement mic capsule would be (don't move the mic!!!) and the other end placed in the center of the source speaker. Forget the tweeter stuff, as the ETC measures the total energy, and there is much more energy content in the low-mids and mods than in the tweeter. Besides, if you want to determine the actual acoustic origin of the speaker, the ETC can be used to identify this as well! But that's another exercise for some other time!. Now, with the endpoints firmly located, at one point in the body of the string, extend the loop body out and see what boundaries/surfaces you can tangentially touch with the string being stretched taut' The point you can touch, is the point of incidence indicated by that particular spike in the ETC. Note the incident spot on the boundary. Repeat for the other energy returns.

Easy, but I suspect you will quickly tire of this - as will your friends who will want to be doing something a bit more exciting!

string test read #2:
http://www.gearslutz.com/board/6397239-post80.html
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post #51 of 292 Old 09-24-2011, 09:25 AM
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Quote:
Originally Posted by localhost127 View Post

if you notice, there's a bunch of clutter right after the original signal as well --- 1-3ms after original signal. could be edge diffraction off an object very close to the speaker or very close to the mic. HUNT IT DOWN AND DESTROY IT.

First, let me say that this is a fantastic thread I've been following since inception. On point: I thought I'd read that reflections within the first few milliseconds (<5) were acceptable because the brain cannot distinguish these from the original event. Have I misremembered or just plain misunderstood?

/Ken
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post #52 of 292 Old 09-24-2011, 09:40 AM
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Originally Posted by RUR View Post

First, let me say that this is a fantastic thread I've been following since inception. On point: I thought I'd read that reflections within the first few milliseconds (<5) were acceptable because the brain cannot distinguish these from the original event. Have I misremembered or just plain misunderstood?

/Ken

they're worse.

ask the guys who design speakers and cabinets if edge diffraction off the cabinet is acceptable in their world

edit: few docs for you:
http://www.sendspace.com/file/rdwtpo
http://www.sendspace.com/file/8z5r57
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post #53 of 292 Old 09-24-2011, 09:42 AM
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mtx,

regarding the 1-3ms clutter - the string test will be difficult so you can always take a blanket and apply it on questionable sources and retest ETC to see if that removes the energy.

Quote:
Originally Posted by mtbdudex View Post

I will remove the side wall panels to see hard drywall comparison, the ceiling ones will stay up though...

good idea - then you'll be able to see how effective your panels actually are at attenuating the signal (eg, -10dB, -20dB, etc) for your own knowledge.
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post #54 of 292 Old 09-24-2011, 10:23 AM
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Quote:
Originally Posted by RUR View Post

First, let me say that this is a fantastic thread I've been following since inception. On point: I thought I'd read that reflections within the first few milliseconds (<5) were acceptable because the brain cannot distinguish these from the original event. Have I misremembered or just plain misunderstood?

/Ken

and it's not just regarding where the brain cannot distinguish the early reflection as a separate event (time smearing, loss of clarity), but you still have frequency response issues from the constructive and destructive interference between the original signal and summation of other specular reflections at the listening position. this is generally why comb-filtering / frequency-response issues above the schroeder frequency (specular region) is resolved with regards to the time-domain (ETC) --- as the comb-filtering is a result of the summation of original signal + specular reflections. use the ETC to destroy early reflections and that should in itself help smooth out the frequency response (since you're eliminating the destructive/constructive interference).

it's always a good chuckle to see people so heavily focused on the frequency domain/response regarding the specular region (and trying to apply eq to smooth the response out) -- when the root cause of those issues are found and remedied in the time-domain. i hope that makes sense.
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I just re-read this thread....getting more comfortable and knowledge.

Some people posting here I'm sure have read this by Peter D'Antonio of RPG Diffusor Systems vis their Library http://www.rpginc.com/news/library.htm.

It helped me grasp initial time delay better, MINIMIZING ACOUSTIC DISTORTION IN HOME THEATERS


I spent lots of time yesterday afternoon (after kids 3rd soccer game) taking measurements, playing with a few extra pieces of OC703, etc.


Moved boom to rear of seat, seats reclined to lower back to minimize reflections during readings


After church will post graphs and thoughts.
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a thought, I took all my readings using sweeps of 200hz-20khz, did I reach too low into bass freq?
Should I have gone higher into the transition region, say 400hz as low and used 400hz-20khz sweeps instead?
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Quote:
Originally Posted by mtbdudex View Post

I just re-read this thread....getting more comfortable and knowledge.

Some people posting here I'm sure have read this by Peter D'Antonio of RPG Diffusor Systems vis their Library http://www.rpginc.com/news/library.htm.

It helped me grasp initial time delay better, MINIMIZING ACOUSTIC DISTORTION IN HOME THEATERS


I spent lots of time yesterday afternoon (after kids 3rd soccer game) taking measurements, playing with a few extra pieces of OC703, etc.


Moved boom to rear of seat, seats reclined to lower back to minimize reflections during readings


After church will post graphs and thoughts.

glad to hear

you may wish to toss a thick blanket on that reflective couch behind the mic, so the couch is not tainting your test measurements (HF reflections off the couch).

if you only have a single couch (row), you can likely get away with placing your side-wall absorbers 48" wide and 24" high, vs multiple panels w/ 48" height (are you moving your head up/down 48" from within the listening position?) - remember, broadband absorption should be applied surgically only where necessary re: early reflections. you'll likely use a lot fewer materials.

since your seating is spread across the horizontal place (eg, a couch), you're more interested in coverage on the side-walls horizontally - not vertically (disregard if there is a seating row that is not detailed in your photos that is set higher than the couch)



edit: and remember, you are focused on the acoustic-center of your speakers (with regards to reflection points), NOT the tweeter - as there is much more energy present in the mid/low specular range than the HF range of the tweeter.
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Quote:
Originally Posted by mtbdudex View Post

a thought, I took all my readings using sweeps of 200hz-20khz, did I reach too low into bass freq?
Should I have gone higher into the transition region, say 400hz as low and used 400hz-20khz sweeps instead?

what specifically are you testing with regards to this post?
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Quote:
Originally Posted by localhost127 View Post

what specifically are you testing with regards to this post?

I was curious if the sine sweep I was running for my room size/geometry was in the LF modal region, or above Fs (Schroeder frequency).

Looking here, http://www.linkwitzlab.com/rooms.htm, and using this spreadsheet with my room geometry and measured T60 = 350ms (on avg) from the ETC charts, http://www.linkwitzlab.com/modes1.xls , it appears my Schroeder frequency fs = 138 Hz , so doing sine sweep 200hz-20khz should not bring LF modal readings (long decay freq) into the ETC charts.
(I've used waterfall charts for the LF modal region analysis already, posted elsewhere)

Quote:


© LINKWITZ LAB 21 October 2000
Replace bold faced values with your own numbers
L = 18.5 ft 5.64 m
W = 14.5 ft 4.42 m
H = 8.8 ft 2.67 m
Floor area A = 269 ft^2 25.0 m^2
Volume V = 2352 ft^3 67 m^3
Surface area S = 1115 ft^2 104 m^2
Edge length Le = 167 ft 51 m

Below frequency fm = 150 Hz
Total number of modes N = 41
Avg. mode spacing df = 2.2 Hz at fm

Estimated reverberation time T60 = 350 ms
Resonance bandwidth bw = 6.3 Hz
Rise time Trise = 112 ms
Schroeder frequency fs = 138 Hz
Monopole reverb distance Rm = 0.78 m 2.6 ft
Dipole reverb distance Rd = 1.35 m 4.4 ft
Avg wall absorption a = 30%

Estimated avg wall absorption a = 30% Speed of sound c = 344 m/s
Reverberation time T60 = 349 ms 1128 ft/s

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Quote:
Originally Posted by mtbdudex View Post

I was curious if the sine sweep I was running for my room size/geometry was in the LF modal region, or above Fs (Schroeder frequency).

Looking here, http://www.linkwitzlab.com/rooms.htm, and using this spreadsheet with my room geometry and measured T60 = 350ms (on avg) from the ETC charts, http://www.linkwitzlab.com/modes1.xls , it appears my Schroeder frequency fs = 138 Hz , so doing sine sweep 200hz-20khz should not bring LF modal readings (long decay freq) into the ETC charts.
(I've used waterfall charts for the LF modal region analysis already, posted elsewhere)

that can't be right...far too low.
focus on 0-300waterfall plot, and ETC (time-domain) above that...

regarding this link:
http://www.linkwitzlab.com/rooms.htm

the gentlemen is quite concerned with rt60 times, critical distances, etc - of which are irrelevant in small acoustical spaces because a reverberant sound field does not exist in small acoustical spaces.

Quote:
Originally Posted by linkwitzlab View Post

When we consider radiation in the reverberant frequency range above 149 Hz, the sound at the listening position is composed of the direct sound from the source and the reverberant sound that is more or less uniformly distributed in the room. The direct sound pressure level decreases inversely to distance from the source and will equal the reverberant sound pressure at distance xr. The ‘reverberation distance’ xr (also called 'critical distance') is calculated from


there is no statistically random incidence in a small acoustical space.

http://www.hometheatershack.com/foru...rt60s-sas.html

you should be wary anytime someone discusses such with regards to small acoustical spaces...

Quote:
To quote from Sound System Engineering, 3rd Ed.[/b] Pp 178-9:

“Small Room Reverberation Times

To quote the late Ted Schultz (formerly of Bolt, Beranek and Newman)
“In a large room, if one has a large sound source whose power output is known, one can determine the total amount of absorption in the room by measuring the average pressure throughout the room. This total absorption can then be used to calculate the reverberation time from the Sabine formula. This methods fails badly in a small room, however where, a large part of the spectrum of interest lies in a frequency range where the resonant modes of the room do not overlap but may be isolated…In this case the microphone, instead of responding as a random sound field (as required for the validity of the theory on which these methods depend), will delineate a transfer function of the room… It does not provide a valid measurement for the reverberation time in the room.”*

What is often overlooked in the attempted measurement of RT60 in small rooms is that the definition of RT60 has two parts, the first of which is commonly overlooked.

1.) RT60 is the measurement of the decay time of a well-mixed reverberant sound field
Well beyond Dc (the critical distance).
2.) RT60 is the time in seconds the reverberant sound field to decay 60 dB after the sound source is shut off.

Since in small rooms, there is no Dc (critical distance), no well mixed sound field, hence no reverberation but merely a series of early reflected energy, the measurements of RT60 become meaningless in such environments.

What becomes most meaningful is the control of early reflections because there is no reverberation to mask them.”


What you have instead is a small acoustic space dominated by room modes (which are not reverberation!) and focused spectral reflections – which by definition are anything but a well-mixed reverberant sound field wherein the arrival of reflected signals from every direction is equally probable.

In the small acoustical space, there is no mixing, nor homogeneous, statistical reverberant sound field! In fact, in a small acoustical space, as the lowest frequency that can effectively develop across the largest dimension can easily increase to ~500Hz – compared to a large acoustical space where such frequencies are often below 30Hz!

In such environments, intelligibility can be degraded by specular reflections that must be isolated and corrected directly, not statistically.


*Note, It is equally proven that the fundamental form of Sabine’s expression cannot be modified so as to become correct for large absorption. Per Sabine’s Reverberation Time and Ergodic Auditoriums, Wm. Joyce, Journal of the Acoustics Society of America, Vol. 58, No. 3, pp. 643-655,Sept. 1975.

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