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post #1 of 288 Old 08-01-2011, 12:39 PM - Thread Starter
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I'd like to read a primer on HT enthusiasts using energy time curve (ETC) charts/graphs for their acoustic analysis.

Not some REW help files or theory in books/online either, I've read all those and .... sorta get it.

What I'm looking for is a thread that details some peoples journey thru the acoustic rabbit hole, jumping in, getting really small once realizing how much info/data there is.
Then, growing and using energy time curve charts/graphs for their acoustic analysis.
The "final step".

I searched but can't find a thread here.
I've taken lots of measurements, freq/H20, for my IB sub, but for some reason I've steered away from energy time curve charts for my side wall/ceiling acoustic treatments.

I'm willing to be "Alice", who will lead me thru the acoustic energy time curve (ETC) charts/graphs rabbit hole?
Show me what you done, how you've analyzed your room acoustics, and made changes until you've said "I'm done".

My (4) LH and (2) RH side wall panels are easily removable, and while the (2) ceiling clouds are sorta big/bulky hanging there, I could cover their facing downside with kraft paper to negate their effects for measurements.

I'm thinking this would help others also as I go thru this acoustic learning journey.....

http://www.santafevisions.com/csf/ht...considerations




[edit Feb-28-2012] Plan to re-boot this thread March/April 2012:
Come spring, when I re-do my 130" screen due to humidity induced warping of the screen, , I will totally remove all my existing side wall/ceiling panels and start from baseline of no treatments on the room
(besides my corner superchunks, which all have front facing reflective treatment on them)

I also will do the following:
-Pure reflective room baseline
-baseline of "the mirror tricK", as I did, with side wall 2" panels + 2" air gap
-Pure ETC approach and using 4" panels + 4" air gap at specific locations verified by ETC
-compare and make objective and subjective assessment
All above is being done utilizing porous absorbers.

Come 2014/2015, when it's new PJ time (4k +LED light engine), and I go AT front screen - with DIY L/C/R main speakers in a baffle wall, then the whole HT acoustic treatment strategy will change; diffusion, membrane absorbers, etc....but that's topic for a different thread.

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post #2 of 288 Old 08-01-2011, 04:52 PM
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ETC analysis was the fad back in the late 70's early 80's. Basically doesn't work. You know that old saying about re-inventing the wheel?
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post #3 of 288 Old 08-01-2011, 05:55 PM
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Quote:
Originally Posted by fastl View Post

ETC analysis was the fad back in the late 70's early 80's. Basically doesn't work. You know that old saying about re-inventing the wheel?

the ETC is a tool that details a room's specular response. it is not a fancy analysis; it merely displays gain vs time. that is all.

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Originally Posted by SAC View Post

Its time more people looked at the time domain ETC and focused on what it shows.Its characteristics are displayed in terms of gain and time. There is no frequency scale. There is a reason the time domain takes precedence here, as the time relationships are causal with regard to the derivative anomalies that appear in the frequency domain.

the ETC is used to identify energy (eg specular reflections that are modeled as rays), and where it impedes the listening position after the original signal, with respect to time.

it also allows you to see how your diffuser (eg if it performs spatial and temporal dispersion - is actually performing and how a particular specular reflection now impedes the listening position as a diffused return.

control rooms and critical listening rooms employ the LEDE (and now RFZ) room models to achieve a particular response. this response is measured with the ETC.

if one is seeking to emulate the Reflection-Free-Zone room model, then early-reflections which must be attenuated are identified (and their incident boundary) with the ETC.

if one is seeking to induce a haas kicker, then this is measured to arrive at the correct time after the original signal with the ETC.

you'll fancy this comment by george massenberg regarding
blackbird studio C's ETC response:

http://www.gearslutz.com/board/6834790-post49.html

RFZ is an evolution of LEDE - and now, blackbird studio c's room response (ambechoic) is an evolution of RFZ. but good luck emulating that response in your room.

for your pleasure (take note to SAC's commentary):

http://www.gearslutz.com/board/studi...c-impulse.html

http://www.gearslutz.com/board/bass-...absorbers.html

http://www.gearslutz.com/board/6202822-post67.html

http://www.gearslutz.com/board/studi...s-trigger.html

http://www.gearslutz.com/board/6118734-post48.html & http://www.gearslutz.com/board/6145795-post60.html (troubleshooting for early specular reflections and their elimination).
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post #4 of 288 Old 08-01-2011, 06:02 PM
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Quote:
Originally Posted by mtbdudex View Post

I'm willing to be "Alice", who will lead me thru the acoustic energy time curve (ETC) charts/graphs rabbit hole?

present ETC's from the listening position, measured one speaker at a time.
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post #5 of 288 Old 08-02-2011, 07:00 AM
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This room;



"The lows and the highs are easier to perceive because they have roughly the same ambience, the same reverberation time. We don't have any fiberglass on the walls it absorbs largely at high frequencies and is less likely to absorb at low frequencies. Any absorption in this room is caused by several processes, including the air turbulence around all these edges, and the scattering of reflective sound from all these surfaces. Dr. D'Antonio has referred to our principle in action as ambechoic.' "

The room is conducive to accurate work because we have taken away the boundary effect by eliminating' the walls. The boundary effect is usually a speaker set away from the wall, which causes comb filtering because the sound hits the wall and bounces back at a different time than the direct signal from the speaker. Certain frequencies are canceled and certain frequencies are enforced, which is not a good idea in a critical listening environment.


Then again;

"the room is hardly anechoic, though. just listening to music in there is extraordinary; ensemble musicians report that it's easier to play live in the room because it's easier to hear in the room.

those that appreciate it use it to it's utmost. the "sound" is more or less like an "acoustic white-out", and the decay is extraordinarily linear with higher frequencies (the Schroeder curves are remarkably linear) having more decay than any other room i've ever experienced. i made the best record i ever made in my life in there (from which that video clip came). unfortunately, Nashville's short-sighted and inept "music" industry will not tolerate innovation of any kind, and i'm told that the room now has heavy curtains hung in front of the diffusors.

i now live in Montreal, by the way.
George Massenburg


Damn, that last sentence in the last paragraph really sucks

Thanks for the links
LL

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post #6 of 288 Old 08-02-2011, 07:22 AM - Thread Starter
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Quote:
Originally Posted by localhost127 View Post

present ETC's from the listening position, measured one speaker at a time.

Hopefully I can get to it this weekend.

Below is my HT and it's current acoustic layout, I have 8 listening positions.....

I will take 3 readings from each seating position, one for each of the mains R/C/L, one at a time, and of course labeling them.
3 x 8 = 24 charts... @ approx 3 minutes/measurement + label graph/etc, this is at least a 2 hour data gathering session.
Upload, organize a post.....yea need to do it at night while the kids are sleeping.



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post #7 of 288 Old 08-02-2011, 07:25 AM - Thread Starter
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Quote:
Originally Posted by FOH View Post

This room;

FOH....my wife would totally negate that........I'd love to listen in it though, with the curtains off of course.

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post #8 of 288 Old 08-02-2011, 09:51 AM
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Quote:
Originally Posted by FOH View Post

Damn, that last sentence in the last paragraph really sucks

stifling innovation at its finest!
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post #9 of 288 Old 08-02-2011, 05:35 PM
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localhost - Thank you for the information. I know what ETC is. Pro sound magazines of the era I mentioned, such as dB, REP and Studio Sound were infested with articles on the topic. There were all these "experts" running around with Techcron analyzers (and the like) making ETC measurements and charging big bucks. I think most folks eventually figured out that it was basically just another audio consultant scam and it eventually fell out of vogue. Doesn't surprise me at all that it gets "discovered" again.
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post #10 of 288 Old 08-02-2011, 05:38 PM
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fastl,
What do you prefer instead of ETC?
Thanks.
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post #11 of 288 Old 08-02-2011, 05:58 PM
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Very nice, Mike! I don't know where you read about it but ETC analysis is one of the things I was going to go through with you.

Are you trying to make every seat in the house equally good? My intention is to make all of the seats good seats, but not amazing seats. Therefore while I may measure ETC from all of the seats, I am not going to fuss over the finer points of achieving the desired response from the cheap seats.

And, as the ambechoic response was mentioned... you need to decide what you are trying to do with the tool.
-Verify your ISD gap?
-Strongly terminate the ISD gap?
-Add to the sense of spaciousness in the room?
-Decrease the sense of "deadness" in the room?
-Improve the quality of the decay characteristic?

I can definitely tell you that the ETC has been getting progressively "better" in my room as I do more. And if you do want to see some "journeys to the center of the earth" I could point you to a couple of studio build threads on gearslutz where people did just that.


@fastl, can you provide a reference for "basically doesn't work"? Or just PM me with _your_ methodology and requirements/expectations for room acoustics? I'm quite interested to hear what causes you to say it "doesn't work".
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post #12 of 288 Old 08-02-2011, 07:00 PM
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Quote:
Originally Posted by aackthpt View Post

Very nice, Mike! I don't know where you read about it but ETC analysis is one of the things I was going to go through with you.

And, as the ambechoic response was mentioned... you need to decide what you are trying to do with the tool.

precisely. the ETC is the tool that details your room's total specular response. you modify your room as needed to achieve the particular room response you desire - and re verify with ETC once treatment has been applied.

it would likely be wise for a starting-point (mtbdudex) to pick a single listening position and focus on the L/R for the time being until the process is fully understood.


take note!:

Quote:
Originally Posted by SAC View Post

Thus if you choose to determine the specular energy paths in the most basic and accurate manner, you can, depending upon the amount of separation in time of the various reflections, more easily determine which boundary surface correlates to each energy spike. And from there, (omitting a few mechanical steps*) you can determine precisely the location of the center of the incident region.

And, assuming one knows the acoustical model** they are working to satisfy; one indeed knows where to place treatment. The type of treatment is then determined by the effect one desires to create.

Absorption will damp the reflection.
Reflection will redirect the energy which will effectively cause incidence later in time at other location(s) - in other words, it will both reduce the gain of the spike and it will effectively be 'moved' to a later time.
Diffusion will do two things. One, it will decrease the gain of the spike. And two, it will break the primary reflection into 'smaller' reflections of lower gain and spread them out in time. Thus you will have a nesting of distributed lower gain spikes, generally with the distribution in time being skewed to a later time.
(See graphics below)

Thus, if you know the target acoustical response desired, reading and interpreting the ETC correctly will indeed provide information as to the precise point of incidence, and this point is where treatment is applied to mitigate said energy in the manner desired. And knowing the acoustical response desired, you can appropriately choose which kind of treatment is useful at the location.

Note, I say what type of treatment may be useful rather than what is necessarily best. The reason is that there are often multiple ways to achieve similar results depending upon the context of your space. For instance, if your acoustical model is that of a NE room, then you will most generally want to use absorption to reduce the reflection. On the other hand, if you are building according to the LEDE model, and you want to preserve the energy while simultaneously controlling its dispersion; then you may want to employ either reflection or diffusion in order to create the acoustical response appropriate for that portion of the ETC response. But a comparison of the actual ETC with the acoustical model template will help you to determine what behavior is optimal with respect to time and gain for the given incident point in the room.

Does that make sense? The ETC provides a total picture of the specular response in the room - from early arrivals to the 'last' of the energy, be it totally damped or a decaying diffuse soundfield. And this is all done with respect to time.

The ETC allows you to see exactly what kind of energy distribution you have currently, and allows you to select and precisely place the treatment you have chosen in order to create the effect you desire - be it damping, redirection, or diffusion. It also allows you, upon repeating the measurement, to see the precise impact the positioning of the chosen type of treatment has. from this you may be satisfied, or you may want to further refine the positioning in order to insure the proper response is accomplished.

Oh, and one more important point here. It will also show you if, and to the degree, that your treatment does not act completely in the manner you suspect. In particular, this is most common with absorption, which to many folks surprise, will often exhibit a stronger degree of reflection than anticipated - especially if the angle of incidence at the boundary is great. you can also determine the actual degree of diffusion versus scattering a diffusor or a scatterer such as a poly-cylinder exhibits. With this information, you may decide to modify or use another type of treatment if , for instance, the resulting reflections are not sufficiently diffuse.

But in any case, if one becomes proficient in using the ETC, the days of blindly assuming a treatment based simply by virtue of its name, performs exactly and solely as one expects, should be over. you will discover that absorbers have a reflective quality. And that diffusors exhibit an absorptive component (often more than one would like if your goal is to diffuse and retain said energy!) and that they may also act as reflectors (especially is the incident signal is perpendicular to the unit). In other words, you will not only know what is happening within the room, but you will quickly learn a great deal about the real, as opposed to ideal, behavior of the various treatments.

And with this combined knowledge of both what is happening in the room, having the information of where to place treatment appropriate to your acoustical response design goals, knowing what specific affects your choice of treatment actually achieves - while becoming aware of any residual artifacts of the treatments, you will be well on your way to creating the response you desire....

Quote:
Originally Posted by SAC View Post

There seems to continue to be some persistence in looking at specular responses in the frequency domain and worrying about EQ.

If you address the time domain issues in the specular region, you don't have to worry about the frequency domain or EQ! EQ is appropriate ONLY for adjusting the direct signal that comes out of the speaker - NOT the speaker-room interaction

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post #13 of 288 Old 08-02-2011, 07:25 PM
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Quote:
Originally Posted by fastl View Post

localhost - Thank you for the information. I know what ETC is. Pro sound magazines of the era I mentioned, such as dB, REP and Studio Sound were infested with articles on the topic. There were all these "experts" running around with Techcron analyzers (and the like) making ETC measurements and charging big bucks. I think most folks eventually figured out that it was basically just another audio consultant scam and it eventually fell out of vogue. Doesn't surprise me at all that it gets "discovered" again.

Maybe it just became well known as a tool so it was no longer media-worthy. There are many tools for acoustics, including your ears. They should all be used judiciously and in ways for which they are fit for purpose.

I'm all for someone gaining another tool in their toolbox. Even more so, I'm for people learning how to do acoustical analyses that include time. What we are actually seeing here is amateurs becoming able to use analyses that professionals were using 20 years ago.
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post #14 of 288 Old 08-03-2011, 07:26 AM - Thread Starter
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Quote:
Originally Posted by aackthpt View Post

Very nice, Mike! I don't know where you read about it but ETC analysis is one of the things I was going to go through with you.

Are you trying to make every seat in the house equally good? My intention is to make all of the seats good seats, but not amazing seats. Therefore while I may measure ETC from all of the seats, I am not going to fuss over the finer points of achieving the desired response from the cheap seats.

And, as the ambechoic response was mentioned... you need to decide what you are trying to do with the tool.
-Verify your ISD gap?
-Strongly terminate the ISD gap?
-Add to the sense of spaciousness in the room?
-Decrease the sense of "deadness" in the room?
-Improve the quality of the decay characteristic?

I can definitely tell you that the ETC has been getting progressively "better" in my room as I do more. And if you do want to see some "journeys to the center of the earth" I could point you to a couple of studio build threads on gearslutz where people did just that.


@fastl, can you provide a reference for "basically doesn't work"? Or just PM me with _your_ methodology and requirements/expectations for room acoustics? I'm quite interested to hear what causes you to say it "doesn't work".

John;
I'll have to take some ETC first then I can answer your q/statements with educated response.
-Verify your ISD gap? Yes - as starting point with my treatments as they are need to know this
-Strongly terminate the ISD gap? Need to see what it is to determine
-Add to the sense of spaciousness in the room? Subjectively I'm amazed at the 3d seemless of the sound as is compared to w/o treatments. Objective chart should help quantify that.
-Decrease the sense of "deadness" in the room? Subjectively my HT room does not appear overdamped to me, but ETC should help quantify that
-Improve the quality of the decay characteristic? tbd

I've got some reading to do @ gearslutz ......

Quote:
Originally Posted by localhost127 View Post

precisely. the ETC is the tool that details your room's total specular response. you modify your room as needed to achieve the particular room response you desire - and re verify with ETC once treatment has been applied.

it would likely be wise for a starting-point (mtbdudex) to pick a single listening position and focus on the L/R for the time being until the process is fully understood.


take note!:

Advice taken.

For now, I'll focus on 2 seat positions.
seat #6 in 2nd row, as that is the best for 10-200hz, so that is where measurements will be taken and any acoustic treatment changes will be initially based on that
seat#2 in 1st row, as reference to seat #6.

Keeping in mind this is a HT, so while seat#6 will be my "learning seat", final goal is overall "improvement" to as many seat positions as possible, or at least coming to terms with the HT acoustic state and then accepting it as "done" and "good to go" for a DIY amateur basement HT.
(some people would stop now, but the OCD inside of me says "get more fact based data".....)

I'm open to adding some strategically placed diffusors in the HT....the DIY ones seem to involve much time to research/build, but I'm ok with that as a ongoing learning project.
Also, I've got "left over" 6 sheets of OC703 2' x 4' board.

This will be a 2-4 month term project, as summer is still here and a few weekend camping trips coming up, then outdoor movie weekends, etc.

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post #15 of 288 Old 08-03-2011, 10:30 AM
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Lolz,

I would LOVE to contribute mtbdudex but this is :



Test,

Mpray1983- "User error due to sneeze or fart occurred during measurement"
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post #16 of 288 Old 08-03-2011, 11:39 AM
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Quote:
Originally Posted by mtbdudex View Post

I'm open to adding some strategically placed diffusors in the HT....the DIY ones seem to involve much time to research/build, but I'm ok with that as a ongoing learning project.

I'd suggest you consider polys as well as Schroeder diffusers. Both have their merits. I'm actually investigating putting up quite a few polys in my theatre. If I don't like them I'll pull some down and put in something else.

As for the room being overdamped... well it depends on what you're going for, to whose acoustic philosophy you prescribe, and what sort of environment you find comfortable. Plenty of people would suggest that near-anechoic is the best response model for a multichannel environment. I don't really have a strong opinion on it personally. Then again I haven't read Toole's book yet so I don't know the details of his philosophy.
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post #17 of 288 Old 08-03-2011, 12:07 PM
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Quote:
Originally Posted by aackthpt View Post
I'd suggest you consider polys as well as Schroeder diffusers. Both have their merits. I'm actually investigating putting up quite a few polys in my theatre. If I don't like them I'll pull some down and put in something else.
additional reading on polys (for those interested):

 

AcousticAbsorbers & Diffusers.10.4.Polycylinders..pdf 462.1220703125k . file
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post #18 of 288 Old 08-03-2011, 05:21 PM
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...What do you prefer instead of ETC?...

It depends on what you're trying to do. Room mode analysis is probably the most effective thing you can use. I'm not aware of any of the real pro acoustics designers using ETC as the basis of their designs, but they do use room mode analysis. Our hearing is basically frequency domain oriented, meaning that we don't directly sense sound waveform energy as presented by an ETC. If you really want to get into ETC, I would suggest trekking down to a library that carries dB or REP magazine back issues and reading through all the old "stories", so you can at least experience all of the wild claims that were being made.
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post #19 of 288 Old 08-03-2011, 07:55 PM
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Originally Posted by fastl View Post
I'm not aware of any of the real pro acoustics designers using ETC as the basis of their designs
Of course not, because they use super-duper simulation packages such as CATT. But they use analyses in it that provide ETC or equivalent (as well as other acoustical measures), plus being able to actually listen to the room sound from the simulation. They're also usually working on a room that isn't built yet, allowing them to tailor the (apparent) room boundaries to do half the acoustic work for which the rest of us rely upon "treatments".

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Our hearing is basically frequency domain oriented, meaning that we don't directly sense sound waveform energy as presented by an ETC.
LOL, that is epically untrue. If the ear/brain system were not time-oriented, we would not hear echoes. Sound is waves, and waves are inherently time-based. FR is actually just one way to characterize a system, and only a part of the characterization. FR is derived from a recording of what really does exist, which is pressure versus time, making it the representation more removed from physical reality. Not making it unuseful, though.

I'm actually somewhat interested to know what wild claims were being made. Unfortunately I'm not interested enough to head to a library. I know that I've seen some making it sound like it's an acoustical panacea on forums, and I actually object to that. Time is important in acoustics, and ETC is one of the tools that helps visualize and analyze it. Just another tool, full stop.

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Room mode analysis is probably the most effective thing you can use.
Please, for the sake of clear discussion define what you mean by "room mode analysis".
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post #20 of 288 Old 08-04-2011, 05:29 AM
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Quote:
Originally Posted by fastl View Post

There were all these "experts" running around with Techcron analyzers (and the like) making ETC measurements and charging big bucks.

Quote:
Originally Posted by fastl View Post

I'm not aware of any of the real pro acoustics designers using ETC as the basis of their designs, but they do use room mode analysis.

so, if i follow your commentary correctly, the "experts" who charge the "big bucks" have simply moved on to another tool?



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Our hearing is basically frequency domain oriented, meaning that we don't directly sense sound waveform energy as presented by an ETC.

Helmut Haas and Dr. Manfred Schroeder would like to have a word with you...
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post #21 of 288 Old 08-04-2011, 12:35 PM - Thread Starter
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I've spent more than a few hours reading the gearslutz links provided by localhost - some I've seen before in my various lurking there over the past 10-12 months.

My subjective observation:

The crew over at gearslutz seem much more into the detailed objective acoustic analysis and tweaking than the HT enthusiast - in general.

I take it they focus mainly on studios, and as such its part of their bread/butter $$$ daily job.
Poor or not desirable acoustics may mean missing a job, or mixing some material slightly off if not compensated for in the mix correctly.

While the HT enthusiast is a recreation / hobby / etc, where we like to learn/know acoustics, and apply some of it to our various spaces, but not at the depth the studio guys do.
(except for certain members here....)

Not meaning to start a flame inside my own thread, but just sharing my observation.

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post #22 of 288 Old 08-05-2011, 03:52 AM
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Quote:


It depends on what you're trying to do. Room mode analysis is probably the most effective thing you can use. I'm not aware of any of the real pro acoustics designers using ETC as the basis of their designs, but they do use room mode analysis.

Likely because they are not "pros" and have got their hands on a nifty spreadsheet, thinking (and claiming), once they get that right, all their problems are solved. Nonsense. I doubt they could afford, or understand, something like Odeon (http://www.odeon.dk/acoustics-simulation-software). Modal response is the least of the issues.

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post #23 of 288 Old 08-05-2011, 09:48 AM
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Quote:
Originally Posted by mtbdudex View Post

The crew over at gearslutz seem much more into the detailed objective acoustic analysis and tweaking than the HT enthusiast - in general.

I take it they focus mainly on studios, and as such its part of their bread/butter $$$ daily job.
[snip]
Not meaning to start a flame inside my own thread, but just sharing my observation.

Didn't seem like a flame to me, Mike. Good observations, and accurate too. The guys over there are Erskines but for control rooms rather than HTs. Most of those guys are into acoustics by passion or vocation, and often both.

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Modal response is the least of the issues.

Thank you for chiming in, Dennis.
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post #24 of 288 Old 08-05-2011, 10:50 AM
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Here are a few more sources that are readily available from around the 'net, that are either illuminating in this area, make the subject somewhat easier to understand, or at the very least explain why we should care. While SAC is knowledgeable and insightful, I find him occasionally difficult to read and digest. A variety of sources is also always a good idea.

http://www.cari-llc.com/pdfs/Mono2Surround.pdf

http://www.fmdesign.com/support/ETC_article.pdf

Also:

Attachment 219308
These are the curves for perception of side-arriving reflections from MHoA. It shows signal levels vs. delays that will result in different perceptual effects, using speech as a test signal. I think it's debatable whether or not we are trying to keep all reflections below the bottom curve, or whether we want to put some into the "spaciousness" region - though the decision to do so might depend on whether one is treating a room primarily for two channel vs. multichannel.

Attachment 219309
This is a "Signal comparison" from MHoA. This shows the changes in the curves from the previous graph based on "impulsivity" of the sound. Note that you would need reflections at a MUCH lower level to be inaudible for a highly impulsive sound source such as clicks (or many percussive instruments).

Attachment 219310
This is "Thresholds for various instruments" which I got from Long - Architectural Acoustics but evidently comes from Olive & Toole originally. It shows a more detailed breakout of how the curve shapes change for various instruments.

I actually intend to take those side-arriving reflection curves and convert them to data so that I can overlay them on ETC responses in REW, eventually. I also want to look into effects of late arriving reflections from other directions. I've had my thoughts of coming up with an arrangement of rigid fiberglass to run an REW sweep capture with only side-arriving reflections in it to know exactly which of the reflections shown in the ETC are side-arriving (and presumably, then, which are not).

Interesting. I just found out that someone named Hugo Fastl wrote a book about psychoacoustics. And it appears to be fairly recent in newer editions. But evidently that guy lives in München.

Another free source that has some great discussions of room design based on psychoacoustic principles is http://alexandria.tue.nl/extra2/afst...unster2003.pdf but it's a long book, I have not remotely read it all myself. It's been sitting in my reading list, but I did just have a quick look at some of its LEDE/RFZ stuff and it looks like a better exposition of control room design evolution than I have seen elsewhere. Also it presents some other viewpoints of control room design than RFZ which is by far the most popular on GS, except among the occasional non-environment room fan.
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post #25 of 288 Old 08-17-2011, 07:38 AM
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double-posting this across threads - due to significance.

SAC has been so kind as to hand this information to us on a silver platter. one must understand the importance and sympathy of the TOTAL specular response (the ISD + the termination + the way the energy decays) as a whole - as measured with the ETC.



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Originally Posted by SAC View Post

the ISD termination should be a maximum of 12 dB SPL below the direct signal level. ... Up to an ISD length of ~25ms.

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Originally Posted by SAC View Post

The preponderance of the termination as well as the exponentially decaying diffuse tail energies should arrive laterally.

To quote from Schroeder: "..research, based upon a subjective evaluation of the acoustics of 20 major European concert halls, has shown that many modern halls have poor acoustics because their ceilings are low relative to their widths. Such halls do not provide the listener with enough laterally traveling sound waves - as opposed to sound traveling in front/back direction and arriving at the listener's head in his "median" plane (the symmetry plane through his head). Such median plan sound, of course gives rise to two very similar acoustic signals at the listener's ears, and it is thought that the resulting excessive "binaural similarity" is responsible for the poor acoustical quality."

Also, to keep it short...
The purpose of the ISD termination is to REMOVE the localization cues of the later arriving energies and to reinforce the localization cues of the direct energy.

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Originally Posted by SAC View Post

Also, in the diagram, note the delta between the separate source and image shift thresholds and compare that with the level of the ISD termination.

Quote:
Originally Posted by SAC View Post

A late arriving reflection, if of sufficient gain, is perceived as a destructive element that skews localization and tonality. (And in a Large Acoustical Space, if after 80 ms, it is perceived as a distinct echo). Hence the use of the ETC in identifying and mitigating them and the use of the Haas kicker - the ISD termination - to reinforce the direct signal localization prominence.

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Originally Posted by SAC View Post

As the intensity of the ISD termination has a significant impact on the sense of liveliness of the space, while the laterally arriving semi-reverberant soundfield and to the sense of space (size) of the room.

The ISD balances between the comb filter interval (CFI) featuring time delays within the first few ms (say a meter) that cause significant spatial misdirection via comb filters, and the ISD where the establishment and support of the time region capable of supporting a partial Haas effect as supported by Madsen's research.

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As far as the two issues regarding orientation of later arriving diffuse energy and the psycho-acoustical requirements of relative levels of arriving energy relative to the direct signal necessary to trigger individual behaviors. (That is not to say that we know everything there is to know! But it is what research we currently have.)

Yes, it is beneficial to increase the degree of later arriving, laterally directed, exponentially decaying, diffuse energy return in that it increases the sense of space in the room. Bu this is optimally accomplished by a combination of controlled dispersion speakers minimizing boundary incidence (and including soffit mounting optimizing Q), limited strategic absorption, and the redirection of energy maximizing its content until it is diffused back into the listening position. Thus it is an issue of maximally conserving energy and minimizing losses in the finite energy supplied. Thus this also implies the judicious use of lossy diffusion as well such that reflection is the choice until such time as diffusion is strategically introduced.

The higher ISD termination relative to the direct signal within the Haas time interval both increases the sense of liveliness in the room while also increasing the Haas effect and direct signal localization process while simultaneously minimizing the destructive localization and tonality cues of any later higher gain reflections.

But as to the basic relationships as defined in the LEDE model, they have all been researched and predicated upon psycho-acoustics principles - not simply assumptions. And there is not much point of debating them anecdotally unless we have new psycho-acoustic research that either invalidates or modifies concepts that are directly supported by the implementation of the model.

With the advancement in the complexity of diffusive treatments, there has been the corresponding advance in the integration of the soundfields, but there has not been a corresponding 'replacement' discrediting of the basic psycho acoustical concepts. Instead the models have simply become more tightly integrated and complex. We are now able to generate much more complex mixed soundfields and this has enabled the various models to become more effective, as evidenced in the extreme low level and extremely complex diffusion incorporated in the ambechoic model.

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Originally Posted by SAC View Post

My general point remains that the issues with retaining sufficient energy to reach the prescribed ISD termination levels requires judicious attention to detail at each stage of the signal process, including a careful management of boundary incidence, reflection/redirection, and a care in the optimal use of lossy diffusive techniques in the process of redirecting the later arriving diffuse energy laterally back to the listening position.

Quote:
Originally Posted by SAC View Post

All of the variables listed as simply steps in the process whereby the energy from the speaker propagates and is either redirected or absorbed.

Our larger task is on the one hand to have only the direct signal arrive at the listening position during the ISD gap, while ALSO, redirecting the indirect energy in such a manner as to preserve as much energy as possible, to return the energy from a lateral orientation in a diffuse manner at a level within at least 12 dB of the direct signal level.

Thus, be it the over application of (broadband) bass trapping, broadband specular absorption, and/or diffusion (and less diffusion, as the process of diffusion has a relatively high 'absorptive' loss component.

Whereas it is common to hear the notion that one cannot apply too much bass or broadband absorption, this is simply is not the case with this acoustic response model! Here we require very judicious application of the treatment sufficient to tailor both the direct response as well as the indirect response.


Thus controlled Q/dispersion radiation from the speakers minimizing the need for absorption, bass trapping must be frequency specific, specular absorption must be surgical, and diffusion efficient (diffusing diffusion is a big energy loss) while also effectively reflecting/redirecting the energy such that after a suitable time it can be redirected laterally in a diffuse manner back to the listening position with sufficient gain is a challenge in energy conservation.

Each step contributes to the losses, and with each step we are faced with economizing to preserve the energy. The best term that comes to mind is "surgical" - to do just what is necessary to prevent anomalous behavior while preserving the later arriving energy.

And while the perils of excessive absorption are rather obvious in this regard, so too are the issues with diffusion - not because diffusion is bad, but rather because it is a rather inefficient process with higher losses than many would assume. And the emphasis shifts a bit from 'are the absorbers large enough', to the balance with low loss reflective and re-directive surfaces, or moving surfaces to avoid incidence entirely as was done in the RFZ implementation of the LEDE.

Its a balancing act at each step in the process.

Quote:
Originally Posted by SAC View Post

The length of the ISD gap maximizes intelligibility during the Haas interval where multiple arriving signals are merged/smeared into one event in what Heyser called 'time smear distortion'.

I don't see that as ever being good or acceptable.

Also, in a control room or critical listening space, the ISD also allows one to experience the reproduction of the recording space in total before the incursion of the control room or listening space.

The termination of the ISD does several things as well psycho-acoustically. It provides the sense of liveliness to the listening space. Second, it aids in localization by 'removing' the localization and tonality shifting cues from later arriving signals - causing the focus and localization to lock onto the direct signal. And the later arriving exponentially decaying diffuse soundfield adds to the sense of space/size to the listening environment. This is the bonus above and beyond what a dead room affords you.

So in fairness, while its hard to say "which one", I guess one would have to acknowledge the primacy of the ISD regarding fundamental localization, imaging and intelligibility of the direct signal itself. As the termination and later arriving diffuse soundfield further augment this.

The termination and latter arriving lateral diffuse sound field further augments the localization and imaging, accurate tonality and provides a sense of space to the listening environment.

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post #26 of 288 Old 08-17-2011, 12:12 PM - Thread Starter
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That's good info, need to read at home instead of tiny iphone4 screen...

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post #27 of 288 Old 08-17-2011, 12:44 PM
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Superb thread

localhost, good lookin' out. A lot to digest there.

Thanks a bunch

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post #28 of 288 Old 08-17-2011, 07:05 PM
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Thanks for the compilation, localhost. To preface the rest of the discussion, when I say "ISD termination" I mean one of the strength that SAC suggests in localhost's compilation. SAC has noted previously that "all ISDs are terminated" which is true, else it could not actually be called a gap. So I make the distinction that when I write of "ISD termination" I actually mean a specific burst of energy intended to strongly terminate the ISD gap, not simply the beginning of the decaying sound in the room. Specifically, the difference between the two responses in Andreas' post here: http://www.gearslutz.com/board/5282419-post1.html.

I hope to read more of the original research that SAC mentioned w.r.t. ISD gap termination. Have you ever heard a a room with an ISD termination vs. a room without? Some of the GS descriptions of how much adding it changed the sound are fairly dramatic. G.E. seemed to think it was an improvement on the same level as getting the ISD at all. But I remain skeptical until I hear a room with an ISD termination, and hopefully can block the device causing the termination to get a feel of the sound with and without it (as well as run sweeps to verify the response with and without said blocking). It's audio and while I don't believe hearing is believing, I also don't think it should be discounted. In other words, I'd LOVE to get the chance to play in Andreas' room!

I also think it is less clear if ISD termination is desirable in a home theatre environment as opposed to a two-channel environment. I also wonder if it is really desirable in a (two-channel) listening room as opposed to a control room, given their subtly different goals. Further, I don't necessarily see being able to provide an ISD termination for more than one or two seats in a home theatre - but if one believes that centredness is absolutely required for great imaging (as do many "audiophiles" as it were) then one might say there is only one seat per row worth trying to provide it anyway - not to mention that I don't really see any way to provide it to multiple side by side listeners at the same time relative to the direct arrival for each of them. Though I'm usually there alone, I like to think of home theatre as more social than the typical two-channel listening room you see with one chair.

I haven't read Toole, but some of the things I've read on forums leads me to believe that some of his stuff may suggest that an ISD gap in a listening room is not critical (or perhaps not as critical). I don't doubt its value at all in a concert hall, where Beranek originally noted it distinguishing good-sounding halls from poorer-sounding ones. Mike has read Toole, so might be able to summarize Toole's views on the matter for us here. In any case the one thing whose value I don't really question is a decent logarithmic decay of diffuse reflections. Also, just because I question something doesn't mean that I don't think doing it has caused a good improvement in my experience. Getting early reflection panels up just on the sidewalls of my theatre room made a significant difference that I really enjoy to the sound. I fully intend to follow through and finish cleaning up the ISD gap for my prime seat, though most of my plans lately run toward increasing diffusion in the room.

I think it's worth noting that an ISD termination of the sort SAC discusses is a feature of an RFZ room, and such actually does not exist in the response of Blackbird Studio C (which is admittedly called ambechoic and not LEDE/RFZ) which we discussed earlier in the thread. Taken directly from post the designer made at GS:
Attachment 220350
LL
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post #29 of 288 Old 08-17-2011, 07:17 PM
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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?
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post #30 of 288 Old 08-17-2011, 08:43 PM
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Quote:
Originally Posted by aackthpt View Post

...

fyi - that's Andreas' room, not Jens! -both healthy contributors and much respect to their public display of their findings ---

and note his commentary:

Quote:
Originally Posted by Andreas View Post

I guess you've seen my LEDE/Haas trigger thread. The current state of the room, nearly a year after, is that the specular Haas triggers are toned down a bit and the diffusion have gone up a bit.

There's a lot of added reflectors on top of the porous traps in my room. One mistake I did, that you can see in the pics, is to use too small pieces for the reflecting parts. The small reflectors lets too much mid through, giving the later part of the verb a bit of a nasty sting in that there's a boost in the lower high end and a bit of lack of midrange. The positioning of the absorbers adds up too, with most of them having a bit of 1/4'th wavelength bump in the same general frequency range. And, not to keep raining or pissing on other peoples parades, but there's also some peculiar absorption properties in the styrofoam I used for the diffuser builds. Not a deal breaker, but worth having in mind.

Lets flip it and look at the reflective properties instead. The styrofoam diffusers will not reflect the low end of the spectra. This means, in itself, that the energy you see returned from the diffusers does not have much low end. This again means that the levels you observe in the ETC graph will be lower than for an equivalent size unit that reflects energy across most of the spectra. Another reason I don't get totally hung up on the absolute deciBel levels as seen in the ETC graphs.. (if you're feeling logical, you may see where I was heading earlier with the general cautions regarding ETC's and levels! I just need to think these things through some more, do some more testing and talk to some more people before being explicit about it..)

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?

Quote:
Originally Posted by aackthpt View Post

I think it's worth noting that an ISD termination of the sort SAC discusses is a feature of an RFZ room, and such actually does not exist in the response of Blackbird Studio C (which is admittedly called ambechoic and not LEDE/RFZ) which we discussed earlier in the thread. Taken directly from post the designer made at GS:
Attachment 220350

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:

Quote:
Originally Posted by GML View Post

note in this ETC that the decay is approximately 0.3 seconds, but more importantly, the direct to ambient ratio is in excess of 30dB, which is also the reverberation spec for an anechoic chamber.

the room is hardly anechoic, though. just listening to music in there is extraordinary; ensemble musicians report that it's easier to play live in the room because it's easier to hear in the room.

those that appreciate it use it to it's utmost. the "sound" is more or less like an "acoustic white-out", and the decay is extraordinarily linear with higher frequencies (the Schroeder curves are remarkably linear) having more decay than any other room i've ever experienced.

the decay and the LEDE model in itself is related to small acoustical spaces (and important factor regarding all of this conversation!!). the intense diffusion which immediately attenuates any reflection to the noise floor creates a sense of a reverberant space (in a large acoustical space).

...more commentary...
Quote:
Originally Posted by SAC View Post

Let's we get lost down the rabbit hole, the is no reverberant sound field in a small acoustical space. Ted Schultz effectively blew that one out of the water long ago.

And if there were, it would be experienced as a masking noise floor, reducing the S/N of the system..

Second, by definition, if you have a reverberant sound field, the arrival direction is equally probable in all directions, and thus there is no energy flow in any direction, and thus it makes no sens to speak of localization of reverberant reflections, not more than it would a relative absolute or a finite infinite.

We have, at best, a diffuse or semi-reverberant soundfield. And RT60's at the practical frequencies with which we are concerned are non-existent, as they too by definition assume (read: require) the existence of a reverberant soundfield.

While many use the term "reverb" and "reverberation" loosely (and the effect commonly described by the term in no corresponds to the actual phenomena!), it is not an appropriate concept corresponding to any appreciable behavior in the acoustical spaces in which we are dealing. Thus it is a case where the improper use of the term implies behavior not in evidence, and thus merely muddies the discussion.

Please do not confuse decay times with RTxx times.

Quote:
Originally Posted by SAC View Post

If you do not have a statistically reverberant sound field, you have no Critical Distance (Dc) - and you have neither in a Small Acoustical Space.

Quote:
Originally Posted by SAC View Post

What "undirected sound" are you talking about? We are dealing with either modl standing waves or specular signals - all of which have 'direction'.

A diffuse sound field (and I use this term to refer to semi-reverberant sound fields) still has signals which have 'direction'! We are NOT dealing with a reverberant sound field which exhibits a statistical equal probability of originating from any and all directions simultaneously. A Small Acoustical Space is a "non-reverberant" space.

And the reverberant sound field, if present, is not perceived as a localization signal anyway, it is perceived as the noise floor relative to the direct signal. And if its level is sufficient, its effect is to simply to mask the direct signal, it is not perceived as a localization specular signal. It impacts the S/N characteristics of the space.

Just as a "live room" has Nothing to do with the existence of a reverberant sound field! ** Instead it is the ratio of the direct sound level to first significant reflection level that determines the perception of "liveness".

A late arriving reflection, if of sufficient gain, is perceived as a destructive element that skews localization and tonality. (And in a Large Acoustical Space, if after 80 ms, it is perceived as a distinct echo). Hence the use of the ETC in identifying and mitigating them and the use of the Haas kicker - the ISD termination - to reinforce the direct signal localization prominence.

As far as the late arriving semi-reverberant/diffuse soundfield that should optimally arrive from the lateral (rear side) direction, it is always going to be at a lower level than the direct signal, and it ideally should follow an exponentially decaying gain curve. This later arriving energy provides the sense of 'space' - of the room being larger than it is, to the experience.


**(Note that this mis-characterization is rampant in the industry! How often does one see someone walk into a space, clap their hands, and then make some assessment of how "reverberant" the space is?!?!?! That single action alone is enough to tell me exactly what level of acoustic understanding one possesses.)

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