Should the center be as capable as the mains? - AVS Forum
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post #1 of 33 Old 02-06-2012, 02:51 PM - Thread Starter
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I'm putting this in DIY since it's based off of a DIY speaker design and builds I've done. Hope it's the right place. I've been doing some thinking about the speaker layout in my theater room build (see sig). My main speakers are the Dayton classic WMTMW I built (see sig again). The surrounds (sides and rears of a 7.1) are going to be Tritrix MTs. For the center, I was just going to use a single sealed tritrix MTM that I had built, but I am wondering if changing it to a WMTMW to better match the mains would be more appropriate. Would I get any benefit of the center being larger like my mains? Everything will be crossed to a sub at 60-80 Hz or so (TBD) and I'm wondering if I would get any benefit of a larger speaker. I would set my AVR to accurately represent the situation of large (WMTMW) or small (MTM and MT) speakers. Thanks,

Brian

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post #2 of 33 Old 02-06-2012, 03:06 PM
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Imho, yes. Whenever possible, have your CC identical to your left and right but if not, at least just as capable.


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post #3 of 33 Old 02-06-2012, 04:33 PM
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The center doesn't need to be quite as large as the mains, as it doesn't get as much low frequency content. But it shouldn't be much smaller, as it carries most of the load.

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post #4 of 33 Old 02-06-2012, 04:42 PM
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IMO, the center is the most important speaker in a system...so don't cut corners.

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post #5 of 33 Old 02-06-2012, 04:48 PM - Thread Starter
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Ok, looks like a unanimous opinion. It would not add much cost considering the total project.

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post #6 of 33 Old 02-06-2012, 04:48 PM
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Quote:
Originally Posted by Scott Simonian View Post

Imho, yes. Whenever possible, have your CC identical to your left and right but if not, at least just as capable.

Agreed
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post #7 of 33 Old 02-06-2012, 05:04 PM
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Yes.

Quote:
Originally Posted by SilentJ20 View Post

I'm putting this in DIY since it's based off of a DIY speaker design and builds I've done. Hope it's the right place. I've been doing some thinking about the speaker layout in my theater room build (see sig). My main speakers are the Dayton classic WMTMW I built (see sig again). The surrounds (sides and rears of a 7.1) are going to be Tritrix MTs. For the center, I was just going to use a single sealed tritrix MTM that I had built, but I am wondering if changing it to a WMTMW to better match the mains would be more appropriate. Would I get any benefit of the center being larger like my mains?

You'll have less distortion from the larger center (where over 60% of the sound track energy ends up) and it will have the same all-pass behavior as identical mains so imaging between center and left or right works correctly.
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post #8 of 33 Old 02-06-2012, 05:56 PM
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IMHO the best solution for a home theater is that the LCR are identical and all behind an acoustically transparent 2.35:1 screen


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post #9 of 33 Old 02-06-2012, 07:54 PM
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I'll go on the record as either design is a BAD choice for a center channel. There's going to be too much horizontal lobing to be effective at dialogue intelligibility. Build a WMTW where the mid and tweeter are vertically aligned instead or you could do the WMTMW where the tweeter is heavily offset to the top of the enclosure and the mids are nearly touching side by side.
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post #10 of 33 Old 02-06-2012, 07:57 PM
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should be identical...and if you're building speakers, you can do it!
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post #11 of 33 Old 02-06-2012, 08:11 PM - Thread Starter
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Quote:
Originally Posted by BIGmouthinDC View Post

IMHO the best solution for a home theater is that the LCR are identical and all behind an acoustically transparent 2.35:1 screen

That's the plan. Though maybe not 2.35:1. Projector limitations (though I think I'll start a thread there or do some searching).

Mayhem, would modifying the placement of drivers that much for the WMTMW require a crossover change? I understand the WTMW would and that stuff is over my head.

How much of an issue is it going to be or is it sort of a "wait and see" thing?

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post #12 of 33 Old 02-07-2012, 01:05 PM
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So, you're building a third WMTMW tower? Great option.

Re-arranging the drivers will change the sound if the XO is not modified accordingly... or maybe not. It all depends on the designer's approach and the thoroughness of their analysis supporting the design. I'm not finding a whole lot of documentation on your towers; almost sounds like an unfinished design.

That's not necessarily bad... a scrunched-together sideways CC with the same XO will make sound and protect the drivers, but you'll be open to linear distortion (FR non-linearity) that varies with listening location. Heck, I bet they don't have a ruler flat FR to begin with, and that's the point!

Conversely, if you have or can get FR and impedence curves for those drivers, you can simulate both the original towers, your cobbled-up CC with that XO, and perhaps with a modified XO... it's all doable.

That said, the best option will always be to build a proven design CC. There are lots of 3-way CC's, but there is one 2-way, MTM CC I can recommend - Jon Marsh's Modula MTM CC. Dual 7's will likelly keep up with your Dayton 8's and that Scanspeak tweeter allows it all to happen (tight C-C spacing, XO at 1,200Hz). About $350 for the parts... and available for both free-standing and in-wall/on-wall applications.

HAve fun,
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post #13 of 33 Old 02-07-2012, 01:29 PM
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Quote:
Originally Posted by mayhem13 View Post

I'll go on the record as either design is a BAD choice for a center channel. There's going to be too much horizontal lobing to be effective at dialogue intelligibility. Build a WMTW where the mid and tweeter are vertically aligned instead or you could do the WMTMW where the tweeter is heavily offset to the top of the enclosure and the mids are nearly touching side by side.

I second this.


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post #14 of 33 Old 02-07-2012, 01:31 PM
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The center really isn't that important because you can run a "phantom center" meaning you have L/R's only. However, if you want a center, it should be identical to the L/R's. So it's either all or nothing.

YID DIY
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post #15 of 33 Old 02-07-2012, 01:50 PM - Thread Starter
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Okay, I think I'm seeing a point of confusion. I would be building it as a vertical-standing WMTMW, not a horizontally oriented one. This would all be behind an acoustically transparent screen, so it can stand tall with the left and right fronts. Am I correct that if stood vertically the lobing no longer is a concern?

Here is a quick pic of my plan:

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post #16 of 33 Old 02-07-2012, 01:51 PM
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You are correct.


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post #17 of 33 Old 02-07-2012, 02:08 PM
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Quote:
Originally Posted by SilentJ20 View Post

That's the plan. Though maybe not 2.35:1. Projector limitations

Then the L/R may not have enough spread and may be better off outside the screen border behind your false wall. Move the subs inside if they are in the way.


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post #18 of 33 Old 02-07-2012, 02:16 PM
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I get the mtm with a woofer...but why 2 woofers?
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post #19 of 33 Old 02-07-2012, 02:24 PM
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Quote:
Originally Posted by SilentJ20 View Post

Okay, I think I'm seeing a point of confusion. I would be building it as a vertical-standing WMTMW, not a horizontally oriented one. This would all be behind an acoustically transparent screen, so it can stand tall with the left and right fronts. Am I correct that if stood vertically the lobing no longer is a concern?

Here is a quick pic of my plan

Yes! Do this.

Quote:
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I get the mtm with a woofer...but why 2 woofers?



Simply... more sensitivity in the bass region, where it's needed. More bass, son!!! C'mon!!!


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post #20 of 33 Old 02-07-2012, 02:36 PM
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but what about the orientation, one top one bottom...why not both on the bottom?
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post #21 of 33 Old 02-07-2012, 02:46 PM
 
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Quote:
Originally Posted by Scott Simonian View Post


Simply... more sensitivity in the bass region, where it's needed. More bass, son!!! C'mon!!!

And if the separation between drivers is , say 3 feet, the highest they can produce without destructive polar lobing is ~93 Hz.

And that is completely ignoring the impact of the spacing between the drivers and the virtual drivers by virtue of their mirror image equivalence off the boundaries. And as each distance is greater than 3 feet, the corresponding limiting frequency will be still lower.

Think modal distribution and near field polar lobing...

There is a bit more to gain than simply sticking a bunch of spaced drivers up there.

For that frequency range there are any number of alternative topologies that would produce a much greater coherent source as well as greater sensitivity.

Folks would do well to research the origins of the D'Appolito /MTM configuration and discover for just what problem it was designed to compensate... (hint: a 15 degree upward polar tilt when used in conjunction with a passive 3rd order LR crossover...) rather than simply assuming its superior due to its being common...
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post #22 of 33 Old 02-07-2012, 02:52 PM
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Quote:
Originally Posted by benareeno View Post

but what about the orientation, one top one bottom...why not both on the bottom?

Oh... well, you didn't say that.

Quote:
Originally Posted by dragonfyr View Post

And if the separation between drivers is , say 3 feet, the highest they can produce without destructive polar lobing is ~93 Hz.

And that is completely ignoring the impact of the spacing between the drivers and the virtual drivers by virtue of their mirror image equivalence off the boundaries. And as each distance is greater than 3 feet, the corresponding limiting frequency will be still lower.

Think modal distribution and near field polar lobing...

There is a bit more to gain than simply sticking a bunch of spaced drivers up there.

For that frequency range there are any number of alternative topologies that would produce a much greater coherent source as well as greater sensitivity.

Folks would do well to research the origins of the D'Appolito /MTM configuration and discover for just what problem it was designed to compensate... (hint: a 15 degree upward polar tilt when used in conjunction with a passive 3rd order LR crossover...) rather than simply assuming its superior due to its being common...

Heh. I know about all this.


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post #23 of 33 Old 02-07-2012, 02:56 PM
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Quote:
Originally Posted by benareeno View Post

but what about the orientation, one top one bottom...why not both on the bottom?

Why not ask the same question about the midranges?

Noah
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post #24 of 33 Old 02-07-2012, 03:29 PM
 
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Why not discover the fundamental limitations of ALL spaced sources used to cover the same passband?????

And then learn a bit more about the fundamental limitations of ALL non-colocated drivers?

Debating one configuration over another without an understanding of the fundamental limitations that govern all of the topologies is really not a worthwhile endeavor.
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post #25 of 33 Old 02-07-2012, 04:44 PM
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Quote:
Originally Posted by dragonfyr View Post

And if the separation between drivers is , say 3 feet, the highest they can produce without destructive polar lobing is ~93 Hz.

And that is completely ignoring the impact of the spacing between the drivers and the virtual drivers by virtue of their mirror image equivalence off the boundaries. And as each distance is greater than 3 feet, the corresponding limiting frequency will be still lower.

Think modal distribution and near field polar lobing...

There is a bit more to gain than simply sticking a bunch of spaced drivers up there.

For that frequency range there are any number of alternative topologies that would produce a much greater coherent source as well as greater sensitivity.

Folks would do well to research the origins of the D'Appolito /MTM configuration and discover for just what problem it was designed to compensate... (hint: a 15 degree upward polar tilt when used in conjunction with a passive 3rd order LR crossover...) rather than simply assuming its superior due to its being common...

Any alignment can be picked apart ad infinitum, but I see your point. What about the lobing that happens between a pair of stereo speakers playing a mono source like a bass guitar or vocals (which seem to be usually centered in a mix, along with kick and snare)? No one complains about the horrible lobing there, that varies quite widely with just a few inches of listener position, especially 'phantom center' supporters....

Symmetrical and non-symmetrical alignments have the advantages and drawbacks. Good reading:

http://www.birotechnology.com/articles/VSTWLA.html

It goes over exactly what dragonfyr was pointing out, that in the right circumstances, an MTM is a good idea, in others not so much. But once you take into account all of the problems with audio playback (the room and it's multitude of problems, separate sources trying to replicate a 'continuous soundfield', varying listener positions, etc) you have to wonder how much difference one choice will make.

I have picked apart AxiomAudio's center channel speakers with widely spaced tweets, but they get consistent critical acclaim. Would I buy them? No. But going with the above arguments, really only fullrange drivers with uncanny off-axis behaviour would fit the bill, or Danley Synergy Horns. Line arrays? HELL NO. Think of the non-colocated sources, the comb filtering, the defects in their impulse response. Yet line arrays have a good following.

In other hobbies I have enjoyed, there is an adage: "It all works". This is no more true than in surfing, where there are some people who would not ride anything other than a 6'0" thruster, yet some folks can take a closet door and ride a wave with it (or something similar). Here's a vid:

http://hydrodynamica.com/totem/lord-board.php

I'm not saying people with a Bose system get the same thing as pnw's Octagon setup, but I have found that the time spent deciding on relatively small choices can detract from enjoyment, unless you are here for the journey, and not the end result.

I've seen people spend more time on the beach switching out fins and debating which one to try than actually surfing....just a thought.

Oh, and I would go with the drawing above, 3 identical vertical WMTMW, especially since 2 of them are already built, making the time from build start to 'fire up the film and get the beer' shorter....

JSS
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post #26 of 33 Old 02-07-2012, 04:53 PM
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Well said.


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post #27 of 33 Old 02-07-2012, 05:42 PM
 
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Quote:
Originally Posted by maxmercy View Post

I have found that the time spent deciding on relatively small choices can detract from enjoyment, unless you are here for the journey, and not the end result.

First, i was not the originator of the obsession over speaker driver topology. I merely suggested a few rather fundamental and simple topics that could be explored that would render the subject rather trivial and a simple issue to evaluate.

Nor do I advocate obsessive evaluation of such issues that once the fundamentals are understood, do indeed become rather trivial - hence my original reply! The larger concern is why such a topic is routine here rather than a focus on the more fundamental concepts that would effectively render the need for such debates moot.

I mean, after almost 40 years, no more than a handful of folks are actually familiar with, or can succinctly and FACTUALLY explain, what all the ACTUAL brouhaha was over that resulted in the all too common routine Bose bashing - that SO MANY still routinely partake in for all the WRONG imagined reasons...

Unfortunately the overly simplistic assessment that 'its all obsessive fussing' or 'it's all good" is an invalid analogy.

With just a little homework, one need not obsess unduly over inconsequential minutia - as so often happens in tangent on this forum to the exclusion of many much more fundamental and consequential issues!

And if one is not aware of what 'that is', that alone should provide a pretty simple metric that a bit of time spent learning this would be time well rewarded both in effectiveness and in the final result.

It is relatively easy to determine and to prioritize the largest return on investment in terms of 'fussing' and 'obliviousness'. And quite frankly the largest areas that dominate ROI are in the area of acoustics and of speaker-room interaction - Not small signal electronics behavior.

Thus one can start by ignoring the routine 50 page threads obsessing over how many decimal place precision is required regrading small signal electronics performance. And with that I can just imagine several hundred folks 'going apoplectic' and fainting...

But if one does not care about making REAL tangible improvements, why in heck are they wasting time here debating silly issues to begin with? Judging from the orientation of the majority of threads, I wonder...
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post #28 of 33 Old 02-07-2012, 06:06 PM
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I did prob go over deep end, but I was simply pointing out that with two already built WMTMW, the choice is pretty obvious. I got what you were saying, but with equal driver complement and optimized WMTMW and MTMWW designs, could someone really tell the difference in practice? enough to say "something is wrong with that center channel" if it were the different one? Maybe.

I agree with you that speaker room interaction is where a lot of attention should be paid, and it rarely gets anything over lip service. Hardly anyone posts Listening Position freq response except for subs. Most of them are horrible. The holes in the upper bass due to the speakers being in the room and not in-wall? Get the smelling salts.....they would make crossover designer guys pass out. To say nothing of the floor and back-wall bounce....blasphemy, all of it! Sidewall comb filtering? It 'adds something'. Yes it does, it adds nice, sharp dips to the freq response at regular intervals on top of the 'spaciousness'.....and so on......

I am also guilty of doing the paralysis via analysis thing....I am not immune. I have put off builds wanting to know if what I was doing was the best way to go, I am currently doing just that on a few builds.....and hope the weather turns warmer soon.

Reason I say all the above is I helped a friend set up his system this past weekend. In-wall speakers that were unknown quantities (bought w/ the house, likely had the grills painted after being installed), L/R about 8 feet off the ground. Surrounds L&R in ceiling. A generic Paradigm or Polk Center, and a 10" sealed Sony sub (with great highpassing, BTW, surprised, no bad sounds with sub-40Hz content). He knew very little about audio, and was sending only red/white RCAs to his receiver from his players. We went and got some optical cables (for his DVD player and XBox360), and re-ran MCACC. Even though he only got 40Hz extension, and the imaging was totally screwed with L/R being 6 feet above Center channel, it wasn't slit-your-wrists bad.....the jump from 2.0 to 5.1 and analog stereo to optical digital out was a big one.

I once built a 30Hz horn with a 6" driver, disconnected the 22Hz 15" loaded horn, and played around. Although losing an octave and 9dB in overall volume, movies for the most part were still pretty good.....

But I did miss the extension and output, and so to the garage the little horn went....it will get finished up and used soon, though.

OK, enough OT. Sorry for the de-rail.

JSS
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post #29 of 33 Old 02-07-2012, 06:16 PM - Thread Starter
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It did get a bit OT, and unfortunately that is some stuff that I would like to learn about. But in the spirit of the forum, I suppose we should stay somewhat on topic.

Here's a question about frequently response at the listening position that was mentioned. With the larger center that matches the fronts, would I measure each channel seperate or would all 3 (or 7.1) play at the same time for an overall system? It seems it would be beneficial to know how each channel was doing, but I'm still learning this stuff.

I guess that's not completely on topic, but its useful to me and it relates to having a more full range center. And it's my thread, lol.

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post #30 of 33 Old 02-07-2012, 06:25 PM
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Quote:
Originally Posted by SilentJ20 View Post

Here's a question about frequently response at the listening position that was mentioned. With the larger center that matches the fronts, would I measure each channel seperate or would all 3 (or 7.1) play at the same time for an overall system? It seems it would be beneficial to know how each channel was doing, but I'm still learning this stuff.

Each one will have its own frequency response dependent on its listening axis and proximity to room boundaries.

That said human timbre perception seems to come from some weighted combination of what our brains believe to be a sound and its reflections. Simple amplitude measurements in reverberant spaces don't reflect this, gated measurements which only see direct sound aren't enough, etc.

The bass area may be more complex, with Siegfried Linkwitz commenting

Quote:


I have investigated possible reasons for the qualitative difference between monopole and dipole bass reproduction by in-situ measurements [6] and scale model experiments. I am convinced that the steady-state low frequency response is a poor indicator of the quality of bass reproduction, other than to point to the one or two modes that need to be equalized. The strongest correlation between measured data and subjective impression appears to come from a modulation-transfer-function measurement which is analyzed in the time domain. For example, when a short length of a 100% amplitude modulated signal with a carrier to modulation frequency ratio of 10:1 is used as stimulus, then the room response reduces the depth of modulation and increases the burst duration for different frequencies and room locations. A pattern seems to appear whereby the modulation envelope is subjectively preserved more frequently with a dipole than a monopole. This correlates strongly with the impression that bass reproduced by a pair of dipole woofers is more articulate and thus more realistic of the recorded source.

[6] Siegfried Linkwitz, "Investigation of Sound Quality Differences between Monopolar and Dipolar Woofers in Small Rooms", 105th AES Convention, San Francisco, September 1998, Preprint 4786.


Sean Olive has a formula which strongly predicts speaker preference based on polar response (and therefore probably best quantifies accuracy because that's what previous studies have suggested people were looking for using similar measurements) although AFAIK it's not published and remains a Harman Group trade secret.
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