Early reflection panel thickness... - Page 2 - AVS Forum
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Old 03-20-2012, 04:30 AM
 
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Originally Posted by Digital_Chris View Post

Just to requote myself here...

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Originally Posted by Digital_Chris View Post

I'm not trying to make the room a perfect sounding room, that would drive me CRAZY trying to figure that out.

...well, it has.

And to be honest, yes, all of this technical mumjo jumbo IS incredibly informing, don't get me wrong, but it is also driving me crazy because I just won't be able to apply the recommended absorbtion, which has been pulled forth only because of theory and others (Toole, etc) testing, it seems. How come I haven't seen anybody post for me that they've tried the thin panels, then went to thicker panels and were,for a lack of a better phrase, blown away? Atleast, that's what I expected, instead of test data only from the extreme audiophile. To me, it's almost like saying, "The new Honda Civic get's 2mpg more than last years model" Will we honestly notice it, I doubt it, but the test data is there to prove it. Bad anology, I know, but it serves the point.

Please, to all of you (Local, FOH, dragon), I'm not getting aggrivated at the awesome knowledge that you guys are providing me with, but instead, I feel like you are missing the point that I have a small room and can't give up the real estate for thicker panels, and instead of trying to find alternatives or give me other recommendations, all I get are facts and statements that, in lamens terms, sound to me like "you must do it this way or else your room will sound terrible" or "the only way to make your room sound good is to follow this procedure for treatment".

Sigh.. I'm sorry

Maybe I can postpone this thread for now untill I do some measuring and see where and what my problems are exactly, maybe that will help you guys get a better idea on some alternatives for me

Sorry, but there is a thing called physics that determines the objective behavior of sound.

What shortcuts and compromises are acceptable to YOU, I cannot ascertain.
And like another thread where we were supposed to determine what another would "prefer", that is Your job to determine.

Determining an acceptable degree before which an analogous ill fitting pair of shoes becomes unbearable is not a goal to which I have spent my time studying nor aspiring.

Nor does the example of Lee Iococa and the Ford Pinto and the 'acceptable' compromise whereby retrofitting them with a $3 plate to prevent the differential from puncturing a gas tank resulting in the incineration of 'a few' where the calculated losses were projected to be less than the cost of retrofitting the cars seem an solution to be emulated, despite the fact that one hopes that no one would spontaneously burst in to flames due to bad acoustics...

And despite Don Davis' observation that "If bad sound were fatal, audio would be the leading cause of death", it is at least hoped that there are not an excessively large number of fatalities as a result of acoustics.

But you seem to feel the need to make improvements, but for whatever reasons, are not willing to do the job completely. And that is of course one's choice. And a very common one at that.

So why don't you tell us at what point the acoustical behavior becomes unacceptable? And please do so in an objective manner that can actually be gauged. (...and yes, this is a common criterion generally referred to as "proof of performance" in major contractual projects.) If you can do that, perhaps we can suggest the biggest return on your investment in terms of time, effort and money spent.

And despite the acknowledged practical limitations of such issues as thick porous absorption, you don't seem very motivated to pursue the design and/or utilization of more complex alternative products as apparently those are also our responsibility to anticipate and to design as well. But if you are a bit curious, I might suggest a perusal of the RPG website in order to get a pretty good overview of some of that alternative treatments to common porous modal and specular absorption that are available.

So exactly what tradeoffs are you willing to make? Please specify this with objectively verifiable limits. As I suspect few are willing to spend their time designing an amorphously stated moving target where the optimal is eschewed, but for which the problem remains, well, a problem.

Treatments can easily be designed to fall short of the optimal. So, please, tell us in what objectively determinable performance areas and to what objectively verifiable extent is it acceptable for the performance to fall short?

Quite frankly, the 'the optimal behavior as defined by physics is excessive, so what is a median acceptable level of dysfunction' line of reasoning is a bit unusual as a design concept. Except to say that life is full of compromises...

The fact is, it seems that your request is ready made for almost any of the one size fits all solutions that are readily available almost anywhere you look. They all in one manner or another deal with modes and reflections to 'some' degree. Some of them are even very pretty and aesthetically pleasing. And if one installs a popcorn machine that may improve on the acceptability all the more. ...At least one needn't worry about having to listen to their stomach growling...a small but tangible advantage!

The treatments most likely will not be optimal. But hey, if one fails to even take the time to objectively measure the behavior in one's space there is the consolation that one lacks an awareness of exactly what needs to be addressed, and thus any shortcomings are apparently acceptable as at least they lie below an objective awareness. One just needs to decide what price point is acceptable.

I just find it rather schizophrenic for one to complain regarding the existing level of behavior, and yet to also object to objective measures as to how and why behavior can be objectively mitigated.

But rest assured, the industry is literally dominated by those who are more than willing to sell you an incomplete solution. Take your pick. The fact is that deciding amongst them simply due to their sheer numbers will make the choice yet one more dilemma you have to face. ...But at least you can't complain of a lack of selection!

That is your alternative.

But if you can, please generate objective analysis of what is actually happening, and from this we can identify the actual issues and establish a hierarchy of issues that should be addressed, and hopefully you can at least cherry pick the most egregious issues to be addressed.

And lest you fault me for the observation, read one of Toole's own conclusions from the document quoted from a bit earlier:

"However, there is no doubt that the majority of people in the audio business do not know most of the facts and techniques you have just read about. They rely on the roll of the dice, or purely subjective trial and error, to get good sound. Often they do not succeed. This is not an acceptable way to run a business, much less an entire industry.

Given that the room is the final audio component, and it is the one over which the audio manufacturers normally have no control, any improvement is a tremendous asset to both loudspeaker manufacturers, and customers..."
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Old 03-20-2012, 09:37 AM
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"However, there is no doubt that the majority of people in the audio business do not know most of the facts and techniques you have just read about. They rely on the roll of the dice, or purely subjective trial and error, to get good sound. Often they do not succeed. This is not an acceptable way to run a business, much less an entire industry.

Given that the room is the final audio component, and it is the one over which the audio manufacturers normally have no control, any improvement is a tremendous asset to both loudspeaker manufacturers, and customers..."


+1 Scary,...depressing

-----------

I may understand digital chris' conundrum. The acoustic science of audio being what it is, I fully understand an individual approaching potential improvements of known issues, and becoming burdened/reluctant to continue viewing it as an all or nothing proposal. That's a mindset that some possess, and some don't.

I appreciate the kind words, however I still feel like a newbie with regard to my knowledge in acoustics. The cliche "the more I learn, the more I discover how much I don't know" is spot on w/me. The science is there, I've read Toole for decades, Keele, AES papers etc. I've been aware of Patronis and Davis' Sound System Engineering for what seems like forever. I've read Everest's work. Point being, as fractal like as this all seems to me, the science is right here before us. A finite, tangible, comprehensible, repeatable base of work.

As I explained in a previous post, I've been pursuing subjective differences of the various interactions of the boundaries around my mains. I've attempted to apply an effectiveness rating to approaches that change imaging characteristics, clarity, envelopment, response smoothness. I've found that one can clearly learn to hear certain aspects, and with repetition, it becomes quite easy to key in on and hear the characteristic you're focused on. This is what has led me to ultimately decide to entirely rotate my primary set-up in our non-dedicated, family room HT. The proximity of the sidewalls, and closed in feeling of my long standing, shoebox style 25x13x8 room, became quite the obstacle. Despite my various efforts, the sidewall proximity, and a measure of acoustic asymmetry, I decided to explore a temporarily setup things firing into the 13' orientation, instead of the long 25' orientation. Although the LF wasn't optimized, and the ceiling had no treatments, I subjectively fell in love with the distant sidewall openness, even with the detrimental rearwall all too close behind me. We'll see,...huge logistical change for a fully functioning family room. That's the big family system, I've got a nice, fully optimized system for my enjoyment, recordings, editing, etc.

Back OT;

I'm beginning to fully understand dragon's complaints that these issues have been identified, solutions are availed to us, yet even a significant amount of knowledgeable enthusiasts oftentimes overlook fundamental issues, and subsequently their efforts fall short. Bosso makes this very same contention with regard to subwoofer design,....oftentimes stating "we've learned these things, let's push forward",... as others re-hash well vetted, solid work. As I've attempted to expand my awareness of optimizing my sound systems, both professionally and in my home, I'm convinced one big mis-step, that stares all enthusiasts in the face is focusing on the frequency domain, and little attention to the time domain. You're always going to have the new-comers and the basic fundamentals will always need to be explained. But it is troubling to see the consistent herd mentality mistakes, that exist even within the knowledgeable segment of enthusiasts.

Ignorance is bliss. As digital chris discovers more and more about acoustics, to attain the theoretical ideal seems to become less attainable. Compromises, as in loudspeaker design, are key here. Tradeoffs,...that's why I went off on my excursion of matching the measurable theoretical ideal, with what's audible at the LP. If you can't possible have a 4" velocity based absorber, accompanied by a 4" gap, that's ok, do what you can, when you can. The advice about examining RPG's solutions is spot on. Perhaps even more drastic, outside the box approaches, ie., a speaker with additional, tighter pattern control. Perhaps structural, or even temporary reflection/re-direction past the LP.

Don't sweat it,... put it to rest Focus on the recorded material.

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

Brief OCD anecdote;
Forget about it and focus on the material.
Having been to Negril Jamaica several times, my wife and I were attending an outdoor, open air, live music establishment one late evening. Upon learning of my FOH prowess, a staffer at our resort suggested we check out this outdoor club because of their killer sound system. We had prior plans, so we sacrificed significantly to go that night.

We get there, susprisingly, this dude is expecting me and gets us this great spot. The system is the most hodge podge, mis-matched group of boxes, horns, traps, etc that I've ever seen. I was really dis-appointed. I wanted to hear a first rate, "A" rig in this great outdoor venue. These speakers were a joke. Very little matched,...tons of bass bins all across the front of the stage. I kept thinking, what a joke. The background, walk in music sounds good however, and we enjoy some cold Red Stripe. Long story short (very eventful evening) the act takes the stage and begins jamming, at a low level,.... into this sweet, percussion heavy reggae instrumental. Not bad I thought. Then they welcome everyone and launch into their first tune. Wow, I was blown away. Full tilt, extremely loud yet ideally spendid and smooth sound. I was slack jawed in disbelief,...looking again at the PA. Wonderfully full bass, yet perfectly tight. The sound was refined, crystal clear, every detail was fantastic,...I was amazed as it washed over me. Clearly, the system was nearly perfect wrt signal/time alignment. The big, huge reggae bass was perfectly delineated. These players, very good,...a pleasure to watch them perform their craft. Even the vocals sounded very good,....this was a genuine treat. Point being, I simply forgot about the gear and spent the next couple of hours enjoying some of the best live music I've ever enjoyed in my life. Clearly, if you've ever heard reggae music, the rythym section,...the bass guitar and the drums, are often featured prominently. Well, live, it's all about the bass. Fortunately, outdoors, you can get away with huge amounts of bass, without muddying up the rest of the mix. It was huge, full, impactful, tight,....I couldn't believe it, truly speacial for me.

At the end of the show, I had forgot all about the system, and why we had come. Prior to leaving, I spoke with the FOH engineer. He was very proud to tell me,...dramatically saying; "the drivers, all new jaayyy-beeeee-ellll" What a night. Does it apply here, for Chris? Hell I don't know, I doubt it,...I just enjoy sharing that story. Under a starry night, killer sound w/superb bass, outdoor live reggae in Negril Jamaica, right at the waters edge.

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

What you can, when you can.

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Old 03-20-2012, 09:40 AM
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Wow.

DC, space limitations are a real constraint. If you don't have the room, you don't have the room. Membrane absorbers may work more effectively in less space but are more expensive and/or more difficult to DIY. Many companies, including RPG, offer products that may help when space is limited. Ultimately, you will have to decide how much space and budget you can allocate, and then use both measurements and your ears to do the most good you can within those constraints. However short that falls from the ideal just is what it is.

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Old 03-20-2012, 11:07 AM
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Originally Posted by Bigus View Post

Wow.

DC, space limitations are a real constraint. If you don't have the room, you don't have the room. Membrane absorbers may work more effectively in less space but are more expensive and/or more difficult to DIY. Many companies, including RPG, offer products that may help when space is limited. Ultimately, you will have to decide how much space and budget you can allocate, and then use both measurements and your ears to do the most good you can within those constraints. However short that falls from the ideal just is what it is.

I agree with Bigus. Also, in the home theater setting, I think you should concentrate on speech intelligibility. First reflections can make it difficult to understand the dialogue, which is very important when watching movies (most movies anyway, maybe not so much with Arnold Schwarzenegger). The typical human voice ranges from 80hz to 250hz. I would concentrate on making certain you cover that range when considering absorption at first reflection points, if you find that absorption is necessary at all.

"Trying is the first step towards failure" - Homer Simpson

 

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Old 03-20-2012, 08:26 PM
 
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Sound in air does not travel as longitudinal waves but as alternating zones of compression and rarefaction. Particle velocity does indeed become zero at a reflective boundary and pressure reaches its maxima. Stop thinking of sound as equivalent to light waves.

But stopped for how long? How long does it take sound to reach the speed of sound after it stops moving?
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Old 03-21-2012, 09:33 PM
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While I also appreciate the vast amount of knowledge everyone here has, I think there is a little tunnel vision going on. For some there is a desire to make improvements however small they may be within the constraints we all have. Take my situation, it is not budget, but rather the visual enjoyment I get from a nicely decorated room. Isn't this whole thing about enjoyment??? So some of us might spend years making the room as perfect as possible, sometimes forgetting to enjoy the movies and music, because we are too busy thinking about how the room could be better. I am working on the initial stages of my 3rd theater and will certainly have questions, but I think the original thread starter, myself and 95% of owners out there take a balanced approach. I want the room to provide a great picture, great sound, and it has got to be somewhere that is absolutely beautiful and welcoming. If I feel like I am in a room of oversized and oddly placed engineered cubes of fuzz, then I am not going to spend as much time in the room and for me that defeats the whole purpose. So more power to those whose balance takes them in one direction over another, but lets not loose sight of the fact that this should be fun. So if a 1" or 2" panel is all that works in the space and provides improvement then perhaps that is the most perfect of all approaches instead of scaring someone into no improvements.

Check out my whole house build HERE
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Old 03-21-2012, 10:39 PM - Thread Starter
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Originally Posted by BrittonYoder View Post

...instead of scaring someone into no improvements.

Thanks Britton, this is exactly what's happening to me, this whole build is becoming less fun because I'm stressing over all of these things.. It's terrible.

I'm hoping that some soon taken measurements show something to look forward to and that whatever I can fit for treatments will be for the better, and not the worse.
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Old 03-21-2012, 10:49 PM
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But stopped for how long? How long does it take sound to reach the speed of sound after it stops moving?

Particle velocity (actual air molecules in this case) does not move at the speed of sound. The wave does, but individual particles move much, much slower. In fact, inside an idealized room if you were to excite a pure mode along one of the room's dimensions, there would be zones within the room where particle velocity remained zero even for sustained tones.

Don't confuse speed of sound with particle velocity. And I probably misspoke earlier... I was probably thinking of transverse waves. I think Soundwaves in a gas or fluid are actually considered to be longitudinal. In any case, I think you have some persisting confusion about how sound propagates.

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Old 03-22-2012, 08:00 AM
 
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Quote:
Originally Posted by Bigus View Post

Particle velocity (actual air molecules in this case) does not move at the speed of sound. The wave does, but individual particles move much, much slower. In fact, inside an idealized room if you were to excite a pure mode along one of the room's dimensions, there would be zones within the room where particle velocity remained zero even for sustained tones.

Don't confuse speed of sound with particle velocity. And I probably misspoke earlier... I was probably thinking of transverse waves. I think Soundwaves in a gas or fluid are actually considered to be longitudinal. In any case, I think you have some persisting confusion about how sound propagates.

Hmmm....I thought the particles moved along with the wave. That would make a huge difference if they do not.
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Old 03-22-2012, 09:39 AM
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Originally Posted by cybrsage View Post

Hmmm....I thought the particles moved along with the wave. That would make a huge difference if they do not.

http://www.acs.psu.edu/drussell/Demo...ves-intro.html
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Definition of a Wave
Webster's dictionary defines a wave as "a disturbance or variation that transfers energy progressively from point to point in a medium and that may take the form of an elastic deformation or of a variation of pressure, electric or magnetic intensity, electric potential, or temperature."
The most important part of this definition is that a wave is a disturbance or variation which travels through a medium. The medium through which the wave travels may experience some local oscillations as the wave passes, but the particles in the medium to not travel with the wave. The disturbance may take any of a number of shapes, from a finite width pulse to an infinitely long sine wave.

here

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Old 03-22-2012, 10:43 AM
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Quote:
Originally Posted by Digital_Chris View Post

Thanks Britton, this is exactly what's happening to me, this whole build is becoming less fun because I'm stressing over all of these things.. It's terrible.

I'm hoping that some soon taken measurements show something to look forward to and that whatever I can fit for treatments will be for the better, and not the worse.


Digital Chris;
For some reason, hardly anyone here uses ETC as a tool for their room.
Further, besides myself and a handful of people (fotto among them), I've hardly seen anyone post their ETC charts for review/discussion/etc.

Why?
What are people afraid of to show/share?

For me, I posted in this ETC thread http://www.avsforum.com/avs-vb/showt...6#post20766356
Quote:


[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.

So, I can't help you with your decision making just yet.
For me, the 2" thick + 2" air gap definitley (subjectively) improved my L/C/R soundstage vs bare walls.
Truly, I love watching and listening to movies in the HT.
Objectively, data driven, until I take those down and measure the room I can't state the same with graphs/charts.

Possible fotto (Floyd) can chime in?
I believe in his thread he had baseline bare walls + with treatments.

You have measurement mic/etc?
Can you take baseline readings of your set-up as is and post those?
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Old 03-22-2012, 12:23 PM
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Quote:
Originally Posted by Digital_Chris View Post

Thanks Britton, this is exactly what's happening to me, this whole build is becoming less fun because I'm stressing over all of these things.. It's terrible.

I'm hoping that some soon taken measurements show something to look forward to and that whatever I can fit for treatments will be for the better, and not the worse.

Perspective


Like I emphasized in my post above, it's easy to lose the proper context.

Typically, these are our homes. Whether enjoying a sporting event, a concert, a motion picture, or simply listening to music, we need to step back and fully understand that the enjoyment of the content is first and foremost. Either totally enjoying a great recording of your favorite artist, or experiencing an incredibly engrossing movie, enjoyment and the suspension of dis-belief is the goal here for many. One can attain that in a variety of low tech, non state of the art environments.

That said, that's the cake. All these additional details we split hairs about here and elsewhere are merely the icing.


Perspective

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Old 03-22-2012, 08:53 PM
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Quote:
Originally Posted by Digital_Chris View Post

Thanks Britton, this is exactly what's happening to me, this whole build is becoming less fun because I'm stressing over all of these things.. It's terrible.

I'm hoping that some soon taken measurements show something to look forward to and that whatever I can fit for treatments will be for the better, and not the worse.

Prior to the (on going) construction of my new room, I've had three dedicated rooms in rented apartments / houses. All of them benefited GREATLY from 2" treatments stuck on the walls at first reflection points. Totally non-scientific and completely DIY. Don't let the pedantry that's overwhelmed this forum lately scare you off trying a few options and trusting your ears. DIY panels are easily constructed, so start off with a few, experiment with placement and see how you like it. It can be a really fun, eye opening and educational process.

Cheers,
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Old 03-23-2012, 12:16 AM - Thread Starter
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Thank you all for helping me up out of my pit a bit... except for dragon, who makes me want to go back to school just so I can understand his post completely... needless to say I had a hard time reading it and understanding if it was a shot at me and my "complaining" even though that was not the intent of my posts.

One of the flaws in my design was that I tried to keep things simple and tried to design everything before testing... If I did testing before I did the designs, the room wouldn't have turned out the way I wanted, or atleast not as easily (if it were so in the first place)

I designed my panels the way I did after reading a handful of posts by some of the major acoustic gurus that 1" fg will be sufficuient (granted this was in the early stages of the "Acoutical Treatment Master Thread")... Do I regret it? Right now, absolutely, because of all of this data that is now available. I don't want to give up the space, but still want pleasable results.

Again, in the next few days, I will do a number of graphs for you guys to see how my room acts, as is. The only acoustic treatment I have ready to be installed are my superchunk bass traps in the rear corners. I will do a few graphs of before the traps and after and then maybe some ETC plots, although I might need some help with those tests.

Any more advice, please jot it down, otherwise, stay tuned for another thread (including my build thread) for some graphs
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Old 03-23-2012, 12:30 AM - Thread Starter
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Quote:
Originally Posted by mtbdudex View Post

You have measurement mic/etc?
Can you take baseline readings of your set-up as is and post those?

I've got the well known Radio Shack meter along with a SoundBlaster external USB sound card. Should get me going. I should be able to post up a few graphs soon, not sure about ETC though.
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Old 03-23-2012, 09:24 AM
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With respect to ETCs and panel thickness, you will find (at least I have found) that 2" treatment is sufficient to make reflection spikes disappear (ie. be mitigated by 10-15dB) from the measured graph. This IMO is a limitation of the nature of an ETC - it shows total energy across the entire spectrum. This does not necessarily mean that you have absorbed all the low frequency energy with only 2" of treatment. You can frequency window the ETC but the resulting graphs at very low frequencies are a bit bizarre to look at (at least in REW).

Let me add that, subjectively, in my experience 2" at first reflections makes a significant improvement. More is better (to some point where diminishing returns occur, whether that be at 8" of treatment or something less)
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Old 03-23-2012, 11:44 AM
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Originally Posted by Digital_Chris View Post

I've got the well known Radio Shack meter along with a SoundBlaster external USB sound card. Should get me going. I should be able to post up a few graphs soon, not sure about ETC though.

Since it sounds like you have a clean slate to work with, you're in a great spot to play around with REW and try to have some fun in the process, in lieu of letting it beat you down. If you haven't seen my thread where I demonstrated the basic concept, take a look here http://www.avsforum.com/avs-vb/showt...om+measurement
Really not much too it once you grasp the measurement set-up.

If I was where you are now, I'd spend a few hours just taking measurements with no treatments, and track down and mark where your early high gain reflection points are. You can do that with little investment other than some fluffy pink and time. Once you have that accomplished, you can experiment a bit with either absorption or diffusion at the previously marked areas and see how you (and REW) like the results.

On a side note, I have yet to treat my very high gain ceiling reflections!
But guess what....I thoroughly enjoy the audio in my space and have no issue with dialog intelligibility etc. I do however intend to treat that area as I'm comfortable there will be some improvement. But in the meantime, I'm having fun watching movies, listening to music, and not stressing any longer about trying to get to some level of perfection that I may never achieve or even know I have achieved when/if I did. (there IS some message embedded within that last sentence)
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Old 03-23-2012, 11:47 AM
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the Radio Shack is an SPL meter - useful for setting gain levels but not useful for the types of measurements relevant to the context of this thread.

a low-cost option:
http://www.parts-express.com/pe/show...number=390-801
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Old 03-23-2012, 09:30 PM
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Originally Posted by kromkamp View Post

With respect to ETCs and panel thickness, you will find (at least I have found) that 2" treatment is sufficient to make reflection spikes disappear (ie. be mitigated by 10-15dB) from the measured graph. This IMO is a limitation of the nature of an ETC - it shows total energy across the entire spectrum. This does not necessarily mean that you have absorbed all the low frequency energy with only 2" of treatment. You can frequency window the ETC but the resulting graphs at very low frequencies are a bit bizarre to look at (at least in REW).

Good point, and one worth reinforcing. An ETC, for all its worth, is spectrally blind, and since we humans hear spectrum, not peaks on an ETC it's worth in my opinion (though others may disagree) looking at things from a frequency perspective. A complete walk through on this approach and the reasoning behind it can be found in this article.

Advocating 2" treatment for all side wall reflection points is an 'ok' general recommendation, given that most speakers are bad off axis and absorbing the off axis sound improves things. However if you are really aiming for top level performance then you should be measuring things and making decisions based on that.

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Old 03-23-2012, 10:28 PM
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Originally Posted by mtbdudex View Post


Digital Chris;
For some reason, hardly anyone here uses ETC as a tool for their room.
Further, besides myself and a handful of people (fotto among them), I've hardly seen anyone post their ETC charts for review/discussion/etc.

Why?
What are people afraid of to show/share?

Can you take baseline readings of your set-up as is and post those?

Arriving late to this thread.

Interesting questions about why we don't see more measurements posted. I can't answer them, but I posted some graphs and my raw REW files in the acoustic treatments master thread.

I would say the reasons are that people don't have the measurements, or don't find the feedback when posting them useful.

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Old 03-24-2012, 11:05 AM
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Originally Posted by Nyal Mellor View Post

Good point, and one worth reinforcing. An ETC, for all its worth, is spectrally blind, and since we humans hear spectrum, not peaks on an ETC it's worth in my opinion (though others may disagree) looking at things from a frequency perspective. A complete walk through on this approach and the reasoning behind it can be found in this article.

Advocating 2" treatment for all side wall reflection points is an 'ok' general recommendation, given that most speakers are bad off axis and absorbing the off axis sound improves things. However if you are really aiming for top level performance then you should be measuring things and making decisions based on that.

what custom treatments are you designing/utilizing based on band-limited ETC measurements? eg, for figure 5 in the article?
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Old 03-24-2012, 12:19 PM
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Nyal, just read your article and have a couple questions if you don't mind:

1) I'd like to understand how a reflection can be of higher gain than the direct one. I seem to recall this was discussed somewhere but can't find it.

2) From the Benade citation "The loudness of the perceived sound is augmented above that of the first arrival by the accumulated contributions from the later arrivals." Could this be one explanation/contribution as to why some folks can listen comfortably at reference level while others find it unbearable, that is, some have better treatment in place which mitigate the perceived higher volume of the accumulation?
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Old 03-24-2012, 05:21 PM
 
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Thanks!

So the particles basically stay put (vibrating where they are), but the energy wave that makes them vibrate keeps moving. That returns me right where I started, though. The wave that makes the particles vibrate moves at the speed of sound. When it hits the wall, it reflects off the wall in a new direction. This energy did not stop or slow down, it merely changed directions. Why would 1 or 2 inches of air do anything to this energy? Isn't the energy that reaches us what we are concerned about stopping?
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Old 03-24-2012, 05:22 PM
 
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Barring any external mitigating effect, a single reflection from a source cannot be higher in gain than the direct signal itself from which it derives.

A 'reflection' can only be greater in gain if multiple signals are reflected back to the same point in time and sum in phase.

A dome or a concave curved proscenium (so common in churches!)are a classic examples. But (thankfully), in most places (except modern churches where architects seem to LOVE such concave curved surfaces) they are exceptions.


And one needs only deal with band limited issues if the acoustical impedance of the walls or applied treatments are so wonky as to create the spectral imbalance in the reflected sound from a broadband source. And one only needs that in order to identify the problem and to get rid of the offending treatment, or to replace it with broadband treatment that will take precedence over the wonky wall surface.

Otherwise, broadband treatment will effectively deal with any off-axis limited bandwidth component that is incident on the surface and will effectively avoid detrimentally altering the spectra of the indirect signal upon incidence.

In fact, to quote Toole from section 9.2.1 of Loudspeakers and Rooms for Sound Reproduction:

9.2.1 Above the Transition Frequency (The specular regions)

• Steady-state in-room measurements may be indicative of certain problems that are audible, but they are of little use in assigning corrective measures. One cannot separate problems in loudspeakers from problems in rooms, and each requires different solutions. For example, a dip in a room curve could be caused by destructive interference from a strong reflection or standing wave, a dip in the frequency response of the loudspeaker, or a reduction in the dispersion of the loudspeaker. Some of these problems require acoustical or electro-acoustical treatment, and others can be corrected by equalization. Equalization schemes based only on room curves involve a risk that the wrong corrective measure will be applied to a problem.


Meaning you need to know the 3space volumetric power response spatial distribution of the speaker with which you are dealing in order to first identify speaker 'problems'.

And a note about broadband treatment that many would do well to read several times:
Toole from section 9.2.1 of Loudspeakers and Rooms for Sound Reproduction:

• Any device inserted into a reflected sound path—reflector, absorber, or diffuser—should perform uniformly well at all frequencies above the transition frequency region, say, 200–300 Hz. This is in order to preserve the spectral balance of the loudspeakers, to uniformly attenuate the full spectrum of reflections, and to ensure that the precedence effect is maximally effective.
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Old 03-24-2012, 08:35 PM
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Originally Posted by cybrsage View Post


Thanks!

So the particles basically stay put (vibrating where they are), but the energy wave that makes them vibrate keeps moving. That returns me right where I started, though. The wave that makes the particles vibrate moves at the speed of sound. When it hits the wall, it reflects off the wall in a new direction. This energy did not stop or slow down, it merely changed directions. Why would 1 or 2 inches of air do anything to this energy? Isn't the energy that reaches us what we are concerned about stopping?

The wave may propagate at the speed of sound, but individual particles collectively carry the energy. The particles at a barrier do not move but are at high pressure. A little away from the barrier they move a little more and exert less pressure. Flow resistivive absorption requires individual particles to move through or within the material to dissipate their energy. Thus this must be done where they actually move. Per the above, moving away from the barrier increases particle velocity and increases resistivive absorption efficiency. This is maximized as particle velocity reaches a maxima. That is not at the barrier. A few inches do matter.

I do not think I can simplify the explanation any further.

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Old 03-24-2012, 09:06 PM
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The particles in the middle do not move, but they transfer the energy.

http://en.wikipedia.org/wiki/Newton%27s_cradle

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Old 03-25-2012, 03:12 AM
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cybrsage;
For a good discussion on 1/4 wavelength and such, read here
why do we insist max particle velocity is always at 1/4wavelength from boundary?

This is directly related to porous absorbers in acoustic treatment application.
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Old 03-25-2012, 03:50 AM
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Originally Posted by GRBoomer View Post

The particles in the middle do not move, but they transfer the energy.

http://en.wikipedia.org/wiki/Newton%27s_cradle


Here are some good YouTube video lectures from MIT that help grasp wave mechanics.
Good verbal + visual explanation here, the other 2 involve some math....


This one is worth watching, just for the beauty of the derivation...with a chalk board.

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Old 03-25-2012, 05:54 AM
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You can also think about how a certain wavelengths are best absorbed at 1", 2", 4", 6" or 8". Within the first 2" of FG you will absorb the 1" and 2" wavelengths. Then the sound will pass through 2" of air, hit the wall and then pass through 2" of air and hit 2" of FG. The 6" and 8" wavelengths will now be absorbed. Mind you, there will be some absorption at different parts of the wave too and even 1" of FG will absorb *some* low frequencies, but not necessarily be significant.

Better broadband absorption will be had by just going with 4" of regular FG insulation than 2" of compressed FG and cheaper also.

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Old 03-26-2012, 12:09 AM - Thread Starter
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Originally Posted by dragonfyr View Post

Any device inserted into a reflected sound pathreflector, absorber, or diffusershould perform uniformly well at all frequencies above the transition frequency region, say, 200-300 Hz. This is in order to preserve the spectral balance of the loudspeakers, to uniformly attenuate the full spectrum of reflections, and to ensure that the precedence effect is maximally effective.

This is true, if I'm after a perfect sounding room, whether my ears realize that the room is perfect or not, but that is not what I'm going for. I'm sorry, but if all you can recommend and enforce is an all or nothing scenerio, then I'm affraid you are no help to me. If I can find a way to sufficiently attenuate freqencies down to that region with my limited space, I will certainly go for it, but I just don't have 8" of space to play with, nor do I believe that it's necessary when coeficient charts show that 3" of FG on the wall will absorb down to about 250hz.
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