What is a MEMBRANE ABSORBER?

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Postby Scott R. Foster » Mon Aug 07, 2006 7:12 am

While this thread has been interesting, it hasn't caused me to come to consider the question of what a membrane absorber an occult issue.

In fact, this discussion has firmly convinced me that the question is no more complicated than I first thought it was.  When you really get down to it, membrane absorption is an action - a way a device or material may behave - and one that can arise in a number of types of materials and devices. Thus any definition of a membrane absorber has to key on a description of the behavior - key on the materials or a particular morphology and you yield a description that will be to specific to be correct.

A pretty common thing really - there are for example more than one ways to skin a cat - as the saying goes.  But that does not make it hard for reasonable men to agree of what a skinned cat is.

The only really interesting point made so far to me is that sometimes membrane absorption behavior pops up in surprising places [Eric's definitions of the term point this out - for me anyway] - so the "common wisdom" of what a membrane absorber is may at times call for special insight into how a particular device or construct might behave in this way.

Nonetheless, exploration of all the ways membranic action might arise in some element of acoustics has done nothing to make the term "membrane absorber" confusing for me, and I doubt that is going to happen in future - unless the person using the term intentionally chooses to cause confusion.  

Avoidance of using the term in a misleading way strikes me as a very easy thing to do - shame on those who don't.
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Postby Eric Desart » Mon Aug 07, 2006 7:34 am

Scott,

This is a very nice post.

Eric
.
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Postby Zaphod » Mon Aug 07, 2006 10:44 am

Indeed, Scott, good post.

but...
hands off from that poor cat  :evil:
:mrgreen:
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Postby bert stoltenborg » Mon Aug 07, 2006 11:22 am

[quote="Zaphod"]Indeed, Scott, good post.

but...
hands off from that poor cat  :evil:
:mrgreen:[/quote

Yeah, what's it with Americans that want to skin cats and beat dogs.....


Watch this, it's so funny. Come here, come on. Heh, heh, good dog, red rocket, red rocket, good dog, red rocket, red      rocket, come on.
Cartman, what the hell are you doing?
I'm milking the dog. They make dog milk.
No they don't.
Yeah they do.
Yeah, just hold on a minute. The fifth graders showed us how to do it. Aha.
Woah, cool.
That's awesome.
I told you guys.
I had no idea, dogs made milk. Do it again.
Dumb ---, you can only milk a dog once every few hours. It doesn't work if you beat off a dog again, right away.
You beat off?
That's what it's called when you milk a dog. Beat it off. Don't you guys know anything?
Wow, you learned all this from the fifth graders?
Yeah, guess they thought we were cool, so they showed us how to do it. Hey, come here dog. Dog, come here, heh, heh.]

:mrgreen:
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Postby Rod Gervais » Mon Aug 07, 2006 12:26 pm

Scott,

Great post (as usual)

Joel,

If  I want to purchase room treatments - and I consider yours and some other company's - in the end - it isn't going to matter to me what you call your treatment - in fact - 99% of the people who buy treatments have no interest in what they are - how they work - what type of mechanisms alter the sound in their room..........

All they care about is that the remedy will fix the illness - the cost per unit to make that happen - and how well they fit in with the room decor...........

I have a lot of work under my belt - and I know that the owners eyes glaze over the minute I have to get technical (which sometimes makes answering a question they ask difficult)..................  all they want as the answer is specifically what is the fix...

If I tell them it's a membrane absorber - they  look at me like I had 2 heads..........  If I told them it was your "sound traps" they'd smile and say "get er done".............

Sincerely,

Rod
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Postby Scott R. Foster » Mon Aug 07, 2006 3:11 pm

...how well they fit in with the room decor...


So true - Rod!

Looking cool should never be underestimated.
SRF
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Postby Scott R. Foster » Mon Aug 07, 2006 3:32 pm

Collateral point on membranic action... I stated early in this thread that when you choose a panel over a wedge for a corner mounted absorber - you get more 100 Hz peak.  In other words, panels are more peaky.

But in a recent conversation over at AVS

http://www.avsforum.com/avs-vb/showthre ... 944&page=2

we take a closer look at the two - and try using an offset factor try to arrive at a rational overlay point for the two devices for comparison of "peakiness.  Now, I have to admit right off that I know this is a judgement call, but when I look closely, I am more and more convinced the panel - assuming the use of 703 - is actually less "peaky" when the concept is meant to be consider in the overall absorptive propertis of the device.

When you overlay at 500 Hz the graphs of the SCA and SuperChunk as shown below you get a nice collapsing to congruency at the 500 band and above, and lo and behold, the panel seems to me to have sharper and modestly lower peak than the wedge.

Image

Thoughts???

PS: If you run the overaly at a lower band, then the devices seem about the same, lower still, and the panel gets more peaky.  Is it more correct to say they are about the same?

Image

Image
SRF
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Postby Eric Desart » Mon Aug 07, 2006 4:04 pm

Scott,

Unless more in-depth investigation it's difficult to tell.
But I find this comparison not completely correct.

The angle of the higher frequent curve (going more down) shows the better impact of the edge effect which points to better low frequent absorption of the velocity friction loss effect.

Can you try putting this curve in % absorption taken the top at 100 % for both?

This absolute comparison for me gives a somewhat wrong picture.
Just calculate a straight line fitting between both and you get another picture.

You can do that in Excel with the trend lines.

Image

Image

And seeing the shape of the peak I assume the SCA will be higher than those numbers than the same effect for the SSC.
But that's all a bit gambling.  My belly tells the SSC is better

This more declining curve of the SSC (less flat than SCA) shows the higher influence of the edge effect which shows a better low frequent absorption in the "above resonant range" for the SSC.

I also assume that the peak of the SSC is better controllable with lower gasflow resistance without significant loss in overall absorption.  It will be less sensitive to the use of arbitrary mineral wool.
But I DO agree, this is my belly speaking. My hands only type it in. I've nothing to do with it.
For me personally, with what we know now it's simple. If budget isn't a limitting factor, or the additional work, I clearly go for the chunks.  And personally, if possible I rate the relative importance of cost versus the impact on a project.
But I DO agree that when cost is a limitting factor the SCA is the way to go (price + energy per sabin much better).

I also must admit that I have other measurements which I can't show, which support my ideas.  As you can see in Rod's book the LENRDs don't show much peak. The RealTraps LENDR measurements a bit more. But I know others too.

For me both trendlines in the above pictures are in favour of the SSC.
.

Eric.
.
Last edited by Eric Desart on Mon Aug 07, 2006 5:07 pm, edited 3 times in total.
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Postby Scott R. Foster » Mon Aug 07, 2006 4:53 pm

Ahhh...

Ok, I'll take a look at that.
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Postby Bob » Mon Aug 07, 2006 5:37 pm

I thought the depth of the dip was at least as important as the height of the peak.
i.e. what we want is smooth absorption from frequency to frequency (monotone increasing).
And based on that criteria the superchunk is better.
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Postby bert stoltenborg » Mon Aug 07, 2006 5:56 pm

When you have a typical home recording control room, untreated, does it have a lineair RT time?
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Postby Scott R. Foster » Mon Aug 07, 2006 6:49 pm

Bert:

No.. but peeps don't mind the LF being a bit longer.  In fact I think we expect and enjoy longer RT for lower frequiencies.

But adding a typical Broadband style treatment scheme isn't going to make the "lower freq. = longer RT" go away. It can smooth out the modal peaks but unless you pull a full "Woodlock" you aint going to yield a flat RT line across the band.

Trying to mix a tight interleaved kik thump in with a slick full sounding electric bass line is a bitch in an untreated small room, but I don't think it is RT related. In my experience, it is the small room's sharp modal peaks that are bothersome - not the trend that lower freq.s have longer RT's.  Smooth the modal peaks a bit and LF clarity greatly increases - no need to pursue a flat RT IMO.

Bob:  I think you have put your finger on the thing that hurts.  We don't have the two curves to compare [velocity versus pressure] for the two devices.  I understand Eric's comment about "more testing" to mean that we really need to gather these elements separately if we want to make firm statements about "peakiness".  We can look at the trends as the Bleg Belly advises - but in the end we desolve to guesswork unless and until we can compare the MS element of the panel behavior to the MS element of the wedge behavior directly and outside the influences of the wavelength behavior.


My $0.02
SRF
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Postby bert stoltenborg » Mon Aug 07, 2006 10:14 pm

Hey Scott,

you catch my drift.
Only nagging for the theoretical part.
:D
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Postby jonessy » Tue Aug 08, 2006 2:49 pm

Scott R. Foster wrote:No.. but peeps don't mind the LF being a bit longer.  In fact I think we expect and enjoy longer RT for lower frequiencies.


According to Beranek, at least for concert halls, the RT should be a bit longer for LF.
He describes it as a parameter called 'Bass Ratio' (BR), which measures the ratio between LF and LMF.
I have the formula somewhere in my library.
Should anybody care about this, I'll look it up.

:)
Jon.
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Postby bert stoltenborg » Tue Aug 08, 2006 2:53 pm

Hey Jon,

I was reffering to the fact that a treatment with perfect linearity applied to a non'lineair room will not automatically mean good treatment.
You know you shouldn't take me serious.
:D
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Postby Rod Gervais » Tue Aug 08, 2006 3:43 pm

jonessy wrote:
Scott R. Foster wrote:No.. but peeps don't mind the LF being a bit longer.  In fact I think we expect and enjoy longer RT for lower frequiencies.


According to Beranek, at least for concert halls, the RT should be a bit longer for LF.
He describes it as a parameter called 'Bass Ratio' (BR), which measures the ratio between LF and LMF.
I have the formula somewhere in my library.
Should anybody care about this, I'll look it up.

:)
Jon.


Also,

According to the ITU the formula would be as folows:

The average value of reverberation, Tm, measured over the frequency range 200 Hz to 4 kHz should be:
Tm  =  0,25 (V / V0)1/3               s
where:
V : volume of room
V0 : reference volume of 100 m3.
The tolerances to be applied to Tm over the frequency range 63 Hz (see Note 1) to 8 kHz are as follows:

(this is specifically for small rooms -For multichannel stereophonic reproduction: rooms sized 30-70 m2)  

From 63 Hz to 200 Hz - a sliding scale from +0.3s (@ 63 Hz) to +0.05s (@ 200 Hz).

From 200 Hz to 4000 Hz - +0.05s to -0.05s -

From 4000 Hz to 8000 Hz - +0.1s to -0.1s.

They do not list a tolerance above 8000 Hz.

This is taken from "RECOMMENDATION  ITU-R  BS.1116-1*" - dated 1997.

Sincerely,

Rod
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Postby Zaphod » Tue Aug 08, 2006 6:25 pm

If you follow the tolerancy masks you find here (click on the first link):

http://www.elac.com/en/products/caraquickb.html#

Flat is always ok, higher RTs for LF is ok, lower RTs for HF is ok.
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Postby Scott R. Foster » Tue Aug 08, 2006 9:54 pm

Z:

That's one spiffy site!
SRF
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Postby Joel DuBay » Sun Aug 13, 2006 2:44 am

Rod Gervais wrote:Scott,

Great post (as usual)

Joel,

If  I want to purchase room treatments - and I consider yours and some other company's - in the end - it isn't going to matter to me what you call your treatment - in fact - 99% of the people who buy treatments have no interest in what they are - how they work - what type of mechanisms alter the sound in their room..........

All they care about is that the remedy will fix the illness - the cost per unit to make that happen - and how well they fit in with the room decor...........

I have a lot of work under my belt - and I know that the owners eyes glaze over the minute I have to get technical (which sometimes makes answering a question they ask difficult)..................  all they want as the answer is specifically what is the fix...

If I tell them it's a membrane absorber - they  look at me like I had 2 heads..........  If I told them it was your "sound traps" they'd smile and say "get er done".............

Sincerely,

Rod




Indeed, and this has been my personal experience as well. I try to avoid such terms at all costs when speaking with a new client, even if I have a hunch that they are somewhat educated on the subject. (I can see it now!).

However, this does not deter me from wanting to know what the term "should" say about the device, and perhaps now more importantly, the behavior. But in the end, the "word" membrane conotes a tangible piece of matter, hardware, etc. So the challenge for me has been to answer the question: "Is your panel a membrane absorber?" and since it contains no physical membrane, (mamybe save for the fabric itself) I have not been able to answer "yes" in an educated and definitive way, even if my particular device exhibits some membranic behavior consistent with that of a true "limp mass" membrane. (I won't open that can of worms by the way).

But to be frank, explaining membranic behavior to a semi-novice (I am slowly counting myself out of this group) surely follows by the glazing over of eyes you mentioned. So, when asked the questions, I now have to reply, "well, there isn't clear definition of what that is", yet.  "But...."


Great discussion. I have learned more about this topic. And I am certain other have too.
is that not the point?

I dare say we'd agree it is.


Cheers,
Joel DuBay

Shshshshshshshshsh......, smell that?......
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Postby Rod Gervais » Sun Aug 13, 2006 2:04 pm

Joel,

I agree 110% - in the end - learning is what it's all about.

Rod
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