New One on me

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New One on me

Postby Rod Gervais » Thu Apr 29, 2010 3:24 pm

I have never run across this before when reviewing test data for room treatments - meaning a negative result for a particular frequency.......

The product is a simple 2" rigid in a bag.
I am aware of test result for products exhibiting Absorption Coefficients of greater than 1 - but never a negative number at any frequency - which would suggest to me that not only is the trap not absorbing that particular frequency - but is actually adding to it.

1/3 OCTAVE........... ABSORPTION.................TOTAL
..CENTER...............COEFFICIENT.............. ABSORPTION
.FREQ (Hz)............................................. (SABINS)
....63...................... -0.11......................... -7.70
....80...................... -0.44........................ -31.87

Has anyone ever run across this before?

Rod
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Re: New One on me

Postby Scott R. Foster » Fri Apr 30, 2010 6:13 am

Looks like someone thought an absorber ought to subtract - thus they listed results as negative numbers?

Makes sense in a crooked sort of "I'm just making this crap up as I go along" sort of way.
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Re: New One on me

Postby Rod Gervais » Fri Apr 30, 2010 1:38 pm

Tests are from Riverbank Acoustical Laboratories........
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Re: New One on me

Postby Scott R. Foster » Fri Apr 30, 2010 1:48 pm

Got link?
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Re: New One on me

Postby jonessy » Fri Apr 30, 2010 3:42 pm

Negative alpha, as a concept, is not all that strange.
It normally corresponds with a physical phenomena called 'acoustic emission', which happens when some materials start to deform.

But I can't see any particular reason why this would happen to a bag full of fluff, and definitely why at such low frequencies...

Can you link to the report?
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Re: New One on me

Postby Rod Gervais » Sat May 01, 2010 12:36 am

Here is the file:
Attachments
2panelspdf.pdf
(206.45 KiB) Downloaded 208 times
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Re: New One on me

Postby Scott R. Foster » Sat May 01, 2010 3:09 am

a panel resonance?
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Re: New One on me

Postby Rod Gervais » Sat May 01, 2010 4:07 am

Scott,

I honestly have no a clue with this........ although i suppose it's possible - these panels do have a plywood backing to stiffen the assembly....... but even then I don't quite get it - not a good thing anyway you look at it....... I am surprised they don't do whatever is necessary to modify the design to rid them of this...... that 80 Hz number is pretty disturbing.......
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Re: New One on me

Postby jonessy » Sat May 01, 2010 4:51 am

I wouldn't be too concerned about this, as these numbers are at frequencies well below the 'confidence limit' for such testing method anyway.

This merely means that, for some reason, inserting the panels caused the decay times to increase, rather than decrease.
I suppose that if we delve into the testing methodology and calculations we can probably find the mathematical 'catch' that caused this.
But I very much doubt that this is due to some unforeseen physical behavior of the bag full of fluff.
Resonance (due to acoustic excitation, not someone hitting the panels with a broomstick) doesn't seem feasible, as it should have increased alpha rather than decrease it.

Surely, I could be wrong too...

If you want I can ask some of the hotshots profs at Uni.
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Re: New One on me

Postby bert stoltenborg » Sat May 01, 2010 9:50 am

Where's that Belg when you need him?

I'll ask our oracle, he's active on some dutch fora.

:D

http://www.geluidforum.nl/viewtopic.php?p=2910#2910
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Re: New One on me

Postby Scott R. Foster » Sat May 01, 2010 11:28 am

Naaa Jonessy... I can't buy that its simply math voodoo, they can get reasonably accurate measurement data down to 80 Hz in that room... something weird is going on like energy from other frequency bands being broadcast back in the 80 band. I suspect that plywood backer is being excited by harmonics... but I await the curmudgeon's opinion with baited breath!
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Re: New One on me

Postby Eric.Desart » Sat May 01, 2010 2:05 pm

Damned Bert waked me from hibernation.

That's a bad measurement.

Scott knows I'have seen it there.
When your absorption becomes very low then you can have following effects:
Your absorption measurement is always a comparison between the empty room (air and boundary absorption) en the room with sample.
When both values are close together they can cross one another causing these negative figures.
If now something happened with either the measurement of the empty room, or the room with sample, or both, then exactly in this area with very low meterial absorption can show ugly things.
An an error could have happend as well as e.g. forgetting to activate the rotating diffusers which can cause model effects to intervene.

Scott you know we had a comparable problem there which was corrected afterwards by redoing measurements.

1) A measurement was made with some strange low-frequent behavior.
2) I confronted that lab guy with it and he redid that measurement which looked (in these lows) quite different.
3) That lab guy asked: from which of both do you want the report? And I said: in fact none of both, we want a correct measurement (I didn't feel save with this deviating data), and also you can't tell what happened with these. I even laughed making clear that I didn't like to gamble about lab data.
4) Then The director there, knowing my comment was right offered to redo the same measurement on their own expense for the 3rd time (which was done then).
:mrgreen: Hence, we had 3 measurements for the price of one. That 3rd measurement confirmed (+/-) the second telling that indeed the first was bad.
What possibly happened (but nobody was sure) that with one of those, the diffusers weren't turning.

For that measurement here by Rod, I should redo such a measurement to find a possible cause. Whatever effect occurred it's just gambling like this. As a lab itself it should even be interesting to go further in depth.
I also don't discuss the lab itself here.

Rod, small negative numbers where the absorption is very low, in the low frequencies is not that abnormal. In other labs the software will just prevent such things to appear on the outprints. In this case (your report) however this is too jumpy (and negative) for my taste en points very possibly to some problem not related to the absorptive properties in itself. Which can as well be caused by the empty room measurement as the measurement with sample, or a combination of both.
Take into account that an empty room measurement happens independent from the sample measurement, at another time, and if some things happened with a different rotating mice position.

This does not means that the lab isn't good. Things can happen .... and basically they are accredited down to what? about 80 Hz?
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Re: New One on me

Postby bert stoltenborg » Sat May 01, 2010 11:38 pm

Eric still rocks.
:mrgreen: :mrgreen:
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Re: New One on me

Postby Scott R. Foster » Sun May 02, 2010 12:08 am

Hi Eric:

Yes, of I recall that erroneous measurement. It was pretty obvious something was wrong - and it was masterful the way you calmly handled the staff and got the problem fixed [a free redo].

That said, it boggles the mind that in this instance the same thing happened, but no one noticed and thought to take another shot at the measurement. If that is what happened, it would certainly explain the result. And, if that is the case, then shame on the lab for publishing the result as though it were valid.
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Re: New One on me

Postby Rod Gervais » Sun May 02, 2010 1:00 pm

Eric,

Thanks for the response my friend - and I do understand.

Scott, you make my point for me - such a thing should never make the public eye........... I have to wonder about the manufacturer allowing this to rest on the face of things without straightening this out- it is the sort of thing that would give me cause to pause as far as using their product.........

Rod
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Re: New One on me

Postby Eric.Desart » Sun May 02, 2010 3:11 pm

Hi Rod,

Towards the manufacturer I don't feel this as a problem, in this that he just publishes what he has.
As a lab I should have more problems. However that COMPLETE report clearly states what the official range is and whatever outside this range is only for info.
When interesting enough the manufacturer can indeed go more in-depth to investigate this low frequent behavior. This costs money and basically what you see within that range is rather logical and dives down from +/- 400 à 500 hz and down.
What further happens are the effects of the panel acting as a membrane of panel-damper which is mounting sensitive anyhow.

As a manufacturer I shouldn't show this data just to prevent such questions based on uncertainty (that's not a matter of hiding stuff anymore). Better of course is investigating and understanding (but that costs money). It's typically not a panel for low-frequent use, and when one wants to use it as a combined panel-demper it calls for much more investigation to find the mounting dependency.

As a lab I should like to know the why, but also here the lab describes the limits of the report. Is it better that the lab just removes it? It makes no part of the official data range.
For being honest it's hard to blame a manufacturer, and it's good that questions as these arise as well to show that these lows aren't that easy and that for reading a report it's good to extend one's background.

I'll tell a nameless secret here. Someone (not ready acoustics here) did a measurement. It showed a 63 Hz value (and lower) which could be used against them by unwilling competitors (making it a hobby emphasizing other's assumed weaknesses and hiding/disguising their own). Clearly this value was some kind of measurement limitation of the lab.
The person in question wanted to publish that measurement after removal of these values from the official report.
But you just can't alter an official report because it looks better in the altered manner preventing difficult questions and annoying remarks (authorship, copyright).
In this case I advised to contact the lab itself which indeed confirmed these numbers and related lab limitations, agreeing that in the explained case the report could better be used removing these numbers confusing layman not used in interpreting such data (and gladly suggestively pointed by competitors). Hence they officially gave permission to alter the report by removing this data possibly misinterpreted by other parties.

So what is better? Showing what is, including question marks, or just preventing the questions possibly being asked?
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Re: New One on me

Postby Scott R. Foster » Sun May 02, 2010 4:12 pm

So what is better? Showing what is, including question marks, or just preventing the questions possibly being asked?


Eric:

I realize your question is somewhat rhetorical - but just for kicks:

When you get squirrely results, ask questions and retest until it aint squirrely cause you know the measurement is as accurate as context allows. There are lots of ways to mess up a measurement - if you get an apparent anomaly retest to confirm this fact and discard. Move forward with data you trust. Just as unfolded in the incident you describe above.

In the end, show data that is honestly representative of your team's ability as scientist to measure the phenomena, together with calculations of uncertainty and detailed information on mounting. just as was produced from the suite of experiments you mentioned.

As to the folks with weird out of bounds data dumping some of the numbers - I have no problem with data of high uncertainty being left out of marketing literature or graphics... and though I would prefer this was not discarded from lab reports I can accept that such could be done honestly due to uncertainty [after all, at some point it is not really data as the uncertainty becomes quite high - in that case its just a number, not a measurement].

my $0.02

PS to Rod: I agree - am still wondering about that -.44 at 800 Hz with you. Doesn't seem right unless there is in fact a panel resonance as we have pondered - in which case see comments above in regard to mounting details - this info would help the user understand what he is looking at.
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Re: New One on me

Postby Rod Gervais » Sun May 02, 2010 10:26 pm

I don't have a problem with the data being in the report if the data included the level of uncertainty as a part of the equation.......

I really need to spend some time thinking this over - and (just to be thorough) I am going to question the manufacturer on the issue......

Thanks for all of your inputs - you observations and opinions are highly valued here - with a certain oracle sitting at the top of the list.......

Rod
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Re: New One on me

Postby Scott R. Foster » Mon May 03, 2010 4:08 am

Well... hats off to the honest manufacturer who published the data... but Bert doesn't trust it and so neither do I.

I suggest a redo on that one - or at least more information about how the test was done.
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Re: New One on me

Postby seb » Tue May 25, 2010 12:19 pm

Just came across this thread and found it rather illuminating. In Australia we have very few acoustic labs but test anomalies pop up from time to time and I am always slightly amazed at the lack of technical understanding shown by some lab staff. Often, unfortunately, it appears they see their job as to move microphones around and write down numbers. Next time I come across this I hope I can be as calm and effective as Eric was.
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