Quote:
Originally Posted by
arnyk /forum/post/21953065
The comparison fails on several grounds.
(1) My comments related to this application and this application only:
Critical points:
(1) This application involves home thermal insulation which has a typical density of 0.4 to 0.6 pounds per cubic foot.
LOL!!!! Let's see, is that mass comprised of one large strand? Or is that mass comprised of thousands of smaller strands? Mass simple tells you how much of something there is, it tells you NOTHING about how the mass is distributed.
Quote:
Originally Posted by arnyk /forum/post/21953065
(2) This application applies to an absorber whose average depth appears to be about 5 inches.
Let's see. A Superchunk style absorber, being 24 inches on each wall boundary and 34 inches on the face - which is typically covered with a minimum of 6 mill plastic in order to retain specular frequencies above 600 Hz in the room, but which can easily be covered by luann or MDF in order to render is more effectively as a bass only low frequency absorber.
The maximum normal depth of the Superchunk absorber using those dimensions is 17 inches. Ironically, that is less important than the effective width of the material.
The irony is that perforated metal covers can easily be used to extend the mid frequency effectiveness, as is so commonly done in industrial application where resiliency is required.
Quote:
Originally Posted by arnyk /forum/post/21953065
(3) This application applies to an absorber that opens up into the space it serves through an apparently acoustically transparent grille cloth.
First, the presence of a perforated membrane is moot as the configuration would be equal for both the compared PF3350 and the OC703 materials.
And furthermore, if one were knowledgeable about the use of perforated materials and their very mature use in noise abatement, one would know that they are effective as mid-high frequency absorbers. And technically, a cloth is a perforated membrane as well. Again, "acoustically transparent" is a misnomer, as it is only pertinent to the degree that it significantly affects the behavior of sound at frequencies of which we are concerned. It does NOT mean that it is truly acoustically transparent at ALL frequencies. Damned that fisiks stuff.
But as we shall see, the perforated covers were not used for the advantages they can offer in terms of increased mid and high frequency absorption. Instead they were used to address matters of resiliency! And the issue of their negligibly small contributory effects with regard to absorbancy are noted in the conclusion as well.
To quote from the oft ignored Acoustic Absorbers and Diffusers:
"Often porous absorbers are covered by a thin membrane; this might be achieved by wrapping the material in thin plastic or similar. This is done to prevent the absorber being damaged or to stop fibers from the absorber being lost. The effect of the membrane will be to reduce the high frequency absorption."
But in any case, their effects would be the same, regardless of the materials chosen, materials whose own effectiveness was rather radically different - which speaks to the very point of the thread and the critical causal nature of the effect of gas flow resistivity on the issue rather than density - ironically, a measure not mentioned once in the paper.
Quote:
Originally Posted by arnyk /forum/post/21953065
The Ames research paper exists in several forms, more complete versions of which reveal the following significant differences:
Hmmm... I am only aware of it in its complete form. Perhaps that is why some might be aware of the effects of gapping and the effects of the perforated panels that contribute only structural resiliency unlike others commenting on the paper who are obviously unaware of the facts. I might suggest referring only to the
complete paper, as apparently much of the substance was missing in alternatively referenced copies whereas the complete paper more than adequately addresses the points raised here.
Quote:
Originally Posted by arnyk /forum/post/21953065
(1) As mentioned above the thickness of the Ames absorber was about twice the depth of the absorber I commented on.
Yes, it was 8 inches with a 2" gap, which, as they observed, performed equivalent to a 10 inch porous absorber. A fact clearly stated in the conclusion.
The maximum effective depth of the Superchunk is 17 inches, and as mentioned, the more critical dimension is the effective width, of which the maximum face is 34 inches.
A point superfluous to the absorptive capacity measure of the porous material itself, assuming it is sufficiently large to be effectively 'seen' by the wavelengths in question. And since 100 Hz is 11.25 feet long, and 1/4 of that equals 2.81 feet or 33.72 inches. And as a material must be at least 1/10 the wavelength to have any effect (p.131 of AA&D 1st ed.), that corresponds to 13.5 inches....
One should note that the lower in frequency that one desires to treat with porous absorption, the larger the treatment must be in order to address the physics of longer wavelengths, right? Too small and the sample is not even effectively 'seen' by the wavelengths in question, and the energy simply diffracts around it as if it were not there. Thus, if the sample is too small it really doesn't matter what material is used. I am assuming that anyone worried about this issue is cognizant that this condition be met...
Quote:
Originally Posted by arnyk /forum/post/21953065
(2) A little research suggests that the PF 3350 material Ames used has a density of about twice - more specifically about 1 pound per cubic foot.
Only someone preoccupied with an inconsequential measure would go to so much trouble to try to extrapolate inconsequential units which are not even addressed in the paper!
They utilized a material whose gas flow resistivity was 4100 mks rayls, as compared to 27,000 mks rayls. Roughly 1/7 the GFR. And the amount of material was sufficient to be 'seen' by the wavelengths at issue.
If one wants to misuse the density figures, so be it. Assuming a linear relationship, if OC703 is ~3 pounds/unit measure, then the PF3350 was roughly .43 pounds/per unit volume. ...As if the density has necessarily anything to do with gas flow resistivity give the differences in mass distribution within the material structure! ...Its that fisiks stuff again...
Quote:
Originally Posted by
arnyk /forum/post/21953065
(3) More complete versions of the referenced paper show more details about the Ames wind tunnel application. Please see:
http://ntrs.nasa.gov/archive/nasa/ca...1988003624.pdf Figures 1 and 2.
I see no effective comparison between the two applications. The Ames wind tunnel application involved multiple layers with spacing between the layers and was covered by a perforated metal cover, not an acoustically transparent grille cloth.
Multiple layers?
It involved ONE layer of porous material and a 2 inch gap of air. The effects of the perforated membranes is discussed at length in the piece, and you can refer to their conclusion for a well stated synopsis of their relative lack of acoustic pertinence here, whereas it was actually employed for mechanical resiliency.
The fact that no comparison between basic material absorption characteristics and the fact that such a causal comparison is dependent upon gas flow resistivity is quite apparent.
We have already addressed that issue of the gapping (which
should be well understood even here on this forum by now) and the effects of perforated material; but apparently some still lack an awareness of such issues as the effects of spacing porous absorptive material from a wall - given its near free lunch effective attributes where the advantage is stated rather clearly in the report.
But rather than explain them once again, here they are in a very neatly summarized form as quoted from the Ames document:
The conclusions can be summarized as follows:
Sound absorption in the low to mid-frequency range can be increased by the use of PF 3350 fiberglass (nominal flow resistivity 4100 mks rayls ) instead of Owens Corning 703 fiberglass (27,000 mks rayls) and by an increase in lining thickness from 6 inches to 10 inches. When PF 3350 fiberglass is used, the acoustic absorption coefficients predicted for 10 inches of material are essentially the same as those for 8 inches of material plus a 2-inch air gap.
The presence of the fiberglass cloth improves the sound absorption of the lining, but there appears to be reasonable latitude in the choice of flow resistance of the choice of the flow resistance of the cloth.
Increasing the thickness of the perforated plate from 22 gauge to 12 gauge, increasing the plate open area from 33% to 40%, or introducing a perforated backing plate has only a small effect on the predicted sound absorption coefficient.
It is recommended that the perforated plate be thicker than 22 gauge for structural reasons.
The installation of a wire mesh screen between the perforated face plate plate and the fiberglass cloth is recommend for erosion protection. The mesh should be sufficiently open that it does not effect acoustic performance of the lining.
Representative characteristics of the wire mesh screen are given in Table 1.
In fact, I would suggest folks take a look at the figure on pages 36-41 and note that the performance of the perforated plate varies only slightly among the various sample changes in the mid-high frequencies. Again, a fact one would easily know if they were familiar with the mature design and use of perforated materials for noise abatement in industry.
AN ANALYSIS OF SOUND ABSORBING LININGS FOR THE INTERIOR OF THE NASA AMES 80x120-FOOT WIND TUNNEL
Quote:
Originally Posted by arnyk /forum/post/21953065
Nice try guys, but no cigar!
Seeing as we do not smoke, a good thing. But I might suggest others lay off smoking stuff until they gain a more complete understanding of the causal relationship between gas flow resistivity and absorption, the behavior of perforated plates for use in noise abatement and control, and jettison the notion of density, which only accounts for how much stuff is available per unit volume but utterly fails to account for the critical factors of how such mass is distributed within said unit volume - unlike gas flow resistivity which via its focus on porosity and tortuosity, and their effects on the acoustical impedance of a material, does.
Do us all a favor, and before trotting out another ill-formed case for density being a determinant causal basis for evaluating the absorptive behavior of porous material, PLEASE read
Acoustic Absorbers and Diffusers and learn a bit about the physics of how the material actually works. And then, if one is really brave, they can actually access the various studies and models upon which the prediction and modelling of the materials performance are based.
This topic is done.