 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Natural Science Forum / Physics / Acoustics / August 2006sound speed in absorption
The speed of sound in a porous absorbing material is lower as in air, AFAIK. Is there anybody who knows if this effect is frequency dependent? Or knows a paper or something about this? Thanks. > The speed of sound in a porous absorbing material is lower as in air, > AFAIK. > Is there anybody who knows if this effect is frequency dependent? > Or knows a paper or something about this? I have some ancient data on sound propagation in fiberglass and in sand (very erratic)... Don't have time to dig it out now, but send me an e-mail with some better details on your needs and I'll respond later.. Angelo Campanella > The speed of sound in a porous absorbing material is lower as in air, > AFAIK. > Is there anybody who knows if this effect is frequency dependent? > Or knows a paper or something about this? > > Thanks. Stuart Bolton of Purdue Univ. has done a lot of research on porous materials. Here is an example of one of his JASA articles: The Journal of the Acoustical Society of America -- March 2000 -- Volume 107, Issue 3, pp. 1131-1152 "A transfer-matrix approach for estimating the characteristic impedance and wave numbers of limp and rigid porous materials" Excerpt from abstract: "The procedure has been used to estimate the acoustical properties of a glass fiber material: good agreement was found between the estimated acoustical properties and those predicted by using the formulas of Delany and Bazley." Acoustical properties may include speed of sound. I have not read the paper. Also, check proceedings of NoiseCon and InterNoise over the past 10-15 years, where he may have published some papers related to sound propogation in porous materials. I don't have a handy link for searching those proceedings. Steve (P.S. - Sorry if this posts twice - wasn't sure if the first mouse click worked) > The speed of sound in a porous absorbing material is lower as in air, > AFAIK. > Is there anybody who knows if this effect is frequency dependent? > Or knows a paper or something about this? > > Thanks. I think the speed of sound lower than in either of the "constructs" (air and absorption material). Can't remember any papers about this right now. Why should the speed of sound not be frequency dependent? Except that in most text books, it is considered/approximated as the speed of sound at 0 Hz. Or is it a question of whether one considers the speed sound being real or complex? One way to look at the case might be to consider absorption of being due to a viscous process which also becomes/is frequency dependent. If I remember correctly, Attenborough (and others) in UK might have done something like this in the field of acoustics. In fluid dynamics similar things exists, for example in Wylie and Streeter, Fluid transients in systems At least mentions frequency dependence. BR, ari Thanks. This question arises from thoughts about aborbing materials in speaker enclosures (virtual volume increase) and for example double wall partitions. We were wondering about effects of altering the air spring, the influence of speed of sound etc. In the literature about walls you read factors from 20 to 40 % alteration of the air spring, in the speaker world we never measure more that 20 %. So we need better insight in this. Thanks again. > > The speed of sound in a porous absorbing material is lower as in air, > > AFAIK. [quoted text clipped - 21 lines] > > ari We know for example that there are adiabatic and isothermal effects wich are of course freq dependent, but were wondering about research providing tables or formulas for this. > Thanks. > [quoted text clipped - 34 lines] > > > > ari Don't know if this far fetched, but... To me a loudspeaker is something like this. It has a vibrating "plate" with not very sound insulation as compared to the rest of the enclosure and possibly an open area (vented ones). Obviously all these things and structures have frequency dependent phenomena. So as a though. Göransson from KTH in Sweden gave a presentation called ADVANCED MATERIALS AND STRUCTURES FOR NOISE CONTROL in Euronoise 2006. Earlier for example Mechel (1980's?) has dealed with optimization on (multilayer) materials. BR, ari > Thanks. > [quoted text clipped - 34 lines] >> >> ari Ben, I think you are now convinced that the speed will be frequency dependent in fiberglass, etc. That will generally be true-by analogy with resistive transmission lines aka telephone lines. Western Electric spent lots of research effort on equalizing the resistive effects out, and it can be done with passive components over a limited frequency range. But I suppose you are interested in solving the problem-so why not use active methods? Such a thing is done in the Isobarik enclosures, where another speaker is driven to make the interior seem (infinite, damped, resonant...pick any one). That is an approach that will let you do it all. Jerry > > Thanks. > > [quoted text clipped - 34 lines] > >> > >> ari Yes, That neat for hifi. For serious speakersystems (PA) that's not always an option. :-) But the question was mend to give better insight in absorbing materials. Also because there are many questions about the behaviour of porous aborbers in rooms and in double walls etc. If you know more about this, you maybe can make better predictions about the behaviour of these constructions. When you see for example tables of measurements or something, you sometimes get a feel for what's going on. Seeing the practical stuff that's written seems to contradict now and then. Bert > Ben, > I think you are now convinced that the speed will be frequency [quoted text clipped - 48 lines] > > >> > > >> ari > The speed of sound in a porous absorbing material is lower as in air, > AFAIK. > Is there anybody who knows if this effect is frequency dependent? > Or knows a paper or something about this? > > Thanks. Yes. The effect is frequency dependent. The lowest speed (usually at lowest frequencies) approach so called isothermal sound speed. Have a look, for example, to Heckl M, Müller H A, Taschenbuch der Technischen Akustik, Springer, 1975, p. 372-382, Tempkin S, Elements of Acoustics, J. Wiley 1981, p.382-389. Zarek J H B, Sound absorption in flexible porous materials, J. Sound Vibr. 61(1978)2, 205-234. Bies D A, Hansen C H, Flow resistance information for acoustical design, Appl. Acoust. 13(1980), 357-391. There are better sources/explanations, but these were the first ones I found from my library. Kari Pesonen Thanks, Kari. > > The speed of sound in a porous absorbing material is lower as in air, > > AFAIK. [quoted text clipped - 21 lines] > > Kari Pesonen > The speed of sound in a porous absorbing material is lower as in air, > AFAIK. > Is there anybody who knows if this effect is frequency dependent? > Or knows a paper or something about this? After reading many posts down the line, I figured what you were asking. When I was hard into speaker building in the early 80's there were things unknown, like the differences between wool, fiberglass, cotton, and Dacron. I started doing my own tests. I also started investigating transmission lines. I could never find any frequency dependant speed changes in the bass region. I found the speed changes much less than common transmission line predictions of others. The more you stuff, the slower the sound. I have also got a 40% increase in apparent volume, under the right circumstances using fiberglass, and polyfill is not as good in doing this. I was reading Bradbury and one other paper, forget the name. greg |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2009 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.