Author Topic: How can a material absorb sound?  (Read 7306 times)

whatismisophonia

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How can a material absorb sound?
« on: January 26, 2012, 02:19:05 AM »
Ive done alot of research on soundproofing, but im still alluded as it's a little complex and people are not always entirely knowledgable or honest -_-  But anyway, Im on the topic of sound absorption.  I know that dense materials block rather than absorb sound because it takes alot of energy to move the particles, and at low frequency the rigidity of a solid thick panel is helpfull, but how does open celled foam or fiberglass work?  Visoelastic substances convert sound energy to heat, resonate at different frequencies than most substances, and i think, appose wave after wave of energy due to a longer time delay in its springyness, and they, like lead, absorb sound energy.  My understanding about foams or other open celled technology is that they work through friction and air resistance (or is air resistance regarded here as the same as friction?).  Is this sorf of like how a gun silencer works?  One final note, i have seen how often times foam is glued to a substrate; Im assuming that this helps the foam to damp the solid material at its surface as the compression and expansion waves attempt to cause the solid material to resonate, but is that the primary way that it helps to reduce echo within a room?  Im asking because, for example, rigid fiberglass is used often in acoustic panels; yet, if its the absorbing ability of the substance with regard to airborn sound that does most of the work, then why is several inches of soft fiberglass in a wall cavity only marginally effective for reducing sound transmission?  Sorry if this is a bit of a big question...

Randy S

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Re: How can a material absorb sound?
« Reply #1 on: January 26, 2012, 05:05:04 PM »
So I have read your multiple posts on our forum, I can see your difficulty in finding peace and quiet due to the condition you stated.
A very Enormous challenge to say the least, there are basic principles applied to sound control in order to address the many variables that are in involved.
1) Decouple from the stcture or vibrating surfaces. This principle is to eliminate or reduce the amount of direct connection for vibration to travel. Now keep in mind that Most All direct connections actually increase the speed of sound your working with. ex; air = 1129 fps   concrete= 10,600 fps wood= 13,000 fps.  This is the path that renders the insulation in the cavity a lessor value than exposed panels to the direct sound source.
2) add mass or barrier, 2 ways to look at this principle is by the mass law or by cavity control in a limpid application(like a diaphragm that can give with LF or SPL).
3) absorption, you always want a cavity filled to prevent standing waves.
4) dampen the final surface to reduce the reverberation time when dealing with LF.

Now that you have control to a degree this is when you see the full benefit of your open cell or fiberglass products which is how you would address the reflection (echo) and Sabin timing of a room.
The value of absorption is viewed as surface area that exposed to sound..now keep in mind this is where everything gets confusing....most panels only slow down the sound waves by milliseconds primarily to avoid the increase of noise that harmonics make in a room...so the amount of exposed absorption material is what silences the sound that is bouncing in a room based on coverage...it does not effect the direct path from source. The more coverage the faster sound is absorbed based on thickness of material exposed to sound waves.

I hope this helps you out to some degree, if you would like give me a call directly and I would be more than happy to spend some time talking with you.

Randy S.
1-760-752-3030 ext 104
 
Randy Sieg

Super Soundproofing Co
www.soundproofing.org
888-942-7723
Ph. 760-752-3030
Fax.760-752-3040

whatismisophonia

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Re: How can a material absorb sound?
« Reply #2 on: January 28, 2012, 02:56:38 AM »
"...so the amount of exposed absorption material is what silences the sound that is bouncing in a room based on coverage...it does not effect the direct path from source. The more coverage the faster sound is absorbed based on thickness of material exposed to sound waves."

Is that how those large triangles in anechoic chambers work?  I was thinking that larger triangles absorbed lower frequencies and smaller ones absorbed higher frequencies; based on what youve said, im guessing that's not the case, though, if it is the case, why? Also, im wandering what the rigidity of the foam, the amount porosity, and material its made of do for different frequencies

Randy S

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Re: How can a material absorb sound?
« Reply #3 on: January 30, 2012, 06:15:32 PM »
remember that the lower the frequency the longer the wave is and therefore requires thicker absorption material to achieve reduction of reflection. the higher the frequency the thinner the material can be...

different density and type of material dictates NRC value
Randy Sieg

Super Soundproofing Co
www.soundproofing.org
888-942-7723
Ph. 760-752-3030
Fax.760-752-3040

whatismisophonia

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Re: How can a material absorb sound?
« Reply #4 on: January 31, 2012, 10:44:32 AM »
" The NRC rating is an average of how absorptive a material is at four frequencies (250, 500, 1000 and 2000 Hz). This rating is appropriate for assessing how well a material absorbs sound within the speech frequencies, but can be inadequate for sound generated by music, mechanical equipment or other low-frequency sounds.
Because this rating is an average, two materials with the same rating might not perform the same in identical applications. - http://www.acoustics.com/ra_nrc_products.asp
 
" just fyi for anyone curious.

Anyway, I found this site, http://www.ethanwiner.com/acoustics.html , to be particularly interesting and full of all kinds of information that bring together alot of the concepts ive been studying; the page is mostly about optimizing sound produced within a space, but the concepts are clearly explained and could be interpreted to understand several soundproofing principles as well.

 

anything