[Snovosel]

I'm thinking of purchasing fiberglass duct board to place on the bottom half of my walls (below the chair rail) but I don't know how to rate the products. Most of the specs are displayed according to sound absorbtion coefficients that compare the thickness of the product to the frequency.


The frequencies listed start at "125" (Hz), and proceed to "250", "500", "1000", and "NRC". The respective sound absorbtion coefficients are: .07, .25, .63, .90, and .70.


How do I translate these numbers into something that's meaningful?


Thank you.


SteveN

 

[mysphyt]

The numbers listed are actually the absorbtion coefficients, they represent the fraction of energy, at the listed frequency, which is absorbed on striking the surface of the material. The Noise Reduction Coefficient (NRC) is the average of these numbers over the frequencies given (measured at frequencies of 250, 500, 1,000 and 2,000 Hz).


How is this meaningful? They show how effective the absorber is at certain frequencies. The absorbtion coefficients are actually more meaninful than the NRC because they show more detail. NRC's are only good for broad comparisons of materials.

 

[Ted White]

And if you like, Chris can do a model response of your room or an actual session to optimize the acoustics of any room. Pretty cool stuff!


Ted

 

[Patrick C]

If you do a search, you'll also find a spreadsheet from pablo which will permit you to use these #'s to help obtain a good rt-60 across the spectrum.

 

[Snovosel]

I take it that RT 60 is what one hopes to achieve?

 

[mysphyt]

RT60 is the amount of time that it takes for a sound to decay to 1/1,000,000 of its original intensity which is the same as -60dB. The RT stands for reverberation time, so RT60 is the amount of time that the sound will reverberate (bounce around) until it decays to -60dB.


RT60 is different for different frequencies. It is often shown as a curve like this one:
http://www.cmcpics.homestead.com/rt60.jpg


A good RT60 measurement for a home theater is less than 0.5 seconds over the critical midrange, with a goal of 0.35 to 0.25 seconds. RT60 is not terribly meaningful for low frequencies in small rooms where the response is dominated by resonant modes.


The absorbers you are thinking about adding will contribute to lowering the rooms RT60, the frequecies with the highest coefficient of absorbtion will be most effected. Normally a combination of absorbtive, reflective, and diffusive surfaces is required to acheive a proper RT60 without making the room too "dead"

 

[Snovosel]

So, Chris, as I understand you, the duct material that I referred to absorbs various frequencies across the spectrum differently. And as stated, 7 percent of 125 Hz gets aborbed, 25 percent of 250Hz gets absorbed, etc. Therefore, depending on my specific room acoustics, I could modify as appropriate?


Now to put my original question into a broader context, am I creating any potential issues, acoustically speaking, by using the before-mentioned duct material (and covering it with fabric) in the manner I described? Or, on the basis of the manufacturer's specs would you recommend I use something with greater/smaller coefficients?


Am I correct in assuming that only after I've installed the carpets, seats, ductboard, etc. would I be able to assess the "true" room acoustics in order to achieve a good RT60 measurement?

 

[Snovosel]

Or, is this discussion related only to reducing the amount of sound that could potentially leak out from the HT to other areas of the house.


I'm not sure I'm getting the disctinction between sound proofing vs. room acoustics. Can the ductboard do both?

 

[mysphyt]

Usually NRC is used in conjunction with acoustic treatments (improving the "sound" of the room). Sound Transmission Class (STC) is a single number used to compare the "sound proofing" capabilities of a material or construction method. The higher the STC the better it is at stopping sound. The ductboard does some of both, but would only be effective at improving soundproofing if you lined the whole room with it.


Since you mentioned putting this material on only part of your wall, I assume we are looking at improving the acoustics of your room. Not soundproofing. The material you mention seems like a good choice, the absorbtion coefficients and NRC are where you would want them for reducing the overall RT60 of the room. I'd suggest covering the whole wall behind the speakers and the side walls up to the point even with the face of the speakers to reduce early reflections, then you can follow your plan of adding the insulation around the lower half of the room.


Here's a link to a simple RT60 calculator: RT60


You can certainly measure, or estimate, the RT60 of your current room, and again after treatment. Your room can also be computer modeled, with and without treatment, to get some idea what the changes will do.

 

[Snovosel]

That's an excellent explanation that clears up a lot for me.


You're correct. I've done all of the soundproofing I can at this point, having built the HT from the ground up. I'll use the material as you've suggested. It sounds (pun unintended) like a reasonable plan that won't have any negative effect on my HT.


The unintended consequence is that it'll provide an uncomplicated way to secure the fabric to the wall all around the room.


Thank you.

 

[Snovosel]

That's an excellent explanation that clears up a lot for me.


You're correct. I've done all of the soundproofing I can at this point, having built the HT from the ground up. I'll use the material as you've suggested. It sounds (pun unintended) like a reasonable plan that won't have any negative effect on my HT.


The unintended consequence is that it'll provide an uncomplicated way to secure the fabric to the wall all around the room.


Thank you.