|Originally posted by Chris White
If I read the chart correctly, at 100 hz, 705 will absorb about 55% and 703 will absorb about 50%. Of course, I have no idea what it means when it says the material sample is "infinitely thick."
Sorry to leave you hanging, Chris. I was hoping that Ethan would have digested some of this material by now.
The point of the Mechel's figure 23, which I gave, was that if you look for a simple correlation between porous material density and absorption coefficient, you don't find it. Density is simply the wrong parameter to compute absorption coefficients from.
Mechel uses in his paper a different quantity, a dimensionless number which he calls E. This is defined as (po f)/ R1, where po is the density of air (sorry, I don't have a "rho" on my keyboard!), f is the frequency, and R1 is the specific flow resistance of the porous material. He gives tables of E vs. normalized impedance which allow you to read off the absorption coefficients for any combination of these quantities.
It's unfortunate the lowest frequency is 100 hz. The relationships appear to be reasonably linear from 100 to 500 hz, but that may not hold below 100 hz. I'm most concerned about what happens below 100 hz, because that's the problem I'm trying to fix.
Yes, that's a problem with acoustical data in general. Until relatively recently, 100 Hz was considered "low frequency." The advent and proliferation of subwoofers has changed this.
What happens as you increase the thickness at a given density? For example, will two pieces of 703 (one on top of the other) be twice as absorbent as one? Would two pieces of 703 absorb less than, equal to, or more than one piece of 705?
Absorber thickness greatly increases low frequency absorption. I'm not sure if "twice as absorbent" has a clear meaning in terms of absorption coefficients.
Spacing the absorber away from the wall has a rougly comparable effect to increasing the thickness. It's an excellent way to save on porous absorber material. Instead of making a 4-inch absorber and placing it against the wall, you can use a 1-inch absorber and space it 3 inches from the wall.