Thanks. Isn't a membrane or panel with some mass attached to the insulation part of the system. Something thin like 1/4" ply? After a little research, it seem that they do. Would this kind of be a hybrid between a broadband absorber and a panel absorber?
I thought you had a physics background? A membrane or panel with some mass attatched to the insulation? For what? Maybe I don't understand the question. Absorbers such as rigid fiberglass need nothing. You could simply stand them on the floor up against the wall and they perform their function. The reason for the airgap between them and the wall, is because the velocity of air particles at a boundary is ZERO, but pressure maximum. At 1/4 wavelength, their velocity is greatest. Hence, thicker panels absorb a longer 1/4 wavelength which translates into a lower frequency. If you place the fiberglass against the wall, you are actually wasting a portion of the thickness for nothing. If you use something to stand it away from the wall 1/2" to 1", you increase the working thickness, which in effect, lowers the bandwidth. Spacing 1" thick material out 3" will increase its absorption to about what 2" material will do mounted directly to the wall.
To answer the other part of your question, no, it does NOT create a hybrid. Either it is a membrane absorber, or a resistance absorber. However, membrane absorber absorbs by resonance, and as such, REQUIRES a sealed air cavity between two membranes, such as the air cavity in a WALL. Although it is a narrow bandwidth. The sheetrock becomes the membranes, with a resonant frequency determined by the density of the membrane, and the depth of the cavity. I have read that the absorption coefficient of 1/2" drywall on studs 16" oc at 125hz is .29. That is almost a 1/3 of a sabine for every square ft of drywall. It even has a higher coefficient at 63hz, which is even lower frequency. Many newcomers to studio building do not realize that they have an abundance of
FREE BASS TRAPS built-in to their rooms already. However, the more layers of drywall, the stiffer the membrane, the higher the frequency of absorption. Increase stud spacing to 24", and it lowers the frequency. This is why studio designers must include drywall absorption calculations for the entire room to predict RT-60 values.
here is the formula. but its hard to write formulas this way
f(freq in hz)=170/square root of (m) (d)
m=surface density of membrane(lbs/sq' of panel surface
d=depth of airspace(inchs)
The use of fiberglass insulation within the cavity, is for dampening the membrane, which lowers the Q( I believe). Also, you can place a resistance absorber on the face of a membrane absorber with a small airgap for High Frequency absorption in the same space. I guess you could call this broadband or a "hybrid" when used together.
A better type of low frequency absorber for homestudio users is Slat absorbers with varied slot and slat widths. These can be built into INSIDE-OUT wall assemblies, or simply as gobos. They are in essence, a Hemholtz Resonator.
fitZ
