Don't confuse broad band with the helmholtz resonators that Ethan has plans for. Helmholtz resonantors by their nature are a narrow band absorber... so they may only cover an octave effectively for example (but they are VERY effective in that one octave). These are broad band absorbers which means they cover the whole spectrum of sound. Of course they are more effective in certain areas of the spectrum than others. However, you really need a large room to make a helmholtz design worth it as you need a bunch of helmholtz traps and in a small room, you generally don't have the space. Broad band absorbers like this are sort of the marriage between effectivness and space efficiency. According to Ethan on his forum, rooms below about 10x16 shouldn't even bother with more complicated traps. (And I can bet most of us home recording folks are in a bed room that size or smaller.)
If you are really interested, here is the NRC of the material I used. Keep in mind, this is just the NRC of the material mounted flush to the wall. (An NRC value of 1 = all sound is absorbed. Higher values are a side effect of the process used to measure the material. Read Ethan's site for more info on this.)
CO-EFFICIENTS AT FREQUENCIES
Thickness 125 250 500 1000 2000 4000
1.5” 0.17 0.58 1.06 1.07 1.00 0.99 0.95
2.0” 0.39 0.84 1.08 1.01 1.02 1.01 1.00
3.0” 0.68 0.92 1.08 1.03 1.03 1.03 1.10
4.0” 1.00 0.95 1.06 1.04 1.06 1.08 1.05
So you can see the stuff I used is rated at 1.00 at 125hz at a 4" thickness assuming it is mounted flush to the wall with no air space behind it. The thing is, there are frequencies above and below that point that may have a difference NRC measurement. This is why we space the traps away from the wall to try to cover an even wider range of frequencies effectively.
As for cost, these were about $35 each once you figure in fabric, hardware, 3M adhesive, and insulation. The 2" mid/high panels are about $12-14 less.
