Originally Posted by benunc
If I understand your charts, it looks like I'm getting 20% absorption at 125hz with 1" spacing off the wall.
Left chart is on-wall. You're getting
about 25% absorption from a 2" panel on-wall; 0 spacing. You gain ~15% from 2" spacing for a 40% total.
...But it doesn't look like spacing and thickness have the same effect - i.e., 3" thick and 0" spacing absorbs 60% at 125hz while 1" thick and 3" spacing only absorbs 20%. What level of absorption should I be shooting for?
Correct, spacing and thickness are not interchangeable.
- thickness increases the amount of absorber present, so it increases the energy absorbed
- spacing it away from the wall just shifts the low frequency roll-off down in frequency.
Level of absorption... no good answer without data. You're not shooting for a level of absorption so much as a decay rate for sound in the room. You want to measure room decay time as a function of frequency, aka a waterfall plot. Typical criteria are:
- 0.3 sec for a -40dB decay at all frequencies
- no modal resonances extending beyond 0.4 to 0.5 sec.
A waterfall plot will show decay at all frequencies.
- add absorption at frequencies that exceed 0.3 sec
- swap diffusion for absorption at frequencies that have much less than 0.3 sec decay
- tell us about any modal issues and ask what folks have done; bass is very hard to absorb.