#1. I thought I read everything there is to read concerning bass traps. However today I came across a thread on GS whereby some of the seemingly more knowledgeable people claim any air-gap between the rear of the trap and wall needs to be SEALED to be effective? I've seen countless pictures of hanging traps, both manufactured products as well as DIY traps and the gap always seems open. Confused!
#2. What's more effective for bass traps. Say a 6" (2 x 3") layer of Safe-N-Sound with a 6" air gap, or a 9" (3 x 3") layer flush against the wall with no air gap?
I. If you are talking about porous absorbers, no, the gap does not need to be sealed. If you are talking about tuned panel or Helmholtz resonant absorbers, yes the cavity needs to be sealed.
2. It would be better if you had asked 6" thick with a 6" gap VS 12" thick up against the wall. You see I used the same total thickness because I think makes a clearer comparison, one with half the thickness plus an equal gap compared to the absorber the full thickness. In this case, the answer is: The full 12" thickness is slightly better at twice the cost. Looking at it the other way: The 6" thickness with a 6" gap is only slightly worse at half the cost.
If money is an issue, you're way better off with double the AREA coverage with gapped panels than HALF the area coverage with full thickness. If money is no object, go full thickness
To answer your 6"+6"gap VS 9" no gap, I'd say the 6"+6" extends 1/2 octave lower than the 9" but the 9" would be a little more effective but start to roll off a 1/2 octave higher. So it might be a toss-up in which case the 6+6 would be cheaper.
To start with and most important to remember is that ALL broadband porous absorbers work at ALL frequencies. It's just that the thicker, the MORE AMOUNT of lower freqs are absorbed. In other words, their effectiveness rolls off gradually.
So I ran some numbers using the formula to get the frequency from a given 1/4 wavelength: f=C/λ where f is Freq, C is the speed of sound = 1130fps and λ is wavelength.
282 Hz has a 1/4 λ of 12" 188 Hz has a 1/4 λ of 18"
Or in other words, one half octave lower with 18" VS 12"
But I emphasize, these are not the cutoff frequencies but the theoretical point where rolloff begins.
One more thing: From a practical standpoint, after about 8" of absorber thickness, using common "fluffy" insulation is as effective as denser "rigid" and is cheaper.
Rock is right about everything. I just want to add that 'diminishing returns' is relative in this case. You nearly immediately will hit diminishing returns depending on what kind of absorbers you've installed first - such a phenomenon tends to be a curve, after all. Even so, I think most people run out of enthusiasm or budget before they have 'too much absorption'. Remember that once you've solved for modal resonances and specular, flat, reflections, you can always add in diffusion.