[gkinberg]

Question regarding acoustical panel construction methodology. Let’s say an acoustical panel is made with two inches of acoustical material and a 1.5 inch air gap at the rear of the panel. Is this air gap between the rear of the acoustical material and the wall more effective, equally effective or less effective than filling that air gap with an additional 1.5 inches of acoustical material that rests flush against the wall?  While it might be a though comparison, I am trying to decide which formulation would give the better low frequency absorption, two inches of material followed by a 1.5 inch air gap against the wall, or four inches of material flush against the wall. I’d appreciate the benefit of your expertise.

Thanks, Garth

[srlaudio]

The air gap benefit behind an absorptive acoustic panel has to do with "doubling your investment" with a panel by having it be effective on the bounce of large waveforms (bass) off of the wall and being attenuated again as they pass through the panel again.  It is a great concept, but to be effective on typical small room bass standing waves the distance is much greater than a few inches, in fact it becomes unmanageable unless you have a large space to work with.  Tom Hidley designed rooms had bass trapping using almost as much floor space as the actual space you actually end up with to use!  Very impractical for most rooms out there.  A more space efficient approach is to use large diffusers that have low target frequency design. We have had great luck doing this with depths from 14.5 to 20 inches deep.  In addition to a low target frequency we use high prime numbers which end up with a wider width.  This is also significant when you are trying to fix small room bass issues by making a larger area for the bass waves to push against.  This concept was explained to me by a top acoustical testing facility that has measured these effects and described it in the terms of hydraulics.  The bass energy is transformed to heat by trying to "compress" the air in the channels of the diffuser.  He did say that they must be "large" panels to achieve this......

[gregfisk]

So, it sounds like the air space behind the panel isn't deep enough to have an effect on the bass waves. gkinberg's question still remains, are we better off making a thicker panel or leaving the air space, or are they basically of the same benefit and the air space saves the cost of a thicker panel?

Regarding diffusers, what type of material works best and why? It seems like a reflective material would work best if we are trying to diffuse the sound waves or scatter them.

Thank You,

Greg

[srlaudio]

Well the real answer is probably not what you want to hear, you need a thicker panel and a wider airspace.  There will be some improvement of leaving a 2 inch gap, but I think that 2 inch thicker material would do better.  in a typical bass trap, nothing is trapped, because it is a fact of physics that you can't destroy energy, it can only be transformed into a different form.  Fiberglas or rock wool offer resistance to the air molecules somewhat like the difference of running through an open field to running through a field with 5 ft high grass.  The high grass is like the bass trap, it slows down the air molecules thereby attenuating their energy by turning it into heat.  Knowing that, and knowing that sound waves have compression and rarefaction, that is, they speed up and slow down.  If you look at a sound wave on a scope, where the wave crosses the zero line represents the slowest speed while the highest + and lowest - represent the highest speed.  "Basstraps" are most effective if placed at the highest speed points of the waveform.  This is also known as the quarter wave.  A 60 hz tone is about 18 ft long making the quarter wave about 4.5 feet.  This how far from the wall you should place your basstrap for maximum attenuation at this frequency.  A 2 inch gap would place the quarter wave about 2500 hz.  Not really helping a whole lot in the bass range.
The answer about diffusers is that the material does need to be reflective of sound.  The bass is affected by them in two ways, the lower frequency that the diffuser will scatter, and the compression effect of the channels.  The guru I spoke with told me it works sort of like shock absorbers.  The bass waves are large, and as they as he put it lay across the surface of a large diffuser, the energy is once again converted to heat, this is by compression, the same way a diesel motor works.  I hope this helps, but on a closing note I must tell you that I have heard great improvement in bass (subwoofer) performance when LARGE diffusers are deployed in the room.  At one RMAF show, I arrived with a six foot wide by six foot tall 10.5 inch deep diffuser we placed on the back wall opposite the speakers.  The designer of the speakers was shocked, because he had to reduce the gain significantly on the sub channel to get the balance right again.  His grin when this was done and he sat down to listen was a special moment!