One more annoying Bass trap Question.

S

stoctony

New member
Just one more guys. I was reading the post about fiberglass below and it got me in the fiberglass mood. I Built a bass trap. I is 2ft wide by 8ft tall and 6in thick. I then packed it(I mean packed) with fiberglass batting. I know it wont do anything for highs because of the front covering is plywood. But, theoreticlly(yeah I like to use big words lol) will this do just about the same as 6in or so of Rigid fiberglass being propped up in the corner of my studio? Just a question to throw out there. Just wondering Thanks in advance

Tony
 
are you sure? I dont have any air gap inside. Its just all batting compressed therefor making it like rigid fiberglass. All the wood is doing is just holding it all together.
 
Tony,

> All the wood is doing is just holding it all together. <

Depending on the thickness of the plywood, it's likely also blocking the low frequencies from getting to the fiberglass.

--Ethan
 
I know it wont do anything for highs because of the front covering is plywood
Click to expand...
Well, I'm quite sure of what you told us.
Why would anyone cover the front of a resistance absorber. I don't think you understand how these things work. I am certainly no EXPERT, but here is what I have been led to believe is a resonable explaination of the absorption principles of these devices.
Rigid fiberglass absorbs by "resistance" to the movement of air molecules within the "interstices" of the fibers. The DENSITY is what determines it's absorption characteristics, and the thickness determines the absorption coefficeint at various frequency ranges. Batt insulation absorbs the same way, only is much less dense, therefore absorbs at lower coeffecients. However, if you place a rigid membrane over the face of the fiberglass, you block the entrance to particle movement, as particle velocity at a rigid boundary is ZERO. That is why MEMBRANE absorbers reflect part of this energy back into the room OUT OF PHASE. Absorption WITHIN the CAVITY of a membrane absorber, also occurrs because of particle movement within the enclosure via RESONANCE of the panel, which has a center frequency determined by the density of the panel, and depth of the cavity. It is the flexing of the panel at resonance, that transforms energy to heat...voila! absorption. However, I believe the purpose of the insulation within, is mostly to dampen the membrane, which "smooths" out the curve of a membranes frequency range as it will absorb plus or minus an octave on each side of the center frequency..but don't hold me to that as my "disclaimer" already states my depth of understanding. However, I think you get the picture. Point being, you stated your unit has a plywood front with batt insulation within. That distinctly gives me the impression you have built a membrane absorber. All I was trying to tell you, is UNLESS this unit is mounted on a wall, or has another rigid membrane on the BACK and is SEALED, it will NOT perform, although the panel may resonate at some unknown frequency. Thats all. But let me add one more thing. A room with gypsum board wall sheithing has FREE absorption, as each area from stud to stud, is actually a Membrane absorber.
fitZ
 
Put another way (with the same no-expert disclaimer :D ), what you have done is taken a very good full band absorber (6" of open fiber) and covered it with the makings of a membrane absorber that can't (may or may not?) move freely to do it's job.
Wayne
 
To clear things up. The front is covered with Compress board. You know the stuff that usually has the holes in it to hang the little hooks on.(can you beleive i have a construction back ground) any way. I asked Ethan awhile back as to what bass would actually travel through so Im not just going on pure judgement here. This stuff is not more than maybe 1/8. And by Packing it with the batting I am going for the flow resistance theory. All the waves should definetly flow through all the material and with 6 in, should be atleast capable off handling upper lows. Right? Thanks for all the help by the way.
 
To clear things up. The front is covered with Compress board. You know the stuff that usually has the holes in it to hang the little hooks on.(can you beleive i have a construction back ground) any way. I asked Ethan awhile back as to what bass would actually travel through so Im not just going on pure judgement here. This stuff is not more than maybe 1/8. And by Packing it with the batting I am going for the flow resistance theory. All the waves should definetly flow through all the material and with 6 in, should be atleast capable off handling upper lows. Right? Thanks for all the help by the way. The reason it is packed also is to increase the resistance instead of just hanging up a peice of Batting that would only be 4 in thick and fluffy.
 
Just poking around, I found a reference to Owens-Corning that says pegboard in front of 6" of glass is .99 from 125 Hz (they don't show lower) to around 1-2k where it starts reflecting, and a Helmholtz resonator which tends to be narrow band but depends completely on perf size, percentage, and depth. You might be closer to the later, but now it's math time to find out. :D
Both are in F. Alton Everest's 'How to build a Small Budget Recording Studio' -a good one for a DIYSr's tool kit.
From what I've gathered, you don't need to stuff the stuff tightly, and using a cloth face keeps it from beaming on the highs. Another way is to use wire crossing over the face to support the soft bats before the cloth covering.
Peace.
Wayne
 
To clear things up. The front is covered with Compress board. You know the stuff that usually has the holes in it to hang the little hooks on.(
Click to expand...

Does it or does it NOT have holes, that is the question. You say "usually", which implies it does NOT. If it does, you have a Hemholz Resonator, IF it has a back, or mounted to the wall. If it does not have a back, or mounted to a wall, you have nothing but a frame full of insulation.
IF it does NOT have holes, you have a membrane absorber, but again, must have a back or mounted on the wall or does nothing, although it may absorb some reflections from the walls on the backside if OPEN. Never heard of Compress board. We call it Masonite. The stuff with holes is single tempered(smooth one face) but is also available Double Tempered(smooth 2S), and is better stuff.
fitZ :)
 
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