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Discussion in 'Home Theater Projects' started by Jones_Rush, Oct 1, 2003.
A rigid material such as hardboard will reflect any sound waves not dissapated on the first pass back through the absorbtion material thereby dissapating a larger percentage of the original wave. This also allows it to dissapate longer waves. In my case I was more concerned about high frequency reflections from the ceiling and elected not to use hardboard backing. The extra weight was also a prime consideration.
Just as a speaker cabinet resonates, so will the absorbers panel. It takes energy to make it resonate, therefore it is dissipating energy. The fiberglass (behind for a true bass panel) in front of the panel absorbs energy coming in and reflecting off the rigid panel, which only gets excited by lower freqs. The idea behind a true resonate panel bass trap is having a rigid panel mounted on the face to reflect higher freqs and fiberglass behind with an air gap in between to absorb the lower freqs that go thru and resonate the panel. Pete
Pete, what I'm asking is, why do you need the absorbing panel to have a rigid back (like pegboard), when you have a rigid wall behind the absorber anyway ?. One of them seems to be simply a waste.
Tim, I don't understand how Jon can say, in the SAME sentence, that putting the panel, 4" from the wall, will dramatically improve the bass absorption, AND, that having a rigid back to the panel, will also dramatically improve the bass absorption. It simply sounds like an oxymoron to me. You have to have the panel Both FAR and NEAR a rigid surface ???...
It's simple physics folks. The thicker the 'rigid' panel and the farther away it is from the wall, the lower the frequencies it will absorb. Look HERE for more info. Look HERE for the DIY version.
Guys, I thinks there's some misunderstanding here. Jon isn't talking about a plywood panel. Here's the whole quote to put things in context.
I think that rigid rock wool also helps reflect sound to double back through the 6" of fiberglass batting, thus improving sound absorption. If John wanted his panels thicker, why wouldn't he have just recommended 8" of fiberglass batting instead. Adding 1/2" or whatever it was of rockwool probably isn't helping the thickness to cut the frequency absorbed in half. I'd say it more acts as a moveable membrane like a resonator, or as a slightly reflective backing. Which is what people attempt to do with hardboard backings to their soft fiberglass absorbers. Spacing the panels from walls generally helps BASS frequencies. The reason to space an acoustic panel from a wall, is because the fiberglass has to slow down "air" movement. The speed of air on the surface of the wall is not as great as the speed of air approaching the wall. By putting the panel in an area where there's the most air speed, the air will get slowed down by the fiberglass mesh, thus reducing the SPL levels of that wave. I believe it may also be related with standing waves and how the nodes of a standing wave are at the walls, while the "peaks" are located away from the wall. The "peaks" are points of high sound pressure levels which is where the absorption should be. I understand the point you are making, and how sound can travel through a soft absorber which then hits the drywall and bounces back into the soft absorber so why bother having a hard backing... When the soft absorber is spaced from a wall, that means sound may not always double back through the absorber. The backing also helps the panel structurally so it doesn't warp. I think this whole acoustic treatment thing is getting complicated. I would just make a panel that has 3.5" thick fiberglass, and includes some type of backing such as sound deadening board, or pegboard. Or make things ultra simple and mount 1" thick compressed fiberglass panels on your walls covered with fabric. You may not get low frequency absorption, but you'll still notice a difference.