one for the real buffs out there

I saw that it's beneficial to leave an air gap behind the 703/rockwool inside a room, between it and the drywalls. Extanding this reasoning, would it be beneficial to leave an air gap behind a superchunk and make it a hollowchunk like such? And the bonus one for the real, real buffs experts: I got that bass trap broadband panels (the ones that are a slab of 4" 703 with a frame around it) are great about 4" from the walls. Acoustic hangers are apparently still a bit of a mystery but they work. What about... hanging broadband panels? :yahoo: Anyone tried?

leaving an airgap between panel and wall is, indeed, better for low end absorption. On corner trapping, however, it's better to fill the whole corner. Tho, not mandatory. Add a sheet of wool accross the corner will leave a gap, so be it. That's why I'm a superchunk lover ;) Sure you can hang the panels and call them hanging absorbers. Let's see what John has to say about that, since he's practically married with hangers :)

Ta Ro! So... no other takers on hanging absorbers? I kinda like the idea...

Given the same piece of material (for example, one 2' x 4' x 4" panel of OC 703), spacing it out from the wall will add to the effectiveness in the mid and low frequencies compared to placing the same panel directly against the wall boundary with no air gap behind it. As I (mis?)understand it this is related at least in part to wavelengths - sound goes through the panel (and some of the energy is converted to heat), bounces off the wall, then goes back through the panel (and some of the energy is converted to heat). Spacing the panel out from the wall thereby increases the low-frequency attenuation a bit. It probably also changes how the panel works, if only slightly, if it's 'swinging in the wind', so to speak, as opposed to 'stuck to a wall', but that's entirely conjecture on my part. :blah: But removing material from behind would make it less effective - a 'superchunk' would be more effective than a 'hollow superchunk of the same exterior dimensions with material scooped out from the back'. And a 4" thick piece of 703 is still more effective to lower frequencies than a 2" thick piece. "Space your home-made fiberglass or rockwool panels a few inches out from the wall" is meant as a way to get 'more for your money' in materials costs, making the same piece of material more effective for the same price (plus any additional materials you're using to hang them that way). But "more" is still more than "less". :mrgreen: Sure hope I didn't just embarrass myself in front of Mr. Sayers, Mr. Gervais, Mr. Winer, et al... :|

chrisrnps wrote:

It probably also changes how the panel works, if only slightly, if it's 'swinging in the wind', so to speak, as opposed to 'stuck to a wall', but that's entirely conjecture on my part.

Thanks very much for your explanation Chris, the one I'm really curious now is the topic above - hanging panels are good, trap panels are good, is a combination of both good or not - but maybe no one has ever tried or thought about it - in that case I'll give it a go and report!

As I (mis?)understand it this is related at least in part to wavelengths - sound goes through the panel (and some of the energy is converted to heat), bounces off the wall, then goes back through the panel (and some of the energy is converted to heat). Spacing the panel out from the wall thereby increases the low-frequency attenuation a bit.

Well... sort of! To understand it better, think of the physics of how a sound wave moves, and what it does when it hits a wall. A sound wave is nothing but a pressure difference that moves through air at a certain speed ("the speed of sound"). There is a fixed amount of energy in each wave front, and in simple terms you can break it down as the sum of the "speed" energy plus the "pressure" energy. So at some places in its travels there will be more "pressure" energy and less "speed" energy in the wave, while at other places the reverse will be true, but the TOTAL of the two will always be the same (OK, it's not actually that simple, but the mental picture helps to understand the principle). So you can think of the total energy in a wave being made up of two components: velocity, and pressure. So think about what the wave does when it hits a wall and bounces back: Exactly at the boundary, the speed is obviously zero: it hit the wall! So it MUST stop. "Stopped" = "zero speed". If you throw a tennis ball against a wall, it slows down, stops then accelerates and comes back. If you could analyze it in very small time slices, really close to the wall, you'd see that the tennis ball slows down from maximum speed to zero speed as it hits the wall, but the collision causes the ball to flatten out front-to-back, and expand sideways, thus storing the "speed" energy as more "pressure" energy inside the ball. That pressure then causes the ball to expand back to its normal shape, which pushes it away from the wall, thus accelerating it in the opposite direction. (simplified explanation). More or less the same happens with a sound wave. As it hits the wall, the "speed" energy drops to zero and is converted into "pressure" energy, which sort of "pushes" the wave away from the wall again. (Once again, it's not actually that simple, but it gives you a good mental picture of what happens). So, the wave slows down as it approaches the wall, stops exactly as it hits the wall, then speeds up again as it comes back. That "speed" energy that disappeared must have gone somewhere! It did: it was converted into "pressure" energy. So exactly at the wall, the pressure component of the wave is at its maximum peak, while the speed component is zero. At the wall, the energy in the wave is entirely pressure, no speed at all. The problem here is that fibrous absorption, like mineral wool and fiberglass, acts on the SPEED component of the wave, not on the pressure component. The fibers resist the velocity of the wave moving through them, but couldn't care less about the pressure. So right at the wall, the fluffy stuff does nothing at all to the sound wave. The further away from the wall you put the fluffy stuff, the more effective it is. This "energy interchange" happens as a function of wavelength. At exactly one quarter wavelength away from the wall, things are reversed: the pressure component is at its minimum, and the speed component is maximum. So that's where you want your greatest thickness of fluffy stuff to be: exactly one quarter wavelength away from the wall. But of course that distance changes with frequency: at 20 Hz, a quarter wave is about 4.25 m! At 100 Hz. it is down to 85 cm. At 500 Hz it is just 17 cm. etc. So you need to make your fluffy stuff as thick as possible, and space it further away from the wall if you want to absorb lower frequencies, or closer to the wall and thinner if you want to grab highs. Of course, the energy interchange function is not linear: it is a sine function, so you still get some effect even at distances that are a small fraction of a quarter wavelength from the wall: (at 1/8 wave you still get 70% of peak absoprtion, and even at 1/16 wave you still get nearly half) but you get MAXIMUM effect exactly one quarter wavelength away. All of the above assumes that the wave hits the wall head-on, and bounces straight back. If it hits at an angle, then even better: it "sees" more volume of fluffy stuff, since it must take a longer path through it, so it is affected more. In that case, the maximum effect is still at the quarter-wave distance, but in the direction of travel of the wave, so the fluffy stuff will absorb frequencies that are even lower than for the "head-on" case, since the apparent distance away from the wall is greater. It gets complicated, but that's why an absorber can appear to work well on frequencies that are much lower than you'd think, just from looking at its depth and distance from the wall. All the above is for flat panels, parallel to the wall. But as Ro pointed out, for superchunks things are a bit different: Superchunks go in corners: so there are TWO walls, not just one. So any given wavefront must be traveling ALONG one of the walls, in order to hit the other wall head-on. Or if the wave is coming in at an angle to both walls, then there is some combination of "head-on" angle vs. "along the wall" angle, but still the wave is moving along at least one of the walls. The point is that the part of the wave that is traveling along the wall, is NOT at zero speed, so the edge of the superchunk where it contacts that wall actually IS having an effect on the wave. Thus, it makes no sense to leave a gap behind the superchunk, since you'd be reducing the absorption for waves that do not hit that wall head on. And since low-frequency sound is not directional, chances are that most waves are not hitting either of the walls head-on anyway, so the superchunk will do a better job if you push it right up against both walls. It will still work with an air gap behind it it: it just won't be as efficient as it could be. At least, that's my reasoning here. Others may have different opinions. Panel traps, of course, work on a different principle: They are pressure based, not velocity based, which is why they MUST be against the wall, and sealed. I wish I could help you with how hangers work, but the consensus from the experts seems to be: nobody knows! They just DO work, but the theory of how and why is yet to be explained from the point of view of physics. At least, that was the case the last time I looked into it. Maybe that has changed... - Stuart -

Stuart, this is a very detailed explanation thanks so much!