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In Reply to: Question for Jon Risch re: acoustic panels posted by Deaf Ear on February 1, 2007 at 22:46:16:
Using the 2" thick OC 703, I would use at least three panels for a wall panel frame, for a total thicknes of 6", with a layer of polyester batting on the front.However, I also recommend a wall panel size larger than 2 X 4 feet. Note that absorption of LF involves not only wall panel thickness, buit the total area/size as well. A panel that is too small in size is just as bad as one which is not thick enough.
You could double up on the panels, or cut them in half and alternate the overlapping seams, to create a wall panel that is larger, so as to match what I recommend, or you could exceed my size recommendation, by placing two panels side by side, and one across the top for a total panel size of 4 feet wide by 6 feet tall. This method requires no cutting of the 2 X 4 foot panels.
With acoustics, you reap what you sow, and if you do not use a thick enough panel, or a large enough panel, the LF absorption will suffer.
I am not saying that the wall panels need to be turned into bass traps (and I still take exception to the oft promoted notion that a couple of 2X4 OC panels with some cloth wrapped around them some how magically become bass traps! It just isn't going to happen), but you do want them to absorb down low enough to be able to properly control the 1st reflection points, as well as when used in the corners or in the center of the front or rear wall, etc.
Follow Ups:
I know this probably cant be a simple either or question, but how would you decide which is more important, thickness of the panel, or total size?For my personal situation, Im trying to kill long reverb in a very large room (50W x 36L x 18H - middle school band room). Should I try to go with thicker panels, but less total surface absorbtion area, or use thiner panels, covering more surface area? BTW, Ill be mounting them off the wall at least a few inches if that makes a difference...
Thanks for your expertise!
Brent
The biggest single mistake many people make in treating a room, is to use too thin a material, and try to make up for it by using more than the optimal amount of area.So in that sense, thickness is more inmportant.
BUT, there is easily such a thing as NOT being large enough, not having enough area, to absorb lower frequencies.It helps to realize that bass waves are LONG, and that in order to absorb them, to keep them from reflecting off of a wall (such as when you place a wall panel at a first relfection point on a side wall), that the absorber has to have enough physical size to absorb the wave, or the wave will diffract around it, as if it weren't even there.
As an extreme example, some folks recomend placing a 12" by 2 foot piece of plywood on the side wall angled so as to reflect the side sounds away from your ears. Imagine a top down room diagram, with the this plywood panel placed at the first reflection point, and horizontally 'aimed' so that the panel is pointing at you, this will cause the HF's that hit the 1st reflection point to be directed away from you. However, due to it's small size, this panel will only be able to 'direct' or 'aim' sounds above approx. 500-600 Hz, everything below that frequency will tend to diffract right around the panel, so that at lower frequencies, it will be as if the panel is not even there, it is too small to direct those longer wavelengths.
By the same token, a wall panel can be too small to help absorb a lower frequency, even though it has enough depth and wall spacing to do some good at a particular frequency. Note that these things are not absolute, there is no "falling off a cliff" kind of effect, it is gradual, and related to frequency and the physical size of the waves involved.
I do try to stress that the typical pre-fab panels that are 2 feet by 4 feet are only good down to a certain frequency, no matter how thick they are. This limiting frequency is approx. 260-280 Hz, below that they are starting to be to small to effectively absorb enough of a low frequency sound wave enough to prevent it from continuing on as a reflection. This coupled with the all too common practice of only using a 2" or at best, a 4" thick panel, means that most of these panels are only working FULLY down to about 800 Hz, and partially down to about 260 Hz. Below that, they will do very little real good.
That is why the use of true bass traps in the corners should go hand in hand with the use of wall panels, so as to extend the acoustic control of the room down to the lower frequencies.
The first thing that size (area) affects is how much absorption you get, as measured in Sabins. The amount of absorption goes up basically proportional to the increase in surface area so double the surface area and you should roughly double the absorption you will get.Size also has an effect on the lowest frequency that will be absorbed. As Jon points out, you need panels of a reasonable size if you're going to have them be effective into the upper bass/lower midrange area.
Thickness really affects the lowest frequency that will be absorbed, with increases in thickness helping the panel to be more effective at lower frequencies.
Mounting off the wall increases effectiveness.
If you have a long reverb problem, then it's probably a lower frequency issue. Increasing panel size and thickness will both be beneficial and you should try for both.
Bass traps from floor to ceiling in all corners may well prove the most effective treatment of all. Bass traps tend to deal with lower frequencies than panels, and you can make them effective at higher frequencies as well, and corner placement ensures that all room modes will be covered and room modes are the longest reverb problem in a room.
Thanks Jon,I recall reading somewhere that you recommend the whole wall frame be a sealed unit. If I butt 2(2x4) together to make a 4x4, do you recommend gluing the edges together with liquid nails?
I am not sure that I am following your train of thought correctly.Wall panels do not need to be "sealed", in fact, it is a mistake to place a solid board on the back of the panel, and even perf board is going to reduce the overall performance of a wall panel, unless certain specific construction and assembly measures are taken, which are uneccessary if you leave the back 'open'.
If you are refering to the frame legs or edges for two panels side by side, no, they would not need not be glued together, but you should take steps to prevent any rattling or buzzing between the two.
Thanks Jon,I was confusing two separate issues.
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