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In Reply to: RE: DIY corner traps: panel-like or tube-like construction? posted by LewinskiH01 on October 14, 2009 at 19:23:25
I have been getting a lot of e-mail lately with folks completely confused about what a bass trap is.
I copy one of my responses down below:
Please realize that the wall panels and the bass traps are two different animals, they serve different purposes, and operate on different principles.
A WALL PANEL, NO MATTER HOW DENSE, IS NOT A BASS TRAP, EVEN IF PLACED IN A CORNER.
The companies that sell simple wall panels as bass traps are doing their customers a dis-service, and creating deep confusion in the marketplace.
A very FEW companies market an advanced design that utilizes a limp membrane in conjunction with a resistive panel to help boost the bass absorption over that of a regular absorbing wall panel, and IF these special types of panels are placed in the corners, they will provide more than ordinary bass absorption. But even these special panels do not match the sheer absorption of the ASC type bass traps, they just don't.
Now, if you really can't use the ASC style bass traps, which my cylindrical traps are a clone of, just a DIY version, then yes, you can beef up the bass absorption of the panel by increasing the density, up to a certain point, and as long as a lower density layer is still present on the front surface to absorb the HF's and upper mids.
The easiest way to achieve this is to compress the regular building grade fiberglass bonded to kraft paper by 2 to 1, and to really do this, you would need to use some chicken wire or animal fencing to deliberately compress the fiberglass on BOTH sides of the 1X4 frame.
OR you can use the higher density semi-rigid panels such as Owen's Corning 705, etc. as an additional layer on the rear of the frame. The more of these, the more low bass absorption UP TO A POINT.
Merely leaning a wall type panel across a corner, no matter how dense (optimally or otherwise), will only absorb so much bass, even the special panels included, although they do absorb a bit more before they 'hit the wall'. In order to absorb any more than this, such a panel would have to be "air-sealed" into the corner, around all edges, so that it created a cavity in the corner, and the only way for the air pressure to get into the corner, was through the panel.
Until you seal a wall panel into the corner like this, it is just a velocity absorber, and not a pressure based absorber, like an ASC style bass trap.
My Super Quick & Dirty bass traps work on the principle of sheer amount of absorbing material of a higher than 'loose' density, but they are still primarily just velocity absorbers. There is some pressure absorption going on, due to the sheer circular diameter of the compressed fiberglass, but not as much as the velocity absorption that is occuring.
I just want you to be aware that no mater how dense, or compressed, you make the wall panels, they are still going to be velocity absorbers, and will not match the effectiveness of the true pressure-absorber bass trap.
END
Jon Risch
Follow Ups:
because at the moment it seems as if you're simply stating that a panel, no matter how effective or how placed, can never be a "bass trap" because it doesn't function as a pressure device.
What is the defining characteristic of a bass trap? Is it:
- mechanism, ie a pressure rather than a velocity device, because a velocity device also reduces sound pressure level—a big problem here is that you have published instructions for a "quick and dirty BASS TRAP that is a velocity rather than a pressure device;
- how it's used, eg a device placed in a particular location—I have some sympathy with this as part of a definition because certain locations are much more optimal placements for a bass trap, whatever that is, than others;
- something which achieves a certain level of effectiveness, but one would need to state the level of effectiveness required as part of the definition.
I'm not trying to be smart but it is possible to build a panel that will be more effective at controlling bass modes when placed in a corner than a tube trap. One simply has to make the panel sufficiently large, thick and dense and the tube trap sufficiently small in diameter and/or height and low in density for the panel to outperform the tube but you're stating here that tube traps are BASS TRAPS and panels aren't, with no qualification of either statement so the implication is that all tube devices are bass traps, all panels aren't, and that bass traps will always control room modes better than a panel when that simply need not be the case.
I'm not trying to ask a trick question. I'm simply saying that your post here doesn't provide reasons to classify tube traps as bass traps and panels as not bass traps and I feel like I'm missing something that's being left implicit and unstated.
I accept that tube traps and panels operate on different principles but so do membrane traps and Helmholtz resonators and at least one other RPG product, all of which are intended to reduce modal behaviour in rooms. What defines one device as a trap and excludes others from being defined as bass traps, especially if the effectiveness of one of the excluded products should exceed that of a product defined as a bass trap?
David Aiken
What is a bass trap?
Is it a wall panel, consisting of a single 2X4 foot sheet of 2" thick fiberglass?
Is it a Helmholtz Resonator, tuned precisely to one frequency?
Or is it something else?
In response to Dave's post, I replied that of the three aspects he cited:
1. Mechanism
2. Placement
3. Effectiveness
I felt that a device that deserved to be called a bass trap would include some of all three.
It would have an effective and efficient mechanism to absorb bass energy, it would be placed where it could absorb and dampen the maximum amount of bass energy, and it would have to have a certain basic level of performance in order to be called a bass trap.
Let's look at the last aspect first. How much bass does a device need to absorb to be called a bass trap.
Let's look at the absorption in Sabin's, standard ASTM conditions, 3rd party data.
Freq in Hz 63 125 250 500
16"dia 3 ft tall
Cylinder Trap 14.9 17.0 15.0 17.8
11"dia 3 ft tall
Cylinder Trap 10.2 10.9 16.7 16.8
2X4 ft 2"
Thick Panel 1.3 1.4 6.9 9.12
(705)
2X4 ft
Specialty 0.7 4.2 12.3 14.2
Panel 1
2X4 ft
Specialty 4.2 9.8 12.8 16.4
Panel 2
Personally, I consider an 11" diameter cylinder trap to be a sort of 'minimum' amount of performance for a bass trap, but that is a bit arbitrary.
However, I think that most people can see that the simple 2 foot by 4 foot by 2" thick fiberglass panel is just NOT a bass trap, by anyone's criteria.
Keep in mind that I recommend the use of a 4 foot tall cylinder trap, this increases the Sabin numbers in the chart by approx. 133% (multiply by 1.33)
In one post about panel traps vs. cylinder traps, it was stated that one 24" (by 48") by 4" thick fiberglass panel (705) was equivalent to a single 20" cylinder trap.
Balderdash.
Freq in Hz 63 125 250 500
20"dia 3 ft tall
Cylinder Trap 14.9 17.0 15.0 17.8
2X4 ft 4"
Thick Panel 3.3 7.2 14.7 13.1
(705)
Also keep in mind that at frequencies below 63 Hz, the cylinder traps are going to be doing something to the bass room modes all the way down to the lowest mode in the room, while the simple panel traps, EVEN IN THE CORNER, will essentially 'go away' for all intents and purposes.
Before we go any further, I know that the panel company folks are going to crawl out of the woodwork and start talking about all kinds of extenuating conditions, and so on.
So, before they cry foul:
Yes, these are ASTM conditions. Not a corner. In some cases, the wall panel data will improve by a factor of 2 at low frequencies when placed in a corner, and at some resonant frequency for the specialty panels, it might even go up to almost three times as much absorption. There, that is out in the open, not to be denied.
But then, let's be fair to the cylinder traps, they will ALSO increase if measured in a corner, perhaps by a factor of 1.5 to 2 times the above data.
This is the point that never get's made in the comparisons of cylinders to wall panels.
This could be because due to ASC patents, they can not offer a cylinder trap for sale, only wall panels. Or it could just be an oversight, a complete coincidence. You decide.
There will also be the question of where the data came from.
The cylinder trap data came from "The Master Handbook of Acoustics", 3rd Ed. by F. Alton Everest. The data is not quite the same as the data on the ASC site, and appears to be a true 3rd party measurement on the entire trap, not just on a "per linear feet" basis, as is the data at the ASC site (in the flyer PDF)
(ASC data: http://tubetrap.com/tubetrap-flyer.pdf
http://www.tubetrap.com/technical.htm )
The data for the 705 panels is widely available, and it has been measured independently from OC, and I did peruse some of these measurements on various acoustics forums, both years ago, and just recently, to make sure they were still out there.
The data I used for the specialty panels comes from the manufacturer's web site, and some of the various brands of specialty panels have been measured by third parties as well. At least the data given at the manufacturer's web site is close to that taken by the third parties.
I still need to explain about aspects #1 and #2, and why these aspects come into play.
And I will explain them, soon, because they explain why even with equal Sabin numbers, the cylinder trap still outperforms a wall panel type. I just thought a few raw numbers first might be fun.
Look for Part 3 soon.
Jon Risch
Comment: " Then please define a bass trap ...because at the moment it seems as if you're simply stating that a panel, no matter how effective or how placed, can never be a "bass trap" because it doesn't function as a pressure device.
Reply: I do make a distinction between simple fiberglass wall panels, and wall panels which use additional means of providing bass absorption, so I am not saying that all wall panel type device's are not bass traps.
I did sort of define what I think a bass trap is in my answer up above
( http://www.audioasylum.com/audio/tweaks/messages/16/166034.html ):
In my opinion, a bass trap is a device that is specifically designed to absorb and dampen lower bass frequencies very effectively. It can either depend on a sheer mass of absorbing material, or it can leverage some other absorbing principal, as do the ASC type bass traps.
The typical wall panel is just not thick enough, nor is it large enough to absorb or dampen very much lower bass frequencies.
****************************
However, you asked some very specific questions, and I will endeavor to answer them as best I can.
You mention three things that might define what a bass trap is:
1. Mechanism
2. Placement
3. Effectiveness
In my mind, I think of a bass trap as having all three aspects:
efficient use of pressure style absorption, placement to maximize that pressure absorption mechanism, and enough effectiveness in terms of bass frequency absorption AND bass frequency damping.
In order to explain these criteria, I will need to cover more thoroughly exactly what is a velocity absorber, and what is a pressure absorber.
That is going to take some time to explain, and I will need to take the time to coherently organize and write down these explanations. It will also involve delving into room modes, room boundary stiffness, and a host of other related issues. Trying to condense that down into a single post that is not heroic in length will require even more time and effort.
For the sake of folks who have been confused and even mislead into thinking a simple wall panel placed in the corner is a bass trap, I will try to do so some time in the next few days.
As a short preview to the other side of the coin explanation, yes it is possible to place a flat panel in the corner and get significant bass absorption, even to the point of being able to legitimately being called a "bass trap", but that is not what will happen when the typical "wall panel" consisting of 2 by 4 feet of a few inches of fiberglass is placed in a corner. To think that such a panel is equivalent to an ASC style pressure-principle bass trap is sadly mistaken.
Jon Risch
Well appreciated here....
-chris
Thanks, Jon. I'll look forward to reading Part 2 when it comes.
David Aiken
> I'm not trying to be smart but it is possible to build a panel that will be more effective at controlling bass modes when placed in a corner than a tube trap. <
Indeed. Bass traps based on rigid fiberglass - whether flat panels, stacked triangles, or filled or hollow tubes - are all velocity absorbers. Rigid fiberglass is a porous material, so sealing the ends or sides etc should have no effect. With velocity absorbers all that matters is they be thick enough, large enough, and you have enough of them placed where bass builds up in the room.
BTW, here's a thread going on now at Audio Circle where a fellow will test some of these theories and post his results:
Comparing "sealed", hollow, insulation tubes to stuffed insulation tubes
--Ethan
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