markus76 said:
There will be a band of frequencies where the corner trap will work better and a band where the plate absorber will work better. The plates peak effectiveness will likely be at a lower frequency than the corner trap, but the corner trap may be broader.
I have found so many ways to add damping in the rooms that i've built that I just look at each room as unique. One guy I helped uses a closet filled with clothes as a tuned absorber. I built a series of cascaded tuned absorbers under a riser in a HT. Almost anything can be made to absorb at a certain frequency, but its not always easy to figure out how, or to tune it. That's where the "pro" stuff has an advantage, it probably works as advertised. But there is always some way to get what's needed in other ways.
Member
Joined 2003
how much of a problem is group delay in the lf's with the filters used. As I understand it, the large majority, if not all of the plate sub amps have built in high pass and low pass filters which are not completely defeatable. These will cumulatively add group delay, and often they are of quite high order, 3rd or 4th electrically isn't uncommon. I asked once before and don't recall if it was answered, but it seems like all this filtering adds an awful lot of group delay. Enough that the box design itself seems to become irrelevant in that department.
No, not really, except one might call my walls "limp mass". Certainly their mass dwarfs their stiffness in the audible band.Paul W said:
pjpoes said:
A very arguable point, but I don;t think that a DCX2496 feeding a conventional amp will have enough group delay to worry about. Its only going to be a few ms worst case. Thats not going to be audible. Some techniques, like long FIR filters will had much more than this and could be a factor however.
I've talked with Peter D'Antonio before, and I don't think he would agree that these work over more than one primary frequency area. His designs and his research seems to show ways to tune the traps to cover a broader range by dampening, but it doesn't it won't give multiple peaks or the type of broad absorption seen by velocity absorption, it just works at much lower frequencies.
I still stand by my view that velocity absorbers, even corner trap types like Ethan builds and recommends, are practically useless below 100hz. He doesn't agree, I know Dr. Geddes has said before that it's still worth having (Peter would agree with me since he is the one who told me this), but I've just not been finding that velocity absorption makes any sense below 100hz. This leaves things like mass resonance dampening, such as Dr. Geddes suggested CLD, or the various commercial options available.
I still stand by my view that velocity absorbers, even corner trap types like Ethan builds and recommends, are practically useless below 100hz. He doesn't agree, I know Dr. Geddes has said before that it's still worth having (Peter would agree with me since he is the one who told me this), but I've just not been finding that velocity absorption makes any sense below 100hz. This leaves things like mass resonance dampening, such as Dr. Geddes suggested CLD, or the various commercial options available.
right, I understand that. This is why I don't see how adding a bunch of fiberglass into the corner of a room will have much impact on the lf's. The only thing I can see happening is that it can add dampening to the boundry, which can act as a pressure absorber. Even normal drywall will act as a pressure absorber right? I don't know if Peter's explanation is truly right, I think it was an over simplification to make it easily comprehendable to a lay person, but he refers to the pressure absorbers as working by converting the high pressure low velocity energy into higher velocity lower pressure (with some energy lost as heat), and then normal dampening can deal with the rest (i.e. fiberfill or fiberglass).
RPG's broad band absorbers, as I understand things, work by combining pressure and velocity absorption techniques in multiple ways. However, as I understand Peter's comments, these broadband absorbers are not effective down low, at so they build pressure absorbers for that range which primarily use dampened steel plates, which have a pretty high Q resonant peak. Dampening, which is adjustable , then lowers the Q, reduces the maximum peak absorption, but increases the range.
RPG's broad band absorbers, as I understand things, work by combining pressure and velocity absorption techniques in multiple ways. However, as I understand Peter's comments, these broadband absorbers are not effective down low, at so they build pressure absorbers for that range which primarily use dampened steel plates, which have a pretty high Q resonant peak. Dampening, which is adjustable , then lowers the Q, reduces the maximum peak absorption, but increases the range.
All I can say is that this is not how Fuchs explains the functionality of a CBA. One of the guys that filed the original patent: http://www.hunecke.de/en/knowledge/absorbers/panel-absorbers.html
Best, Markus
Best, Markus
markus76 said:
Markus
That is a marketing writup and I would put a lot of faith in its accuracy.
john k... said:
The pressure moves a panel and the panel's velocity is dissipated. But of course you know that, so I don't understand the comment. Or should I have said "pressure BASED absorbers, like panels"? Seems kind of picky to me.
markus76 said:
The link wasn't actually about his products, it was a broad, simple, overview of concepts about room acoustics. I was interested in things like the designs for panel traps:
"One of my favorite types of bass trap is the membrane absorber, also called a panel trap because it's made with a wood front panel. One huge advantage of membrane traps is that they do not have to be very thick to absorb very low frequencies. Because the bass range spans about four octaves, most panel traps are designed to work over only part of the bass range. Therefore, you will need an equal mix of trap types, with one intended to absorb the lower bass frequencies and the other for the higher bass range. Besides absorbing low frequencies very well, the wood front on a panel trap is reflective at higher frequencies.... "
And this was an answer to my question about the absorption:
"You may notice that absorption coefficients sometimes have a value greater than 1.0. Although it is impossible for any material to absorb more than 100 percent of the sound, measurements can yield a value greater than 1.0. The main reason this occurs is that all material has a finite thickness, and the edges - which are not included in the stated surface area - absorb some of the sound. So for a piece of 703 fiberglass that is two by four feet and four inches thick, the real surface area includes the four-inch thick edge around the material. If included in the measurements, this would add four square feet to the stated surface area of eight square feet - a 50 percent increase!..."
In the current discussion which might apply to "CBA", he said this about designing plywood panel bass traps:
"The closer the fiberglass is to the plywood panel, the more effectively it damps the panel's vibration. But it is important that the fiberglass not touch the panel because that would restrict its movement. For a panel trap to absorb as efficiently as possible, the panel must be free to vibrate with no restriction other than the damping action of the nearby fiberglass"
This seems to contradict the CBA design. However this panel trap is a sealed cavity and the CBA is open, so it is different. I just don't know how to base a comparison of effectiveness if they use different test methods for the results.
markus76 said:
I meant the test results to be compared, not direct comparison.
They don't use the standard tests:
"Many published low frequency absorption coefficients are misleading, because the accuracy of the absorption efficiency decreases frequencies below 100Hz in standard ISO 354 and ASTM 423 test chambers, due to inadequate diffusion and modal density. Therefore, additional testing has been carried out at the Fraunhofer Institute in large scale impedance tubes with a cross section of 1.6 x 1.2 m and a special approach monitoring the decay times of the lowest axial modes in a 5x4x3 m room, with and without the sample present on one of the opposing surfaces. You can trust these data to be reliable and accurate."
http://www.rpginc.com/products/modexplate/index.htm
TRADERXFAN said:
Nevermind, I found it in chapter 4...
I am a little unclear though. It sounds like you are saying if you treat one opposing wall, it would not increase the total absorption if you treated the other opposing wall. Did I read that correct? Or does it say that if you take the same amount of damping material that you have on 2 opposing walls that it would equal putting the same amount of material on only one of the walls?
So question is, does it increase the total damping if both opposing walls are entirely damped rather than just one?
-Tony
TRADERXFAN said:
No, that is incorrect. Its the total absorpotion that counts, but it makes no difference which wall its on. That was the point. In my room only one of two opposing walls is damped. The other is rigid (corner of a basement). But thats fine because it works the same as if the same TOTAL damping were on both walls (but that, of course, would be impossible).
markus76 said:
Hehe, like your websiteJust wanted to give a link in case anybody wants to get in contact with someone that was actually there when the CBA was "invented".
Best, Markus
Markus
Sometimes you are a real pain in the axx! There is nothing on my web site that is untrue. Perhaps it is the exception, I don't know, but the CBA stuff in your link was very "misleading". That's either through ignorance or deliberate. I'll leave that distinction to you to sort out.