Sensible Riser Design?

Interesting design, I'd like to see what the bass trap engineers have to say.

I'm with BIG. I don't see the downside, but I don't have any confidence in my ability to predict the function at this level.

With all of that exposed 703 surface on the back wall, you're going to suck the life out of the room.

6" of 703 will also reduce the air flow into your under decking pressure absorber and you'd really need to do something on the left/right wall as well.

Best part I like of the design is the software used to make the pretty pictures! What software is that?

That's the point of the holes isn't it ... to act as a pressure absorber. When placed near the side/rear wall boundaries, you've created a pressure absorber which is most effective on modal frequencies. By covering your 6" of 703 you've created a diaphragmatic pressure absorber and prevented airflow into the under platform area. The predicted frequency range of effectiveness for your diaphragmatic wall absorber is likely over a narrow, but unknown range of frequencies. If I were to swag a guess, I'd think between 50-60Hz.

I made a nearly identical riser, with the difference being that the opening was on the surface and the riser butted up to the rear wall. So, I had, in essence, a register running along the entire back width of the riser that was 4.5" wide. This opening was covered with the 3 layers of 2" 703 (and 5 mil plastic) like you have. This way, I mimic, what several registers would have achieved. My theater is still being built, so I have not tested the results. I put 6" of pink insulation in the riser cavity, and kept it elevated with plastic twine that I zig-zagged between joists. Make sure that the pink insulation does not drop to the floor - you want to maintain easy air flow at the bottom of the riser.

There are two professionals (that I am aware of) who use the riser as a method of resolving low frequency issues in rooms. Those are Tony Grimani and myself and no, neither of us have published test results, neither of us are going to. Further, of the various approaches we have used, none of them is "better" than the other but each one of them individually is "best" for the specific room. If you're telling me your openings into the platform have 6" of OC703 blocking them, my response to you is don't waste your time cutting the openings.

Let me re-phrase the first sentence ... There are only two professionals who independently developed methods of using the riser to address low frequency room response and have refined those various methods over tens of years designing and calibrating small rooms.
Edited by Dennis Erskine - 1/15/13 at 1:32pm

Similar to what one above. Have I measured the effect of the ones we build and install? The answer is yes.

What if you stopped the OC703 about 8"-10" from the top of the riser? That way you don't have the restrictive element blocking the opening to the cavity.

I agree. However, I don't know the specifics of the designs that Dennis is referring to (his or Mr. Grimani's), and I'm only assuming that the fluffy insulation is immediately under the vents. if the insulation is suspended, and the vents are open to the floor........ Well, you get the idea. Unfortunately, without measured data or a valid model, we're just grasping at straws. There's really no way to know for sure without building and testing it.

There are a lot of variables that I don't have a feel for their importance in the design. I'm just rambling here, but one question that comes to mind is does the cavity within the riser provide enough volume to allow the wave to propagate freely within it? Does the wave develop a velocity component within the riser, or is the energy dissipated very near the openings? If we suspend the insulation, does the few inches between the floor and the insulation provide enough space to allow the wave to propagate again (how does a 20' wave act in a 12" tall riser)? I suspect there is more going on than meets the eye, though. I know in the EM domain, the waves only "see" features that are approaching a fraction of a wavelength. The point being, will a 60 Hz wave even "see" a 6" wide opening in your riser? I could ramble for hours about this; but unfortunately, I don't know the answers to even the few questions that have come up since starting this post If only I had unlimited time and resources, we could get to the bottom of this

J_P_A, I'm about to step out of my depth here, but I think you're hitting on the issues I have tried to bring up in threads of this sort before. It seems to me that you and most people here want to think of riser traps in the same ways you think of superchunks and other velocity-based absorbers. As Dennis has said (though I think is it generally missed by most in casual reading), his designs are pressure-based absorbers. Pressure-based absorbers do not, I think (again I'm out of my depth), require complete wave propagation. The sound wave, as we normally think of it, does not "enter" the cavity. Pressure changes at the mouth of the cavity induce resonance within the cavity.

If I continue to try to describe this, I'll be wrong and sound overly pedantic, and I don't want to give the impression that I'm right and I'm correcting you are anyone on any particular details; my point is that the proper conception of a riser as an effective bass trap is not the same as common resistive absorbers, and requires an entirely different approach and mindset.

Of course, I could be wrong too...

Fred

I understand what you're getting at, and you may be absolutely right. My thought, however, is that if there is no propagation of the wave inside the riser, we will see a pressure differential at the opening. There will be a high pressure region at the boundary, and a pressure gradient across the insulation at the opening back to atmospheric. With no velocity component remaining the wave can't propagate into the riser and take advantage of the extra insulation there. That raises another question. How far does that gradient extend into the riser? If the gradient extends all the way to the next rigid boundary, then BINGO, you've got it nailed! However, if that's the case, why would covering the openings into the riser with insulation prevent it from functioning? There should still be a pressure gradient across the OC703 just as there is with regular old fiberglass.

Now, perhaps you are correct that there is some resonant phenomena going on here that I don't see yet. That's certainly an interesting thought, and I'm trying to see how the energy at one frequency can be "moved" to other frequencies by inducing a resonance in the cavity. I'll have to think on that a while.

Again, I really don't understand exactly how these things work. On the surface it seems simple, but when you get down to the details it seems to get complicated pretty quickly.

Again, I resist presenting physics here as though I were well-informed. I believe that Helmholtz is the proper fundamental concept to apply here. This web-page seems thorough - but I'm not sure what sorts of changes to this theory are necessary to adapt the device to absorption in such a large device. http://www.phys.unsw.edu.au/jw/Helmholtz.html

Relevant in particular to your remarks here J_P_A I think is this section I've copied. Again, I'm not sure how having multiple openings or low GFR porous material changes the particulars of any of this.

Hmm. Interesting. It seems like I remember reading that we use several large holes in order to prevent inadvertently building a helmholtz resonator. However, I can't argue with the similarities here. Wouldn't a resonance like that become audible, though?

http://www.audioholics.com/education/acoustics-principles/helmholtz-resonant-absorber
As a corollary, look at the design of intake air ducts in lots of modern cars. Many of them will have dead end canisters built on the side of the pipe. http://www.deicon.com/Helmholtz%20Resonator%20a%20tuned%20acoustic%20absorber.pdf

To answer your question specifically, I don't know. I suspect either it doesn't matter, or it's not audible. Maybe I'll find out if I go forward with my plan to house the cavity of a Helmholtz absorber outside my theater, with the neck coming through the wall.
Edited by HopefulFred - 1/18/13 at 6:37pm