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Sensible Riser Design?

post #1 of 22
Thread Starter 
Planned riser at the back of my 21' long x 13' wide x 7.5' high room is shown below. I'm hoping to get some broadband trapping (none of that helmholtz craziness). I have basically removed the face of the rear riser perimeter to provide as much open area to the pink fluffy (a la swiss cheese). The platform is also approx 6" away from the rear wall. Is this structurally sound? Is this a good way to open up the riser rather than cutting a bunch of holes on the platform covered with unsightly registers? Thanks for any feedback.

The riser as viewed from the back of the room (rear wall removed to illustrate):

Showing the rear wall treatments and how the 6" thickness covers the gap left between the rear wall and the riser:

Side angle showing openings for bass trapping:
post #2 of 22
Interesting design, I'd like to see what the bass trap engineers have to say.
post #3 of 22
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.
post #4 of 22
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.
post #5 of 22
Thread Starter 
Originally Posted by BIGmouthinDC View Post

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

Thanks Big, so structurally at least no issues?
post #6 of 22
Thread Starter 
Originally Posted by Dennis Erskine View Post

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.

Thanks Dennis, however with my question focused on the riser I neglected to indicate the acoustic design calls for the 703 to be covered with 5 mil plastic on the back wall.

If the 6" 703 reduces airflow then perhaps in doing so it has already done part of the work to knock down those lower frequencies. What does make it through could then be further handled by the riser absorption? If I went with the alternative of cutting holes in the decking in front of the 703 (6" inches away from rear wall) then wouldn't the advantage of maximum pressure, minimum velocity at the wall boundary be minimized?
post #7 of 22
Best part I like of the design is the software used to make the pretty pictures! smile.gif What software is that?
post #8 of 22
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.
post #9 of 22
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.
post #10 of 22
Thread Starter 
I have seen various conflicting opinions from highly respected individuals on these forums when it comes to riser designs. I suppose that's a sign that no published tests of various approaches has ever proven one better than the other. I guess I'll go with the feedback from the acoustic expert I'm paying for which means sticking with the design I came up with above and also completely filling the riser with pink stuff.
post #11 of 22
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
post #12 of 22
Thread Starter 
In the 10 years of testing and refining have you ever measured the effect of a riser similar to the one above?
post #13 of 22
Similar to what one above. Have I measured the effect of the ones we build and install? The answer is yes.
post #14 of 22
Thread Starter 
If the riser design shown at the top of this thread has never been tested then perhaps it shouldn't be so hastily dismissed as ineffective. If anyone out there has some spare time then maybe they could cover their rear wall vents with OC703 and measure the differences in LF between the covered and uncovered vents.....biggrin.gif
post #15 of 22
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.
post #16 of 22
Thread Starter 

Hmmmm, interesting thought J_P_A !

I'm just having trouble with the whole "remove the restrictive/absorptive element" so that the LF waves can get access to..........'more restrictive/absorptive material' in the riser.
Is the absorption on the back wall a 2nd class citizen to the absorption in the riser?
post #17 of 22
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 rolleyes.gif If only I had unlimited time and resources, we could get to the bottom of this biggrin.gif
post #18 of 22
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...

post #19 of 22
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.
post #20 of 22
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.
Now let's get quantitative:

First of all, we'll assume that the wavelength of the sound produced is much longer than the dimensions of the resonator. For the bottles in the animation at the top of this page, the wavelengths are 180 and 74 cm respectively, so this approximation is pretty good, but it is worth checking whenever you start to describe something as a Helmholtz oscillator. The consequence of this approximation is that we can neglect pressure variations inside the volume of the container: the pressure oscillation will have the same phase everywhere inside the container.
post #21 of 22
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?
post #22 of 22
Excited by a frequency relative to it's tuning, the air mass in the port tries to leave. It's motion, in turn, lowers the pressure of the air mass in the enclosure - which consequently attempts to draw the port's air mass back into the cavity. These motions are, naturally, lossy - the activating frequency (which, in reality, is a room mode), in perpetuating these motions, itself becomes lossy - hence the absorbent action.
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
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