Riser Bass Trap, not against walls - Page 3 - AVS Forum
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post #61 of 71 Old 11-15-2011, 08:46 AM - Thread Starter
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Interesting product longshore... Clearly that would support the weight and has plenty of open space. I'd need 5 packs of the 16" square panels which would come pretty close in cost to the birch plywood I purchased. Not sure how well the carpet would hold up over top of it but it looks like about 20 bars in 16" so probably about 7/8" holes. With the right mesh over top it might hold up pretty well. Certainly much less effort than cutting plywood.
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post #62 of 71 Old 11-15-2011, 10:03 AM
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What if you were to do a fabric covered panel as the front face of the riser instead of carpet? This would be much more porous.

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post #63 of 71 Old 11-15-2011, 12:28 PM - Thread Starter
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Yeah that would be an option, and I'd pursue that if I were still thinking to do the Helmholtz resonator. For a broadband trap, it's just about whether the sound will be reflected or not. I'm quite sure that carpet will either absorb the sound or let it through; very little will be reflected. Whether it's the carpet absorbing energy or the insulation in the riser, I'm happy so long as it's being absorbed.
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post #64 of 71 Old 11-15-2011, 01:24 PM
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Felgar,
Any chance you can take a couple pictures of how you slot the wood?
I found that product is also carried at home depot in larger packets. However, I found some aluminum grating available on Craigslist that I might try as well.

My plan is to make the top and front out of some sort of grating and then fill the whole riser with insulation.

I was under the impression that low frequencies (like below 250hz) would penetrate any carpet and pad placed on the riser so long as there are plenty of holes for them to enter.

If I have an 8' x 12' box made of grating, filled with pink fluffy stuff, and then carpeted(with pad) would that be a great bass trap?
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post #65 of 71 Old 11-15-2011, 04:39 PM - Thread Starter
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Quote:
Originally Posted by longshorejl View Post

I was under the impression that low frequencies (like below 250hz) would penetrate any carpet and pad placed on the riser so long as there are plenty of holes for them to enter.

If I have an 8' x 12' box made of grating, filled with pink fluffy stuff, and then carpeted(with pad) would that be a great bass trap?

That's my understanding as well, and I hope it makes a great bass trap because that's pretty much what I'm doing also. I'll certainly take pics and document the build here as I go, and I'll also do before/after room measurements so the effect is well-documented. This coming weekend will probably be dedicated to painting but I hope to start assembling early next week.

As for the grating, what are your thoughts on hole sizes and how large of a gap can be spanned by carpet and underlay? I picked up a small scrap from the carpet place to test but haven't had time to yet. I'm expecting that with the 1/4" mesh, probably 5/8" or 3/4" should be able to be spanned, but we'll see.
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post #66 of 71 Old 11-15-2011, 05:31 PM
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I am considering this:


ALUMINUM BAR GRATING
3/16 X 1" BARS
MEASURE 27.5" X 30"

My thought is I will build a frame that allows the grating to sit flush with a wood edge. Beams will be 24" OC. The Aluminum bar grating will get cut down to 24" x 30". I will build it such that the carpet can be stretched to the edges and fastened to wood. I think with the right pad the carpet will not go down into the individual holes. I'm going to do some searching on the net though. I'm still considering the Attic Dek mentioned earlier just because it seems easier.

My big concern is we're the only ones that are doing this. There have been so many risers built that a calculator exists yet know one has tried this? The only ones I've seen have used vents at the wall... and they appear to be successful. Good luck with your build.
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post #67 of 71 Old 11-15-2011, 09:23 PM - Thread Starter
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Same to you longshore! Definitely look into the wire mesh. I got mine at a local Rona (canadian home depot type store) so I expect Home Depot or Lowe's will have it also. Comes in rolls, with 1/4 or 3.8" grid. With that over the grating the carpet should hold up well.
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post #68 of 71 Old 11-15-2011, 09:36 PM
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Lol, this would be a great time for local or dragon or ethan et al to stop by and comment on the basic theory of broadband absorption I have described.

But I do believe this is correct, and I do believe the conventional wisdom given by erskine and pretty much everyone else on avs over the years just doesn't make sense. I can assume only that either I am off my rocker, or simply that no one has stopped to think about what is going on for a moment.

The worst case that I think you might encounter is having too good of a broadband absorber. Its likely that the carpet will normalize the aabsorption to what a non trap riser might do in the upper frequencies. In the low midrange and down, it is possible that you'll get too much absorption if say the entire riser is open grating. A facing membrane over portions could vary that. Or you could just membrane all the grating. I dont have handy measurements of the absorption, reflection and transmission spectra for typical carpet and pad. That would help. And I guess its possible to be overly absorptive at bass frequencies but I think unlikely.

Optimally, you would build the riser and measure before final carpeting. Any indication of over absorption could be dealt with by simply "plugging up holes" to one degree or another.

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post #69 of 71 Old 11-29-2011, 08:00 AM
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Quote:
Originally Posted by Bigus View Post

Felgar, no need to worry about the riser not contacting the walls. If you are trying to make the holes as big as possible so as not to create a resonant absorber, what could be better than just moving the riser away from walls?

Can you Swiss cheese the faces of the riser facing but not against the walls? Do this as well as the forward face and you should be golden.

If a riser is not close to any walls, and all four sides of the riser are vented, is there any need to vent the top deck under the chairs? My riser will be 8" tall. Also if the inner wood supports are off the floor leaving a 1" air gap ( causing it to be 1 big chamber) is there any need to cut large square holes in the left to right mounted floor joists? With the riser only being 8" high, not sure it's worth venting anywhere? Venting the sides, front, back or the internal joices will weaken the structure due to the small sized 2"x6" being used with 2" top deck? Maybe vent top deck under chairs only?
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post #70 of 71 Old 11-29-2011, 09:42 AM
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Quote:


I can assume only that either I am off my rocker,

Do you even own a rocker?

By definition, modal frequencies are at their highest pressure and lowest velocity at a wall boundary. Hence, platforms against the walls can use surface vents into the platform to directly affect ("knock down") room modal response differences in the seating locations. Frequencies creating nulls in the seating locations would be the first focus. Those creating peaks in the seating area are secondary since other mechanisms are available to resolve peak issues. (Clearly, a frequency creating a null in the seating area, could also create a peak in a different position in the seating area.)

Platforms can be, and have been, used as low pass filters or for broadband absorption. Typically, you'll see large areas of the platform surface in steel grating where broadband is desired, or a hole pattern (ie, 1/4" holes every inch of the platform surface) where the platform is to function as a low pass filter. Very clearly, the nature of the carpet and pad is a very important consideration where the intent is "broadband" or "low pass". The restriction of air flow can just as easily work against you as for you.

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post #71 of 71 Old 11-30-2011, 04:19 PM
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Pertaining to the sides, front or back of a riser, it seems that some use unblocked metal vents so air flow is best. Others carpet over vents (I assume not using any metal vents, just an open hole) I would think it would be best to not use a carpet pad in front of the holes. Aesthetically, carpeting over the holes certainly looks best. Not sure how much of an acoustical function hit it takes.
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