Riser Bass Trap, not against walls - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 72 Old 11-04-2011, 02:04 PM - Thread Starter
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I've searched through a bunch of the threads that discuss turning a rear-row riser into a broadband absorber. A common strategy is to add floor vents along the rear and side walls where the base is accumulating. In the attached image you can see that my planned room design calls for a riser that isn't against any walls.

Given that, I'm thinking that making a Helmholtz Resonator may be more effective; I was thinking that making one with fairly wide-band absorbtion would allow me to address the floor-ceiling mode (given that I have a large flat floor-parallel surface to work with) and that the length modes will be somewhat addressed by my planned superchunk traps in the front wall corners.

2 questions though:

1) The HR calculations account for End Effect, which to me indicates sufficient sensitivity that covering port holes with carpet (and possibly underlay) will dramatically alter the frequency targetted by a given hole size. True? How can I compensate for this? By hiring some consulting from Dennis Erskine perhaps?

2) Although reduced in total effectiveness can I still make the riser into a reasonably effective broadband bass absorber? Floor vents probably won't look great out in the open, but the sofa will cover a large portion of the riser. Perhaps swiss-cheesing the entire area under the sofa and on the front and sides will make it basically as good as it can be??? I've ran into many references about Dennis Erskine's article that apparently discusses the swiss-cheese approach but can't find an active link to it; does one still exist?

A final question unrelated to the bass trap is whether that step is creating too large of a tripping hazard, as it's been said that a cut-in step is less than desirable. What would be an alternative to that?
LL
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post #2 of 72 Old 11-07-2011, 11:02 AM - Thread Starter
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I had another thought as a possible construction alternative... Does anyone know of a material that could be used for the face and floor of the riser that's mostly porous but that could be carpetted over? Like some sort of metal grating perhaps, which would basically allow any sound hitting it to pass right through to the large block of insulation beneath... Something like this perhaps: http://www.mcnichols.com/products/tr...ontentproducts
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post #3 of 72 Old 11-08-2011, 01:45 PM - Thread Starter
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I think McNichols would have some grating products that would work, but it's looking like the cost would be around $1500 for what I think would work best (fiberglass mini-grid), so I'm going to revert back to basics and try to build a broadband absorber as effectively as possible.

New plan is to use 1" OSB or MDF, and then while it's still in a 4x8 sheet, use a skill saw to slice it into a grating while leaving the perimiter in tact for fastenning. I'll test how wide of a gap I can have while having my high-density 10lb underlay and carpet able to be laid over top of it and take foot traffic. Then hopefully the 1" MDF will have enough strength I can get up to about 50 or 60% open space through the whole platform.

My goal is just to let as much sound into the riser and insulation as possible. Does anyone have any thoughts? Is there a material that would let frequencies < 150 hz or so through that I could layer on top of the MDF to help the carpet bridge a larger gap? Maybe some kind of vinyl or something???
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post #4 of 72 Old 11-08-2011, 01:54 PM
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Wish I had an answer for you (and myself). This question gets asked a lot and I do not think there is a boilerplate answer for bass trapping in risers. I would also like to use my riser as a trap but its hard to get any answers on this topic.
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post #5 of 72 Old 11-08-2011, 03:23 PM
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Quote:
Originally Posted by stockmonkey2000 View Post

I would also like to use my riser as a trap but its hard to get any answers on this topic.

After I read the question laid out in this thread yesterday, I did an hour or so worth of searching. I agree with you entirely: there is not much conclusive in forums about this.

What I gleened from my reading is that people have a lot of trouble tuning resonators. Especially if you read at gearslutz, you'll find a number of people who have built tubes intended to work as Helmholz resonators and many of them have failed. It seems a lot of people conclude that the physics just doesn't play out to work in the real world. I don't personally buy that; I think they've just done a bad job tuning the thing. There are two or three things that I haven't figured out the impact of in designing a tuned resonator for bass trapping. I know that if you're looking at multiple opening or slits in your riser the formulae are different, but this was what I could find most readily.

Here's a conceptual description of how they should work: http://hyperphysics.phy-astr.gsu.edu...es/cavity.html

Here's what seems to be a good article with some important technical details and tips for construction: http://www.audioholics.com/education...onant-absorber

Some of the things people suggest matters, though I haven't quite worked out how, are cavity volume (because it's impractical to tune a resonator by changing the volume in most cases - unless you build it like a trombone I suppose), positioning (within a room), and the shape of it. Also, as an observation, the physics is clear that port length compensation must be carried out. If I remember correctly, you can find discussion of it at hyperphysics, and it amounts to adding about 73% of the port diameter to the actual length when calculating the functional length.

I would assume for bass, the larger volume the better to start with.

People suggest placing the absorber where the sound is loudest. I think that makes good sense.

It seems that shape and wadding or insulation both act to change the tuning of the absorber through it's Q. A Helmholtz resonator can have a very high Q, with an effective range of only 5 or 10 Hz, but if the shape is more angular or convoluted or if insulation is added the Q comes down rapidly. The maximum potency decreases and the bandwidth increases at the same time.

So it seems like if you want to use the space ocuppied by your riser for broadband trapping, you want as much free movement of air into it as you can get and you should just fill it loosely with fluffy pink insulation. Ethan Winer would probably endorse this approach, as he seems to always believe that more bass trapping is better. On the other hand, if you have one particular room mode you need to deal with, tuning a resonator is not for the faint of heart.

For my personal take-home message so far: If you build a riser with open sides or edges, it can be used as a bass trap just like a soffit or corner trap. On the other hand, for Helmholtz applications a riser is a foolish choice.

For recommendations to the OP based on my reading: I'd suggest opening the sides (the riser, in stair terms, not the tread) as much as possible, and leave the covering as thin as possible - definitely no carpet pad. If you fill it with insulation, it will tune the same way as a corner or wall trap - the larger and thicker the better.

Fred
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post #6 of 72 Old 11-08-2011, 03:36 PM - Thread Starter
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Thanks for the thoughts Fred; I think I've probably read all the same stuff on it that you have (it's a limitted pool after all). At the end of the day, it's all about the air flow through the ports and the speed at which that will happen. Clearly putting carpet over a large hole will make it act as though it were a smaller hole, but who knows to what extent? I flirted with the notion of leaving the front bare paint so I could change the ports as needed but in the end I'm leaning against the Helmholtz resonator as well. Even the diagram here http://www.bobgolds.com/HelmholzRiser.GIF is flawed because it doesn't account for the fact that the surface area of each cavity determines what sound inside a room will even strike the surface to be absorbed. Area 4 is inherently only 1/8 as effective at absorbing it's frequency as area 1 is, yet in the chart they are added together equally.

So bottom line, I'm trying to get as much free air movement as possible and I'll just fill it with fluffy pink. I'm going to pursue the horizontal slat concept I posted about earlier, and I have a question to Ethan on his forum about whether I can expect 50% of all frequency to travel through a grating that has 3/8" of wood and air alternating...
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post #7 of 72 Old 11-08-2011, 05:32 PM - Thread Starter
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New update... The horizontal slat concept won't work. After subscribing and downloading this article http://apl.aip.org/resource/1/applab...sAuthorized=no and understanding about 5% of it, that 5% leads me to conclude that certain resonant frequencies will actually penetrate 100% through the surface, but with the size of the slats being smaller than the wavelength, most of the wavelengths will get reflected...

Hmmm... So I'm down to the simplest option which will be to make openings as large as possible in the front and side verticals, and then also make openings underneath where the seating will be...
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post #8 of 72 Old 11-08-2011, 06:19 PM
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If to make it work you have to tune to a frequency then I can see why there is not much in the way of constructive advice on how to make these. The problem I would have is I have no way of knowing what the response of my room is going to look like until I have seating and everything done - By then its too late to do anything to the riser. It seems like it is anyone's best guess as to how these will actually perform.
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post #9 of 72 Old 11-09-2011, 12:20 AM - Thread Starter
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Actually, I might have to revise my last line of thinking that nothing would get through. Reviewing the Transmission graphs, it actually looks like in a scenario where 11.1% is open space, as the frequency of sound moves toward being 5x longer than the "on center" width (or Period of grating), the Transmission percent climbs toward 10 or 11%... Those graphs are heavily affected by the thickness of the panel though, which I guess is what would be interesting to a physicist...

Geez, I'd probably have better luck setting up a speaker, an MDF board, and ETF outside on a dry day and testing than I would understanding the theoretical physics behind this. Are there any mathematicians here? From the article, one should be able to calculate the expected effects at sizes and frequencies which mean something to use trying to trap bass in a riser.

If anyone wants to check it out, you can sign up for a free trial and at least give the article a read through...
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post #10 of 72 Old 11-09-2011, 12:24 AM
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Quote:
Originally Posted by Felgar View Post

New update... The horizontal slat concept won't work. After subscribing and downloading this article http://apl.aip.org/resource/1/applab...sAuthorized=no and understanding about 5% of it, that 5% leads me to conclude that certain resonant frequencies will actually penetrate 100% through the surface, but with the size of the slats being smaller than the wavelength, most of the wavelengths will get reflected...

Hmmm... So I'm down to the simplest option which will be to make openings as large as possible in the front and side verticals, and then also make openings underneath where the seating will be...

Felgar, I'm in the same boat as you are. I've had difficulty getting a solid answer to building a bass trap riser. My riser, recently built, has holes cut in the back facing the wall and cut out vents on the back part of the platform. Also my middle support beams do not touch the ground so that sound can pass through the whole riser. I'm hoping that will take of some of the bass. So do you plan on cutting holes in the front part of the riser facing the screen and speakers? I still have time to do this since it's not carpeted yet.

Sunny Surrey, B.C. Canada
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post #11 of 72 Old 11-09-2011, 05:08 AM
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There have been many attempts to purpose a seating platform into a tuned resonator. I've seen one design where the platform was designed as a low pass filter. It can be done; but, the number of variables are staggering. A Helmholtz type resonator requires assurance the air flow is exactly as predicted. A tough job without knowing exactly what the carpet and padding will do ... and what effect the seating will have.

A broadband approach is easier and my suggestion is you stick to a broadband type that is most effective on the modal frequencies in the room. Such a design, however, requires the platform be directly adjacent to the back wall and at least one side wall.

Look at the attached photo ... look along the left wall, on the platform just beyond the steps ... you'll see a long black "thing" ... that is a nailor bar diffuser that opens the air cavity in the platform to the room. There is one on each side and two in the rear. I could adjust the vents in each diffuser. In the end, all of the modal frequencies were knocked down and with only very minor PEQ, ended up with smooth bass response in all seating locations.

If your platform is not adjacent to a side and rear wall, you can only count on some impact on height modes.
LL

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post #12 of 72 Old 11-09-2011, 07:15 AM
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Dennis, couldn't you find a better looking room to illustrate your point? : )

That room is just ridiculous. Never get tired of looking at it.
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post #13 of 72 Old 11-09-2011, 07:28 AM
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Sorry .... it's one of the few where the vents are actually in the photo.

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post #14 of 72 Old 11-09-2011, 07:47 AM
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Dennis,

Is the air space under the platform one big volume or was it partitioned into smaller volumes?

An Aspen Woods Theater - Under Construction

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post #15 of 72 Old 11-09-2011, 09:35 AM - Thread Starter
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Quote:
Originally Posted by STP88 View Post

My riser, recently built, has holes cut in the back facing the wall and cut out vents on the back part of the platform. Also my middle support beams do not touch the ground so that sound can pass through the whole riser. I'm hoping that will take of some of the bass. So do you plan on cutting holes in the front part of the riser facing the screen and speakers? I still have time to do this since it's not carpeted yet.

By my understanding you've done what you can in the rear which is where most of the benefit is, and in a broadband application you've built it best where the beams don't touch the ground.

Yeah, I'm definitely going to cut holes in the part facing the screen; this will actually be my area of greatest effectiveness because I can leave large open holes that are carpetted over. It's a relatively small surface area though, but 9 square feet of near 100% absorbtion (passing through 5' of insulation from front to back) can't hurt...

Quote:
Originally Posted by Dennis Erskine View Post

A broadband approach is easier and my suggestion is you stick to a broadband type that is most effective on the modal frequencies in the room. Such a design, however, requires the platform be directly adjacent to the back wall and at least one side wall.
...
If your platform is not adjacent to a side and rear wall, you can only count on some impact on height modes.

Yeah, in my case I'm looking to maximize the effectiveness of the absorbtion while not having it against the wall. I guess in this case maybe the gains aren't worth the effort and most people just build it as a solid box. Right now my main consideration is in how I can best construct it with having as much open area on the top as possible; the largest question being the physics behind a much large wavelength passing through a small hole (say, 3/8")... That's where that article comes in...
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post #16 of 72 Old 11-09-2011, 11:13 AM
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Perhaps you want to consult your local public library? A good reference librarian can help you find the literature you want.
http://www.sciencedirect.com/science...22460X69901758
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post #17 of 72 Old 11-09-2011, 01:48 PM
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So Dennis, just to double check, it's okay for me to cut holes in front of my riser to make it a broadband absorber.

Sunny Surrey, B.C. Canada
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post #18 of 72 Old 11-09-2011, 02:15 PM
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Quote:
Originally Posted by Dennis Erskine View Post

There have been many attempts to purpose a seating platform into a tuned resonator. I've seen one design where the platform was designed as a low pass filter. It can be done; but, the number of variables are staggering. A Helmholtz type resonator requires assurance the air flow is exactly as predicted. A tough job without knowing exactly what the carpet and padding will do ... and what effect the seating will have.

A broadband approach is easier and my suggestion is you stick to a broadband type that is most effective on the modal frequencies in the room. Such a design, however, requires the platform be directly adjacent to the back wall and at least one side wall.

Look at the attached photo ... look along the left wall, on the platform just beyond the steps ... you'll see a long black "thing" ... that is a nailor bar diffuser that opens the air cavity in the platform to the room. There is one on each side and two in the rear. I could adjust the vents in each diffuser. In the end, all of the modal frequencies were knocked down and with only very minor PEQ, ended up with smooth bass response in all seating locations.

If your platform is not adjacent to a side and rear wall, you can only count on some impact on height modes.

Dennis,
I cannot see the vents very well. Are you referring to the vents being installed on the plywood/platform of the riser. I was planning on putting holes in the framing underneath the platform, exposing the riser to the small gap between the framing and the wall itself.
I am not sure if I am answering my own question but the only issue I see with this is that the holes in the framing are not directly exposed to the surface of the wall and the top platform of the riser (areas of higher particle velocity).
Also, assuming the corners of the room are the areas of highest particle velocity, wouldnt it be beneficial to expose the back corner (of the platform) as well. Using corner traps are common for broadband absorption. Why not make a nice size triangle cut out (the size of the trap itself) underneath the trap?
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post #19 of 72 Old 11-09-2011, 02:27 PM
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Also, assuming the corners of the room are the areas of highest particle velocity,


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post #20 of 72 Old 11-09-2011, 03:34 PM - Thread Starter
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Dennis,
I cannot see the vents very well. Are you referring to the vents being installed on the plywood/platform of the riser. I was planning on putting holes in the framing underneath the platform, exposing the riser to the small gap between the framing and the wall itself.

I believe the boundaries are where pressure is the greatest, and velocity is very low (how can something move when it's contacting a fixed wall)... At any rate, the concept of creating holes to allow air flow and sound flow from the room boundaries down into the absorbtion-filled riser is a correct one. The holes must be very large though, so as to avoid accidentally building a helmholtz resonator.

Basically vents are added on top of the riser along each of the walls that the riser is nearly in contact with. You can see the process documented here in this thread where there are even measurements of the positive results. https://www.avsforum.com/forum/showt...6#post15441346 Also correct is the concept that you want the gaps in the framing beneath inside the riser to basically turn the whole thing into one large container. The easiest way to do this is probably to build the "joists" (floor supports) with less height than the perimiter boards so that they are hanging and have a large gap beneath them.

I think I have the general concept down quite well but I stand available to be corrected by Dennis or local or anyone else. My particular complication (and the impetus for this thread) is that my riser doesn't extend to any wall, so I'm not sure how to maximize sound penetration into the riser when I don't have the benefit of a nearby boundary like you do.
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post #21 of 72 Old 11-09-2011, 03:54 PM
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Quote:
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Dennis,
I cannot see the vents very well. Are you referring to the vents being installed on the plywood/platform of the riser. ?

I'm with you - I'm not sure where the vents are in the photo.
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post #22 of 72 Old 11-09-2011, 03:57 PM
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I meant pressure. Sorry about that!!!
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post #23 of 72 Old 11-09-2011, 07:38 PM
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Dennis put these traps in the plans for my theater but I havent started the riser construction yet so will be following along way maybe he wont have to explain it to me right after explaining it to you all.
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post #24 of 72 Old 11-09-2011, 08:26 PM
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Dennis may have good results with air flow openings along room boundaries, and I have no measurements to prove that the following statement will always translate into better performance, but...

Broadband absorbers are porous velocity based absorbers. That is, they are only effective when air particles are in motion. At room boundaries, velocity is zero and pressure is at maximum. Absorbtion placed at room boundaries are effective only at wavelengths short relative to the absorber thickness. This is shown clearly with testing of spaced absorbers... move them further from the wall and their effective frequency range is extended lower.

Risers can leverage this behavior since they are large even relative to bass frequencies. But to be most effective, there must be free flow of air into the absorptive material. Flow is zero at wall boundaries, so this is probably the worst place to locate vents. Won't hurt, but the potential riser effectiveness just isn't achieved. Vents well away from walls, such as along the front face and perhaps under seating should be most effective. I'd place them here as well as along walls to give the best opportunity for air to move feely through the absorption.

Metal grating would work fine if its an area people need to walk on. Otherwise, big holes are the ideal.

And indeed there is no reason why a riser well spaced from all walls wouldn't be effective. In fact, for a broadband absorber that would be ideal. No disrespect to Dennis intended.

A tuned Helmholtz type absorber on the other hand would have to abut at least one boundary with vents placed as close to the boundary as possible. I wouldn't suggest trying it with a riser. Too big a risk that the whole attempt will be a failure and do more harm to the acoustics than no riser at all.

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post #25 of 72 Old 11-09-2011, 08:36 PM
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Quote:
Originally Posted by Felgar View Post

I believe the boundaries are where pressure is the greatest, and velocity is very low (how can something move when it's contacting a fixed wall)... At any rate, the concept of creating holes to allow air flow and sound flow from the room boundaries down into the absorbtion-filled riser is a correct one. The holes must be very large though, so as to avoid accidentally building a helmholtz resonator.

Basically vents are added on top of the riser along each of the walls that the riser is nearly in contact with. You can see the process documented here in this thread where there are even measurements of the positive results. https://www.avsforum.com/forum/showt...6#post15441346 Also correct is the concept that you want the gaps in the framing beneath inside the riser to basically turn the whole thing into one large container. The easiest way to do this is probably to build the "joists" (floor supports) with less height than the perimiter boards so that they are hanging and have a large gap beneath them.

I think I have the general concept down quite well but I stand available to be corrected by Dennis or local or anyone else. My particular complication (and the impetus for this thread) is that my riser doesn't extend to any wall, so I'm not sure how to maximize sound penetration into the riser when I don't have the benefit of a nearby boundary like you do.

I like the way you explain the concepts!!! I understand your explanations a little better. Regarding the thread that documents the riser done with vents, I wasn't sure if this was gold or not because of some contradictions from other specialists. The member himself was questioning whether or not his method was a good implementation for broadband absorption.

The information Dennis posted seems to compliment the ideals of that thread though.

I plan on putting 4" holes on the face of the riser, 3" in the braces as well as holes in the joists. I didn't want to cut vent holes on the top surface of the riser but it makes sense to do so because those areas are higher pressure.
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post #26 of 72 Old 11-09-2011, 09:06 PM
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Originally Posted by Felgar View Post

I believe the boundaries are where pressure is the greatest, and velocity is very low (how can something move when it's contacting a fixed wall)... At any rate, the concept of creating holes to allow air flow and sound flow from the room boundaries down into the absorbtion-filled riser is a correct one. The holes must be very large though, so as to avoid accidentally building a helmholtz resonator.

That's a pretty fair description. If a hole is adjacent to the wall, there will be a pressure maximum on both sides of the opening. The pressure inside will result in particle motion and flow through the porous material. The effect of the viscous losses will be to reduce the pressure inside the riser opening, and thus outside the opening as well. Acoustic energy will thus be sucked out of the room by way of lowering the maximum pressure at the boundaries.

But I would be very careful. I'm pretty sure that you are also correct in that what this creates is in fact a Helmholtz resonator. As the size of the holes increase (away from the boundary), more particle flow will enter them and the broadband effect will increase as the resonant effect decreases. Again, if you follow this through the conclusion is to simply place the holes relatively far from the boundaries.

Think of it this way: the ideal broadband absorber is a huge chunk of porous material with roughly the right gas flow constant (loose pink fluffy stuff probably as close as needed) placed somewhere in the room with at least some portions far from boundaries. Now, this is exactly what the filling of your riser is. The structure of the riser can only degrade the effectiveness from this ideal unless you truly are building a tuned absorber. And as the riser can only get in the way of the absorptive stuff, you want it as "Swiss cheese" as possible, especially for the parts far from room boundaries.

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post #27 of 72 Old 11-09-2011, 09:12 PM
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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.

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post #28 of 72 Old 11-09-2011, 11:27 PM - Thread Starter
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I'm very encouraged Bigus and I feel like I'm coming out the dark knowledge void I was in just 2 days ago with respect to all this. My intention was to do even better than swiss cheese on the front and back where I'd probably leave the whole face almost open, but maybe run two 1" vertical posts in each 16" joist section just to have a place to fasten carpet to. That would make about 5" for the carpet to span. Not sure on the sides but I'll open them up somehow as well...

My main question now is how best to open the top up as much as possible, understanding that it must support a sofa and also a walking path. Under the sofa I can certainly open up some nice big holes but if I do that it will be at the cost of future flexibity in terms of changing to individual seats or a different sofa. To the front and sides of the seating, I was contemplating the concept of making long slits. Suppose I cut out a 3/8" strip almost the whole length of the riser. Perhaps 1" thick OSB (worried about spills on mdf so I dont think I'll do MDF) and then I'd have alternating 3/8" space, 3/8 wood for the whole platform, or at least until the part directly under the seating with larger holes.

I guess I'm trying to figure out how to make a porous surface that can be carpetted over and then walked on. I ordered a high-density 10lb underlay that's made new (not recylced) and is a single piece of foam; I figure that will hold up way better being carpetted over small holes...
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post #29 of 72 Old 11-10-2011, 12:10 AM
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Lots of small holes is probably better than a few bigger ones as far as walking surfaces that are going to be carpeted over. Its possible I guess that there ccould be some hemholtz effect, especially if all the holes are equally sized and spaced, though most will not be in pressure maxima for most frequencies so I probably wouldn't worry much about it. One thought is to take a couple of drill bits in the 1/4 to maybe 3/4 range and go crazy. Or spend lots of time cutting slots. Or, something like the mc-whatever products you linked too earlier. The 3# load bearing expanded grating is $300 for a 4 x 8 sheet and has 60% open area. Of course it weighs 100 pounds too so who knows about shipping. Could probably find some suitable expanded metal grating at a local supplier depending on where you live.

Of course, I've never carpeted over something like that, nor have I tried to keep it quiet (rattling at contact points), and have no measurements to show. But should work... better than solid wood anyway.

But how tall is your riser? If it isn't very tall, it may not be very effective for height modes, and openings in the top decking may not be that critical.

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post #30 of 72 Old 11-10-2011, 03:53 AM
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