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Discussion Starter #1
About to start construction on a 9.5' x 6.6' riser for second row seating. Frame will be constructed of 2x8's and decking will be three layers of 3/4" plywood (RTD Sheathing). Will be close to 10" in height once carpeted. Room is 18'x12'x9'. Two questions:

(1) Should I use wood glue, liquid nails, or roofing paper in between the layers of plywood?

(2) I've seen examples of the riser having 2" holes cut in the front frame for bass response I assume. I'm not installing butt kickers or anything like that and the interior will be insulated with R30 insulation. Are the holes going to make any difference?

Thanks all!
 

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1) roofing felt
2) maybe maybe not. The proven location for "holes" is along the perimeter at the wall riser intersections. Not holes but large 4x 36 to 48 inch vents. For this to work the interior of the riser needs to be one connected air space. Use 2x8s for the perimeter boards but 2x6s for the stringers, add extra support legs in the middle of the 6.6 long stringers, use the scraps you cut from the 8 ft boards.
 

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Discussion Starter #3
1) roofing felt
2) maybe maybe not. The proven location for "holes" is along the perimeter at the wall riser intersections. Not holes but large 4x 36 to 48 inch vents. For this to work the interior of the riser needs to be one connected air space. Use 2x8s for the perimeter boards but 2x6s for the stringers, add extra support legs in the middle of the 6.6 long stringers, use the scraps you cut from the 8 ft boards.
Thanks!!
 

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About to start construction on a 9.5' x 6.6' riser for second row seating. Frame will be constructed of 2x8's and decking will be three layers of 3/4" plywood (RTD Sheathing). Will be close to 10" in height once carpeted. Room is 18'x12'x9'. Two questions:

(1) Should I use wood glue, liquid nails, or roofing paper in between the layers of plywood?
Many folks also recommend using Green Glue (or Quiet Glue, etc. - a viscoelastic dampener of some sort). However, I'd still use roofing felt (you can do both).

The roofing felt (30# variety) will add ~1/16" depth per layer, or 1/8" total in your case (3 layer sandwich).

Also make sure you use roofing felt under the entire riser. Technically, you only need it under any perimeter joists, stringers, or stringer support blocks that touch the floor beneath it. This is to prevent squeaks. If you're placing the riser on carpet then disregard.

If you plan on utilizing your riser for secondary duty as a bass trap, unless you want a taller riser you should consider 2x6 stringers. The reason why is you need an air gap beneath the stringers. It allows the entire volume to become a bass trap. Fill with loose insulation or mineral wool.

Space your stringers 16-24" O.C. if going the bass trap route. It will make laying the insulation easier.
 

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Use 2x8s for the perimeter boards but 2x6s for the stringers, add extra support legs in the middle of the 6.6 long stringers
Big,

This reminds me... I haven't seen anyone provide detailed recommendations on the minimum or maximum stringer gap-to-floor when designing a riser. I would think that a larger gap is better than a smaller gap, from the standpoint of allowing sound waves to be absorbed by the insulation and make their way out the diffuser vents (if installed). OTOH, too big a gap and short stringers could make for a bendy riser floor.

So, question for you is what are your thoughts on the height of the gap below the stringers? What's your typical go-to height and why? Is there a rule-of-thumb???
 

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Use your scraps for legs
Jeff,

Why is it that your riser picks always (I believe) show the stringer supports adjacent to the stringers and attached horizontally, versus placed underneath the stringer?

Is there a reason that you do that versus simply placing small blocks beneath the stringers?

David
 

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I usually do what is the quickest, and I rarely have 2 inch blocks sitting around. Usually I have a pile of board cut ends. Many times the floor is uneven so it would be hard to cut them all the same size. However this time I just ripped some 2 inch strips for this beast.

 

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(2) I've seen examples of the riser having 2" holes cut in the front frame for bass response I assume. I'm not installing butt kickers or anything like that and the interior will be insulated with R30 insulation. Are the holes going to make any difference?
Hi rkb,
I didn't see anyone address your second question. Here is a comment from Nyal Mellor on a thread of mine asking about floor grill styles when using a riser for bass pressure absorbing. He indicates the holes do indeed help -- "as many as possible."

http://www.avsforum.com/forum/19-dedicated-theater-design-construction/2256306-riser-vents-most-open-space-linear-style.html#post39992634

Before I carpet my riser I'll be embarking on a drill-fest with a 1 1/4" bit and the vacuum handy ... my top layer is 2 layers of 3/4" OSB with Green Glue in between, so that will be a lot of fun LOL
 

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Here is a comment from Nyal Mellor on a thread of mine asking about floor grill styles when using a riser for bass pressure absorbing. He indicates the holes do indeed help -- "as many as possible."

http://www.avsforum.com/forum/19-dedicated-theater-design-construction/2256306-riser-vents-most-open-space-linear-style.html#post39992634
Plan carefully before going to town with a drill. One should to take into consideration the structural integrity of the riser floor. You don't want to go too crazy making holes (even 1" holes) or you could run weaken the floor too much, especially if they are in close proximity to one another. For instance, if you have 24" O.C. stringers, it's going to be more of a concern than if you have 16" or 12" O.C. stringers. Likewise, one obviously shouldn't carpet over vents that are large enough to place an HVAC register into.

The purpose of using a riser as a bass absorber boils down to pressure relief, which is what all the holes are for. The greatest pressure build-up is at the room boundaries. So, while making holes all over the top of the riser might help, the greatest benefit is making holes in the right places - where the pressure is greatest.

The key is to focus on corners and floor/wall intersections first, as that is where the greatest pressure build-up occurs.

Dennis Erskine has provided insight into riser vents on several occasions over the years, and I have yet to see any comments by him discuss making holes all over the top of the riser. Not that there's anything wrong with that (within reason) but just sayin'. Here are the high points that I've gathered from multiple Dennis' AVS comments on this subject:

• Riser needs to touch at least one wall in both directions (front:back and side:side)
• Riser should not touch walls (built approximately 1/4" from walls)
• Cut holes completely through the decking (for vents)
• Holes should be centered between the joists and 4" x 10" in size or larger
• Be absolutely sure you do this in the corners (where the bass response waves are most intense)
• Get several pieces of lightweight black fabric and place it in the holes; spread flat to hide fiberglass
• After carpet goes down, insert standard bar (linear) HVAC registers (e.g. Nailor)
• This will reduce high pressure of some modal freqs (mostly bass)
• Corners first, then walls
• Height matters (i.e. don't bother if riser
 

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The reason the edge is where the holes must go is that this is the area of highest pressure and lowest velocity. This is where resonant absorbers work best. The Riser absorber approach is a resonant absorber, at least that is my take. It must be, even a leaky box with a small opening resonates. If it was not a resonant absorber but rather a velocity absorber, then the holes would actually be better placed farther from the walls where velocity is highest and pressure is lowest.


The reason small holes are bad is that the tuning frequency becomes very low to the point of being ineffective. Technically if you had enough small holes it actually would be fine. There are ways to calculate it, but it would be a lot of small holes all over the riser. Using relatively long narrow vents is the best option in my opinion, and you really do need a lot of them. As in along every perimeter. When I say long and narrow, I don't necessarily mean 2 x 60 either. Wider is still better, its just that if the riser is 120" wide, then you want as much of that, say 110" of the length, to be open as possible. The wider the vent the larger the port area the better because it raises the tuning into a more appropriate area.


By my calculations on a typical single riser, they often end up being around 1000 liters and if you add in 1000 square cm of port area to the perimeter, that creates a tuning in the 30hz range or so I believe (I'm recalling this from memory, it may have been lower, but certainly not higher). The Q was also very low with such a design meaning the bandwidth was quite wide relative to typical pipe style Helmholtz resonators. If you reduce the opening, then two things happen. First, the tuning drops to an unreasonably low level. It would be reducing bass output in a region where you don't need to add such damping (below 20hz). In addition, it would raise the Q creating a smaller bandwidth and increasing the potential for this riser to actually create a suckout in the bass rather than generally helping to absorb. The insulation will mitigate this however, so I don't want to create the idea that this is a huge concern, its not.


Another thing to consider is that these openings are ports and as such their depth matters. If the top panel is 3 layers of plywood, mdf, subfloor, whatever and you add in a vent that has a depth of say 2" or so, that lowers the tuning. The port then is effectively a 2" deep port. Let's say its 4" by 30" each. The depth is 2". Let's say that the total number of ports is 6. The total mouth area is 720 square inches, or about 4600 square cm. Then with a 1500 liter riser, we end up with a tuning frequency in this scenario of about 37hz and Q of about .3. This makes the bandwidth huge, more than 120hz of bandwidth, but the overall effectiveness small due to the small rise at tuning. Think of it as if the absorption coefficient was something like .4 from 26hz to 100hz, and then falling off after that on either side. Compare that to a high Q design where it might have a coefficient of 1 at 37hz, but where its only .3 at 50hz and 25hz. If we use no diffuser on the vent and just one layer of plywood on top, then we have two problems. One is that the tuning is now over 100hz, too high, and the bandwidth is now a Q of .1 or so, too low. it basically won't do much. Because its a high pressure zone, it won't offer a lot of absorption via velocity either because velocity absorbers are least efficient in this location and the surface area is too small. The top is also now a flexible resonant structure and so the entire top panel between the spans will be able to resonate like a panel trap, but with somewhat unknown tuning and Q. In general the port tuning and panel resonance are not in phase and so what happens is unpredictable and unsmooth absorption. This is essentially what happened in the BBC tests of Helmholtz absorbers, which they abandoned testing. Again, this is my take.
 

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Another thing to consider is that these openings are ports and as such their depth matters. If the top panel is 3 layers of plywood, mdf, subfloor, whatever and you add in a vent that has a depth of say 2" or so, that lowers the tuning. The port then is effectively a 2" deep port. Let's say its 4" by 30" each. The depth is 2". Let's say that the total number of ports is 6. The total mouth area is 720 square inches, or about 4600 square cm. Then with a 1500 liter riser, we end up with a tuning frequency in this scenario of about 37hz and Q of about .3. This makes the bandwidth huge, more than 120hz of bandwidth, but the overall effectiveness small due to the small rise at tuning. Think of it as if the absorption coefficient was something like .4 from 26hz to 100hz, and then falling off after that on either side.
Questions:

1. Are you suggesting this is a good end result?

2. Regarding the port depth, would creating a deeper port (with the same x, y axis lengths) make a difference?

3. When you measure the port size, does the distance between the interior port opening and the floor matter? Let's say you have a 8" riser vs. a 16" riser. Is the height from the bottom of the port to the floor significant?
 

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Questions:

1. Are you suggesting this is a good end result?

2. Regarding the port depth, would creating a deeper port (with the same x, y axis lengths) make a difference?

3. When you measure the port size, does the distance between the interior port opening and the floor matter? Let's say you have a 8" riser vs. a 16" riser. Is the height from the bottom of the port to the floor significant?
I'm not sure what you mean by, is this a good thing. If the goal is to reduce LF ringing by absorbing some of the LF energy, and the hope is that the riser serves that purpose, then if its effective its a good thing. Tests of small Helmholtz bass traps have not shown great results in the past so they often aren't recommended, not even as problem solvers, but they could have their purpose. The main problem being that with no stuffing the Q is extremely high, so you end up with a notch basically, which may not match the peak you are trying to address (or the notch, I mean, its impacting a mode, so what it does at your listening position can vary). More importantly, making a Helmholtz resonator big enough to have even a small meaningful effect means it needs to be really large, like as in...the size of a riser. I see it as a good thing only in as much as its making the riser useful. However, I think there is potential to do more harm than good if designed carelessly. The ones I've seen coming from pros appear to be designed well enough as to be unlikely to cause a problem (they either are likely to do nothing, or do something good). A DIYer who doesn't understand the principles could in fact do something bad and do more harm.


Making ports longer lowers the tuning frequency. In the case of my example it also lowers the Q to the point of the response being useless. It isn't about good or bad, the tuning frequency needs to be at a meaningful point. You may intentionally lengthen the ports to lower the tuning, but you need to know what you are doing.


As per your third point, technically yes but it won't in practice. The ports should never be that long. If you have a 1000+ liter riser volume with sufficiently large ports then if the length exceeds 2 inches the tuning is likely to be far too low. The port would need to be within an inch or so of the floor/bottom to become a problem and you should never be doing that.


I don't consider Helmholtz tuned bass absorbers a good thing if they are something like a large tuned pipe. The main reason is they are too peaky and too ineffective to be worth the effort, and the likelihood that you would hit the right perfect tuning frequency and place it in the right perfect place is low. I think an expert using appropriate measurement equipment is in a different position and might be able to make use of such a tool, but typically we should not. I think that a riser is different because the Q is lower. In general, we benefit more from broad and even LF absorption rather than peaked absorption. A very low Q tuned absorber like this does that. Its principle mode of operation I believe to be Helmholtz which means they are tuned, but because the proposed designs lead to such low Q designs, they have fairly broad margins of error. Where this isn't true is if people make the ports too deep, too big, or too small. They have to be really big to be too big or really small to be too small. What is really small, if you have two 4x10 openings, that is way too small. What is too big, if you drilled 4" holes in the entire top surface, that was too large. If you opened the entire perimeter out to 12" that is probably too large as well.


There is an Auralex design floating around that is questionable, but I have to be honest I've never seen someone build it and measure, so I have no idea if its bad or not. They divided the large riser into smaller chambers and then tuned these small chambers with relatively long pipe ports. My concern with that approach is that each chamber now is just too small to be effective and each chamber is also likely a high Q resonator. While each was tuned to a different frequency to spread the effect, tests of this in the past haven't yielded the expected results. I am not saying it would do nothing, but I think it has a higher probability of doing one of two things: Not working well at all or creating strange nulls in the response.
 

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I'm not sure what you mean by, is this a good thing. If the goal is to reduce LF ringing by absorbing some of the LF energy, and the hope is that the riser serves that purpose, then if its effective its a good thing. Tests of small Helmholtz bass traps have not shown great results in the past so they often aren't recommended....
My take on this thread is yes, it's about utilizing a riser for the secondary purpose of absorbing some LF energy. There have been discussions in other threads about designing a riser to act as a Helmholtz bass trap, but as you said that approach has been strongly recommended against from the standpoint of DIY riser/bass traps.

So, by "a good thing," I was referring to the goal of damping LF energy.

My whole point a few posts above was with regards to poking holes in the top of the riser (the part you stand/sit on) runs the risk of a) making the riser platform weak from a rigidity, stability, and weight bearing standpoint; and b) although some pros might have indicated it's a great idea, that seems (to me) contrary to the advice I've pooled together from others - primarily Dennis Erskine. Since Dennis seems to be the most highly respected pro on this forum, I would think his opinions and observations ought to carry significant weight.

Anyhow, point is the prevailing information I've read on AVS seems to suggest if one is to poke holes in the riser at all, the best use/placement of this strategy is to position the holes at the room boundaries (corners and wall/floor intersections), where the wave pressure will be highest. That said, there are various opinions on whether or not it's counter-productive for DIYers to try this on their own (in terms of guess-timating where to place the openings), or whether it makes any difference at all for a typical/standard riser.

And Mpoes12, as you mentioned in your posts above, the concept relates to pressure and not velocity; and your comments relative to the size of the openings in the riser correlate to other threads on that subject (i.e. they need to be relatively large). It seems to me the size of the openings is most often arrived at from a practical standpoint - relative to the size and shape of available HVAC vents - so that the holes can be covered and don't create a trip or fall hazard while still providing pressure relief vis-à-vis air flow. Furthermore, very elongated vents require notching stringers, which requires forethought prior to installing a multi-layer top to the riser.

What I was looking for clarification on from your statement was more along the lines of whether or not you believe the effort is worthwhile (to place holes and HVAC vents in one's riser), versus not doing it, and your opinion on making lots of small (let's say 1" diameter) holes on the top of a riser. That's all presuming the end user's goal is to reduce LF energy in a HT room by damping that energy via an insulation-filled riser.

The OP asked a question about drilling holes in the front (stage-facing, vertical) portion of the riser. I don't believe this question really got addressed well. What are your thoughts on that approach?
 

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just realized my calculations are wrong! Ah, I reused a bad formula. It's missing Pi! ignore the tuning frequencies I cited! They are much higher.
 

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Ok you make a good point and it would help if I read the full post first. I read some responses and felt like clarifying.


The pressure is at a minimum at the middle of any room. This means holes in a riser toward the middle of the room are doing the least for pressure absorption. However they would allow sound in for velocity absorption. There is a problem though in that the total surface area is small. The riser is near a boundary, the floor, so its still not a pressure minimum meaning its probably still helping.


I guess without knowing more I'm not sure, but...If you drill 1" holes in the top of a riser all over and then cover it in a carpet pad and carpet, doesn't that plug the holes? Maybe not completely, but mostly? If you take a ported speaker and fill the ports with foam, the speaker measures and acts as a sealed speaker. If you take a Helmholtz resonator and cover the mouth opening with closed cell foam, it won't resonate. If you fill the port area with absorbent foam that is open cell in nature, then it will pass some air, but the tuning effectively lowers and the Q effectively is reduced. The amount would depend, I think, on the flow rate of the material, and I'm not so good to figure that out. My assumption is that drilling holes in a riser and covering it in a pad and carpet would make the riser useless as a LF trap. The HVAC vents are totally open so that's obviously different.


Anyway, the opening near the boundaries of the room are where this works the best, it already doesn't do a lot, so you should focus on the perimeter, not the middle. It's not a velocity absorber, while all absorbers technically will operate in different ways depending in the frequencies and position in the room, we should treat this as a pressure absorber. That means fully open vents near the perimeter.


Did I address it this time?
 

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Oh right and should people poke holes in a riser without understanding what they are doing? NO! That was one of my bigger points earlier. I especially like Erskine's design because its so fool proof. Not that you can't screw it up, but if you place your ports at the perimeter, and they are a reasonable width, then the longer the better. You could go too far and create a useless riser trap. The bigger risk most people have is not having enough openings.


The way I understand these to work, be it a resonant panel or a tuned Helmholtz chamber is that there is a mass that the acoustic pressure is acting on, namely that it is resonating. This slug of air in the mouth of the Helmholtz chamber becomes like a solid, in a way, and resonates in the mouth. The pressure then is converted inside the chamber from high pressure and low velocity to high velocity and low pressure. Same is true with a panel trap. The stuffing inside of the chamber then converts the velocity waves to heat effectively dissipating them.


The efficiency of this conversion/dissipation process is dependent on how much of the boundary pressure can be acted upon, and that depends on the total surface area of the opening. The more the better, otherwise it won't work. If someone installs just a handful of standard available 4x10 vents, it just won't do enough. The tuning will be very low, the total pressure that can be acted upon will be very low, so you wont dissipate much LF energy.
 

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Ok you make a good point and it would help if I read the full post first. I read some responses and felt like clarifying....

Anyway, the opening near the boundaries of the room are where this works the best, it already doesn't do a lot, so you should focus on the perimeter, not the middle. It's not a velocity absorber, while all absorbers technically will operate in different ways depending in the frequencies and position in the room, we should treat this as a pressure absorber. That means fully open vents near the perimeter.

Did I address it this time?
Yes (answering your last question first). :D

Your summary corroborates the comments I've read from Erskine and perhaps 1 or 2 other folks - i.e., that the perimeter of the room is where you want the venting (on the riser). You just explained the concept in more detail.
 

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Oh right and should people poke holes in a riser without understanding what they are doing? NO! That was one of my bigger points earlier. I especially like Erskine's design because its so fool proof. Not that you can't screw it up, but if you place your ports at the perimeter, and they are a reasonable width, then the longer the better. You could go too far and create a useless riser trap. The bigger risk most people have is not having enough openings.
That makes sense.


The way I understand these to work, be it a resonant panel or a tuned Helmholtz chamber is that there is a mass that the acoustic pressure is acting on, namely that it is resonating. This slug of air in the mouth of the Helmholtz chamber becomes like a solid, in a way, and resonates in the mouth. The pressure then is converted inside the chamber from high pressure and low velocity to high velocity and low pressure. Same is true with a panel trap. The stuffing inside of the chamber then converts the velocity waves to heat effectively dissipating them.


The efficiency of this conversion/dissipation process is dependent on how much of the boundary pressure can be acted upon, and that depends on the total surface area of the opening. The more the better, otherwise it won't work. If someone installs just a handful of standard available 4x10 vents, it just won't do enough. The tuning will be very low, the total pressure that can be acted upon will be very low, so you wont dissipate much LF energy.
So, what about multiple small holes made along the perimeter of the riser? Granted, that may be impractical for a number of reasons, but hypothetically... wouldn't that serve the same purpose as long as the total square cm of space were sufficient? Put another way, imagine if instead of let's say a group of 4" x 24" vents that one had 96 1" holes. Same result? Or no?

Is the key the total square footage of venting/pressure relief, presumably relative to the total cu ft of the riser's internal volume? Or does the shape of the vents matter? I'm more or less presuming the shape does not matter, but that it's most often rectangular out of convenience.

Is there a simple formula to calculate the square footage your vents should be, relative to the volume of the riser? (kinda guessing the answer is 'no' or I'm would think someone would have mentioned this before).
 

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Shape doesn't matter, a holes a hole at these frequencies. I think the reason for use of diffusers is as you said, it's safer and looks better. If you just want it to be open, that is fine. The mouth of the port or ports is the resonator. It doesn't matter if you have 100 little ports or one big port. The only issue at all is that very big ports have their own natural resonance, a pipe resonance, and larger ports have lower pipe resonances. These ports are so shallow I don't think it's an issue though. I know in designing a speaker, like a subwoofer, you want to avoid having a port resonance within the usable range as much as possible. It's usually not an issue. In a device like this, I'm just not sure what happens if the port resonance and tuning resonance are close together, but again, I think it's not a major issue to concern over. However if it was, many smaller ports would be better than one big port.
 
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