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post #151 of 229 Old 10-20-2014, 12:07 AM
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Quote:
Originally Posted by granroth View Post
But we want to put a sub or subs on the riser... and here's where it starts to get dicey. Is there any measured proof that just placing a sub on a riser (with no sand at all) would result in the vibrations resonating through the riser?
- The purpose of the rubber underneath is to decouple the riser from the house slab for soundproofing benefits.
- The purpose of the fiberglass and the green glue is to damp the vibrations resonating through the riser.
- The purpose of the sand is to give the subwoofer enough mass to push off of without moving the riser. The subwoofer-riser/rubber/slab forms a mass-spring-mass system -- i.e. the sand improves the frequency response output of the subwoofer. The 10 times the weight of sand to the weight of the subwoofer, sounds good to me for this purpose. If the riser joists are perfectly flat and continuously touching and possibly glued to a perfectly flat concrete slab (as opposed to rubber, or raised joists to give airflow under them for a bass trap), the need for sand is probably removed as long as the sub is atop a joist.

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post #152 of 229 Old 10-20-2014, 12:28 AM
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Originally Posted by granroth View Post
Multiple tests were done using REW both with the vents close and with them open, to see what kind of impact the riser was having as a broadband absorber and graphs of each were posted
That was interesting.
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Originally Posted by link
Vents Open



Vents Closed

Well, that works.

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post #153 of 229 Old 10-20-2014, 01:43 AM
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Modes



The optimum place to get maximum pressure is at the walls.
If the axial mode is moving left-right, then the place to deal with that mode is the entire left or the entire right wall.
Axial modes move not only left-right, but also up-down, and front-back. So, a win-win place to deal with multiple perpendicular modes is at a corner (e.g. a floor wall corner, where a floor intersects a wall).


At no other place in the room do multiple modes conveniently line up -- so there are no other win-win spots in the room other than the corners.



As a side note, the optimum places to put a subwoofer for smooth response by not energizing room modes in phase, are pretty much the opposite of the optimum places to put a helmholtz absorber.




Ridiculous Absorber

The oft mentioned but rarely done, is to put a huge 6' x 10' x 8' fluffy fiberglass pink broadband absorber at one end of a room.


But what if we moved that under the room, and gave it a vent.
At the vent its no longer a pressure spot because there's no wall there to build up pressure against; its more like a water spill into another room volume with some velocity as if the room were that much bigger in that direction.


To me, that's essentially what an Erskine riser absorber is -- a several foot deep absorber (good for bass absorbtion), that covers extremely little wall area (less effective from a sabine equation point of view).
The fiberglass making the portion of the sound wave that hits it be turbulant and deconstructive, i.e. quieter and absorbed.




Vents

You want the vents on top of the riser (aka the 'floor'), as close to the walls as possible -- the 'win-win' spots.


There are two schools of thought about how big to make the vents.


The first is to use great big HVAC return air registers, each about 6" x 24" each, and run them continouously about the perimiter of the riser giving 6" by whatever the width/depth of the riser is.
A variation on that is to use linear bar grilles, basically the same just 3" where the previous was 6" and it looks better.



The second is to use fewer smaller vents, rather than a continuous or practically continuous number of them -- but still far more than would turn it into a helmholtz.
The idea here is that you might want to reduce some of the high frequency absorption, and take advantage of that low frequency sound, as long as there aren't flow/impedience restrictions, ignores objects significanly smaller than the wavelength but high frequency bounces off. So fewer vents makes tunes it a bit by making it less of a higher frequency absorber. I haven't done the math, but my gut tells me the difference would be small -- and that bigger vents within reason are better because they're already a tiny area compared to a wall surface.



The vent placement makes no difference -- there's no practical point of looking for modal spots and putting tiny vents there. Put as many vents as you can along the wall, don't try to pick spots along the wall that are 'optimum'. If you're good enough to pick where they should be, you don't need to be reading this low-acoustical-science post.






Room in a Room in a Room


There's no reason this idea is limited to risers at both ends of the room, or even the two sides or even a whole raised floor.
It could also be done on the ceiling.
And it could be done on the four walls.
If done on all six surfaces (floor, ceiling, four walls), then you get a room-in-a-room-in-a-room, with a sort of silver square outline of vents on each of the six -- just make sure you don't triple leaf your soundproofing.



Helmnoltz
There are two problems with helmholtz absorbers.
#1 ) you can't predict what your room modal response is going to be, unless you're building 1' thick concrete walls. Even then, the moment you start stuffing furniture and stuff into the room the thing changes.
#2 ) Its really hard to build a helmholtz absorber that works. Either it does nothing (doesn't resonate), or its the wrong frequency, or its got a Q so wide its pointless.


Number 1 means there's no point in putting a helmholtz absorber into a riser -- stick with the idiot proof Erskine Figure 5 pictures above.


But there is a place to consider helmholtz, and that's in non surround-speaker columns and floor steps. If these can be placed after the room is built, and ETF measurements taken and an active mode discovered,
then an opportunity arises to design a helmholtz absorber with a tuned vent at that location, including the ability to tune/replace/buy it when it doesn't work as intended.
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post #154 of 229 Old 10-20-2014, 07:08 PM
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Great post! A few questions:

Quote:
Originally Posted by BasementBob View Post

But what if we moved that under the room, and gave it a vent.
At the vent its no longer a pressure spot because there's no wall there to build up pressure against; its more like a water spill into another room volume with some velocity as if the room were that much bigger in that direction.

To me, that's essentially what an Erskine riser absorber is -- a several foot deep absorber (good for bass absorbtion), that covers extremely little wall area (less effective from a sabine equation point of view).
The mention of the vent being where there is no pressure jumped out at me, because Erskine has very consistently described this style of riser as working because the sound waves have low velocity but HIGH pressure at that spot. What am I missing?

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Originally Posted by BasementBob View Post
You want the vents on top of the riser (aka the 'floor'), as close to the walls as possible -- the 'win-win' spots.
I want to clarify "as close as possible" to a reference point. When we talk about being close to walls, are we saying that regardless of any acoustical treatments on the wall? That is, say I had five inches of some kind of absorber on the back wall and a 4" vent. If we mean the physical (reflective) wall, then the closest we can get is 2" (1/2" gap and then 1-1/2" stud). Would that mean that the vent would be mostly covered by the absorber? Or would we put the vent just outside of the absorber, so 5" from the physical wall?
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post #155 of 229 Old 10-20-2014, 08:24 PM
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Quote:
Originally Posted by granroth View Post
The mention of the vent being where there is no pressure jumped out at me, because Erskine has very consistently described this style of riser as working because the sound waves have low velocity but HIGH pressure at that spot. What am I missing?
Go and read Erskine's posts again in light of my pictures and see if your interpretation refines.
Most of the wall would remain a 'high pressure no velocity' point, except near the vent (where the room sort of stretches out).
If Erskine's quotes still conflict with mine, then (a) his are probably correct and mine are trash, and (b) please link them here.


Quote:
Originally Posted by granroth View Post
I want to clarify "as close as possible" to a reference point. When we talk about being close to walls, are we saying that regardless of any acoustical treatments on the wall? That is, say I had five inches of some kind of absorber on the back wall and a 4" vent. If we mean the physical (reflective) wall, then the closest we can get is 2" (1/2" gap and then 1-1/2" stud). Would that mean that the vent would be mostly covered by the absorber? Or would we put the vent just outside of the absorber, so 5" from the physical wall?
When I wrote "as close as possible" what I was thinking of was the 2x8 framing inside the riser moving the vent at least 2" from the wall.
But from a practical point of view, if you look at a sin wave most of the peak is within (guestimate) 1/8 of a wavelength so anywhere in there is fine -- the closer to the edge is more efficient and the closer to 1/8 wavelength the less efficient. Anything greater than 300hz you don't care about for this, the wavelength of 300hz is about 43 inches so 1/8 of that is 5.3 inches. So if you have a vent between the wall and 5.3 inches you're fine for all rooms. A 50hz wave is 22 feet long, 1/8 of that is 33 inches, so the vent must be within a yard of the wall. Nonetheless, leaving the vent 2" from the wall is the most efficient -- the closer you get the vent to the wall the more you get for free so put the vent as close to the wall as possible.

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post #156 of 229 Old 11-21-2014, 11:59 AM
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I thought I’d post up in this thread regarding a very simple riser that I want to build for my media room. I’ve looked at a few other threads here on AVS and a few other websites out there, but just wanted to get a few comments and/or minimum recommendations here. And after reading thru this entire thread about people using risers also for bass trapping / broadband sound absorption, it kinda made my head spin…something I wasn’t aware of.


My goal is to build a simple riser for one basic purpose – add an elevated second row for four basic director’s chairs so that I can host more people for media room gatherings. I’m not too concerned with the sound absorption capabilities as my room acoustics/response are pretty good, at least per the AccuCal calibration report back from May 2013. The riser will sit right on the carpet and not be screwed into the wall or floor, which allows it to be easily removed/shifted if needed. Plus it’ll make it easy to build the riser in the garage, carry it in and plop ‘er down...


First, a few quick facts about the current room setup:
  • Basement media room w/ PJ + 135” screen, located above crawl space (post and beam, currently uninsulated if that matters).
  • Room dims. approx. 20’L x 16’W x 8’H, but is open at left rear with connecting hallway to other rooms (sound control out of the media room is not a concern).
  • Main seats are four powered Berklines, currently located just over 3+ feet from the back wall.
  • Subwoofer is in the right rear corner of the room (per sub crawl tests) and will be adjacent to (but not located on) the riser. Sub currently faces to the left but most likely will be rotated to face towards the front of the room..
I’ve attached a picture of my temporary/testing setup (spare 4x6’s with single layer OSB), along with some PDFs of the intended 8" high riser design/dimensions and general room layout after riser installation. The riser will be perimeter 2x8s and 2x6 stringers.


I plan to shift the Berks forward about 9”+ or so, which will allow a riser depth of about 42” +/-, with an inch or two clear behind the Berks and at the back wall. Yes, this is a short riser, but my room limitations just do not allow for a proper two rows of recliners. I just want a place for the simple director’s chairs to sit when there are five+ people downstairs, maybe be able to extend your legs forward. Yes, the people on the two end chairs might have to get up to let the people in the two middle chairs get in and out, but it is what it is…


As mentioned, I plan to construct the riser perimeter with 2x8s, the stringers with 2x6s on joist hangars. I had planned for a single layer of 5/8” OSB on top (glued/screwed down) with self-stick carpet tiles to finish. The riser frame will be a single assembly and painted flat black, but the OSB will have to be two halves due to the ten-foot width. So my essential questions are as follows:
  • Do I add insulation to the riser, strictly for controlling any sound abnormalities that could result from a drum-like effect of a hollow riser? The sub will be only a few inches away from the riser. FWIW, the sub is an AV123 MFW-15 with the MCCA/Seaton upgrade kit, so it does have some decent output.
  • Do I add a second layer of OSB? The stringers will be about 1’-3” on centers (this could be changed to 12” max.), and the only weight on the riser without people will be the director’s chairs, which can’t weight more than 10 lbs each. I expect the riser to have people sitting there maybe once or month or so.
  • If I redo the string spacing to be no more than 12” on center, could I get away with using 2x4 stringers, or just stick with the 2x6s for only a couple more $$?
I’m assuming rotating the sub and adding the riser will slightly change the overall sound response of the room at the MLP, but will live with it short-term until my scheduled AccuCal re-calibration sometime in May or June 2015.


Thanks in advance for any suggestions!
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post #157 of 229 Old 11-22-2014, 10:11 AM
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Quote:
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So my essential questions are as follows:
  • Do I add insulation to the riser, strictly for controlling any sound abnormalities that could result from a drum-like effect of a hollow riser? The sub will be only a few inches away from the riser. FWIW, the sub is an AV123 MFW-15 with the MCCA/Seaton upgrade kit, so it does have some decent output.
  • Do I add a second layer of OSB? The stringers will be about 1’-3” on centers (this could be changed to 12” max.), and the only weight on the riser without people will be the director’s chairs, which can’t weight more than 10 lbs each. I expect the riser to have people sitting there maybe once or month or so.
  • If I redo the string spacing to be no more than 12” on center, could I get away with using 2x4 stringers, or just stick with the 2x6s for only a couple more $$?
Yes on adding insulation. Or if you have spare cotton or down or something (in the form of pillows, maybe), then that could be used as well.

As far as adding a second layer and/or changing the stringer spacing, just treat it as if it was a floor. That is, floors often have just one layer of plywood or similar as a subfloor, so that would be fine if all you are wanting is to hold up your chairs and people. For the stringer spacing and lumber size, spend some time on the span calculators:

http://www.awc.org/calculators/span/...rcalcstyle.asp

Generally, you'll find that for small spans like your riser, you can get away with relatively small lumber sizes and wide spacing. 2x4s spaced 12" O.C. should easily be able to handle your load in a riser of that size. But... it wouldn't actually be stronger. That is, using 2x6s spaced 16" O.C. would actually provide a more solid base, even though the spacing is wider.
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post #158 of 229 Old 11-22-2014, 12:04 PM
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Gran, thanks for the reply and comments. Assuming my wood selection on the linked calculator is accurate, my design with 2x6s is way more than sufficient. I'll redo the string spacing to be able to staple some standard-width insulation between. 4" sufficient for the insulation, or should I go thicker? I'd just buy a roll, two if needed.

I'll stick with one layer of OSB/plywood to keep it simple. I still have to figure out where to mount my power strip/ surge protector, and to be able to route the Berk's power cords and plug into the power strip after the riser is in place. And I want to keep whatever I can out-of-sight. I also want to add some soft lighting on the end of the riser, like those hard-wired night lights that mount directly in a electrical box.

My simple project isn't entirely simple, but will be fun to build nonetheless. I've already bought all the 2x's at HD last night and had them all cut to size. Everything's in the garage, waiting for me to get started!

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post #159 of 229 Old 11-22-2014, 12:48 PM
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FWIW - Go to 25min50 sec of the podcast with Dennis for riser as a broadband bass absorber discussion....

When I re-do my HT in 2016 (total re-do, 4k PJ , AT screen, DIY speakers, 11.2 to 10.x.8, etc) I'll re-visit my acoustic strategy also....and re-take those riser vent open/close measurements also as another data point to share, since I've added multiple subs since then, and may add a few more subs by 2016......
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post #160 of 229 Old 11-22-2014, 07:13 PM
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Quote:
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Gran, thanks for the reply and comments. Assuming my wood selection on the linked calculator is accurate, my design with 2x6s is way more than sufficient. I'll redo the string spacing to be able to staple some standard-width insulation between. 4" sufficient for the insulation, or should I go thicker? I'd just buy a roll, two if needed.
Definitely thicker. You'll want to completely fill in the interior - couple layers of R-13 would do it. If you left a 4" gap (or so), then you'd still effectively have a resonance chamber.
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post #161 of 229 Old 11-23-2014, 12:18 AM
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Quote:
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Definitely thicker. You'll want to completely fill in the interior - couple layers of R-13 would do it. If you left a 4" gap (or so), then you'd still effectively have a resonance chamber.
If a 6" cavity, I would have said that 3" to 4" of insulation was enough, as long as the entire floor is covered.
What have you read that says it has to be filled?
(I'm thinking of the various experiments with insulation filled walls benefits, and damped membrane absorbers -- where even a little bit of insulation was effective)

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post #162 of 229 Old 11-23-2014, 08:42 AM
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Quote:
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If a 6" cavity, I would have said that 3" to 4" of insulation was enough, as long as the entire floor is covered.
What have you read that says it has to be filled?
(I'm thinking of the various experiments with insulation filled walls benefits, and damped membrane absorbers -- where even a little bit of insulation was effective)
That's an interesting question. The first place where I've seen it stated unambiguously to completely fill the riser is in Erskine's legendary riser article, transcribed here: Bring Your Home Theater to New Heights ("The Riser Article", HTB, Erskine 2003)

Quote:
We don't fill the platform with sand, but every space between every 2x8 must be completely filled with fiberglass insulation. For this you want to use the non-backed fiberglass batt. Whatever thickness you use, just make certain you've completely filled the space. Johns-Mansville has formaldehyde free fiberglass batts that are excellent for this application.
That design is absolutely assuming using a riser as an absorber, though, so it might not be applicable to treating a riser as a simple cavity. That said, I don't recall ever seeing a riser build where they left a gap. I just rifled through a half-dozen more riser threads this morning and in each, there were a multitude of examples of the riser being completely stuffed and none where it was not.

Still, maybe this is another one of those cases where people are assuming a rule that doesn't need to exist.

Maybe it's more accurate to say that filling the riser is a safer option rather than it being a required option.

mak99, why don't you run some tests? Fill up half of the riser with insulation and throw REW at it and then fill up the rest of it and repeat the tests. Are there any differences?
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post #163 of 229 Old 11-23-2014, 08:53 AM
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The portions of my riser that are not filled with sand are only partially filled with insulation. I used hangers to suspend a mixture of R-19 and R-13 batts in a 2x12 riser. I have no data, but I was working under the same rationale that Bob outlined.
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post #164 of 229 Old 11-23-2014, 11:21 AM
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Quote:
Originally Posted by granroth View Post
The first place where I've seen it stated unambiguously to completely fill the riser is in Erskine's legendary riser article, transcribed here: Bring Your Home Theater to New Heights ("The Riser Article", HTB, Erskine 2003)
If you're building an absorber (riser with vents), then yes fill the riser with insulation, every bit of space filled.

If you're building a sealed riser (no vents), then just enough insulation to damp the resonance should be fine. Filling it is also fine.

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post #165 of 229 Old 11-23-2014, 07:07 PM
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I constructed the riser framework yesterday, and I attached a picture in case anyone was curious. Stringers spaced at 16" from the ends, but slightly less than that for the two stringers adjacent to the dead-center stringer. I bought the OSB for the top today, but will finish that up tomorrow along with adding insulation.

Quote:
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Definitely thicker. You'll want to completely fill in the interior - couple layers of R-13 would do it. If you left a 4" gap (or so), then you'd still effectively have a resonance chamber.
Since my riser will have an open bottom sitting on the carpet, I'll probably use 6" of insulation (R-19) between the 2x6 stringers, and maybe staple some lightweight screen to the bottom of the stringers so no chance of insulation ever drooping down onto the carpet over time (I hope this makes sense). If I buy faced, I assume the paper side would go on top, directly underneath the OSB? FWIW, I have an extra roll of R-30 (~9" thick) sitting in the garage, would it be okay to compress just enough to fit into the ~6" stringer depth? I want the 2" space at the bottom of the riser for routing the power cords from the Berks under/thru the riser, to the wall outlet that's directly behind the riser.

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mak99, why don't you run some tests? Fill up half of the riser with insulation and throw REW at it and then fill up the rest of it and repeat the tests. Are there any differences?
I'l be honest, I've never used REW, though I downloaded it many moons ago and know it has quite a learning curve. Once I get the riser completed and moved into the media room, I don't think I'll want to make any changes to it. For now I'm planning on just filling the riser with insulation, screw down the OSB top, then carry it in from the garage. If someone local were able to help me run the REW tests, I'd definitely be willing to do comparisons.
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post #166 of 229 Old 12-18-2014, 08:54 AM
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Vents should be as close to the walls as possible... but is that relative to the drywall wall or the effective wall, if you have thick acoustic treatments?

Okay, so the vents need to be close to properly capture the low velocity waves (as the theory dictates). That suggests that the ideal location would be flush against the wall. However, practical considerations say that you can't get closer than 2", since there's your 1/2" gap between the riser and the wall plus the 1-1/2" taken by the dimensional lumber used for the riser frame.

But now let's consider the fact that quite a few theaters have reasonably deep acoustic treatments on their back wall. It's not at all uncommon to have a 4" or 6" thick bass trap on the back wall.

How does that affect the placement of the vents?

Are they still 2" from the back wall, and thus partially or entirely covered by the bass trap? Or do we now consider the bass trap to be the "wall" and put the vent flush with that?

I tend to think that you still want the vent as close to the actual wall as possible, since the bass trap may slow some of the frequencies down, but it won't stop it like drywall will. The area of lowest velocity would still be at the drywall and not at the surface of the treatment. That's just my guess, though! What's the real answer?
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post #167 of 229 Old 12-18-2014, 11:50 AM
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the riser as basetrap proponent on this forum seldom uses acoustical treatments greater than 2 inches which pretty much lines up with your theoretical best placement, in reality you also have to deal with any baseboards mounted proud of the treatments and the actual overhanging lip of the vent grill and your ability to install the grills. Now if you are doing 8 inches of treatments, you really don't need a riser trap, the back wall is the trap.
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post #168 of 229 Old 12-18-2014, 12:06 PM
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Is there a minimum depth the riser needs to be in order to act as a bass trap? Meaning, I am planning a riser which will only be ~ 6 1/2" from floor to bottom of plywood/OSB. I'll also have a further elevated Seating Platform in the middle of the riser, but that won't be at the perimeter of the theater.

I didn't know if there needed to a minimum height or if any height would work.
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post #169 of 229 Old 12-18-2014, 04:53 PM
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Quote:
Originally Posted by BIGmouthinDC View Post
the riser as basetrap proponent on this forum seldom uses acoustical treatments greater than 2 inches which pretty much lines up with your theoretical best placement, in reality you also have to deal with any baseboards mounted proud of the treatments and the actual overhanging lip of the vent grill and your ability to install the grills. Now if you are doing 8 inches of treatments, you really don't need a riser trap, the back wall is the trap.
Hmm... Erskine has been very adamant that using a riser like this isn't a bass trap at all, but rather a broadband absorber that is naturally tuned towards modal frequencies. I would assume, then, that it has a very different purpose than a traditional bass trap.

BUT, since you've built quite a few Erskine designs, if you're saying that they rarely have big bass traps in the back, then I'm going to run on the assumption that the riser might have bass trapping capabilities after all!
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post #170 of 229 Old 12-18-2014, 05:09 PM
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I call it a bass trap because it is there to tame standing bass waves. I actually think he was amendment that it wasn't a tuned Helmholtz bass trap but rather a broad band absorber.
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post #171 of 229 Old 12-20-2014, 12:02 PM
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Measuring the Effects of Riser Vents

Thoughts on Riser Vents

Erskine has been very adamant that risers with vents are not bass traps, but are instead broadband absorbers that target modal frequencies. Since modal frequencies tend to be in the 20-300Hz range, though, that does essentially mean that the riser becomes a bass trap for all practical purposes.

There's quite a bit of evidence that the riser vents actually work (unlike the "sand in a stage" dogma which had little to no empirical testing until recently). What's still missing is any kind of range of tests to try and figure out what kind of results should be expected, and what are the sweet spots of vents vs performance.

@mtbdudex published his results in this thread, which was linked from this thread earlier. They absolutely showed that the riser vents were very effective. The results might not be fully transferable, though, since mtbdudex also constructed his riser to be a series of Helmholtz absorbers rather than have one large cavity. The typical advice for building a riser is to have one large cavity with the vents, so its unclear how much of his results were due to the vents vs how much due to the multiple cavity sizes.

I'm going to try and add a few more points to the existing corpus of data with the hopes that we can learn some more from this.

The Setup

My theater is now at the stage where it has a sand-filled stage and a fiberglass stuffed riser, but no soffits, columns, screens, or any acoustic treatments of any sort. In theory, I could be sure that any results I see are the results of just the changes I'm making now, since there is so little in the room. It looks like this:



The main riser frame is made of 2x10's with 2x6 stringers spaced 12" O.C. and then stuffed with R-30 pink fiberglass. The main riser cavity is 72 cu ft, with a foot print of 94 sq ft.

I tested using three speakers made of some SpeakerCraft CRSOne 6 ceiling speakers I stuffed in some enclosures, and then my M&K subwoofer. Measurements were done using a Dayton Audio UMM-6 microphone situated in the "listening position" just in front of the riser, 1 meter off of the ground and facing the speakers on the stage.

I did a full 0-20,000Hz measurement using REW, but only the 20-300Hz range is shown here since that's where the riser should be most effective. All REW waterfalls have settings for 50-100dB, 20-300Hz, and with a window of 500ms.

Pre Testing

I ran some tests while I was building the riser just to see how the measurements would progress as I went. I don't have any measurement for the room with no riser at all, for reasons that I don't remember. Maybe there wasn't any reason and I just forgot.

The first measurement was done with the riser frame was completed, but with no fiberglass in it and no top at all. I would sort of expect this to work as a completely un-tuned diffuser. It looks like so:



Ringing galore! Practically speaking, it was a very echo-y room at that point.

I then stuffed the riser with fiberglass and re-ran the test (still with no top attached). I expected that the decay times would be far better since I essentially had a 94 ct ft trap on the floor. Indeed:



It was a night and day difference. But yeah, that's not practical. You can't actually sit on fiberglass and nobody wants that much fiberglass exposed to the air.

The Actual Tests

The riser top is made of two layers of 3/4" OSB, glued together with PL-375 construction adhesive and screwed regularly to the framing using 2-1/2" screws. I made my first measurement with no holes for the vents cut at all:



Some things to note. First, the frequency peaks are notably different with the top on. The peak at 52Hz was around 105dB with no top, but is around 98dB with it on. The frequencies between 100Hz and 300Hz are all a good 5dB higher, though. I can't say I was expecting that. It was reproducible, though. Second, it does very notably better than an empty riser with no top, implying that there is some benefit to a fiberglass stuffed riser even without any vent holes at all.

After that, I cut a 4" x 36" hole centered on the back wall, and offset 2-1/2" from the wall. Its measurement looks like this:



Next up is the vent hole on the right wall, which is also 4" x 36" but offset 2-1/2" from the back wall and 2-1/2" from the right wall. I tried to get it as close to the corner as possible.



And then I cut out the hole on the left wall, which is a mirror of the hole on the right wall:



Analysis

There is a clear improvement between the beginning and ending measurements, especially around 52Hz, maybe 175Hz, and somewhere around 250Hz. In fact, you can see incremental improvement with each hole that was added, although it really seems to kick in with all three holes.

I will say that I was expecting a bit more of a change, though. This knocked off maybe 30ms from the ringing. Based on earlier tests I've seen, I was thinking it would be more dramatic than that.

My vent holes combine to total 3 cu ft of access to the underlying cavity. Maybe more importantly, they take up 23% of the back wall and 40% of each of the size of the riser on the right and left walls. All told, there are 27'-5" of riser cavity facing a wall and 9' (33%) of that is opened via the vent holes.

I wonder if we could say that if your riser has 33% open areas to the walls, that you could expect similar performance gains to what I saw.

Up Next

Would I get notably better performance if I created a few more holes? What if I had two more 4" x 14" holes on the back wall and one more of the same on each of the side walls. That would increase my coverage to 50% of the open area (41% on the back wall and 58% on the side walls).

What I wonder is if placement matters more than amount. Like if I put the two back 4x14s as close to the corners as possible, would it have a bigger impact than if they were closer to the existing hole? Not sure how I could test that without jumping through a lot of "fixing it up later" hoops.

Another thing I want to test is what kind of vents are appropriate. Linear Bar grilles are the recommended type, but those can get decently expensive. Since I can only find Dayus grilles online (no idea where I could buy Nailor or others), it looks like a 4x36 grill would run me $55 not counting shipping. I can get decorative floor grills for maybe half that both locally and online. Would a decorative grill suffer in performance compared to a linear bar grill? I need to figure out some way to test that.

And if you guys have anything else you want me to test or any other details I've forgotten, let me know!
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post #172 of 229 Old 12-20-2014, 03:53 PM
 
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That post was strong as muthafawkingchit.
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post #173 of 229 Old 12-20-2014, 09:12 PM
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The whole effect of the riser on the response plots seemed very subtle. I suppose any little bit makes a difference and the process was simple enough to do.

I currently have a plan on a flat floor to add a riser under 5 seats. There will be two rows of 5 seats each. The row will not be allowed to recline due to space constraints but most of the time the front row will be used.

IOIOIOIOIO
IOIOOOIOI

I am assuming that about 7" high is enough but are steps necessary? Almost every configuarion of steps would be either a risk of tripping (notched) or you would have to climb over a wider platform to get to the front seats. We enter the room from the center back if the room.
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post #174 of 229 Old 12-21-2014, 03:24 PM
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Check your local building code, but much more than seven would need a step. I would think eight would be okay.
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post #175 of 229 Old 12-22-2014, 12:49 PM
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Measuring the Effects of Riser Vents - More Vents

Adding More Vents

I saw only modest improvements by adding one 4x36 vent to the back wall and one more on each side wall. Presumably more vents would improve it even more, but does it do so linearly; compounded; or maybe even diminishingly?

I started by putting two 4x14s on the back wall, in the corners. I then added two more 4x36s on the side walls. They look like so:



These additions mean that 83% of the side walls of the riser cavity is now open, as well as 41% of the back wall. All told, three are 17'-4" of vents along the 27'-5" possible, or 63%. My previous tests covered 33%.

On a practical note, I reformatted the graphs of these waterfalls to hopefully be more relevant.

Refresher

Since the graph parameters are different, here are two of the original results.

The first is my baseline of a constructed riser, but with no vents at all:



54Hz stops ringing at roughly 575ms while it takes 650ms for 234Hz to finally stop.

After I cut out the original three 4x36 vents, I found a notable if modest improvement:



54Hz now stops at 500ms and 234Hz is down to 550ms. Not bad, but not phenomenal, either.

MORE!

So what happens if I cut out the two back/corner 4x14 vents?



It's a little harder to get specific numbers, but it's looking like maybe 475ms @ 54Hz and 525ms @ 234Hz -- roughly a 25ms drop.

Now I take out the big guns and cut two more 4x36 vents, one on each side:



234Hz is now down to 500ms flat, while 54Hz now only takes 450ms.

Analysis

The big question I wanted answered is if the addition of more holes was roughly linear or not. I acknowledge that the placement of the vent holes likely matters a lot, but since I couldn't play with that variable, the only one I could use was total linear feet.

I graphed that out and applied a linear trend line to it. Each X coordinate adds roughly 3'. The two final measurements are both off by 8" and I did put a placeholder in at 15' to keep the scale roughly linear. The results:



Each of the frequencies had their own "sticking points (234Hz at 650ms and 54Hz at 550ms) that illustrated that the positioning of the vents does make a difference for specific frequencies. Nonetheless it's at least roughly linear. I don't appear to be getting diminishing returns, nor does the cumulative addition of more vents compound the effect.

All told, I saw a 125ms drop in decay times @ 54Hz and a 150ms drop @ 234Hz. Rounded up, I'd say that's a 25% drop in ringing, overall.
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post #176 of 229 Old 12-22-2014, 12:58 PM
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Very cool. You could probably recover some of the holes with scrap, just temporarily to compare some other combinations.
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post #177 of 229 Old 12-22-2014, 01:25 PM
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Measuring the Effects of Riser Vents - Cost Analysis

Is It Worth It?

My previous tests have shown conclusively that with a 25% drop in ringing for the modal frequencies below 300Hz, the riser vents absolutely work. But are they worth doing when cost is inserted into the equation?

The reason that costs matter is because while the holes in a the riser are free, the grilles to put on top of them are not. If we use these linear bar grilles HERE then we'd be looking at a $320 (not counting shipping) outlay for the vent holes in my riser.

Is $320 a fair price to pay for those kind of results?

A Target

We first need to establish what a good result is, to be able to make a comparative analysis. If we go off of the recommendation (found HERE, on Nyal's site) that the ringing should decrease by 40dB in 350ms for bass frequencies above 35Hz, then IF we can hit that target using only the riser vents, then we know that it will only cost us $320 total.

Do we?



No. This graph was adjusted to have a range of 100dB to 60dB and has a window of 350ms. Note that at least five frequencies are still ringing at 350ms. Indeed, at least two of the continue until about 400ms.

Based on this, I can say that just adding the riser vents is not enough, and I will need further bass treatments in the room. If that's the case, though, then could I get the same or better results using an alternate type of treatment in the first place? Maybe "superchunk" stacks or similar?

I don't know. Hopefully somebody else can answer that.

Reducing the Price

Our results aren't pointless, though, so if we can reduce the cost somewhat, then the cost-performance ratio swings in favor of the vents. What if we didn't use grilles at all, and instead just covered the holes with some kind of simple home-made frame and fabric? I can make a frame out of spare bits and since nobody steps that close to the wall, it's unlikely that anybody will step on it anyway.

The big question for me was if covering the holes with some kind of fabric would degrade the performance. I don't have any GOM or speaker fabric or anything like that, but I do have some landscaping fabric. It's very breathable and you can even see through it a little:



I put a piece of that over the original three vents (before I cut the rest):



I then ran measurements with and without the cover:




In short, there was no consistent difference between the results. The graph does show 230Hz getting mildly better when covered, but that could be a measurement fluke.

Conclusion

The landscape fabric didn't impede the performance of the riser vents at all. If I could build frames to cover those holes using the fabric and scraps, then my cost for the holes would be very minimal. In that case, the 25% improvement is a clear win, since I get that for next to nothing.

(As an aside, I wonder why nobody uses landscape fabric in their theaters -- that seems like some pretty handy stuff)
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post #178 of 229 Old 12-22-2014, 01:25 PM
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Great timing on getting the wife to approve the construction of the room we have called the "Theater Room" for the past 13 years. (long Story).
Ok, so I don't have awesome pictures or drawings yet ( I know AutoCAD but my dwgs aren't as pretty).
What program did you use to do the drawings?

Also, if so far there is no "law of diminishing returns" on the vent spaces....what if you built the riser to where it is approx 3" off the walls on the sides and rears and framed it to where the joists form an "L" shape void as you start in the corner and then move out every 12"-16". That way the Entire unit is a "trap". Once the seats are on it, you're not going to walk around the back of the seats and most likely not too close to the walls to where you can "fall in". Maybe the sides could be 2" gaps so there is NO way to fit your foot in there.

_l l l l l
___l l l l
_____l l l
_______l l
_________l

So that is what it looks like in my head for half of the riser framing using 2x whatever you need, stuffed with pink stuff and mounted 2"-3" away from the wall.
Thoughts?

Last edited by Laidback; 12-22-2014 at 01:27 PM. Reason: formatting messed up my crappy drawing,lol
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post #179 of 229 Old 12-22-2014, 01:28 PM
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Quote:
Originally Posted by Laidback View Post
Great timing on getting the wife to approve the construction of the room we have called the "Theater Room" for the past 13 years. (long Story).
Ok, so I don't have awesome pictures or drawings yet ( I know AutoCAD but my dwgs aren't as pretty).
What program did you use to do the drawings?

Also, if so far there is no "law of diminishing returns" on the vent spaces....what if you built the riser to where it is approx 3" off the walls on the sides and rears and framed it to where the joists form an "L" shape void as you start in the corner and then move out every 12"-16". That way the Entire unit is a "trap". Once the seats are on it, you're not going to walk around the back of the seats and most likely not too close to the walls to where you can "fall in". Maybe the sides could be 2" gaps so there is NO way to fit your foot in there.

_l l l l l
___l l l l
_____l l l
_______l l
_________l

So that is what it looks like in my head for half of the riser framing using 2x whatever you need, stuffed with pink stuff and mounted 2"-3" away from the wall.
Thoughts?
Formatting screwed up my little drawing. I'll draw up something shortly
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post #180 of 229 Old 12-22-2014, 02:35 PM
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Quote:
Originally Posted by Laidback View Post
Formatting screwed up my little drawing. I'll draw up something shortly
Here it is. ZERO vent costs.
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