Riser Broadband Bass Trap Review - AVS Forum
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post #1 of 52 Old 05-23-2013, 08:18 AM - Thread Starter
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I am building a dedicated theater and doing the design work myself. The first task is to design a riser bass trap. After a bunch of research, I created the design below, but I was hoping to get a little advise from people with more construction and acoustic experience.

I am doing the theater design primarily in AutoCAD, but also created this Sketchup model for the riser.

It is not shown in any of the drawings, but I plan to cover the riser in carpet including the grate. Luckily the grate is going to be behind seats and won't get much traffic. I plan on not doing a pad over the grate

So here is what I am looking for help on:

1. General construction comments - Is there something I should be doing differently? Any products I should consider?

2. Joist separation - They are about 16-17" apart so that 15.5" insulation fit easily. There will be (2) 1/2" plywood layers on top. Are the joists too far apart for a floor?

3. Materials - Is pink insulation the right thing to use?

4. Grating - I have seen a couple suggestions for grates like this. Anyone have any experience with similar product or known where to buy it? Since there will be no carpet pad over the grate, should the grate be a little taller than the surrounding plywood?

4. Isolation - I might put a sub on the riser. What can I do to mitigate rattling/vibrations? Maybe some builders felt between plywood? Maybe a gasket under the grate?

6. Acoustics - Is the grating big enough? I could also put some holes in the front face of the riser, but they would be a couple inches behind a couch.

Thanks for the help!





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post #2 of 52 Old 05-24-2013, 08:50 PM
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*not and expert, just been involved in theater construction and read thousands of post regarding theater construction

A riser built for a bass trap MUST be engineered to be tuned to your room. You could make your bass response WORSE, by accidentally having a trap tuned to a certain frequency. I looked into doing this early on, but gave up on it as the cost for it to be designed (holes, placement of holes, baffles inside riser, ect...) and I was impatient! Please do more research (search the forum) regarding this idea. There are posts about this.

I would vote no regarding placing any sub on a riser platform. The riser will act as a giant drum. This is exactly why we fill out stages with 1000's of lbs of sand, not insulation, before placing our subs and speakers on it. Sand filled cavity = dead, insulation filled cavity = muted drum.

16" on center joist framing is just fine, especially if your decking with 2 layers of 3/4". Screw it down, don't nail it. Place tar paper in between layers OR spring for Green Glue and apply that in between.

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post #3 of 52 Old 05-24-2013, 09:05 PM
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Use the calculator on this website based on the dimensions of your riser base: http://www.mh-audio.nl/acalculators.asp#showcalc

No sand, use pink stuff. 16" oc is OK, but I would do two layers of 3/4" T&G with Green Glue in between. If not Green Glue, then skip the tar paper and stick with glue and screw using construction adhesive like PL Premium.

And why do you feel you need such a beefy lad-bearing grate when nobody will be able to walk in that area? I am sure you can find standard HVAC diffusers or return ducts in the appropriate number of total square inches for your bass trap.


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post #4 of 52 Old 05-24-2013, 10:15 PM
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Use the above calculator to attempt a tuned trap if you know what your problem frequency will be once the riser and seats are installed.

Or, make the riser as porus as possible, sides and top, and fill with insulation to create a broadband absorber.

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post #5 of 52 Old 05-25-2013, 11:06 AM
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Is that 8 chairs I count in back row ?

How wide is your theater ?

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post #6 of 52 Old 05-25-2013, 01:10 PM - Thread Starter
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Bronco - I am trying for a broadband bass trap. I think what you are talking about is a helmholtz. I originally considered this, but many people recommended against it. Appently it is very difficult to realize theoretical designs. I think I will stay away from the sub on the riser, thank you. Green glue looks like what I looking for, too bad it is so expensive. Probably worth it though, I don't want to open this thing back up.

TMcG - Pink stuff it is. I wanted to do a thicker grate to match the height of the double layer drywall. I also thought that there would be less chance of a rattle in a more substantial grate. I thought it would be easier to do something that installed end to end rather than cut individual holes.

Bigus - Are you sure about making it that porous? I know that you want a decent amount of grating near the edges, but there has to be a point where there is too much open space. I'll have to research this more.

Mfusik - I am currently planning for seven chairs in the back row, but might remove one for an equipment rack spot. The 3D model shows 8 chairs because the chair block I imported was too narrow. The theater is 190.5" wide x 184" deep. I am planning to use 24" Milan Rockers.

Thanks for all the comments.
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post #7 of 52 Old 05-25-2013, 01:34 PM
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The reason people put a great number of openings near the edge is to allow the openings (ports) to resonate in response to the high pressure at the boundaries of the room. Notice the words pressure and resonate. This was originated as a pressure-based tuned absorber. I'm not saying that this design is inherently incompatible with broadband absorption. I'm saying that because I don't have any evidence, but I think a broadband design should follow a different design strategy.

Keep reading, do your research, and keep asking and thinking.


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post #8 of 52 Old 05-25-2013, 01:38 PM
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Also, Bigus has done a lot of careful reading on this and related topics. If you're going to take someone's advice without doing enough research to come to your own independent conclusion, I recommend you choose his advice.


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post #9 of 52 Old 05-25-2013, 08:29 PM
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fulminis, think of the problem in its basic form. What is a broadband absorber? It is a velocity sensitive absorber. What does that mean? It means the absorption of sound energy is due to resistance to gas flow through a porus material. What does this require? Free flow of the gas through the material for maximum benefit.

It gets a little confusing because people tend to place broadband absorbers near room boundaries. More on that in a sec. The alternative as you note are Helmholtz resonators, which are pressure based absorbers. These by necessity must be place at room boundaries where pressure is at a maximum and velocity is near zero.

Back to the confusing with velocity based broadband absorbers near room boundaries. Notice that key word "near." Since all frequencies will have a pressure maximum and velocity minimum at a boundary, we know that velocity for all frequencies will increase as we move out from the wall, to a maximum defined by their wavelength. A broadband absorber might well work in the middle if a room, keyword might. If the problem frequency in your room happens to be at high velocity in the location if your absorber, great. But it could be that there is a velocity null there. Since broadband absorbers by virtue of being broadband are typically shotgun approach without explicit knowledge of precise problem frequencies and their maxima/minima mappings, we choose to put them where they are guaranteed to work some, even if not optimally. So "near" boundaries. Corners are nice because they are near multiple boundaries, and thus work on multiple axial modes simultaneously. But only near, which is why the recommendation to space them off the wall a bit if possible, and/or use a thick chunk if absorptive material.

Oh, and I suppose most people would object to a chunk of fiberglass in the middle of their room! wink.gif

Back to risers. Broadband absorbing risers are just corner traps that can support some seats. Both are broadband, and are placed near room boundaries. Would you think it was odd if someone suggested you cover a corner trap in plywood and cut some holes in it near ceiling and walls? Then why would we do that to a riser? Bottom line, you want your riser to mimic a corner trap as much as possible. That means making the wood as much "not there" as possible. And that includes sides and front, so it can act on multiple axial modes at once.

Some do intentionally design for Helmholtz resonators or damped versions of them. I believe Erskine's group usually does this. They also have tons of experience doing so, and all the simulation tools and measuring tools necessary. Most diyers do not.
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post #10 of 52 Old 05-26-2013, 06:39 AM
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Nice post Bigus.

I built my dedicated theater a few years ago after researching various ideas on this site. This included using the riser as a broad band bass trap. The criteria I had to satisfy for myself was the following: If I turn my riser into an attempted broad band bass trap, will I either improve or maintain the acoustic performance of the space? I found no evidence that I could worsen any acoustic issues.

My riser is similar to yours, and spans the back 1/3 of the room. I used 2x12's for the 4 edges, and 2x6's running parallel to the short walls of the riser, 16" OC, on joist hangers. So the whole cavity is open at the bottom. It's lightly filled with pink fluffy insulation. I just put as many registers at the edges as I could fit. The edges of the riser are about 1/2" from the walls, and the whole riser rests on this dense closed-cell foam with an adhesive on one side, mainly because the concrete basement floor is a bit uneven. Top deck is two layers of OSB with green glue in between.

It's hard to say how effective this is as you can't really test your room with and without it very well. One guy a few years ago on the forum did a pseudo test where he did measurements with and without the registers open. The room response was more favorable with the registers open. That was enough to sell me. I have pretty extensive bass trapping elsewhere, with superchunks up front behind the screen and pink fluffy in false fabric covered soffits all the way around the ceiling perimeter. I can only say that the bass in my room is excellent, and I use 2 Submersives.

So all in all I think you could make the riser with less wood, but still plenty supportive, by getting rid of the two long 2x's on the bottom in the middle, and just supporting the ends better on the short side. You'd also have less impedance of airflow into the trap, though I'm not knowledgeable enough to know if that would make a difference.
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post #11 of 52 Old 05-26-2013, 07:09 AM
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Quote:
Originally Posted by uscmatt99 View Post

So all in all I think you could make the riser with less wood, but still plenty supportive.
You can do this other ways as well.

BIGmouthinDC tends to build risers with what these extra supports, midspan. Link
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post #12 of 52 Old 05-26-2013, 07:55 AM
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Found a photo during construction. It looks like they were actually 2x8s or 2x10s now that I look back at the images.


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post #13 of 52 Old 05-26-2013, 08:18 AM
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I've never been able to convince myself that a riser trap does not act as a velocity based trap over some bandwidth. A riser generally covers a good portion of the room. Putting vents around the perimeter allows the high pressure region to be "seen" by the trap, and if the internal volume is open and large enough, why wouldn't a wave propagate in there? I'm sure that over the entire bandwidth it will have some resonant point that it operates better at, but it also seems like it should work to some degree over a bandwidth where the riser is on the order of 1/4 wavelength or more.

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post #14 of 52 Old 05-26-2013, 08:28 AM
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Quote:
Originally Posted by HopefulFred View Post

Quote:
Originally Posted by uscmatt99 View Post

So all in all I think you could make the riser with less wood, but still plenty supportive.
You can do this other ways as well.

BIGmouthinDC tends to build risers with what these extra supports, midspan. Link

That's just for extra support ? Why not just have the wood touch the ground ?

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post #15 of 52 Old 05-26-2013, 08:37 AM
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You want each bay open to the next. You are trying to create a large open volume.

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post #16 of 52 Old 05-26-2013, 09:35 AM
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That's just for extra support ? Why not just have the wood touch the ground ?

There are a number of reasons that I like this design. First the concept of open bays communicating across the entire riser was encouraged by Dennis Erskine designed bass trap risers which I have built for clients.
Second, it saves money
Third, is that it generally is easier to build if you have any irregularities in the basement floor as you only have to level and shim the perimeter.
The mid span supports are just to minimize bounce. Often the riser is less than the required board length so you just put the scraps to use.
Lastly it speeds up the process of running wires in the risers.
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post #17 of 52 Old 05-26-2013, 08:36 PM - Thread Starter
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Wow, great comments. Sorry I haven't responded, dealing with a dead hard drive on a holiday weekend.

Let me try and summarize the acoustical comments. There seems to be three options.

1. A tuned or hemoltz resonator - Where the ports are near the borders in order to capture bass at its highest pressure and minimal velocity. The chambers inside the riser resonate at specific frequency and can be tuned by the size of the cavity.

2. A broadband absorber - The riser acts like giant acoustical panel by slowing the movement of air and damping its sound energy. It should be as porous as possible to allow the maximum amount of air/sound energy to enter the riser..

3. A hybrid of the previous two - The riser captures the bass at the optimal location, along the walls, but the riser is one large cavity with velocity damping material. This method obviously won't work as well as a more porous approach since it won't capture as much energy, but it does seem to offer a decent amount of damping and simplified construction. It will also act as a helmoltz resonator for one frequency and possibly harmonics.

Obviously I was shooting for number 3. It seems like the design could only benefit from more ports. I plan to add some behind the couch and explore other area. Also, BIGmouthinDC's center support design makes sense on many levels, including making the riser height a little more flexible. I have some low window sills at the back of the room that riser needs to sit flush with. I will update the design and repost. It is going to take me a few days to get a new computer up and running, so it might take till the end of the week.

Thanks to all for the comments. This has been very enlightening, especially Bigus' post. I definitely wasn't clear on the difference between a resonator and absorber before.
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post #18 of 52 Old 06-01-2013, 03:14 PM
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I have been exploring the idea of using my riser as a bass trap as well.  It is perhaps 50 cu. ft. in volume and the bottom is open so that all the joist bays communicate.  By necessity, it will be completely enclosed at the back, but I could cut a channel 3-4" tall at the bottom of the front face of the riser to allow airflow into the riser cavity.  But after reading various threads about the potential pitfalls of non-measured/non-engineered bass traps, I am now thinking of being more conservative and just designing the riser to not act as a drum and resonate.  If my riser ends up completely sealed on all 6 sides, would filling the joist cavities with pink fluffy insulation bottom to top, side to side serve to prevent the riser from behaving poorly as an 10th speaker in my 7.2 audio system?  

 

Secondarily, what is the general behavior of a riser cavity with only a large front slot cutout (4" x 16')?  I've read the terms broadband and Helmholtz which both seem to require cutouts at the back and/or top of the riser. 



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post #19 of 52 Old 06-02-2013, 05:05 AM
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RD, the secret to a quiet riser is multiple layers of decking with either 30lb roofing felt or Green Glue between layers. Most contractors laugh at that suggestion thinking it a waste of perfectly good material. The quietest risers I've built involved three layers of 5/8 material. Yes, stuff with pink fluffy. As for using it at a bass trap the risers I've built based on Dennis Erskine's plans all had the vents at the wall riser junctions. Usually at the back of the room and along the sides against a wall. From what I remember of your riser and the shape of your room with the riser in the middle of the room I am not sure if such a design has any merit.


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post #20 of 52 Old 06-02-2013, 09:54 AM
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One of the biggest factors in an effective riser trap is the volume. Calculating the right volume is essential to an effective bass trap or not so effective bass trap. Another factor is sub placement. One is never enough, and when you have multiple subs in the room, where you actually place them is also essential. Openings for the riser are calculated based on size and placement. Everything is a cohesive whole. There are no separate elements in a theater. Everything in that room works together. Whenever I draw, it usually takes me a couple of hours to come down from my brain working overtime trying to keep calculations straight in my head. biggrin.gif. Anyway, approach it from the cohesive whole approach.

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post #21 of 52 Old 06-02-2013, 01:34 PM - Thread Starter
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OK, finally got a new computer setup, i7, 32GB RAM and SSD, here we go. I revised the design and here are a list of changes:

1. Changed support structure to bigmouth's design of a middle support.
2. I used 4x4's at either end, but I bet I could get away with 2x4's.
3. The front of the riser is now open. It will be obstructed by a couch and stairs, but it should allow more sound to enter and be absorbed.

Is there anything I should change? I explored the idea of adding more venting along the sides of the riser, but it complicates things and didn't seem worth it. Any thoughts on this?

I need to figure out the detail at the grate. The plywood will be 1.5" thick, plus felt or glue. The carpet pad will probably be 1/2" thick. So the easiest thing to do would be to find a grate that was 2" thick to support the carpet evenly. I know this is behind chairs, but I have small children and I am sure it will get walked on.







Thanks again for all the help.
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post #22 of 52 Old 06-03-2013, 02:14 AM
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Just a thought, but what does it matter if you have a center support of the joists when the entire front end is open and you plan on having the entire back end open as well? I can see the uneven aspect of the flooring, but the main reason those joists are elevated is to provide for an open cavity. You've already done that by entirely eosing the front and rear. Further, you may want to provide some dimensions, because it looks as if those center supports arent even needed. Appearances can be deiving however. Just my two cents.

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post #23 of 52 Old 06-03-2013, 04:34 AM
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Quote:
Originally Posted by BIGmouthinDC View Post

RD, the secret to a quiet riser is multiple layers of decking with either 30lb roofing felt or Green Glue between layers. Most contractors laugh at that suggestion thinking it a waste of perfectly good material. The quietest risers I've built involved three layers of 5/8 material. Yes, stuff with pink fluffy. As for using it at a bass trap the risers I've built based on Dennis Erskine's plans all had the vents at the wall riser junctions. Usually at the back of the room and along the sides against a wall. From what I remember of your riser and the shape of your room with the riser in the middle of the room I am not sure if such a design has any merit.

 

I could vent it in the back corner of the riser by the electrical closet.  But probably only 1.5-2 sq. ft.  

 

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post #24 of 52 Old 06-03-2013, 07:59 AM - Thread Starter
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Originally Posted by SierraMikeBravo View Post

Just a thought, but what does it matter if you have a center support of the joists when the entire front end is open and you plan on having the entire back end open as well? I can see the uneven aspect of the flooring, but the main reason those joists are elevated is to provide for an open cavity. You've already done that by entirely eosing the front and rear. Further, you may want to provide some dimensions, because it looks as if those center supports arent even needed. Appearances can be deiving however. Just my two cents.

Sierra, I bet you are right that the center supports might not be required in a riser this size (currently 52.5" Deep). They don't server an acoustic function, but are purely for support and removing them would allow for another row of insulation. The center supports would make the structure more rigid, but the effect would be minimal on such a shallow riser. Maybe someone with riser construction experience can comment on what depth supports are recommended.

I added carpet, carpet pad and a 2" grate to the model.

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post #25 of 52 Old 06-03-2013, 11:21 AM
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Just seat of the pants estimates,

2x6s maybe 4 ft
2x8s maybe 6 ft
2x10s maybe 8ft

before I would add mid span supports, I like the riser to feel rock solid, not like a deck, certainly no bounce allowed.

In reality if I have an 8 ft span and I have some wood handy. I'd be banging in supports, I over engineer.


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post #26 of 52 Old 06-03-2013, 11:30 AM
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Any thoughts on what a riser such as the one being discussed here most recently with just an open front would do acoustically?



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post #27 of 52 Old 06-03-2013, 12:05 PM
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Wow, a lot of discussion since I last saw this thread.

Fulminis - here is a link for maximum spans for your reference: http://www.engineeringtoolbox.com/floor-joists-span-d_1479.html With your depth, pretty much anything you use will be sufficient without the middle supports. But they won't hurt anything if you keep them in.

In reference to Bigus' post above.....are you (Fulminis) still trying to build the hybrid between a broadband absorber or a Helmholtz resonator? I'm not understanding the science between making something as open to soundwaves as possible while simultaneously trying to tune a very specific frequency. One of the necessary inputs of the Helmholtz calculation is cubic volume. But are you really able to calculate cubic volume if essentially what you have is 4 feet of plywood structurally supported 15" off the floor? In reality, wouldn't the volume underneath the riser be inclusive of volumes in the main room itself since you are building such a porous structure? I am a bit confused with the approach. And as Mike said above, the whole room functions as an interdependent system, so it is difficult to have definitive conversation on your plans without discussion of the whole room's acoustics, in general.

One other side note - this is complete conjecture, but with that much open space in the riser, it seems as though the frequency you would be trying to tune is sub-sonic. Ports on the largest subs have barely a tenth of the amount of ported space you have in your riser - and these subs go sub-10Hz. Given this comparison, it seems like you would be trying to tune out plate tectonics and exploding nebulas. biggrin.gif


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post #28 of 52 Old 06-03-2013, 12:15 PM - Thread Starter
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I currently am showing 2x8's and the riser is a little too tall. I had planned on being flush with some window sills, but now I can either use 2x6 and be under the sill or 2x8 and go over it. I'll probably do the latter.

I think I will play the supports by ear and maybe just do a few so that I can get an extra row of insulation.

Here is the updated design:





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post #29 of 52 Old 06-03-2013, 12:45 PM - Thread Starter
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Quote:
Originally Posted by TMcG View Post

Wow, a lot of discussion since I last saw this thread.

Fulminis - here is a link for maximum spans for your reference: http://www.engineeringtoolbox.com/floor-joists-span-d_1479.html With your depth, pretty much anything you use will be sufficient without the middle supports. But they won't hurt anything if you keep them in.

In reference to Bigus' post above.....are you (Fulminis) still trying to build the hybrid between a broadband absorber or a Helmholtz resonator? I'm not understanding the science between making something as open to soundwaves as possible while simultaneously trying to tune a very specific frequency. One of the necessary inputs of the Helmholtz calculation is cubic volume. But are you really able to calculate cubic volume if essentially what you have is 4 feet of plywood structurally supported 15" off the floor? In reality, wouldn't the volume underneath the riser be inclusive of volumes in the main room itself since you are building such a porous structure? I am a bit confused with the approach. And as Mike said above, the whole room functions as an interdependent system, so it is difficult to have definitive conversation on your plans without discussion of the whole room's acoustics, in general.

One other side note - this is complete conjecture, but with that much open space in the riser, it seems as though the frequency you would be trying to tune is sub-sonic. Ports on the largest subs have barely a tenth of the amount of ported space you have in your riser - and these subs go sub-10Hz. Given this comparison, it seems like you would be trying to tune out plate tectonics and exploding nebulas. biggrin.gif

TMcG, Thanks for the link. I am moving ahead without supports, but may add some during construction.

I am not trying to do a Helmholtz resonator at all. I want to do a broadband absorber, but am trying to simplify construction through limiting the 'ports' to areas without foot traffic. Of course a semi enclosed riser will have some Helmholtz properties, but like you suggest, it will probably be tuned to a subsonic resonant frequency and the effect will probably be negligible.

I have two M&K subs that I am planning to use. An MX-700 with dual 8" drivers capable of 20Hz and an MX-350 with dual 12" drivers capable of 18Hz. I don't have too many options for placement, so they are going a long the front wall. They might not go incredibly low, but between the riser bass absorber, push/pull sub design and using multiple subs, I am hoping for a tight response and even coverage.
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post #30 of 52 Old 06-04-2013, 12:42 AM
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Quote:
Originally Posted by fulminis View Post



I have two M&K subs that I am planning to use. An MX-700 with dual 8" drivers capable of 20Hz and an MX-350 with dual 12" drivers capable of 18Hz. I don't have too many options for placement, so they are going a long the front wall. They might not go incredibly low, but between the riser bass absorber, push/pull sub design and using multiple subs, I am hoping for a tight response and even coverage.

Where specifically are you placing the subs? Corners? In the middle? What did you use to determine that? I don't feel the riser is going to be as effective as you are hoping. You will need additional help in the form of electronics. Also, how far away from the back wall is your rear seating?

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