Riser Broadband Bass Trap Review - Page 2 - AVS Forum
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Dedicated Theater Design & Construction > Riser Broadband Bass Trap Review
fulminis's Avatar fulminis 07:43 AM 06-04-2013
Quote:
Originally Posted by SierraMikeBravo View Post

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?

Definitely. I was going to save sub placement and electronics for a new thread, but might as well do it now. I had planned on using the Onkyo TX-NR818 or TX-NR828 with Audyssey XT32, but it looks like the 828 won't include XT32. So now it will either be the 929 or 818. It will definitely have a XT32 and individual sub channel calibration. I am a little tentative about this calibration though as it can only be as good as that little microphone that's included. Can a mic that small even be sensitive to the lower frequencies? I do like the idea of one comprehensive calibration system rather than a separate one just for the sub. It will be a 9.2 system and I have a B&K 200.5 S2 to drive the main channels. Sides and front highs will be poweedr direct from the receiver.

For sub placement, I have heard that you want them at 1/3 and 1/4 of the way down a wall and preferably on separate walls. Unfortunately, my theater is super tight and separate walls is not an option. I'll stay away from the corners since they seem to cause a more uneven response.



fulminis's Avatar fulminis 05:19 AM 06-11-2013
I wanted to follow up. I am off to InfoComm today and probably won't be able to work on this for a week.

I think the riser design is pretty much settled. I had to lower the height a little to fit under the sills. That involved moving to 2x6's and adding an extra layer of 1/2" plywood. I also picked a grate, Fibergrate 878872. I need about 16' and it comes in a 48"x48"x1".

Quick question. Should the backing on the pink insulation removed? It seems like it would reflect some sound and degrade it's acoustic properties. Do they sell it without backing?

My sketchup model is coming along nicely. Now that the riser design is done, I will probably start a new thread for other more general design challenges. Here is the current revision of the model. Hopefully someone finds it useful.

Here is the updated design. I'd appreciate any thoughts.










TMcG's Avatar TMcG 05:34 AM 06-11-2013
Removing the backing isn't necessary if you've already bought the insulation, but why not just buy the unfaced R19 if you have the chance?

I'd say your biggest concern would be the acoustic impact of having a square shaped room.

EDIT: If you buy the faced, it will give you the opportunity to secure it to the sides of your 2x6 joists and thereby prevent it from ever falling down. I say the sides because that still leaves you the ability to glue and screw the first layer of decking to the top of the joist and not the insulation paper.
J_P_A's Avatar J_P_A 07:59 AM 06-11-2013
I'm not really qualified to make this statement, but I'm going to anyway biggrin.gif, my first thought was I bet that sectional is going to make a pretty good bass trap. Big and fluffy, and it takes up a lot of space out in the middle of the room. It would be interesting to know the before and after modal response with the sectional in and out of the room.
RossoDiamante's Avatar RossoDiamante 08:10 AM 06-11-2013

For some reason, faced rolls are so much cheaper than unfaced batts around here.  Something like $0.25/sq. ft. vs. $0.55/sq. ft.  Needless to say, I went with the rolls and peeled off the facing when needed.  Otherwise I just used them faced even though they don't need to be faced in my application.


fulminis's Avatar fulminis 08:35 AM 06-11-2013
TMcG, the room is not perfectly square, but that is a good point. I don't remember the exact dimension, but I think it is 190.5" wide and 182" deep. Hopefully that is unsquare enough.
HopefulFred's Avatar HopefulFred 11:51 AM 06-11-2013
Quote:
Originally Posted by J_P_A View Post

I'm not really qualified to make this statement, but I'm going to anyway biggrin.gif, my first thought was I bet that sectional is going to make a pretty good bass trap. Big and fluffy, and it takes up a lot of space out in the middle of the room. It would be interesting to know the before and after modal response with the sectional in and out of the room.
I agree that it would be interesting to see numbers, but I am sure there is some real benefit. Measurements like that are on my list. wink.gif
SierraMikeBravo's Avatar SierraMikeBravo 02:39 PM 06-11-2013
Honestly guys, it won't amount to much. we NEED to remember how big these wavelengths are AND what REALLY causes the problems in this room. Is it the velocity of the wave or the pressure the wave creates? Think of this in terms of tsunami. When the wave is translating in the ocean at 400 mph, and a ship is out in the middle of the ocean, do they feel it? Or is it when the wave slows down as it reaches shore and that energy is then used to create a large wave. Does the ship feel it closer to shore of out in the ocean?
HopefulFred's Avatar HopefulFred 02:48 PM 06-11-2013
Most LF absorbers are pretty useless aren't they Shawn? I mean taking 3dB off a 12dB peak at a primary mode with traps is doing really pretty well isn't it? What kind of impact could a well-designed resonant riser trap really have?

(I hope that doesn't sound argumentative. I'm just trying to confirm my observations match your experience)
SierraMikeBravo's Avatar SierraMikeBravo 03:10 PM 06-11-2013
Actually...the impact can be very large depending on what issues were present to begin with. Where you place the subs is a critical part of that. Dennis and I have designed rooms where the actual measured result of the effect the riser trap was such that there was virtually NO resonating peaks below 100 Hz at the listening position. A properly designed trap can have enormous effect.
rabident's Avatar rabident 06:57 PM 06-12-2013
How critical is implementation? I have a proper design. It says to construct using 2x6's. It doesn't say how. How do I frame a 2' riser using 2x6's ? Does it matter for the performance of the riser, or is whatever the local construction crew comes up with fine?
SierraMikeBravo's Avatar SierraMikeBravo 08:06 PM 06-12-2013
PM Sent
Dennis Erskine's Avatar Dennis Erskine 09:32 AM 06-13-2013
Quote:
How do I frame a 2' riser using 2x6's ?
This comes to mind.... 2footriser.pdf 15k .pdf file

or this
2FootPlatformB.pdf 17k .pdf file
Attached: 2FootPlatformB.pdf (16.7 KB) 
Dennis Erskine's Avatar Dennis Erskine 09:41 AM 06-13-2013
You are not going to have a Helmholtz and broadband absorber in one. You cannot have a 200mph Ferrari and 100Ton loader in a single vehicle. If you want that platform to be broadband, you will want the entire top decking to be grating..or you can make a low pass filter by drilling x dia. holes y distance apart on the entire surface of the decking...you then must have a carpet and pad which will allow air flow. What you have now is a pressure absorber (period) which will have its greater impact on length mode frequencies.

BTW, those 7 seats against the back wall will have rather horrid sound quality (presuming those sitting in the seats didn't pay for the room, I suppose they can suffer). More to the point, however, is those 7 seats add 3,500 BTU's to your cooling requirement and 105 CFM/hr of air exchange.
fulminis's Avatar fulminis 11:13 AM 06-21-2013
My rudimentary understanding of a pressure absorber is that it has to be enclosed, so that sound gets trapped and mode frequencies are reflected repeatedly. But this current design with the open front really wouldn't do that, so is it still a pressure absorber? Or is it partially effective velocity based absorber limited by the small grill area?

It sounds like I need to go back to the drawing board and figure out a way to open up the design more. Maybe I will just drill a bunch of holes and try to find a porous carpet pad. Drilling 200 holes through three layers of plywood sounds pretty tedious, but doing it all with grate is going to be cost prohibitive. Does anyone have a suggestion of what hole size and spacing should be?


I am aware that the back row is difficult audio wise. I have a plan with some dipoles that I think will help and was planning to review it in a new thread once I had the surround speaker in the model. The use of this theater is mainly for family and I prefer more of a family room feel, hence the sectional. The back row is really for sporting events when we need extra capacity. Translation, I will rarely sit there.


fulminis's Avatar fulminis 02:24 PM 06-22-2013
Here is the surround speaker design. I currently have (2) M&K SS-150's tripoles on the back wall and (2) M&K MP-150's on the sides.

I realize that traditionally, the diffusive speakers are usually used on the sides. Placing the tripoles on the back wall seemed like the only way create a soundfeild behind the back row. I have drawn a 90 degree dispersion in the elevation, which I know is inaccurate, but it helps to show who will be in the direct sound field. Also, the SS-150's are a three way design with a forward facing HF and LF drivers. It should act like a directed radiator for the people on the sectional.

I also included an overlay with Audessey's recommended angles to each speaker from the primary position.

Any comments on positioning or type? Is there a way to get a better soundfield for the back row without sacrificing the front?
















Dennis Erskine's Avatar Dennis Erskine 07:20 AM 06-24-2013
Quote:
I have a plan with some dipoles that I think will help ...

Won't help the problem.

Not sitting there ... will help. smile.gif
fulminis's Avatar fulminis 08:23 PM 06-24-2013
Quote:
Originally Posted by Dennis Erskine View Post

Won't help the problem.

I'd be interested to know what you are defining as the problem. One of my biggest home theater pet peeves is when a surround speaker is too close and becomes 'localized'. I find it very distracting and that is really what I am looking to avoid. I realize that the back channels will be overly diffuse/muddy for the back row, but the direct radiating side channels should help a little. Directionally it won't be great, but tripoles should help to make that better.

Is there some other acoustical issue that I am missing?
rabident's Avatar rabident 01:34 AM 06-27-2013
Quote:
Originally Posted by Dennis Erskine View Post

This comes to mind.... 2footriser.pdf 15k .pdf file

or this
2FootPlatformB.pdf 17k .pdf file

Thanks Dennis. The cross sectional view helps. Do I need to do anything to hold the insulation in place along the underside of the floor?
Dennis Erskine's Avatar Dennis Erskine 04:47 AM 06-27-2013
Those were cut/pasted from your plan set.
Home Depot (and others) have the spring steel wire used by contractors to hold the insulation in place. They are slightly longer than the distance between 16" O.C. joists. You just press them up between the joists and they hold the insulation in place.

Here's link:
http://www.homedepot.com/p/Simpson-Strong-Tie-16-in-OC-Insulation-Supports-100-Pack-IS16-R100/100375163
sulakd's Avatar sulakd 12:07 PM 05-27-2014
If using two layers of decking with green glue between, does the top layer just get glued to the bottom layer, or is the top layer also screwed down?
TMcG's Avatar TMcG 12:20 PM 05-27-2014
Quote:
Originally Posted by sulakd View Post

If using two layers of decking with green glue between, does the top layer just get glued to the bottom layer, or is the top layer also screwed down?

Screwed down using screws that are long enough to penetrate both layers into the riser's joists. Use the grey colored exterior deck screws with minimum 3/4"+ screw length beyond your combined deck board thickness.
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