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Re: Major sub issues
chesseroo #279911 11/20/09 02:51 PM
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 Originally Posted By: chesseroo
One Hz out of so many, or a small range and depending on the dB down may not make an audible difference in music/movie playback with everything else that is going on sonically.



Uh-hu, I suppose 1 out of 20,000 ain't bad! ;\)


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Re: Major sub issues
Potatohead #279913 11/20/09 03:02 PM
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 Originally Posted By: Potatohead
I do have about 10 dB lulls at about 56 and 78 Hz, which was the same pattern no matter where I put the sub, so it has to be my room. According to SVS's graph the sub does dip a bit at these points, so that probably contributes a bit as well.

One thing I did find odd, was that I am down a decent amount as well at about 95 - 110 hz... I don't know if this is just from coming out of the range of the sub, which level was probably set a bit too high, but I don't really have another explanation for it as the W22's should handle that frequency with ease.

I think the next step is going to be a couple bass traps in the back corners of the room and maybe some panels on the walls. Can't hurt I guess.

As I was saying, you get the hole around 60 Hz from the length, and the 93 Hz because of the width. It isn't only the sub's bass which is affected by the room. Any bass which is produced will interact, even that from the W22s. Oh, and an 8 foot ceiling is contributing one more major room mode at 140 Hz.

Definitely start with traps, I wouldn't even worry about the wall panels unless they are nearly two feet thick, they won't be able to capture that low of frequency. Wall panels are used more to control treble reflections.


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Re: Major sub issues
ClubNeon #279960 11/20/09 06:47 PM
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Chris,

Would something like this (the 4" thick, 24" x 48") be a good choice for the corners?

http://www.atsacoustics.com/cat--ATS-Suede-Acoustic-Panels--109.html

I would bridge the corner with them, leaving a gap behind. I could do three of the 24" squared ones as well and stack them, leaving a few inches between, which may look nicer and cover more area.

I am also considering building my own, I have read cutting the fibreglass in triangles and stacking it up in the corner under a frame/fabric works well. Those ones are cheap enough though they just wouldn't look as nice.

Last edited by Potatohead; 11/20/09 06:48 PM.
Re: Major sub issues
Potatohead #279965 11/20/09 07:07 PM
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Treatments labeled as "acoustic panels" will usually have no effect in the bass range. You want something actually sold as a "bass trap".

You're on the right route with building your own. From that site, try the Roxul Rockboard 60, cut into half and then diagonally, so you get 4 triangles which are 2 feet long on the short sides, and 2 inches thick. The $49 gets you 6 sheets by 4 triangles by 2 inches is: 4 feet thick. So you'd need $200 worth to do the 2 rear corners (really you only need to do 2 corners, as you're trying to break up standing waves between a pair of walls, break the pair, break the waves) from floor to an 8 foot ceiling.

If you want to cover the diagonal between the walls with their acoustic panels, that'll be a good use for them. But you'll need a custom size being 2'10" wide to cover the long side of the triangle.

EDIT: I just found their absorption coefficients for the 4" thick panels. Just as I figured, they're good down to 250 Hz, but fall off quickly after that. If you do want to do the custom one to cover the rockwool, just go with the 1" thick and request that they be made with an open back (even though they don't usually do that for the less than 4" panels).

ADDITIONAL: Their 96"x34"x1", open backed, panels run $212 each. You'll probably just want to stretch your own fabric over a frame. The panel is just for looks anyway, because the rockwool is doing all the work.

Last edited by ClubNeon; 11/20/09 07:21 PM.

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Re: Major sub issues
ClubNeon #279977 11/20/09 08:28 PM
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 Originally Posted By: ClubNeon
Treatments labeled as "acoustic panels" will usually have no effect in the bass range. You want something actually sold as a "bass trap".

You're on the right route with building your own. From that site, try the Roxul Rockboard 60, cut into half and then diagonally, so you get 4 triangles which are 2 feet long on the short sides, and 2 inches thick. The $49 gets you 6 sheets by 4 triangles by 2 inches is: 4 feet thick. So you'd need $200 worth to do the 2 rear corners (really you only need to do 2 corners, as you're trying to break up standing waves between a pair of walls, break the pair, break the waves) from floor to an 8 foot ceiling.

If you want to cover the diagonal between the walls with their acoustic panels, that'll be a good use for them. But you'll need a custom size being 2'10" wide to cover the long side of the triangle.

EDIT: I just found their absorption coefficients for the 4" thick panels. Just as I figured, they're good down to 250 Hz, but fall off quickly after that. If you do want to do the custom one to cover the rockwool, just go with the 1" thick and request that they be made with an open back (even though they don't usually do that for the less than 4" panels).

ADDITIONAL: Their 96"x34"x1", open backed, panels run $212 each. You'll probably just want to stretch your own fabric over a frame. The panel is just for looks anyway, because the rockwool is doing all the work.


If I was going to do a DIY trap I would probably just build my own panel for the front, some wood, fabric and staples is pretty simple.

Problem I have though is I cannot really put this type of trap in the one rear corner, as my equipment is there. Is it possible I may not even need one there as there is nothing to really bounce off of? I could do it above on the bulkhead, and along the bottomside of the bulkhead against the wall instead if I had to.

Lastly, is the Roxul stuff you find in the store (safe and sound) which I used to insulate the room, no good for this? Not dense enough perhaps?




Last edited by Potatohead; 11/20/09 08:29 PM.
Re: Major sub issues
Potatohead #279980 11/20/09 08:43 PM
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To be effective the corner needs to be filled from bottom to top.

Are you saying you'd build a horizontal trap? That'd cover the resonance modes for the length of the room, and the ceiling to floor, but not the width. Imagine literal waves rolling back and forth between the parallel surfaces of the room. When you have a stack up and down in a corner of the room, waves going front to back, and side to side will come into contact with the most volume of the trap. But waves moving up and down will only catch it slightly. That's not to say that a horizontal trap doesn't help with the up to down mode. To control the two worst room modes though, you need two vertical stacks in two corners. You could put them in the front of the room.

To build a stack you really need the rigid board, so it can support 8' of it's own weight and not compress over time. If you built support shelves every foot, you could go with the flexible stuff, and not lose too much total volume over time.


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Re: Major sub issues
ClubNeon #279981 11/20/09 08:49 PM
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I just went back and looked at the pictures of your room. Yeah, that doesn't leave you many options. A trap in just one corner, and then a false bulkhead full of rockwool opposite the real one may be your best bet.


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Re: Major sub issues
ClubNeon #279999 11/20/09 10:52 PM
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Ok, hopefully the last question.

I think I am going to build a simple frame, and fill it with Safe 'n Sound. I can build it up to about a foot thick, so it should be dense enough.

Should I do the triangle thing, or should I build it so that I can stand the SnS up inside the frame, and make it about three rows deep (about 10.5"). This would allow me to leave an air gap behind the trap.

I don't know if building a soffit on the other side is something I want to do, it would make it pretty claustrophobic in there. Is there somewhere on the bulkhead side that you can see may benefit from the same treatment? I am thinking the bulkhead to back wall connection may be a good idea, although it's kind of short. I could also build about an 8 - 12" deep trap between the equipment and the sub(just to the right of the equipment), which is about 18" wide space. Being a couple feet from the corner though I don't know if it'll help.



Last edited by Potatohead; 11/20/09 10:54 PM.
Re: Major sub issues
Potatohead #280004 11/21/09 12:12 AM
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An air gap isn't as good as material all the way to the wall. The only reason some products are designed to employ a gap is to have the sound pass through them twice. But if they were solid all the way to the wall they'd be even more effective. Just be less flexible in their installation and cost more.

The triangle into a corner has the largest volume for the smallest floor space. Instead of a full squared off soffit, picture a triangle which comes 2 ft across the ceiling and 2 ft down the wall. It shouldn't intrude too much.

That small of a trap won't do too much. When dealing with very small traps they need to be tuned exactly to the frequency you want to absorb, and make use of some advanced techniques like Helmholtz resonators. The reason the traps need to be big is because the wavelengths at these frequencies are that long. A 60 Hz sound wave peak to peak is 18.6 ft long.


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Re: Major sub issues
ClubNeon #280020 11/21/09 03:19 AM
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Chris, better cut those room mode frequencies in half to allow for the round trip.


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