# Bass traps and first order reflections..



## Zeitgeist (Apr 4, 2009)

I have a 10'x20' room (Really comes out closer to 16x20 because of the screen wall).. Ceiling is a drop ceiling w/2x4 panels. Need to measure height.

Behind the screen wall - I covered the drywall floor to ceiling with R13 fiberglass. At the time I didn't know what I was doing - and just wanted something absorptive. It actually does pretty well at absorbing HF - which was all that I really expected out of it.

Now, I'm getting a little more serious about doing some treatments. I would love to go all out (front, back walls, side treatments to ear height, etc) - but since the room needs to remain multi-purpose and not pure HT - I have to make some compromises.

My thoughts:
1. 2x4 2" OC703/705 panels at the first reflection points. The challenge is I have a bookshelf on the left hand side where I think that reflection point is. Most of what I've read suggests that absorbing the first reflection point is very important?

2. Corner bass traps. I was thinking cutting 2" panels into triangles (maybe 4 triangles / sheet). I need to measure my corners to see what I can fit. One of the 4 corners of my room is unfortunately the bottom to the stairs. I can't really put a trap there without getting in the way. What is the best height of a bass trap? Many that I've seen are 4' or so. I wasn't sure what the logic was in the height. 

Would I maybe be better served by good placement if 4-6" OC705 as bass traps on the wall - rather than just building corner traps?

Thanks for any advice.  I've spent too much time skimming through posts about acoustics, it's time to actually make/buy some treatments.


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## bpape (Sep 14, 2006)

703 will be fine for what you're looking to do. No need to spend double the money on 705

If you cut the 2x4 sheets into 4 triangles each, the resulting triangles are 24x24x34". If you cut them into 8, they'll come out to 17x17x24" which is a bit more manageable and will take 1/2 the material of the larger ones. 

Also, with a drop ceiling, take advantage of it and insulate above it at least 2' around the room perimeter and again over the seating area. This will help with any height issues as well as giving you a lot of broadband bass control without having it visible or taking up floor space. It won't address specific things like reflections, cancellations off the rear wall, any boundary issues, etc. but it's still a good thing to have.

Bryan


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## Zeitgeist (Apr 4, 2009)

Very interesting about the drop ceiling. Thank you. I'll see if I can do that. The original installer of the ceiling only left a couple inches of clearance between the celing grid and the floor joists - so it's a challenge to move stuff around. I've thought about replacing the drop ceiling with another grid system (I can't remember the name) where the grid is actually attached to furring strips - or the joists themselves. That way you can remove the panels by removing a portion of the trim -- rather than having to shove the panels UPwards.

Any thoughts on the height of corner traps? Any gains over floor-ceiling versus half height? I imagine - there has to be more absorption with twice the volume --- but I doubt it's twice the absorption?

Other than first order reflections, bass traps, anything else that you'd recommend, if you could treat a third item? Treating the rear wall (or at least a portion of it might be doable). 1" or 2"?


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## bpape (Sep 14, 2006)

Actually, double the surface area does give double the absorption. Same area but thicker just extends how low they'll help.

I would consider thick panels on the rear wall with potentially a facing on them to help with bass cancellations off the rear without killing the surround field.

Bryan


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## Zeitgeist (Apr 4, 2009)

Thanks Bryan. 

One last question -- I've noticed in most panels I've looked at, 2" is pretty common, 4-6" for bass absorption... but 1" seems to be generally only used when you're covering broad areas - say the side of a room? (1" so as to prevent the room from being overdamped and too dead?)

Is that accurate?

Just trying to get a better grasp of what's overkill - and what's...underkill? (Is underkill a word??  )


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## bpape (Sep 14, 2006)

1" is usually used purely for the sake of aesthetics and saving space. The less 'thin' absorption you have and the more thicker, the more balanced the decay time curve will be. Obviously, everything has a limit and you need to calculate what the target time should be, what the existing time is, and what the treatments you know you need are going to do to it so you know what's left.


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## Zeitgeist (Apr 4, 2009)

Good to know. Thanks.

I guess that says a lot about the ineffectiveness of 1". I never knew that. I've just seen it recommended so many times (on other forums) to "cover the walls" more or less.


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