Want to use space inside wall as bass trap... how? - AVS Forum
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post #1 of 8 Old 07-30-2013, 12:55 AM - Thread Starter
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I am building a theater room in my basement.

I have an odd space about 12"x16" that can't really be part of any room in the front left corner of the theater room-to-be.

The space seems perfect for a bass trap! wink.gif Maybe?

Here's my room:



Notice in the bottom left is the "space" between the stairs and the bathroom on the left (which you can't see) - it is to be slightly behind the screen and extends to the corner. The gray is my concrete foundation.

Here's that corner a little closer up:



Notice that the screen covers part of it. (I might go AT with a 14" deep AT space, so screen could be further forward. For now, it's on the wall)

Here's even closer. The orange blocks are the studs. White flat rectangles are drywall locations. Black circle a 4" drain pipe. All drywall is already present except the drywall in the theater itself.



Squiggles show wall spaces (which have insulation batts in them)

Current plan: Leave a 4.5" gap with no drywall between the corner stud and next stud. This "slit" runs floor to ceiling. Cover corner (actually whole screen wall) with GOM fabric panels. Or just use velvet or something. I would fill the space with loose pink fluffy. I want to trap 30 Hz up to 300 Hz or so in here or more. But I want to trap as low a frequencies as I can up to 300-400 Hz.

Question #1: Can this space be used as a bass trap? Or should I just drywall over it and forget the whole idea?

Q2: Do I need more of it to be open to the room? I was planning just to leave a 4.5" gap in the corner open (not put drywall there) but cover the rest. If general consensus is that I need more opening to the room, I can cut some holes in strategic places in the rest of the drywall on the other side of that 2nd stud to open up more area.

Q3: What should I fill the space with? OC703? OC705? Loose pink fluffy? Packed pink fluffy? Something else"

Q4: Do I need to cover it with GOM or some other AT material, or will any material work if I'm doing just 300 Hz or so and below?

Thanks SO MUCH! I am drywalling in about a month and want to fill up this space with the right stuff and do the drywall in this corner best to support bass trapping. Other in-room bass traps can come later but I need to figure this one out now.

A couple other questions for the group I had during my research today.

--Why do most DIY base traps on this forum use OC703 and not OC705? Isn't OC705 denser and a better bass absorber? Or am I missing something?
--Isn't tightly packed pink fluffy better bass absorber than loose pink fluffy? More lbs per cu. ft. (pcf), right? Or wrong?

Again, thanks everyone. I want to get my OC70x ordered soon if I need some!

Electrical engineer by education. Currently a system engineer. I like home automation, theaters, and blinking lights.
Someday it'll be more of an actual hobby - if my employer would ever let me go home!

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post #2 of 8 Old 07-30-2013, 10:37 AM
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A small space like that with an even smaller opening might give a tiny bit of bass trapping. So yeah, fill it with fully fiberglass and cover the opening with any type of fabric. But in the end you'll need far more bass trapping all around the room for best results.

--Ethan
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post #3 of 8 Old 07-30-2013, 08:12 PM - Thread Starter
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Quote:
Originally Posted by Ethan Winer View Post

A small space like that with an even smaller opening might give a tiny bit of bass trapping. So yeah, fill it with fully fiberglass and cover the opening with any type of fabric. But in the end you'll need far more bass trapping all around the room for best results.

--Ethan

Thanks Ethan. I read your whole tutorial yesterday on your website, great stuff. I'm sure I will need more trapping and I'm looking at my plans to think of where I can place them. So you certainly got me thinking about it!

Would it be better to open up more space on the wall to the insulation, or will the 4.5 inch "slit" be enough? I could make it more like 9 inches wide if I had to. If only I had pockets like this in all the corners of the room!

Electrical engineer by education. Currently a system engineer. I like home automation, theaters, and blinking lights.
Someday it'll be more of an actual hobby - if my employer would ever let me go home!

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post #4 of 8 Old 07-31-2013, 01:17 PM
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Second try... first reply disappeared never to return!

Ethan and I may use different terminology, but our goal is the same - uniform decay times with frequency and location in the room. The best way to achieve that is with "advice guided by in-room measurements."

I see an opportunity for a tuned absorber in your nook, but it's a solution looking for a problem! The first step in acoustic treatment is to assess the room's acoustic needs, and that can't be done properly in small rooms until you've added all the furniture, drapes, wall hangings, etc. planned or desired in the room. It's an itterative process, and one Ethan and others can guide you through, once room measurements are available.
- Install what you want in the room
- measure room decay rate vs. frequency
- determine the problems and then start looking for solutions to those specific problems.
- install those solutions and repeat measurement to a) assess effectiveness and b) identify residual problems.

Your little nook could be used any of several ways, the trick is knowing what you want it to do, based on room needs.

BTW, you don't need a terribly sophisticated measurement system. For most users, a Radio Shack SPL meter, with analog output is all you need. Plug the analog output into the "line in" port on a PC sound card, install freeware measurement program and you're all set, save for the learning curve. I like REW. there are others. Just be sure to use C weighting and FAST response on the Radio Shack meter when doing PC-based measurements.

HAve fun,
Frank
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post #5 of 8 Old 07-31-2013, 02:37 PM
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Quote:
Originally Posted by danielrg View Post

Would it be better to open up more space on the wall to the insulation

Yes, I think so. As Frank explained, it's possible to tune a cavity to a specific frequency. But that's a lot more difficult than the broadband absorption you'll get from a larger opening.

--Ethan
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post #6 of 8 Old 07-31-2013, 09:46 PM - Thread Starter
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Thanks for the additional advice! I'm probably going to stuff it with pink fluffy, not packed for now, then let the drywallers have at it, then cut the drywall back out later, maybe after measuring initially.

I have been learning a little about REW and measurements from reading around, and it sounds fun and interesting (and time consuming) to measure/treat/measure. A tuned absorber might be really hard with the space I put above because it does have some pipes, wires, and I forgot to mention it has some conduit running through it also, so the space isn't completely empty, and the conduit crosses through at some odd angles. Best chance was to just decide on the best material to fill it with before drywall. When tuning the room I can cut it back out and maybe change out or pack tighter or whatever with the space.

I'll probably at most do my best to maximize the opening when the time comes as well.

I'm also planning to put in a 5.5' x 9' wide platform after the fact and might want to make that into an absorber as well. The room's a tight fit so I want to take advantage of any empty cavities I can.

I appreciate your responses. Thanks!

Electrical engineer by education. Currently a system engineer. I like home automation, theaters, and blinking lights.
Someday it'll be more of an actual hobby - if my employer would ever let me go home!

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post #7 of 8 Old 08-01-2013, 09:47 AM
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Tuned absorbers have a very specific application, and must be engineerred for tuning-in-place to be really effective.

The application is a single strong resonance over a narrow frequency range, a "finger" on a spectrogram or waterfall plot. A "finger" is a narrow frequency range that doesn't decay as fast as higher/lower frequencies thi
Spectrogram: contour plot of amplitude on a frequency x time chart)
Waterfall: 3D plot of amplitude as the z-axis on a frequency x time chart)

Your cavity is ideal for a Helmholtz resonator, as corners are pressure maxima where the device is most effective. Think in terms of a ported speaker without the driver - a cavity and port matched to resonate at a specific frequency. Fiberglass insulation is just what you want inside. All you'd do is add a port in the drywall near ceiling and/or floor, then adjust it so the cavity resonates at the desired frequency. Port diameter for coarse adjust, length for fine adjust.

But first build your HT, then measure to identify needs, if any, and ignore all this if there's no need!

Have fun,
Frank
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post #8 of 8 Old 08-01-2013, 06:10 PM - Thread Starter
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Perfect. Sounds like it could be used broadband or specific.

Should I measure the cu. ft of the space accurately before I drywall it? That way I'd know the size - if it could be useful in any calculations?

Electrical engineer by education. Currently a system engineer. I like home automation, theaters, and blinking lights.
Someday it'll be more of an actual hobby - if my employer would ever let me go home!

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