Bass Trap Advice... before I make a mess - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 9 Old 09-08-11, 12:30 PM Thread Starter
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Bass Trap Advice... before I make a mess

I just fumbled the ball on installing acoustic panels... so I want to make sure I'm heading down the right path before I start constructing and installing some bass traps.

I have a very challenging space with limitations on how intrusive the traps are in the space I have. My plan is to construct 4 ft high triangle corner bass traps out of 703 cut into 17X17X24 triangles stacked 4 ft high in four corners of the room (see a picture of my HT space below... these triangle stacks are highlighted in the corners as a purple circle encircled by black).

Further more, I wanted to construct tri-corner triangular traps (for ceiling corners)... these will go in the corners of the room that are encircled with aqua.

I know my room diagram is pretty bad... but just follow along if you can!

I've also included a spectrogram of my room. you can see I've got some heavy stuff going on down low... and then a null...

I've read that you can never trap enough of your room... I just want to make sure that I won't be wasting my time by constructing these traps and placing them were I can. it would be a huge disappointment to go through with this and have negligible (or negative) results!

Thanks in advance
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post #2 of 9 Old 09-08-11, 01:17 PM
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Re: Bass Trap Advice... before I make a mess

You're going to have a very difficult time taking care of that problem centered around 30Hz. I would try moving subs and seating to get out of it as a better solution. Not saying you don't have a need for at least the front corners but don't expect miracles down that low.

You can certainly overdo any sort of absorption in a room. It's harder to do in the bottom end but it can certainly be done.

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post #3 of 9 Old 09-08-11, 01:35 PM Thread Starter
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Re: Bass Trap Advice... before I make a mess

I've tried moving the sub... seating position and monitor are locked into place.

My solutions to tame that deep lower end have been:

1) moving sub, which didn't seem to help
2) Parametric equalizer, which helped some by not a tremendous amount

Do you think I should only put bass traps in the front of the room?? Not the back? Just curious as to why you suggest that.

It seems like, compared to some the images I've seen online of corner traps, that my plan couldn't possibly over do it.... ?
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post #4 of 9 Old 09-08-11, 06:50 PM
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Re: Bass Trap Advice... before I make a mess

"Overdo it"???

The problem is that your proposed solution is not substantial enough to even begin to effect a solution.

Bryan is right.

You are trying to use porous absorption to do what it is least capable of doing, stopping the lowest 2 octaves while seeking to restrict its size and placement - which directly limits that which makes it effective.

This is restrictive for precisely the restrictions you cite: intrusive space limitations.

In order to be effective at twice your problem frequency you would need to have, at least, all four corners trapped with Full size Superchunk style corner traps (2'x2'x34"x8') filled with 'pink fluffy stuff' - as the gas flow resistivity of the lighter fill is superior to the more expensive semi-rigid 703 style material. And better still, you would probably need to install additional corner traps in the horizontal wall-ceiling corners as well.

There is not magic to these type of velocity traps. They work by virtue of size and internal travel volume and by moving the absorptive material out from the boundaries to the 1/4 wave position of the lowest frequency of interest.

Seeing as how a 30Hz wave is ~11.4 feet long, the 1/4 wavelength is 2.85feet away from a boundary.

A half-height porous corner trap may fit your aesthetics, but it will not be significantly functional at the frequency of interest.

And building upon Bryan's excellent point, while such a proposed trap's low frequency extension will not be very effective at 30 Hz, it is effectively a broadband trap - and in the meantime it will be sucking up the mid and high frequencies unless effort is made to make the trap more frequency selective. This can be partially achieved by placing a facing that is reflective to the shorter wavelengths on the front of the trap, such as a material ranging from Kraft paper to 1/4" MDF sheeting covering the front.

But while doing so will decrease the amount of mid-high energy absorbed, it will not increase the absorption at the frequencies where you need it.

So, given your stated restrictions, this does not sound like a very feasible plan.

This problem and these restrictions are the precise recipe for tuned resonant absorbers. And while these can be made, they are of sufficient complexity that I will not recommend them to a first (or 2nd or 3rd) time designer, as their design is very specific and their application and tuning (assuming they are effective at the design frequency) as their presence is interactive with the room and causes a shifting of both frequency and positional distribution of the modes, requiring iterative tuning upon installation.

If you want to pursue these smaller more efficient pressure based tuned resonators, I would suggest contacting RPG and inquiring about their Modex 30 Hz Tuned modules. Note, you will need multiple small units or a large unit. The precise total size is not an exact science, so don't be shocked if you need more than a few. Be prepared to use more than one!

The available practical solutions are as Bryan suggested. As the modal distribution is established by the massive boundaries of the room space, you can partially adjust the spatial distribution by repositioning the subwoofer. Perhaps the easiest way to do this is to do the proverbial sub crawl where you put the subwoofer (literally) at the position of the primary seating position at head height (SECURELY!!!!), and you crawl abut at the level of the sub until you find a location where the perceived bass is as balanced as possible.

To the degree that is not possible, you move your seating position forward or backward until you are at a cusp between a peak or null.
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post #5 of 9 Old 09-10-11, 08:39 AM
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Re: Bass Trap Advice... before I make a mess

One thing I will add.... You could do every single corner in the room and if your problems are related to things like height, cancellations/reinforcement off the rear, boundary interactions, etc - all of those problems will still remain.

Corners are an efficient place as they're at the end of 2 boundaires - they are not a be-all, end-all solution for every problem.

If you want to try to build a tuned trap, certainly go for it. I would just strongly recommend a sealed membrane type rather than a Helmholz type. When trying to tune Helmholz absorbers down that low, they can cause their own problems - sometimes more severe than what you're trying to fix.

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post #6 of 9 Old 09-10-11, 09:20 AM
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Re: Bass Trap Advice... before I make a mess

I'll go MUCH further!

Porous traps are a major compromise below 100 Hz, wherever you put them!

Tuned resonant traps are definitely the optimal way to go there (assuming you do not have the luxury of being able to vary the 'complaintcy' of the boundary surfaces).

But, as correctly stated, they are not for those looking for a non-thinking quick fix.
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post #7 of 9 Old 10-02-11, 06:41 PM
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Re: Bass Trap Advice... before I make a mess

Personally i'd go with Panel absorbers... but, and this is a big but. they are going to be big.

I've been working on stretched membrane panels using pond liner and vinyl backed with blow-in insulation suspended by a poly grid, stretched over a tuning frame instead of ply or luan with pretty good results. (patent pending seriously), but for diy use it.

the tuning frame is adjustable and the membrane will flex with much lower frequencies than a solid will with much less surface area. Although 4ft by 6ft seems to be a sweet spot for sub frequencies. 4'x4' has narrower bandwidth and 4'X8' more bandwidth but kinda sloppy.

I got the idea from looking at damped concert bass drums, thinking... what if i put that in a corner of a room??? ta-da idea.
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post #8 of 9 Old 10-02-11, 08:05 PM
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Re: Bass Trap Advice... before I make a mess

What are the dimensions of your room?

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Bill

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post #9 of 9 Old 10-06-11, 09:50 PM
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Re: Bass Trap Advice... before I make a mess

The simplest way to determine the nature of the problem and the effects of treatment are with simple waterfall measurements as well as with test tones allowing one to easily map the physical distribution of peaks and nulls within the space of the space - aiding in potential listening position adjustment forward or backward (while maintaining to avoid either peaks or nulls while maintaining L/R symmetry.

Measurements ALWAYS trump ideal modal calculators that neglect the actual acoustical impedance of the boundary surfaces and assume a perfectly reflective rectangular space with NO surface variations.

And large porous corner traps are more effective than wall mounted porous panel traps for a number of reasons, among them the positioning of the absorbent material further from the boundary surfaces closer to the lower frequency ⅛ wavelength optimal for velocity based absorbers, as well as taking advantage of the adjoining wall surfaces directing energy laterally across and through the comer traps absorbent material face - which if a standard 2' x2' by 34" face construction is used, is a much greater distance than a spaced porous wall panel.
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