# Bass Trap Questions



## fractile (Mar 15, 2009)

I'm continuing to evolve and 'finalize' the layout of my Urban Microstudio. In the process of discovering the space, I'm deciding to invest in some DIY bass trapping to reduce low frequency resonance at the back wall, and probably also on the front wall behind the speakers (primarily to keep the sound inside the room).

I'd like to get some feedback before I do actual construction, to see if my idea will work and/or improvements that can be made to the concept. Ok, let me try and keep it simple. This is basically a square 11'x11' room, slightly wider than deep, with truncated rear corners forming a bay window...\____/, sorta.

Many are familiar with low frequency resonance at the rear boundary of a room. At this boundary I will build a 'window seat' about 6'x3' and ~17" high in the center section of the bay window. For the front of this I have some panels made of ~2x6 frames with fitted slats that can be modified to be slotted (ported). I intend to add some padded cover on the front of these and 4" mineral wool on the back. I may put another panel of 4" wool, or something, over the window.

I've done some reading and still don't understand low frequency acoustics fluently enough. I just read something about how a 16" space behind a trap is a big improvement over placing it against the wall. So, is twice this distance even better? I'm assuming that atmospheric dispersion between the panel and the reflective wall adds to the attenuation. Also, should I line the interior of this space with some nonreflective material like carpet, to assist? I figure that any sound resonance in this room will get transmitted into adjacent spaces, and I want to minimize this. Your thoughts appreciated.

Ok, for the front wall behind the speakers:
In a continuing attempt at maximizing efficient use of space, and despite the laws of physics, I've decided to build an 8 foot wooden shelf below the 8 foot steel frame rack (see pics in the DIY Urban Microstudio thread) for the 2 stereo monitors, plus another center speaker I am building.

I haven't done much listening tests on this, but I assume that not much low frequency sound is hitting this front wall. I have put 4'x8' panels of that ~5/8" woodpulp sound bat under ~3/8" HDP panels, rough side out on this wall, and intend to put a 1/2" jute woven decorative mat on top of that. I've thought about soffitting the speakers in, but I don't know if there'd be any benefit. Well, maybe to extend the edge boundaries to cut down on comb filtering.
lddude:


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## Wayne A. Pflughaupt (Apr 13, 2006)

Hi there,

I'd suggest contacting Ethan Winer at Real Traps. He has a lot of experience with traps for studio applications. Tell him Wayne sent you. 

Regards,
Wayne


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## fitzwaddle (Aug 25, 2010)

Wayne A. Pflughaupt said:


> Hi there,
> 
> I'd suggest contacting Ethan Winer at Real Traps. He has a lot of experience with traps for studio applications. Tell him Wayne sent you.
> 
> ...


Summary: as many traps as you can fit, using corners where possible (including tri-corners), and as thick as practical, medium density rigid fiberglass (faced, unless you're room needs additional mid/high trapping as well).


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## fractile (Mar 15, 2009)

I did some more reading to improve my comprehension of all the aspects of low frequency room modes, axial, tangential and oblique, and that the rear bay window can behave as a lens to amplify sound. A design option mentioned in one article on a similar room configuration was to place a panel absorber on the front wall, since it is a resonance issue between the front and back walls.

Also discovered was the design formulas for panel absorbers and Helmholtz traps: http://www.soundonsound.com/sos/aug98/articles/practicalacoustic.html
The layout and configuration of the room have been pretty much figured out, so it's basically now a matter of constructing the basic bench then experimentally fine-tuning it. And then adding a loft over that and adding in the subwoofer will give a few more problems to solve. It looks like all of these things have acoustic benefits and also difficulties.

The side walls have book cases above door height to the ceiling. The back corners I'll deal with as needed; maybe some corner absorbers and a cloud. Thanks for the feedback.


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## fractile (Mar 15, 2009)

There is a new 5th edition of Everest's _Master Handbook of Acoustics_, updated by Ken Pohlmann (I have his _Principles of Digital Audio_). I keep waiting for it to arrive in the mail. I studied the contents and it looks like a good practical middle ground for me to review basic concepts. The architectural foundation for this issue is still being finished out, so I still have time to think.

Meanwhile I looked at the Vicoustic Super Bass Extreme design [ http://vicoustic.com/vn/MusicBroadcast/ProdutoInfo.asp?Id=71 ]. An interesting composite structure.


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## AudioDH (Jan 25, 2011)

Hi Fractal,
well sounds like lots of fun! I know I am late to reply on this, but thought I would jump in somewhere. ;-)
And before I get started - ditto to Wayne's comment about contacting Ethan at Real Traps.

There are some points of interest before getting to where you are. 
I'm currently involved in a studio rooms design and I had to bone up a bit in those regards.
If you haven't tried one of the Mode programs to determine your problem areas, such as the Mode Calc from Real Traps, another interesting online calculator offers good and extended information as well.
Search:Bob Golds room mode calculator. He does a nice job here.
"Well the links I tried to post for you got me busted by the cyber police.
Do a search on Google for Real Traps Mode Calc and Don Golds" Sorry dude!!

Ethan will tell you to use as much "bass trapping" as possible...and I agree.
However, do not apply broadband attenuation, as your room will be tough to work in and mixes will be flat!
I recommend some reflections except for RFZ absorbers on either side of the mix position and ceiling. You can use a mirror to position them - plenty of web post on that.
What is interesting to learn from the mode calculations is that all surfaces are part of modes and in difference to your statement "but I assume that not much low frequency sound is hitting this front wall" is incorrect and ALL your walls play an important role in causing problems associated with bass trapping. Especially small rooms.

I would be interested in knowing how your project came out and what results and conclusions you've reached!
Dave


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## fractile (Mar 15, 2009)

AudioDH said:


> And before I get started - ditto to Wayne's comment about contacting Ethan at Real Traps...
> 
> ...Do a search on Google for Real Traps Mode Calc and Don Golds"...
> 
> ...


Yeah, reading in that acoustics handbook made me aware that the lower the sound frequency, the less the sound travels like a ray and more like a bubble.

In addition to the mode calcs I can use a mic and get an acoustic map of the room. I'm still doing some plaster and paint repairs, giving me more time to think about the design of the bench/basstrap. I'll keep it as a modular prototype to allow fine-tuning as I go. And most of the other room treatment I plan to have convertible to tune the room for recording or mixing.

By the way, I found a slab of some kind of 6", dense polyester fiber batte to try in the bass trap. So I now have about all the materials to begin putting that together, and I want to get that done because that will be the main listening position that I need functioning to work on setting up the monitors and the front wall.

I've been trying to pick up some theory of acoustic treatment and have to just dive in and learn the practical application. That and trying to build in all the equipment to have it both accessible and efficient with space is a challenge. This is not a built-to-order space and I'm breaking a lot of rules that will have to be worked around. Thanks for your interest.
-Victor


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## fractile (Mar 15, 2009)

Your mention of results and conclusions got me thinking about frequency mode excitation. A recording and mixing room is not the same as a reference listening room set at 85 or 87dB. It would be nice if my situation could be developed to accommodate extended listening at this level, but that is another story.

What I want to point out is frequency mode excitations at various listening levels (specific room excitations dependent on acoustic output) and how this relates to individual room tuning compared with standard 'by the book' room treatment.

I'll admit I have an imagined philosophy of preserving and developing the sound of a room, without letting detrimental coloration ruin it for some type of objective listening. I think what this means is being about to hear the sound from the monitors of how the reproduction would sound in the room you have, under somewhat ideal conditions.

That's just some thought. I listen to things at a variety of levels. As the level rises I think the mode most immediately noticed is the fundamental mode at a listening position close to a boundary, the low frequency. There are other modes all along the frequency band, depending on the configuration of the room. It takes more practice to hear them. My co-axial monitors are more directional than many people would like, but I think it allows me to hear the room better, since reflections will be so many dB lower. I forget. A 10ms delay is 6 or 8 feet, what they call the limit of correlated coherence.

Yeah, in the process I come full circle to conventional acoustic theory on sound rooms; to bring both the spectral reflections and resonant decay down to some continuum in that 10ms window. In this can also be implied a coherent time-constant consistent with the dimensions of the actual room.

Some thoughts. Hope it helps insite.

V


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