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Old 24th November 2012, 02:31 AM   #1
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Default bass traps

When you make a basstrap using a membrane such as plywood, should the plywood never be fixed so that it vibrates? If I suspended a sheet of pylwood in the corner of the rooms with some damping behind it, would that work?
Otherwise how should it be constructed?
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Old 24th November 2012, 02:58 AM   #2
lgreen is offline lgreen  United States
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Look at my website, there are a lot of links for bass traps.
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Old 24th November 2012, 05:16 AM   #3
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An interesting thing happens when you have a loosely supported surface that takes up a significant portion of the walls surface. When a wave encounters a rigid boundary most of the energy is reflected back in phase. This sets up a standing wave in some cases. When a wave encounters a non-rigid boundary, such as a plywood board supported by springs for example, some of the energy will be reflected out of phase and some energy is lost to the boundary. In the case of the board supported by springs that energy would likely cause it to vibrate at it's resonant frequency introducing another wave. This is undesirable so the board must be very well damped. The interesting thing is the difference in phase of the reflected wave. I think this could be utilized to your advantage.

A simple way of demonstrating this principle to yourself is to find a metal pole and tie a rope(10-15 feet is good) tightly to it, hold the far end of the rope and move it up and down one time quickly to send a pulse down the rope. Observe that the pulse bounces back to you in the same upward fashion as you sent it. Now retie the rope but this time fairly loosely so it can slip along the pole and repeat the same motion you did before. You should see the pulse reflect back to you below the line of the rope, opposite of the way it traveled toward the pole, if you've done it correctly.

Anyway, I know that's not an outright answer to your question but I hope the info proves useful.

-Matt
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Old 24th November 2012, 07:16 AM   #4
Baldin is offline Baldin  Denmark
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To make an efficient bass trap using a membrane, it must trap some air behind and be fairly airtight. It's more or less like a speaker enclosure, where the plywood sheet is acting as the membrane. The plywood sheet should be fixed all around, and the membrane should be fairly large and undupported in in the middle to be able to flex and move. This way it will act as a resonator, where the resonator freq is determined by the weight (thickness) of the sheet (membrane) and the amount of ait traped behind.
If you want it to work over a larger frequency range, the plywood sheet can be perforated.
Buy the "Master Handbook of Acoustics" by F. Alton Everest before you start ... it is large constructions you need to make for it to make a difference in your room, so it takes time and money, and good preparation is therefore needed. See also this link:
Build a Better Bass Trap

There are also other ways of improving the rom and bass resonances.
What is your goal with the treatment? ... and what is the problem? .... how big is the room?

See also my website for information on my own treatment in my home theater:
Building a Home Theater and Listening Room

And I can only say that room treatment is essential to make good sound .... so keep it up, it is worth it
-Baldin
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Old 24th November 2012, 02:54 PM   #5
tvrgeek is offline tvrgeek  United States
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Baldin,
When tuning my living room, I had a strange null at about 80 Hz. It did not make sense from any calculated distance. It turned out to be the center channel speaker acting like a trap. The box is stuffed, ported and of course the two drivers. Seemed to be a pretty effective single frequency trap.

I also had a spike that turned out to be the back of the china cabinet. It is the unsupported thin membrane with no dampening to absorb the energy. I stuck a piece of foam between the back and wall behind it and that problem went away.

All this says is every room is different and may take different fixes. Many may cringe, but I recommend spending as much time and money on treatments, both HF and bass as you do on your speakers.
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Old 25th November 2012, 06:33 AM   #6
Baldin is offline Baldin  Denmark
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Hi tvrgeek

Can only say I totally agree. Yes all rooms are different, and do spend time and money .. it is really worth it.

Yes your speakers also will work as resonators, and to treat single frequency problems, you can build Helmholz resonators (box with tube ... like bass reflex without the driver) ... though mine didnt really work that well

But another important thing is your goal with the room. How dampened do you want it? ... I like my room now. It is somewhat dampened, but I have used diffusors and reflectors much more than absorption, not to kill the sound and reverbration.

BR Baldin
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Old 25th November 2012, 12:11 PM   #7
tvrgeek is offline tvrgeek  United States
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Right on. This leads to psycho-Acoustics. How dead a room is and how that is distributed across the spectrum in relation to what your eyes see and brain expects does matter in how long it takes for your brain to establish a "correction curve" for the room. A room that is pleasant to sit in and read is usually pleasant to listen in. Obviously, that is not much help to those who can build a dedicated room.

As I have access to my attic above my listening room, I was thinking if a Hedgman resonator would work. Basically, it was a collection of resonators to give an effective broad band filter. Just think. Leaving from the ceiling joint, a series of 20 or so 8 inch cardboard tubes all cut to a slightly different length from maybe 10 to 30 feet. Lightly stuffed. The other extreme I tried in a past place was a 2 foot by two foot by 12 foot chamber at the rear ceiling inside the attic, filled with rolls of insulation. Worked pretty well. Not cheap or easy. If I remember when I get ready to insulate my garage attic, I should bring in half a dozen rolls and pile them in the corner to see if the help this room. If they do, maybe I'll do it again. My biggest problem is a broad dip at 1.5K that really throws off the vocal balance. I may have to resort to eq for this one.

The first step in tuning is to use position of the subs first. I have seen subs place over to the side so many times. I just have to laugh. If possible, subs should be as close to the mains as possible and between them. Otherwise, you get gigantic nulls that no amount of traps or eq will resolve. I also like very steep crossovers which helps this just a bit.
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Old 25th November 2012, 04:03 PM   #8
Baldin is offline Baldin  Denmark
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This about tubes to the attic, is a good idea, but you need to have separate enclosures for each different frequency. It you make more ports into an enclosure, there will be only one reaaonance at something like the mean of the different tubes.

If you have problems at 1.5 kHz, then it is something different than standing waves I believe. I would think it has to do with reflections, which must be treated with either difusion or absorption. (I would pick difusion to help sound stage).

As for sub placement ... a lot talks for more than one sub, placed in different places of the room ...
BR Baldin
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Old 25th November 2012, 08:16 PM   #10
lolo is offline lolo  France
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Quote:
Originally Posted by Baldin View Post
To make an efficient bass trap using a membrane, it must trap some air behind and be fairly airtight.
Yep, very much so. I built 19 BBC modules that aren't sealed and now have to redo the whole thing as they don't work properly.

check this out, the limp mass has serious advantages, space saving being a great one. I will myself go for that soon.

Tim's Limp Mass Bass Absorbers - Gearslutz.com
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