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How about an active bass trap?
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AMelbye
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22nd April 2008
Old 22nd April 2008
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How about an active bass trap?

I've been thinking... why not design an active bass trap system?

By placing a subwoofer in each corner, and make them output a signal of opposite phase from the loudspeakers, delayed to match the timing relationship of the incoming soundwaves, and filtered to only work within the bandwidth of the modes that are in need of attenuation. It could even be possible to do an impulse response measurement of each corner, and let the subwoofer reproduce the opposite of this.

The software for this should be able to run on any old Pentuim3 or better laptop you have lying around, and the subwoofers wouldn't have to be too powerful, as they'll all be placed in room corners.

What do you think?
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22nd April 2008
Old 22nd April 2008
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This has already been done at least twice, and Bag End (speaker maker) currently sells this as the E-Trap. But there are downsides, and I explain more in this thread:

Room Treatment for Mastering

See my Post #21.

--Ethan
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AMelbye
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22nd April 2008
Old 22nd April 2008
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I agree that the measurement data provided by bagend isn't detailed enough. I'm also unsure of wether a microphone based solution is the way to go, as it means that the unit will work on a range of frequencies, not only the modal ones that you yourself define. Also, such a circuit may be more difficult to get to work without designing specific software/hardware.

What I suggested should be doable using any software sequencer and combination of subwoofers as long as the software/hardware allows relatively fast audio processing and pass-through.

Obviously acoustic treatment will still be neccessary to deal with early reflections, SBIR etc.

But this may pose a solution in rooms (like mine) where there is severe modal build-up, and the time I plan to live here doesn't justify buying or building resonant traps to cure the specific issues.
AMelbye
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22nd April 2008
Old 22nd April 2008
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Sorry, I just realized the E-trap works on two target frequencies. I still believe direct connection to a processed version of the audio output will be more efficient, though
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22nd April 2008
Old 22nd April 2008
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Quote:
Originally Posted by AMelbye View Post
What I suggested should be doable using any software sequencer and combination of subwoofers as long as the software/hardware allows relatively fast audio processing and pass-through.
The main problem is any improvement will be localized. Possibly not even for both ears at the same time. And things may well be worse at other locations, which rarely happens with passive bass traps.

--Ethan
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22nd April 2008
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I just realized there's a very basic question I need an answer to:

When you add a secondary loudspeaker to cancel the signal of the first, will these two completely cancel each other out, or will the waveforms just 'move on top of each other' making peaks and dips dependant on position of the room?

If the latter is the case, I'm with you :p
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22nd April 2008
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Quote:
Originally Posted by AMelbye View Post
the waveforms just 'move on top of each other' making peaks and dips dependant on position of the room?
Basically yes. It depends on the spacing and the wavelength of the sound.

Andre
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22nd April 2008
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K+H has a similar product. See pages 6-8 in this.

Andre
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22nd April 2008
Old 22nd April 2008
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Quote:
Originally Posted by Ethan Winer View Post
The main problem is any improvement will be localized. Possibly not even for both ears at the same time. And things may well be worse at other locations, which rarely happens with passive bass traps.

--Ethan
From my limited experience, I agree with Ethan that acoustic solutions are the best for acoustic problems. It seems to me that the Bag End device will cause problems elsewhere. Absorption and damping treat the problem correctly, but they do require space... however so does the E-trap. If a room is so small you don't have room for trapping or absorption, you are going to be hard pressed to find a spot for the E-trap imho. You can't fit one into a ceiling corner, (or maybe you can)
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23rd April 2008
Old 23rd April 2008
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Quote:
Originally Posted by Russ (Al) Prat View Post
From my limited experience, I agree with Ethan that acoustic solutions are the best for acoustic problems. It seems to me that the Bag End device will cause problems elsewhere. Absorption and damping treat the problem correctly, but they do require space... however so does the E-trap. If a room is so small you don't have room for trapping or absorption, you are going to be hard pressed to find a spot for the E-trap imho. You can't fit one into a ceiling corner, (or maybe you can)
In my case, space is not the problem. the problem is time. The problems I have are of such a low frequency, that tuned traps are the only real option, which is not practical, as I will only live in this house for another couple months. buying more porous absorption doesn't make much sense either, since I already have the amount that will be needed for a room with reasonable modal distribution.

I was hoping that a subwoofer based solution would work, since that would mean that I could just recalibrate it for the next place I inhabit.

Now I think the best solution is living with the space the way it is.
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23rd April 2008
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15th December 2012
Old 15th December 2012
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The "double bass array" does work, basically a 2D line array or plane array. The technique is a strategy that's been used successfully with live sound rigs in tricky untreated venues, you try to make sound at one end of the venue, and then suck it up at the other end, using the side reflections to reinforce the wave to get the sound to carry better. If you work wherever possible with planar waves rather than spherical ones, they're often much more controllable, and also decay less sharply over distance (people further away from the sound system get a more similar volume compared to those close to the sound system). That can cause trouble, though, if the mids and tops have a spheroid – cardioid or omnidirectional – pattern. Ideally in a listening room you'd use something like corner horns or planar speakers for the mains, so that the dispersions would match a bit better, and avoid getting volume discrepancies between the subs and mains at different distances.

The category of techniques falls into "phased arrays", http://en.wikipedia.org/wiki/Phased_array
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26th December 2012
Old 26th December 2012
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