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Old 20th January 2011, 01:18 AM   #1
hjf is offline hjf  Argentina
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Default Is this a room mode?

Hi,
I finished my sub a while ago, and I've been testing it with some video games. At my listening position (bed) which is near the middle of the room, the sound is crap. I get no "rumble" and an almost constant "WOOOOOOOOOMMMMMM" (it's not common-mode noise. if I pause the game it goes away). Same with movies... "dramatic" bass makes this wooom appear.

At the end of the room, if I sit against the wall, the sound is great. Things shake (even at low volume!), and I can "feel" the game. If a bomb explodes near me, I can feel it. Scary!

Is there a way to solve this annoying thing? I've tried different placements for the speaker (down-firing, back-firing, front-firing), different locations, etc, but this damn "wooommmm" keeps coming back.

My LPF cuts at 100Hz at 24dB/octave so I'm not sure if the "wooom" is actually an harmonic or a fundamental.

The room is almost L shaped. It's really a 6x2,75M room where a 2x2M bathroom was built in one of the corners.

Will a parametric EQ solve anything? I don't have one but I guess I can build one.
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Old 22nd January 2011, 08:56 AM   #2
djk is offline djk
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What is the ceiling height?

This is what usually sounds bad, an 8' ceiling will have a nasty 71hz mode.

This is really a time/energy problem, and simple EQ may make it measure flat, but it may still sound poor (because of the time domain issue).

If your ceiling height and another room dimension are close (or an even multiple of each other) it just makes things even worse.
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Old 22nd January 2011, 12:49 PM   #3
hjf is offline hjf  Argentina
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The ceiling is actually 2.55M, which is I think 4" taller than 8'. So the mode should be a little lower, 68Hz.

BTW, I did a simple experiment: just turned the volume down a bit. I was able to hear the rest of the frequencies more clearly, and the "wooom" wasn't there. Is this a textboox example of a standing wave? You put more energy into it and it takes longer to disappear after you remove the source. It's the only explaination I can find.
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Old 22nd January 2011, 01:01 PM   #4
djk is offline djk
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I think you are correct.

The gain is in the time domain, and drive level dependent too.

Also, the 2.55M and 2.75M dimensions are close enough for the resonances to add up, and 6M is close to an even multiple.

RealTraps - Bass Waves in the Control Room
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Old 22nd January 2011, 02:56 PM   #5
hjf is offline hjf  Argentina
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Looks like my eye-measurements were a little off. I found this model I made a while ago on Sketchup. I checked the measurements and they're right. Here's the image:

Click the image to open in full size.

I noticed I do have two "infinite" bass traps the door and the window, both facing to the outside. I'll try that tonight... it's noon and it's too hot outside.
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