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Author Topic: Slot Absorber -- where did I go wrong?  (Read 10229 times)

Ethan Winer

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Re: Slot Absorber -- where did I go wrong?
« Reply #15 on: May 03, 2007, 04:21:50 pm »

Barry,

> Putting both slot absorbers in the room, we still have our 120 Hz room mode, with maybe a dB or two drop in level. <

I have to admit to no experience with slat absorbers. So all I can offer is this:

Passive bass traps made from 4 inch thick rigid fiberglass are known to be excellent at 120 Hz when mounted straddling corners. Since this type of bass trap is so easy to build (or cheap enough to buy), maybe just cut your losses and switch to that type of trap?

Also, adding only two bass traps to any room is not enough to make a big improvement at any frequency. A room has hundreds or even thousands of square feet of reflecting surface, and each trap is considerably smaller. To make a meaningful improvement in the response and ringing you need to cover a meaningful percentage of those reflecting surfaces.

--Ethan

Barry Hufker

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Re: Slot Absorber -- where did I go wrong?
« Reply #16 on: May 03, 2007, 08:34:56 pm »

Ethan,

Again, great advice.  It may be time to cut my loses -- as you say.  And I'll look into building traps with rigid fiberglass.

Thanks for your continued interest and advice.

Barry
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franman

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Re: Slot Absorber -- where did I go wrong?
« Reply #17 on: May 06, 2007, 10:34:25 pm »

Barry,

How did you measure (quantify) the amount of reduction in your modal peak??? Steady state tests are difficult when trying to quantify what is effectively a time (ringing) based problem... The modal decay may be significantly reduced and the mode tightened up but it can be hard to measure in the frequency domain.... just curious??
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Barry Hufker

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Re: Slot Absorber -- where did I go wrong?
« Reply #18 on: May 07, 2007, 12:40:02 am »

Well,  I was probably doing everything wrong!  The test to determine the modes went like this:

1. Pink noise played run through the main console outputs so that it read 0 on the peak meters.

2. Pink noise then played through the monitors, measured by an SPL-meter so that the level was 85 dB-SPL.

3. A DAP 4006 (omni) mic at the mix position.  It's output run into a console bus with the level on the meter matched to 0.  To my mind this calibrated the output and return through the microphone.

4. Playing a series of low frequency tones, one at a time (0 on the meter), the microphone's level on its meter was then checked to see how it had changed.

5. Increases and decreases in level were marked on a graph at each frequency.

6. The two greatest modes at the mix position were 120 and 240 Hz. with peaks at +12 above the microphone's 0 reference.

Figuring those were the peaks to tame, I did some room mode calculations to determine which room dimension(s) were likely to be supporting the modes.  That result indicated it was the length and height generating the modes.

I then built two 6' x 3' Slot absorbers using both formulas presented on this thread -- the Everest and "correct" versions.  The results of these two formulas differed by only 1 Hz.

Bringing those two large absorbers into the room resulted in absolutely no change in the mode test no matter where the absorbers were placed.

That's the method.  Any insight would be great.

Barry
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rankus

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Re: Slot Absorber -- where did I go wrong?
« Reply #19 on: May 07, 2007, 09:48:45 pm »



Did you test the speakers themselves?  (By sticking the test mic right in front of one) ...

I am certainly no expert , but I'm thinking the speakers / chain may be partly to blame?


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Rick Welin - Clark Drive Studios http://www.myspace.com/clarkdrivestudios

Ive done stuff I'm not proud of.. and the stuff I am proud of is disgusting ~ Moe Sizlack

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Barry Hufker

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Re: Slot Absorber -- where did I go wrong?
« Reply #20 on: May 07, 2007, 11:01:54 pm »

Rick,

I think you make an excellent point about speaker response.  Maybe I should have tested the speakers outside first.  And in other situations I have had an omni on the "source" monitor with a second omni in an opposing corner while finding room modes.  This way I could compare the amplitude at the source and in the room.  I admit I slacked a bit because the speakers are within 5 feet of the listening position and didn't do that this time.

I've thought about the frequency bump being a bounce off the console control surface but that is pretty well tamed.  And to double check, I figured out the quarter-wavelength of 120 and 240 Hz.  Sure enough there was a null and peak at every predicted spot (which I found by listening and/or moving the test mic as its level peaked and dipped).  And the boost is highly significant at these two frequencies -- at least 12 dB.  I would even allow 4 dB for the speaker (which to my mind would be a pretty shitty speaker), but I have to count at least the remaining 8 to the room.

Barry
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rankus

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Re: Slot Absorber -- where did I go wrong?
« Reply #21 on: May 09, 2007, 02:41:17 pm »

Barry Hufker wrote on Mon, 07 May 2007 20:01

Rick,

And the boost is highly significant at these two frequencies -- at least 12 dB.  I would even allow 4 dB for the speaker (which to my mind would be a pretty shitty speaker), but I have to count at least the remaining 8 to the room.

Barry


Wow. Yes, 12db is probably not the speakers!  That is quite a bump for sure... best of luck Barry

Another long shot idea:

Is something resonating in that frequency perhaps? Paneled walls, big empty box like structures, ceiling tiles, etc.


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Rick Welin - Clark Drive Studios http://www.myspace.com/clarkdrivestudios

Ive done stuff I'm not proud of.. and the stuff I am proud of is disgusting ~ Moe Sizlack

"There is no crisis in energy, the crisis is in imagination" ~ Buckminster Fuller

Barry Hufker

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Re: Slot Absorber -- where did I go wrong?
« Reply #22 on: May 09, 2007, 05:02:00 pm »

Rick,

I admit that never crossed my mind and it's a good idea.  I'll have to think on that and then investigate.

Thanks!

Barry
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