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R/E/P => R/E/P Archives => Acoustics in Motion => Topic started by: Barry Hufker on April 26, 2007, 03:37:43 am

Title: Slot Absorber -- where did I go wrong?
Post by: Barry Hufker on April 26, 2007, 03:37:43 am
OK... I am trying to absorb two room modes -- 120 Hz, and its harmonic, 240 Hz, by means of a slot absorber.  Following the forumlas in F. Alton Everest's books, come up with the specifications.

I build two absorbers, both tuned to 120 Hz.  Both traps are roughly 6' tall by 3' wide by 1' deep.  The slats are "2x4"s, which are actually 1 3/4 by 3 3/4.  The slot (space between the slats) is .26" (or approximately .25 inch).  

To make the trap a little more wide-band, I placed 1" of the most porous (I could find) styrofoam "insulator" board directly behind the slats (touching them).  This board was then run the entire height and width of the trap.  Then the boxes are tightly sealed at the sides and rear.

Today while testing, not only were 120 and 240 not absorbed (no matter where I put the traps), other frequencies, such as 30 and 60 Hz were now reinforced, whereas before they'd been no problem.

I tried the traps in the corners, vertically and horizontally.  I found the peaks of the wavelengths in the room and put the traps there with no improvement.  I laid the trap flat on its back in a location in the room where there was a mode peak.  Absolutely nothing improved.

What on Earth have I done that this should fail so miserably?

With thanks,

Barry
Title: Re: Slot Absorber -- where did I go wrong?
Post by: Ethan Winer on April 26, 2007, 01:34:53 pm
Barry,

> To make the trap a little more wide-band, I placed 1" of the most porous (I could find) styrofoam "insulator" board directly behind the slats (touching them). <

If you mean the rigid type of foam plastic, like what's used for packing things to ship, that's the problem.

--Ethan
Title: Re: Slot Absorber -- where did I go wrong?
Post by: Barry Hufker on April 26, 2007, 06:50:01 pm
Thanks Ethan.  That's what I suspected (feared).  It was the most porous stuff I could find but it obviously didn't do the job.  I appreciate your confirmation.

We'll rip that stuff out and give it another go.

Best,

Barry
Title: Re: Slot Absorber -- where did I go wrong?
Post by: maarvold on April 28, 2007, 02:40:09 pm
Barry,

I 'feel your pain'.  There's also a guy I came across on the 'net who says the formula in the Everest book, and also on virtually every on line Helmholtz calculator, is wrong.  It fails to take some gray area/difficult to calculate parameter into account (like the friction of air in the mouth of the slot or something like that).  

Here's some info about it, although it's not the specific page I was looking for:

http://forum.studiotips.com/viewtopic.php?t=94

Fran: care to comment?  

Thanks in advance.  
Title: Re: Slot Absorber -- where did I go wrong?
Post by: Barry Hufker on April 28, 2007, 04:00:01 pm
Thanks Mike!

Now I'll have to calculate to see what frequency we really are tuned to.  I'm kinda scared as to what I'll find!

Barry
Title: Re: Slot Absorber -- where did I go wrong?
Post by: jimmyjazz on April 29, 2007, 02:16:36 am
Eric Desart is very meticulous.  I'm glad he spoke up about this particular error.  I listen to what he says as a general rule.
Title: Re: Slot Absorber -- where did I go wrong?
Post by: jfrigo on April 29, 2007, 02:26:56 am
maarvold wrote on Sat, 28 April 2007 14:40

Barry,

I 'feel your pain'.  There's also a guy I came across on the 'net who says the formula in the Everest book, and also on virtually every on line Helmholtz calculator, is wrong.  It fails to take some gray area/difficult to calculate parameter into account (like the friction of air in the mouth of the slot or something like that).  

Here's some info about it, although it's not the specific page I was looking for:

http://forum.studiotips.com/viewtopic.php?t=94

Fran: care to comment?  

Thanks in advance.  


Actually, it says the Everest book has correct in the forumla, but fails to take into account the mouth correction. The other net resources and another text are listed as being the wrong equation altogether (addition instead of multiplication). I'm out of town so don't have Everest handy to check, but it's not surprising that he'd put the basic formula up without the mouth correction. It's not a math-centric book. It just gives you the plain English concepts.

As far as the mouth correction, I've always done an additional calculation that was called "effective length" when I learned Helmholtz: effective length = physical length + 0.8 times the sq. root of the surface area of the opening. This calculation is done first (don't forget to convert inches to feet before plugging it in!), and that gives you the proper, adjusted figure to enter into the regular Helmholtz equation.

The point is that the column of air moving in the opening is not a solid, like a cork in a bottle would be. As the column moves in and out, some air alternately from inside and outside is involved, making the strict, physical measurement of the depth of the opening not what you need for an accurate prediction.
Title: Re: Slot Absorber -- where did I go wrong?
Post by: Barry Hufker on April 29, 2007, 11:23:53 am
Thanks Jay!  I'm learning so much from everyone.

Barry
Title: Re: Slot Absorber -- where did I go wrong?
Post by: franman on April 29, 2007, 10:03:34 pm
Sorry for the late reply on this one... We do not typically make helmholtz slat traps tuned to particular frequency.. We use them in front of varying depth trapped spaces as a quasi broad-band absorber.. I'm more partial to Helmholtz resonators or membrane traps as we find them easier to design and build properly.... a penny late probably, but....

I'll have to look more closely at the references to the formulas but I suspect the Everest reference is correct. The air velocity at the mouth must be considered, but how much of an effect??? I'm not sure.
Title: Re: Slot Absorber -- where did I go wrong?
Post by: Barry Hufker on April 30, 2007, 12:18:47 pm
We have built membrane absorbers before with success.  This effort was to try something different.

Barry
Title: Re: Slot Absorber -- where did I go wrong?
Post by: jfrigo on May 01, 2007, 01:13:01 am
Barry Hufker wrote on Mon, 30 April 2007 12:18

We have built membrane absorbers before with success.  This effort was to try something different.

Barry


For something similar, but a little different, I happen to be a big fan of perforated panel absorbers. Everest has some references, and Russian acoustician Mankovsky did a lot of work on the subject.
Title: Re: Slot Absorber -- where did I go wrong?
Post by: jimmyjazz on May 01, 2007, 02:02:59 am
Ever talk to or work with some of those Russian acousticians?  I did in my earlier days when I did a bit of sonar work for ARL-UTexas.  

Good God, those people shat math.  They were so much more skilled than their American counterparts it was scary.  On the other hand, their computer skills were very lacking.  You give, you get.
Title: Re: Slot Absorber -- where did I go wrong?
Post by: Barry Hufker on May 02, 2007, 01:29:02 am
Having used the formula from F. Alton Everest (and don't ask me which version 'cause I don't remember) and having recalculated with the "correct" formula (not doubting it as the correct formula), our original design frequency of 120 Hz (Everest) changed to 119 Hz using the correct formula.

Does that make sense?  In other words, each of these two formulas made a difference of only 1 Hz.

And tomorrow, the pink insulator board gets ripped out!

Barry
Title: Re: Slot Absorber -- where did I go wrong?
Post by: johnR on May 02, 2007, 05:58:58 am
If the foam you used is closed cell it won't absorb anything, it will just block the slots. Only open cell foam works. Foam for thermal insulation tends to be closed cell to prevent air flow, but foam for acoustic absorption needs to allow air to flow in and out so that frictional losses can occur.
Title: Re: Slot Absorber -- where did I go wrong?
Post by: Barry Hufker on May 02, 2007, 06:55:20 pm
OK, insulation board removed.  A frequency response test with no slot absorbers in the room -- we have our 120 Hz room mode.  Putting both slot absorbers in the room, we still have our 120 Hz room mode, with maybe a dB or two drop in level.

No matter where we put the absorbers, there is no significant reduction in the mode's level.  We've tried corners.  We've tried walls.  We've tried floor ('cause it was easier than ceiling).  We have tried different points in the room where there was a measurable crest in the mode.  No luck.

Exactly how rigid and sealed (back and sides) does this box have to be?  And shouldn't there be some significant level loss somewhere?

Thanks for your help, folks.

Barry
Title: Re: Slot Absorber -- where did I go wrong?
Post by: Ethan Winer 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
Title: Re: Slot Absorber -- where did I go wrong?
Post by: Barry Hufker 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
Title: Re: Slot Absorber -- where did I go wrong?
Post by: franman 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??
Title: Re: Slot Absorber -- where did I go wrong?
Post by: Barry Hufker 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
Title: Re: Slot Absorber -- where did I go wrong?
Post by: rankus 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?


Title: Re: Slot Absorber -- where did I go wrong?
Post by: Barry Hufker 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
Title: Re: Slot Absorber -- where did I go wrong?
Post by: rankus 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.


Title: Re: Slot Absorber -- where did I go wrong?
Post by: Barry Hufker 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