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Author Topic: Slot Absorber -- where did I go wrong?  (Read 10238 times)

Barry Hufker

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Slot Absorber -- where did I go wrong?
« 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
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Ethan Winer

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Re: Slot Absorber -- where did I go wrong?
« Reply #1 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

Barry Hufker

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Re: Slot Absorber -- where did I go wrong?
« Reply #2 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
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maarvold

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Re: Slot Absorber -- where did I go wrong?
« Reply #3 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.  
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Barry Hufker

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Re: Slot Absorber -- where did I go wrong?
« Reply #4 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
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jimmyjazz

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Re: Slot Absorber -- where did I go wrong?
« Reply #5 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.
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jfrigo

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Re: Slot Absorber -- where did I go wrong?
« Reply #6 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.
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Barry Hufker

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Re: Slot Absorber -- where did I go wrong?
« Reply #7 on: April 29, 2007, 11:23:53 am »

Thanks Jay!  I'm learning so much from everyone.

Barry
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franman

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Re: Slot Absorber -- where did I go wrong?
« Reply #8 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.
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Barry Hufker

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Re: Slot Absorber -- where did I go wrong?
« Reply #9 on: April 30, 2007, 12:18:47 pm »

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

Barry
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jfrigo

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Re: Slot Absorber -- where did I go wrong?
« Reply #10 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.
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jimmyjazz

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Re: Slot Absorber -- where did I go wrong?
« Reply #11 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.
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Barry Hufker

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Re: Slot Absorber -- where did I go wrong?
« Reply #12 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
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johnR

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Re: Slot Absorber -- where did I go wrong?
« Reply #13 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.
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Barry Hufker

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Re: Slot Absorber -- where did I go wrong?
« Reply #14 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
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