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 Freddy G 27th August 2010 12:27 AM

New hybrid ESL Build

Hi Everyone!
I'm going to build a pair of hybrid ESL speakers. I went to my local metal supermarket today and they have two types of perforated steel sheets.
The first one is a 16 gauge with 0.125" holes staggered on 3/16" centers. It has a 40% opening area.
The other sheet is 18 gauge and has 0.156" holes. It has a 63% opening area.
Will either of these be suitable? Or should I try for something more ideal from somewhere else?

 Few 27th August 2010 01:37 AM

I'd go for the 0.125" holes staggered on 3/16" centers, but don't have a handy way to calculate or look up the thickness of that gauge of material. Can you convert it to a thickness in inches or mm?

Few

 Freddy G 27th August 2010 01:46 AM

Few,
According to Wikipedia, 16 gauge steel sheet is 0.0598" or 1.52mm. That's just a couple of thous shy of 1/16".

 CharlieM 27th August 2010 01:57 AM

Quote:
 Originally Posted by Few (http://www.diyaudio.com/forums/planars-and-exotics/172594-hybrid-esl-build-post2284900.html#post2284900) I'd go for the 0.125" holes staggered on 3/16" centers, but don't have a handy way to calculate or look up the thickness of that gauge of material. Can you convert it to a thickness in inches or mm? Few
16 gauge equates to .063" or about 1.6 mm

The Cookbook recommends hole sizes should not be smaller than 2x the stator thickness so 16 gauge/.125 holes would be at the limit before coating the stators and the holes would get a bit smaller still after coating.

That's a tough call-- opinions, anyone?

 CharlieM 27th August 2010 02:02 AM

Forgot to mention, I'm using 18 mil (.048") thick stators with .125 holes/40% open and they are very efficient and sound great. Personally, I would opt for 40% open versus 63% open but I think I'm in the minority on that.

 Freddy G 27th August 2010 02:33 AM

hi Charlie,
Maybe if I give the 63% perf some good heavy coating that would reduce the opening enough?

 Calvin 27th August 2010 09:08 AM

Hi,

Quote:
 The Cookbook recommends hole sizes should not be smaller than 2x the stator thickness
Yes thats true. If the hole wands are straight, every hole forms a small cylinder. The summed up volume of air trapped in all those tiny cylinders forms a Helmholtz resonator together with the air volume which is encapsulated between diaphragm and stator surface. As every Bassreflex system it performs a bandpass amplitude response which restricts the upper bandwidth limit. If the cylinderīs length is short compared to its diameter the effect is less pronounced and the upper bandwidth limit is high. Similarly rounded or tapered wands of the holes reduce the effect too. So with smaller hole diameters there goes a thinner sheet unless You are capable to round the holesīs wands.

A second explanations is based on the behaviour of absorbants.
Punched metal sheets are useable as porous absorbers with a frequency dependant degree of transmittance.
The formula for a transmission degree of 50% reads:
f0.5~1.500*eta/I [Hz]
with:
eta the openness of the sheet in % and
I the acoustically effective thickness of the sheet, which depends on the sheetīs thickness X and the holeīs diameter D.
I=X+pi*D/2 [mm]
Example: eta=33%, X=1mm, D=3mm
I=1mm+3.14*3mm/2 --> 5.7mm
f0.5~1500*33/5.7 [Hz] --> 8.7kHz
This implies the choice of thin sheets, small holes and high openness and distinct rounding/tapering of the holes.
A calculation via the helmholtz-formula leads to similar but slightly lower results.
The calculations are not precise and include a high degree of tolerance .. in the dimensional values in... the air volumes.... in the shape of the holes etc. etc.
So regard it as a rule of thumb and only basic design guide.

jauu
Calvin

 CharlieM 27th August 2010 10:21 AM

Freddy,
There you have it !
(I couldn't have said it myself, lol)
Choosing a stator material is a tough call for most of us-- you study all you can and go with your gut feeling on the hard calls and hope you made the right choices.

Thanks Calvin -- I was hoping you would jump in and bail me out on that question.

 Calvin 27th August 2010 11:52 AM

Hi,

I just copied the formulas from my notes. Rethinking about I wonder if they can be right though. Itīs the formula for effective thickness which seems a bit weird to me, because larger diametered holes would lead to a lower transmittance factor. This canīt be right. Thinking in extrems...a single huge hole would show a lower transmittance than a myriad of tiny holes with same openness!?!? Too, a smaller hole increase losses, which allows for microperforated film absorbers to omit with additional damping material alltogether, which larger holed absorbers still need.

jauu
Calvin

 Freddy G 27th August 2010 12:12 PM

Thank you Calvin for the concise explanation. I think I will search for a more ideal material...I don't want to take any chances. (too bad, because this material was local to me and very cheap)

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