Size of perforations related to frequency response on electrostatic speakers??? - diyAudio
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Old 24th February 2002, 07:18 PM   #1
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Default Size of perforations related to frequency response on electrostatic speakers???

Hi,

I believe that I discovered an interesting relationship between the size of the perforations used in electrostatic loudspeakers and their overall frequency response. My first set of DIY ESLs used perforations of 1/16" diameter, while the second set uses perforations of 3/16" diameter. The ESL with the larger diameter perforations appears to have a better midrange and upper-bass response than the ESL with 1/16" perforations. No other variables, such as distance between the diaphragm and stators, were changed.

What I need is information on the acoustic principals that would proplan to construct a small anechoic chamber which will house a full range ve my hypothesis. Diffraction of sound waves seeming would be an important part of the explanation, but how would different frequencies effect the diffraction characteristics of sound waves passing through perforated metal?

I am planning this as an experiment for a science fair project. I speaker, a slot to hold a sheet of perforated steel, and a test microphone. I know how to conduct the investigation, but my knowledge of theory is lacking. Does anyone have any suggested reading material (high school or college level)?

Help me prove that "Frequency response of an electrostatic loudspeaker is directly related to the size of its perforations."

Thanks in advance for your time! You can either post replies or email me through this website.
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Old 24th February 2002, 10:29 PM   #2
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The size of the perforations doesn't effect the frequency response directly--it effects the damping. This is a known principle. The damping is the perceived mechanical load of the air on the diaphragm of the electrostatic speaker. It is directly comparable to damping in a vented or sealed enclosure for a dynamic driver.
Use a speaker program to design a critically damped enclosure (.707 for a Butterworth alignment, .5 for Bessel, etc.), then vary the enclosure size up and down and watch the tail end of the bass response. As the enclosure gets larger, the bass drops more slowly, but begins dropping earlier. As the enclosure gets smaller, the bass humps up just before rolling off, and rolls off more quickly.
There are a number of factors that can effect the damping in an electrostat, but you're quite correct, the size of the holes is important.

Grey
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Old 25th February 2002, 07:30 AM   #3
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Is it the size of the holes in the electrostatic loudspeaker or the percentage of the loudspeaker area that the holes occupy that is important?

I would think that in an electrostatic loudspeaker-I have not built one, though I have studied them-if the holes take up 50% of the space of the loudspeaker, I would prefer to have smaller holes. That is because the metal spaces between the holes will also have to be smaller, and the sound traveling around the metal will therefore have a smaller percentage of it's wavelength to travel before it hits the hole and escapes into the room. The smaller the percentage of the wavelength the sound gets displaced, the less interference.

Did you have an equal percentage of "hole space" to "metal space" on your two electrostatic loudspeakers with different size holes?

Since I have not built an electrostatic, I freely yield to the experience of those who have. It just seems to me that this is the way it would work.
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Old 25th February 2002, 01:05 PM   #4
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I've had a discussion on this subject on the Audio Asylum, and a few guys there seem to agree that the most important factor would be dampening. The stats with the larger perforations have a larger open surface area; therefore, the dampening factor appears to decrease. An electrostat with many small holes would have a smaller open surface area than the larger perfs. because you cannot purchase perforated metal with many small holes in relation to the amount of metal used in the sheet.

My question is, how could I simulate these conditions using a full range magnetic driver to produce the test tones that I need to pass through the the perforated metal. I would like to try to avoid building many small electrostats. Could I use a magnetic woofer to move a mylar diaphragm?

Sorry about my first post. My mouse button is going bad, so I botched the cut and paste.

Thanks everyone!
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Old 25th February 2002, 03:20 PM   #5
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The size of the holes and their depth (thickness of the plate) definitely has a profound affect on the frequency response of the electrostatic speaker, though primarily at high frequencies. This is because the acoustic equivalent of a hole is an inductance in series with the air load impedance, resulting in high frequency rolloff. This effect is discussed fully by Peter Baxandall in the book "loudspeaker and Headphone Handbook" edited by John Borwick. Focal Press ISBN 0 240 51371 1
This is the most thorough work published on Electrostatics, and I would highly recommend that you read it prior to starting your project.
With regard to the question of damping, the hole pattern rarely gives sufficient damping alone. The Quad electrostatics use additional damping to control the Q, and speakers such as the Martin Logans use differential tensioning or split the diaphragm into multiple, different sized areas in order to spread the resonant frequency and therefore reduce the effective resonant peak.
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Old 25th February 2002, 06:23 PM   #6
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I've gone ahead and checked that book out through the OhioLink College library system. Thanks a lot for your suggestion. I always thought Roger Sanders's book was the definitive title on the subject.
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Old 25th February 2002, 07:09 PM   #7
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Roger Sanders book is indeed an excellent source book for electrostatic loudspeakers, but the theory is not as in depth as the Baxandall work. In addition, Baxandall discusses great detail the design of the Quad electrostatics, both old and new. Baxandall was a good friend of Peter Walker, and spent considerable time researching, experimenting and talking to Walker prior to publishing the chapter in the book.
Roger Sanders book seems to be aimed more at the DIY enthusiast, with more of the book devoted to the practicalities of home built electrostatics, but with sufficient theory to help out if one wishes to deviate from his design.
Another book that is indispensable is "Electrostatic Loudspeaker Design and Construction" by Ronald Wagner, TAB Books, ISBN 0 8306 2832 0
All three books are required reading in my opinion. (I have them all )
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Old 25th February 2002, 08:56 PM   #8
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I own both Sanders's book and Wagner, but neither of them really go into acoustic theory.
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Old 23rd April 2002, 08:36 PM   #9
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Hi folks, sorry to chime in on an older thread.

I've seen some mention of using aluminum window screen as a stator material. It's pretty flimsy, but very 'open' of course. In Wagner's book he mentions that it's a promising material and more experimentation is needed.

Does anyone have any definite experience with window screen?

It looks like I'd have to support it externally, so I'm concerned about the size of the 'holes' in the external material. Eggcrate has been mentioned, but as I'm planning on building the curved ESL mentioned in Wagner's book, I was thinking of a series of thin 'blades' of styrene. I don't know what spacing I should use, however.

(edited to removed double sig)
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== John ==
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Old 23rd April 2002, 11:18 PM   #10
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The problem with using aluminium window screen as a stator is that it is very flimsy. Since the audio signal is applied between the two stators, there is a large force of attraction between these stators, and it is one always of attraction, so if they can move they will add distortion to the signal on two counts: since they are not 100 percent open they will directly radiate some sound of their own, which will be highly resonant ( like the panels of a box speaker!!), and because the plate spacing is being modulated they will alter the voltage gradient that the diaphragm sees and so modulate dynamically the force acting on the diaphragm.
This is always the compromise for the stators, the more open they are the less rigid they are likely to be.
In my view the better compromise is to go for less open area and improve the stator rigidity. The effect of less open area is to alter the frequency response by adding mass and resistance in front of the diaphragm, a linear effect that can be compensated by equalisation. In contrast, the resonances and distortion due to non rigid stators cannot be compensated.
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