ESL as a compression driver?

[tade]

In my various searchings and musings I may I had a brainstorm; I really like horns for their low distortion and high efficiency. Also I find that using a horn as an EQ device allows a boxless sound without electronic equalization. Couple with that the predictable and useful dispersion characteristics of some horn designs, I think that that is winning technology for hi fi. The problem I see may lay in the inability for most transducers, whether they be cone, dome, or compression, to produce a true planar wavefront. Most transducers save ESLs.
I would like to design a small panel, maybe 5" X 5" in an assembly such that it is purpose built to load some sort of front horn. The benefit I believe will be to apply the benefits of horn loading to what may be a textbook transducer. The design goals will be a limited bandwidth ESL capable of producing a planar wavefront with as high an spl efficiency as possible.
Because there will be no "compression" there will not be any efficiency increase from this effect. This will be an open backed driver with some absorptive material to attenuate the backwave. The ESL should have as little spacing as possible to provide system response to 300hz. To do this diaphragm tention should be high but not above 150hz, or an octave below tuning. The high voltage bias should also be as high as possible. Physical limitations of this will be the close spacing of the diaphragm and stator and the diaphragm tention. I am thinking a spacing of 1.5mm and a bias of 2kv. because of the modest excursion i think lots of diaphragm support strips or dots may be used to increase the bias further.
With the production of a planar wavefront I am thinking of loading this with an oblate spheroid waveguide whose design is to transition a planar wavefront to a spherical one.
Is this a worthwhile endeavor? I have built ESLs before, so that presents no challenge. Please, tell me your thoughts!

[Calvin]

Hi,

the idea and sensitive use of a Horn is to make a small membrane virtually bigger. The better coupling to the air (acoustic impedance matching) is responsible for this. What is often forgot is, that the transformation is not lossless. A horn is neither theoretically nor practically an ideal transformer. Upsizing the membrane on the other Hand is ´lossless´. And since with ESLs it is always better and very easy to have a larger membrane.
So the horn-like contour of this baffle shouldn´t give a better efficiency than an equally big sized membrane, but rather the opposite.
You can easily shape the distribution character of the ESL by curving or electrical segmentation, so the waveguiding function of the horny baffle is not used, or shows any advantage against such a panel.
Acoustically its always better to have as much membrane as possible and as less housing, baffling, chambering as possible to avoid reflections at the Horn-/Baffle-surface.
Optically I find a transparent ESL design -even if it is large- much more pleasing than a same sized solid Baffle.

The forces acting on the membrane/air in an ESL are very small. Adding considerable air mass with the Horn will lower the bandwidth. But it is one of the major superiorities of an ESL to have wide bandwith. You´d have to add another driver and crossover which makes the things more complicated, expensive and which looses on basically every acoustical aspect without gaining on one.

With ESLs it is as such:
Think big, build Big, smile BIG -- nothing else

Just my thoughts. Maybe I´m totally wrong

jauu
Calvin

[ak_47_boy]

IMO you might as well use around 7kv bias and 2mm spacing. This will enable the esl's drive to lower frequency's if desired while not loosing any efficiency. It should be easy to tension a small panel tight. Make the mylar as tight as possible. Bowing should not be an issue.

Don't section any more than 100 times the d/s spacing. Anything lower than a 2:1 LxW ratio and you may loose bass response.

A flat esl may beam the sound right out of the horn, i have never made a horn esl.

[Calvin]

Hi,

if You want an ESL with acceptable or even good efficiency don´t don´t DON´t ever think of increasing the d/s!! 2mm is already too much for freqs above 300Hz. You will loose nearly on every point!
Dynamics, sound, livelyness, efficiency are getting poorer with increasing d/s (hence increasing voltages).
Always think of using as less d/s as possible! Nothing else! When You need more dynamics, increase the area and only the area!
If Your membrane touches the stator, increase mechanical tension by tightening the membrane or reducing the distance between the spacers. Only this way You´ll get an panel with horn-like dynamical sound.

Maybe everyone interested in building a good panel should learn this ESL-Mantra first.
- build big
- use small distances
- use lowest voltages

jauu
Calvin

[moray james]

Josef Merhaut published a design for an electrostatic compression driver and matching horn in the Journal of Acoustics Society back in the eighties. I think that there is a reprint in the JAS anthology.

[p3142]

If you are interested in details on the Merhaut device, check US patent US 3,590,169.

Peter

[Calvin]

Hi,

Merhaut describes basically just what a Horn does. Nothing really special regarding how long Horns are already in use.

What he doesn´t do -imo his greatest failure- is to check wether a electrostatic device is capable to drive this load (the Horn) properly in first case!
I´m quite convinced that this is not the case!
The forces acting upon the membrane are very small, so the bandwidth will be very small too (in contrary to Merhaut´s claims).
The membrane will undergo significant stress because of high pressures. I doubt, that there will be (ideally) translatory movement of the membrane. Instead there will be local bending and flexing, because of the low number of transmission channels between membrane and horn throat and the softness of the membrane.
Just waste a thought of how long the typical membrane coating might last under these conditions!
The claimed raise in efficiency only applies to a small membrane coupled to the Horn. A membrane the size of the Horns mouth area will have equal efficiency (it should be even higher, because the horn transformtion is not without losses).

Funny enough that all claims of Merhaut can be reached far easier by upsizing the ESL-device to the Horn´s mouth area.


jauu
Calvin

[Coffee]

Calvin, for sure you are not right at this moment. Josef Merhaut did that and he was a very clever guy, a university profesor of electroacoustics and so on.I was talking to guys in my country who listened to his system years ago, a they say that they were totally amazed. Using a horn in between an small electrostatic panel is a good idea, but you must be sure that you are doing it in the right way

I read an article in one of our magazines from 1972, and there were plans and measured data for his tweeter system, that he designed, and I´m sure, it was a prodigy, because this type of construction solves REALLY a lot of problems. Only problem that it has, as people say, was very little maximum loudness.

They sold it to KEF the same year i think

[tade]

I will look into that patent.

I can see the SPL response changing in Three ways; First of all, the ESL will be placed in a much larger baffle than normal, which should augment the low frequency response due to dipole cancellation. second of all, if the OS waveguide functions as I want it to it should spread the high frequencies along with the low frequencies providing constant coverage. This could cause a loss of high frequencies due to the lack of beaming common with ESLs. Third, the fact that there is a large curved baffle in front of a radiator will certainly cause internal reflections (HOM- Earl Geddes) or high frequency roll off.

I plan for the next step of my system to be a limited bandwidth transducer to go in between a midbass horn (70-300hz) and a ribbon tweeter (4khz and above); so limited bandwidth is acceptable.

The horn load is my big wonder. I know that there is a limit on the force (BL) which an E stat transducer may provide, based on arcing etc.

I suppose my main query is whether the special loading demands placed on a driver by a horn will allow some beneficial design requirements on a specially made driver such as not requiring a large xmax which will allow a small d/s. Also, I am willing to utilize the super low distortion provided by a properly designed ESl with no problems of beaming and the ability to match the radiation pattern in between the midbass horn and a tweeter.

My backup plan is horn loading an eminence alpha six midrange. this will probably be a much simpler and more rugged solution but in my quest for the best I am trying to flush out all possibilities.
Thanks!

[Coffee]

only for information, his system was reproducing only range from 1kHz up. And the active area was quiet small, about 15*15cm.