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Spasticteapot 28th February 2013 09:16 PM

Electrostatic speakers as microphones?
 
Has anyone tried using an electrostatic speaker as a microphone? There are many parallels between the two in function; however, while the unimpeded exposure of the diaphragm condenser mic is great, the inherent asymmetry requires extremely close tolerances in construction.

I don't have an electrostatic speaker handy, but for the sake of science, would anyone mind connecting a scope to the output of theirs and seeing if some sine waves could be reproduced?

Kindhornman 28th February 2013 09:57 PM

Spasticteapot,
Theoretically all speakers would be capable of an electrical output that could be used as a microphone. What it would sound like is another story but they will all function this way. Moving a diaphragm that has a coil attached in a magnetic field will produce an electrical signal. Many cheap speakers have been used this way but not typically for high fidelity audio.

bolserst 28th February 2013 10:42 PM

Quote:

Originally Posted by Spasticteapot (Post 3390145)
Has anyone tried using an electrostatic speaker as a microphone? There are many parallels between the two in function; however, while the unimpeded exposure of the diaphragm condenser mic is great, the inherent asymmetry requires extremely close tolerances in construction.

Push-pull dipole ESLs make great microphones: low distortion and flat response.
You will need a high impedance low noise pre-amp.
HF bandwidth is limited by diaphragm mass, you will need to use 3.5uM or thinner or plan to fix with EQ.
LF bandwidth is limited by fundamental diaphragm resonance. This is tricky as you need to get resonance very low < 20Hz using very light tension, yet enough tension to avoid diaphragm instability. Also, you will need to damp the resonance with a thin acoustic mesh applied to the stators.

Single ended ESL pressure microphones (like B&K measurement mics) are a completely difference animal.
Flat response is only had below resonance, so you have to use a very thin small diaphragm that you can apply high tension to.

cyclecamper 28th February 2013 11:14 PM

Of course, a large planar ESL would make a very directional mic, as off-axis high frequencies would cancel; again the speaker model running in reverse.

No, don't try to use your ribbon mic as a speaker, it doesn't get very loud LOL. But if anyone ever wanted to bug my house, they could certainly just tap into the wires to my newform ribbons (actually line-shaped monopolar planar dynamics). At least until I turn the stereo on.

cyclecamper 28th February 2013 11:15 PM

I'd imagine my electrostatic headphones could make acceptable mics with the right power supply and preamp.

Marik 3rd March 2013 09:07 AM

Because of reciprocity of transducers it is possible to use electrostatic speaker as a microphone. In fact, there are some fig8 dual stator capsules with very similar construction (but of course, way scaled down), so in principle you should be able to hear "at least something". However, because of difference in implementation (after all, in a microphone we do not emit, but receive the signal) the speaker would not be optimized for critical music recording and practicality of such "microphone" would be very questionable.

In any microphone besides low distortions we are interested in a few things: frequency response, polar pattern, noise, good transient response, lack of resonances (or IOW, well controlled). Neither of those can be achieved in such a big transducer as a speaker.

The frequency response--while probably that would be perfectly possible to get a more or less usable bottom end, since this is a dipole (fig8) mainly mass controlled system the top end will be limited mainly by the microphone/speaker width. Say, if it is 12" then we will be lucky to get the top response up to some 800Hz (!!!) on a good day.

Considering there is no top end, the polar pattern in our case won't make any sense, other than it is symmetrical fig8, with good 90* rejection. Really, for a good and usable polar pattern we need a small capsule.

Noise most likely won't be optimized, either. Unlike speakers, in microphones there is no need for such a HUGE spacing between diaphragm and stators. In most of condenser microphone it is some 12-25um for maximum efficiency, with bias voltage at some 40-200V, depending on a mic. In a speaker the area is much larger, so it is good, but the distance is so much bigger that I suspect it is not an equal tradeoff. You can easily scale things up--I am too lazy to make calculations...

The main problem in fact, might be very complex resonant modes, which are completely undamped by the loading air, leading to very poor transient and frequency responses, besides high distortions. You will need to add some damping (say to put felt screens right in front and on the back), but then it will create all kind of different problems.

We could also create an enclosed volume, with a certain air compliance (which BTW, would help with top frequency response), however, since we have a mass controlled system in order to get flat response we will also need to create an infinite volume (in practice, that would be something like tapered and stuffed labyrinth of considerable length). That would be quite a complication, which is hardly worth an effort considering all other things...

Quote:

Originally Posted by cyclecamper (Post 3390296)
No, don't try to use your ribbon mic as a speaker, it doesn't get very loud LOL. But if anyone ever wanted to bug my house, they could certainly just tap into the wires to my newform ribbons (actually line-shaped monopolar planar dynamics). At least until I turn the stereo on.

(True) ribbon speaker should be more successful as a microphone, but still the top end response will be very limited and undamped resonances too big of a factor to be able to talk about some kind of sound quality...

Hope this is of help.

Best, M

Bazukaz 3rd March 2013 10:03 AM

Hi,

I think large surface of an ESL should have an interesting property in a way it treats interaction with the room. It is no longer a point target like a small mic or a single ear. So there should be a considerable averaging in frequency response over area IMO as it would capture the sound like lots and lots of multiple mics. Transient response should be rather strange. This does not correspond well to what we are used to in hear from small microphones, but perhaps could have use in some applications?

Regards,
Lukas.

Kindhornman 3rd March 2013 01:50 PM

Government agencies do spy on people by listening through the vibration of walls from exterior to a building, there are many ways that a flat panel would have similar properties as a microphone but the equalization of the sound field would be the problem more than likely. A ribbon speaker and ribbon mic follow very similar construction methods, size and mass is the dominant difference here.

Marik 3rd March 2013 06:48 PM

Quote:

Originally Posted by Bazukaz (Post 3393671)
This does not correspond well to what we are used to in hear from small microphones, but perhaps could have use in some applications?

From top of my head the only application I could think of would be something like kick drum, where many engineers use large woofers as microphones. Still the size of usual wide band ESL will be too large and we get into the bleeding problems. Something like smaller ESL tweeter would be a better match here.


Quote:

Originally Posted by Kindhornman (Post 3393882)
Government agencies do spy on people by listening through the vibration of walls from exterior to a building, there are many ways that a flat panel would have similar properties as a microphone but the equalization of the sound field would be the problem more than likely.

This is my understanding they use windows (not the walls), with laser sensing the vibrations. The major difference with ESL panel is the window glass is very stiff, and works into enclosed volume, so the entire system is stiffness controlled and the overall frequency response will be quite flat and wide (at least for the purpose of intelligibility of recorded speech). Most likely, there is even no need for EQ.

Best, M

Bazukaz 3rd March 2013 07:23 PM

Quote:

Originally Posted by Marik (Post 3394280)
The major difference with ESL panel is the window glass is very stiff, and works into enclosed volume, so the entire system is stiffness controlled and the overall frequency response will be quite flat and wide (at least for the purpose of intelligibility of recorded speech). Most likely, there is even no need for EQ.
Best, M

Hi,

I do not think so. The glass has a very considerable mass. So most of high or even mid range content will be just reflected back instead of setting glass into motion. Also the motion of glass should be very small and perhaps there are some limitation in laser measurement equipment as well. As I understand laser measurement system accuracy is fundamentally limited by laser's wavelength. And movement of glass is perhaps in the order of fraction of a micron which could be similar to wavelength of a laser.

Regards,
Lukas.


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