Electret microphone inside hermetic capsule...

Thread Starter

Externet

Joined Nov 29, 2005
1,667
Hi.
An electret microphone inside a sealed compartment/chamber would need a method to magnetically inject ~2V bias in and deliver its audio out to a preamplifier trough the chamber wall.

A pair of coupled coils can supply AC for the bias power with rectification inside. Another coupled pair of coils could extract the audio out of the container. What to consider to avoid interference from powering and audio coils ? Well separate frequencies ? If power goes in on 1MHz; could a single coil serve as transferring both waveforms bidirectionally trough the chamberwall ?

Filters could manage separating the bands. Sort of RFID, Acting onto the transducer and receiving its data. What else to consider ? It will not be a simple project... Is there an existing gadget using such method that you know of ? :confused:
 

SamR

Joined Mar 19, 2019
3,532
Just what are you trying to listen to? The wall of the sealed container would be quite an attenuator for sound...
 

KeithWalker

Joined Jul 10, 2017
1,935
What are the chamber walls made of and how thick are they?
The power signal going in will be much larger than the audio signal coming out. You certainly will need to use two different frequencies for the two signals. The most efficient signal transfer will occur if the transmitter and receiver coils are tuned to be resonant at the signal frequencies so I would recommend using seperate power and audio coils running at frequencies that are not harmonically related.
Regards,
Keith
 

MisterBill2

Joined Jan 23, 2018
9,482
It is also possible to have hermetically sealed pass-through assemblies, which could be far simpler than the complex magnetic coupling arrangement mentioned. Such a method would also avoid the limitations on frequency response that is unavoidable with magnetic coupling arrangements.
In addition there are a number of hermetic multi-pin connectors available in a number of sizes and materials.

What sort of pressure is involved, and what sort of environment? Wet, humid, chemical, or gas??
 

Thread Starter

Externet

Joined Nov 29, 2005
1,667
Thanks, gentlemen.
Unavoidably attenuated:(; wall can be mylar, cellophane, silicone, low density polyethilene, plastic film, 0.1 to 0.5mm. Flexible to avoid implosion and unfriendly to sealing glues for attaching pass-troughs :(
Tuning audio spectrum to resonance does not smell proper. Resonance implies a single frequency, or very narrow span.
Not passive radio bug, active microphone under 150psi corrosive humid atmosphere. Been checking other methods already and had to get opinions on the not-pretty induction method.
 

AnalogKid

Joined Aug 1, 2013
9,386
active microphone under 150psi corrosive humid atmosphere.
I think I speak for many on this forum:

W ... T ... F ...

Well, I feel better ...

Size limits?
Weight limit?
Power sources available?

I fber optic an option? Wireless power in, fiber audio out.

ak
 

KeithWalker

Joined Jul 10, 2017
1,935
Thanks, gentlemen.
Unavoidably attenuated:(; wall can be mylar, cellophane, silicone, low density polyethilene, plastic film, 0.1 to 0.5mm. Flexible to avoid implosion and unfriendly to sealing glues for attaching pass-troughs :(
Tuning audio spectrum to resonance does not smell proper. Resonance implies a single frequency, or very narrow span.
Not passive radio bug, active microphone under 150psi corrosive humid atmosphere. Been checking other methods already and had to get opinions on the not-pretty induction method.
I did not intend to suggest that the audio channel should resonate at audio frequencies. I assumed you would amplitude modulate a higher fixed frequency carrier. I like the idea of using amplitude modulated light for the audio, now that we know that the walls could be transparent.
 

AnalogKid

Joined Aug 1, 2013
9,386
Infrared cordless headphones are cheap on ebay. the guts could test the concept without having to design and debug the hardware.

ak
 

MisterBill2

Joined Jan 23, 2018
9,482
I was going to suggest some sort of optical transmission scheme for the outgoing audio because very good response over a very wide range. And there are multi-pin hermetic connectors available. take a look at astronaut suits for examples. A compression seal against a mylar membrane could easily withstand 150 PSI, or is that the pressure inside the capsule?A slit-section transformer could even be a mains powered one with a bit of effort. Is there a pressure port on the capsule? Heat sealing is an option.
 
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