Author Topic: Obscuring microphone recordings  (Read 3080 times)

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Offline delicadoTopic starter

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Obscuring microphone recordings
« on: February 24, 2017, 11:25:03 am »
Hi

To obscure electret microphone recordings, I read that one could use two high-frequency signals above 20 KHz (about 1 KHz apart) that beat together. One or both of those frequencies was frequency-modulated with a noise signal. The idea is that the frequencies mix together on the mic's diaphragm and thus create an audible masking noise that makes it impossible to record anything else that is a lower level.

How come at least one of the signals needs to be frequency-modulated with a noise signal?

Is this meant to be a plausible approach?

Any thought appreciated.
« Last Edit: February 24, 2017, 11:30:52 am by delicado »
 

Online Fungus

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Re: Obscuring microphone recordings
« Reply #1 on: February 24, 2017, 11:31:24 am »
Is this meant to be a viable approach?

If you're trying to create an inaudible signal that stops microphones from working than I doubt it will work.

It's not difficult to test though.

PS: Just talk in the bathroom with the shower on.
 

Offline delicadoTopic starter

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Re: Obscuring microphone recordings
« Reply #2 on: February 24, 2017, 11:35:01 am »

If you're trying to create an inaudible signal that stops microphones from working than I doubt it will work.


But the claim is that the masking noise will be audible because of some "mixing" happening on the diaphragm. Essentially, two inaudible signals are emitted, but somehow they're turned into an audible masking noise by passing through the diaphragm - if my reading is correct.
 

Offline AlfBaz

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Re: Obscuring microphone recordings
« Reply #3 on: February 24, 2017, 11:50:33 am »
Wouldn't tha same noise be formed on the ear drum? After all diaphram and ear drum use the same mechanism it just whats on the other side is different
 

Online Siwastaja

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Re: Obscuring microphone recordings
« Reply #4 on: February 24, 2017, 11:55:58 am »
Wouldn't tha same noise be formed on the ear drum? After all diaphram and ear drum use the same mechanism it just whats on the other side is different

Exatly.

Potentially, the idea might be that some specific poor-quality microphone suffers from this effect more than, say, a proper microphone, or the ear.

This should be tried in practice.
 

Online Fungus

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Re: Obscuring microphone recordings
« Reply #5 on: February 24, 2017, 11:56:38 am »

If you're trying to create an inaudible signal that stops microphones from working than I doubt it will work.


But the claim is that the masking noise will be audible because of some "mixing" happening on the diaphragm. Essentially, two inaudible signals are emitted, but somehow they're turned into an audible masking noise by passing through the diaphragm - if my reading is correct.

We're missing the "why" from this question.  What's the goal? :popcorn:

(I guessed it would be 'inaudible masking' because of the 20kHz+ frequencies combined with "electret mike")
 

Online Fungus

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Re: Obscuring microphone recordings
« Reply #6 on: February 24, 2017, 12:16:58 pm »

If you're trying to create an inaudible signal that stops microphones from working than I doubt it will work.


But the claim is that the masking noise will be audible because of some "mixing" happening on the diaphragm. Essentially, two inaudible signals are emitted, but somehow they're turned into an audible masking noise by passing through the diaphragm - if my reading is correct.

I suspect it's just a theory, an academic paper. If it works at all it will depend on the diameter and/or resonance of the microphone's diaphragm, ie. factors that you can't control.

Quick reality check: If it worked then the CIA wouldn't be worried about bugs. They'd simply put one of these in all their buildings.
 

Offline delicadoTopic starter

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Re: Obscuring microphone recordings
« Reply #7 on: February 24, 2017, 12:17:51 pm »
We're missing the "why" from this question.  What's the goal? :popcorn:

(I guessed it would be 'inaudible masking' because of the 20kHz+ frequencies combined with "electret mike")


Yes, that's right. I'm trying to block/obscure recordings on a working electret microphone. Not necessarily inaudible, but less intrusive than playing a loud noise.
 

Offline Richard Crowley

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Re: Obscuring microphone recordings
« Reply #8 on: February 24, 2017, 12:29:30 pm »
Wouldn't tha same noise be formed on the ear drum? After all diaphram and ear drum use the same mechanism it just whats on the other side is different
In fact, this was (is?) a project several years ago with military applications.  Using ultrasonic transmission, they could project audio a considerable distance using the "beat" between two otherwise inaudible acoustic signals.
 

Offline Buriedcode

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Re: Obscuring microphone recordings
« Reply #9 on: February 24, 2017, 12:50:08 pm »
There have been several' products' using ultrasonics to project sound onto a surface at a distance, one that spring to mind (pretty dodgy):

https://www.kickstarter.com/projects/richardhaberkern/soundlazer
https://www.indiegogo.com/projects/a-speaker-the-speaker-that-only-you-can-hear-sound#/

I vaguely remember reading about such devices from an 80's book by Bob Iannini. The book was full of alarming (and potentially extremely dangerous) high voltage stuff, but the principles were sound - pardon the expression.  So non-linear mixing of ultrasonics doesn't appear to be a new idea, but applications are limited, and since ultrasonics are rather directional one would have to know where the microphone is in order to try to 'jam' it. Range is limited - I'm guessing a few feet, tops.

So what is the application?  I can only assume its to jam 'bugs' or recording devices in a specific area. It can't be to prevent people from recording live music shows because the music would trump any 'acoustic' jamming system.


 

Offline madires

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Re: Obscuring microphone recordings
« Reply #10 on: February 24, 2017, 01:24:59 pm »
I thought that EEs are familiar with non-linear V-I curves  >:D  Please see https://en.wikipedia.org/wiki/Sound_from_ultrasound for an overview.
 

Offline delicadoTopic starter

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Re: Obscuring microphone recordings
« Reply #11 on: February 24, 2017, 01:35:26 pm »

So what is the application?  I can only assume its to jam 'bugs' or recording devices in a specific area. It can't be to prevent people from recording live music shows because the music would trump any 'acoustic' jamming system.


I'm just trying to jam my webcam's microphone. The mic can't be disabled in software, so I can only make an external source for that. What I don't quite understand is why does one of the signals need to modulate a noise signal? Couldn't they just be, e.g., a 25kHz and a 26kHz signal?
 

Offline cyr

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Re: Obscuring microphone recordings
« Reply #12 on: February 24, 2017, 01:42:08 pm »

So what is the application?  I can only assume its to jam 'bugs' or recording devices in a specific area. It can't be to prevent people from recording live music shows because the music would trump any 'acoustic' jamming system.


I'm just trying to jam my webcam's microphone. The mic can't be disabled in software, so I can only make an external source for that. What I don't quite understand is why does one of the signals need to modulate a noise signal? Couldn't they just be, e.g., a 25kHz and a 26kHz signal?

Disable it in hardware (desolder it or cut cable/track and put a switch inline).
 

Offline delicadoTopic starter

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Re: Obscuring microphone recordings
« Reply #13 on: February 24, 2017, 01:44:16 pm »

Disable it in hardware (desolder it or cut cable/track and put a switch inline).

I'm hoping to not void the warranty though.
 

Offline bradl

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Re: Obscuring microphone recordings
« Reply #14 on: February 24, 2017, 01:48:01 pm »
So non-linear mixing of ultrasonics doesn't appear to be a new idea

I wonder why a microphone's diaphragm is non-linear whilst our ear drum isn't?
 

Offline DBecker

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Re: Obscuring microphone recordings
« Reply #15 on: February 24, 2017, 05:58:52 pm »

So what is the application?  I can only assume its to jam 'bugs' or recording devices in a specific area. It can't be to prevent people from recording live music shows because the music would trump any 'acoustic' jamming system.


I'm just trying to jam my webcam's microphone. The mic can't be disabled in software, so I can only make an external source for that. What I don't quite understand is why does one of the signals need to modulate a noise signal? Couldn't they just be, e.g., a 25kHz and a 26kHz signal?

You want noise so that a simple notch filter doesn't remove the interfering sound.  And because your target might happen to have a response notch at the frequency you pick.

But I doubt that this approach will work.  A modern small device will have several MEMS microphones.  They usually have excellent ultrasonic response, and equally excellent filters to remove those signals.  They are packaged in miniature sardine tins with digital outputs, so they are more resistant to magnetic and electrical interference than the surrounding circuits.
 

Offline delicadoTopic starter

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Re: Obscuring microphone recordings
« Reply #16 on: February 25, 2017, 12:53:21 am »
A modern small device will have several MEMS microphones.

It's probably not going to work for MEMs, but I'm trying to block just electret mics.
 

Offline DaJMasta

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Re: Obscuring microphone recordings
« Reply #17 on: February 25, 2017, 04:07:03 am »
Without intentionally damaging the microphone, access to the signal path coming out of it, or noisy blocking... your solutions are going to be pretty limited.

I think the best easy bet would be a noise canceling circuit (like one ear of a noise canceling headphone) and then playing white noise on the microphone side.  If you've got the active canceling and decent insulation, the noise won't have to be that loud to make sure you don't have usable sound on a recording.


The much simpler method would be disabling in software, like uninstalling drivers and such, but this may be more difficult for just the mic element of a audio codec or under a lot of more closed operating systems.

If you are fine with a permanent/damaging solution and can't cut the wire/trace, then maybe a few drops of very thin super glue through the port would do the trick.  It shouldn't take a lot on the diaphragm itself to stop it from moving, though you may need to try an acoustic stimulus or something to get the glue to seep in far enough.
 


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