Obscuring microphone recordings - Page 1
Forum section: electronics / projects

author: delicado
author_rank: Contributor
timestamp: February 24, 2017, 11:25:03 am
content: HiTo 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.

author: Fungus
author_rank: Super Contributor
timestamp: February 24, 2017, 11:31:24 am
content: Quote from: delicado on February 24, 2017, 11:25:03 amIf 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.

author: delicado
author_rank: Contributor
timestamp: February 24, 2017, 11:35:01 am
content: Quote from: Fungus on February 24, 2017, 11:31:24 amBut 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.

author: AlfBaz
author_rank: Super Contributor
timestamp: February 24, 2017, 11:50:33 am
content: 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

author: Siwastaja
author_rank: Super Contributor
timestamp: February 24, 2017, 11:55:58 am
content: Quote from: AlfBaz on February 24, 2017, 11:50:33 amExatly.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.

author: Fungus
author_rank: Super Contributor
timestamp: February 24, 2017, 11:56:38 am
content: Quote from: delicado on February 24, 2017, 11:35:01 amWe're missing the "why" from this question. What's the goal?(I guessed it would be 'inaudible masking' because of the 20kHz+ frequencies combined with "electret mike")

author: Fungus
author_rank: Super Contributor
timestamp: February 24, 2017, 12:16:58 pm
content: Quote from: delicado on February 24, 2017, 11:35:01 amI 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.

author: delicado
author_rank: Contributor
timestamp: February 24, 2017, 12:17:51 pm
content: Quote from: Fungus on February 24, 2017, 11:56:38 amYes, 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.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 24, 2017, 12:29:30 pm
content: Quote from: AlfBaz on February 24, 2017, 11:50:33 amIn 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.

author: Buriedcode
author_rank: Super Contributor
timestamp: February 24, 2017, 12:50:08 pm
content: 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/soundlazerhttps://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.

author: madires
author_rank: Super Contributor
timestamp: February 24, 2017, 01:24:59 pm
content: I thought that EEs are familiar with non-linear V-I curvesPlease seehttps://en.wikipedia.org/wiki/Sound_from_ultrasoundfor an overview.

author: delicado
author_rank: Contributor
timestamp: February 24, 2017, 01:35:26 pm
content: Quote from: Buriedcode on February 24, 2017, 12:50:08 pmI'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?

author: cyr
author_rank: Frequent Contributor
timestamp: February 24, 2017, 01:42:08 pm
content: Quote from: delicado on February 24, 2017, 01:35:26 pmDisable it in hardware (desolder it or cut cable/track and put a switch inline).

author: delicado
author_rank: Contributor
timestamp: February 24, 2017, 01:44:16 pm
content: Quote from: cyr on February 24, 2017, 01:42:08 pmI'm hoping to not void the warranty though.

author: bradl
author_rank: Newbie
timestamp: February 24, 2017, 01:48:01 pm
content: Quote from: Buriedcode on February 24, 2017, 12:50:08 pmI wonder why a microphone's diaphragm is non-linear whilst our ear drum isn't?

author: DBecker
author_rank: Frequent Contributor
timestamp: February 24, 2017, 05:58:52 pm
content: Quote from: delicado on February 24, 2017, 01:35:26 pmYou 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.

author: delicado
author_rank: Contributor
timestamp: February 25, 2017, 12:53:21 am
content: Quote from: DBecker on February 24, 2017, 05:58:52 pmIt's probably not going to work for MEMs, but I'm trying to block just electret mics.

author: DaJMasta
author_rank: Super Contributor
timestamp: February 25, 2017, 04:07:03 am
content: 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.