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Most microphones are designed to emulate the human ear, hearing sounds that we hear, and not hearing ones that we don't. Scientists from the University of Illinois at Urbana-Champaign, however, have created a new sound that we can't hear but that is picked up by mics of all kinds. It could have some valuable applications, although there's also the potential for misuse.
The university's Coordinated Science Laboratory states that the sound is produced by combining multiple tones that interact with a microphone's mechanical workings, creating what is known as a "shadow" – this is a type of white noise that is detectable only by the microphone, as it's formed within the mic itself.
Transmitted by ultrasonic speakers within a room, the sound could be used to keep confidential conversations from being clearly picked up by hidden listening devices. The people talking would still have no problem hearing each other, as the sound would be inaudible to them.
It could also thwart illegal audio recordings in movie theaters or music venues, plus it could be used in place of Bluetooth for wireless communication between Internet of Things (IoT) devices.
(Score: 0) by Anonymous Coward on Tuesday June 27 2017, @08:53PM (3 children)
TFA is too vague, but I'd guess that this technique would have less effect the better quality microphone you use. Obviously studio mics with metallic mesh, foam batting and the nylon mesh screen might combat this phenomenon better than the little pinhole microphone on your cell phone or most portable recording devices. The intent seems to be to stop covert eavesdroppers. I think you'd spot the one with the boom-mic.
(Score: 1, Interesting) by Anonymous Coward on Tuesday June 27 2017, @09:50PM (2 children)
Great if you want to be blasting ultrasonic white noise at jet engine SPL (~120db) to have any chance of stopping someone recording a conversation. The interference caused by the ultrasonics has to be at least 9db louder than everything else in the recording to successfully mask it and it's trivial to effect a 12db reduction above 20kHz (17mm wavelength in air). I just took an SPL meter reading of myself talking in a small room, the meter pointed away from me and it was hitting 92db (C weighted). So it's not really a question of the microphone, just physics.
(Score: 0) by Anonymous Coward on Tuesday June 27 2017, @11:24PM (1 child)
Way to cherry pick one sentence, pull it out of context and use it as a straw man argument complete with buzzwords. I'll reiterate what I meant since it wasn't obvious. This technique will work on the microphones most often found in mobile and thus covert recording devices. you don't need ~120 db sound when any decent crosswind has the same effect as this sound emitter TFA talked about.
(Score: 1, Interesting) by Anonymous Coward on Wednesday June 28 2017, @09:12AM
Yes you do, as I have explained. 92dBC (SPL of my voice in a small room, meter pointed away) + 12dB (attenuation at 20kHz) + 9db (rule of thumb by which a louder sound will mask a sound at similar frequency) = 113dB. The recommended limit for prolonged exposure to ultrasonic frequencies in the UK is 115dB [hse.gov.uk] and the absorption coefficient of air at 20kHz yields roughly 0.5dB/M attenuation. Placing your "covert" recording device in a rolled up pair of socks will easily yield in excess of 3dB attenuation at 5kHz, 6dB at 10kHz and 12dB at 20kHz. What are you not understanding?