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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: 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?