Can a sound be created that only a machine can detect?

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Offline thedoc

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Alain asked the Naked Scientists:
   Can a sound be created, that only an instrument(machine) can pick-it up ?
What do you think?
« Last Edit: 17/05/2016 07:21:05 by chris »

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Offline evan_au

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Ultrasound (above human hearing) and infrasound (below human hearing) can be detected by instruments, but not by humans - unless they are of a dangerously loud amplitude.

Small earthquakes < 2 on the Richter scale are not detectable by humans - most of this energy is infrasound.
Some bats use ultrasound ranging, and this is not audible by humans.

The normal vibration of air molecules produces continuous wideband noise (including the audible range) which we normally ignore.

See: https://en.wikipedia.org/wiki/Ultrasound
https://en.wikipedia.org/wiki/Infrasound

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Offline evan_au

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Quote from: Alain
Can a sound be created, that only an instrument (machine) can pick-it up ?
The MP3 sound processing technique often used for podcasts reduces the amount of data to be downloaded and stored.

It does this by analyzing the sounds picked up the microphone; it detects and drops those sounds that cannot be heard by humans.

The discipline of psychoacoustics has identified a number of scenarios where a sound that would be audible by itself becomes inaudible in the presence of other sounds; for example when it is an harmonic of a lower frequency of higher amplitude, or if it is close in frequency or time to a sound of higher amplitude.

See: https://en.wikipedia.org/wiki/MP3#Development

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Offline Colin2B

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The discipline of psychoacoustics has identified a number of scenarios where a sound that would be audible by itself becomes inaudible in the presence of other sounds; for example when it is an harmonic of a lower frequency of higher amplitude, or if it is close in frequency or time to a sound of higher amplitude.
In most acoustic masking a sound is masked by another which occurred first. There are situations (backwards masking) where the second sound maskes the one which occurred shortly before, it's as though the brain erases memory of the first.
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Offline evan_au

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Data can be hidden in audio such that it can be detected by a computer, without being audible to a human. Hiding information in this way is called steganography.

When the US telephone network was first being digitized, a technique called "bit stealing" was used to communicate between telephone exchanges, by overwriting the least significant bit of the voice signal. This increases the background noise slightly, and is not audible (especially if the transmitted signal is encrypted so it appears random).

See: https://en.wikipedia.org/wiki/Steganography