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If we have an object which produces sound with approx. 120 kHz (i.e. bat). And let's say that we record this sound with sample rate of 44,1 kHz (assuming that we do not know about Nyquist theorem).

With which frequency we will hear the recorded sound when we play it?

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What you are dealing with in this case is aliasing. As mentioned here : https://dsp.stackexchange.com/questions/2456/alias-frequency-formula, Aliased frequency is the absolute difference between the actual signal frequency and the nearest integer multiple of the sampling frequency.

In your example :

  • actual signal frequency = 120000 Hz
  • nearest integer multiple of the sampling frequency = 3 * 44100 Hz

The aliased signal will therefore appear as a (3 * 44100) - 120000 = 12300 Hz signal.

Note that to record such a 120 kHz signal, you will need an ultrasonic microphone that can handle such high frequencies, which is not easy to find.

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