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SESK: Stuff Everyone Should Know · sesk.info/eng-aliasing

What happens to a frequency above the Nyquist frequency when it is sampled?

It aliases: it shows up as a false, lower frequency.

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  1. It is simply recorded perfectly
  2. It folds back as a fake lower frequency ✔
  3. It becomes silence

Why: Too few samples cannot distinguish a fast wave from a slower one. Depending on input frequency and sample rate, it can appear as a false lower frequency. A low-pass filter removes such tones before sampling.

Huh, didn't know that: The wagon wheels that seem to spin backward in old films are aliasing in time.

The dad joke
That high note went undercover with an alias. Now it's pretending to be a low note.
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References

Answer: It folds back as a fake lower frequency. Too few samples cannot distinguish a fast wave from a slower one. Depending on input frequency and sample rate, it can appear as a false lower frequency. A low-pass filter removes such tones before sampling.

  1. Indiana University, Music Technology Text, Chapter 5: Nyquist cmtext.com/digital_audio/chapter5_nyquist.php
    Frequencies above the Nyquist frequency cause aliasing (also sometimes called foldover or biasing).
    Checked 2026-10-10.

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