Alias frequency formula: f_alias = |f_in - (n x f_s)|, where f_in = input freq, f_s = sample rate, n = nearest integer multiple
Example: a 30 kHz tone at 44.1 kHz folds to |30 - 44.1| = 14.1 kHz fake tone you can hear
Human hearing tops out near 20 kHz, so 44.1 kHz was chosen to leave headroom above it
The fix: an anti-aliasing low-pass filter BEFORE the ADC (analogue-to-digital converter) removes content above Nyquist
Pro rates: 44.1, 48, 88.2, 96, 176.4, 192 kHz; higher rate = higher Nyquist = more headroom
Bit depth (16/24-bit) sets dynamic range/noise floor, NOT aliasing; aliasing is a SAMPLE-RATE problem
Aliasing is inharmonic, so it sounds gritty/whistly and is never musically related to the note played
How it works
A microphone signal hits the converter as a continuous analogue wave.
The ADC takes snapshots (samples) at a fixed rate, e.g. 48,000 times a second.
If the wave wobbles faster than half the sample rate, snapshots miss its true motion.
The maths then 'folds' that high frequency back down into a lower fake tone.
An anti-aliasing low-pass filter blocks the too-high content before sampling, stopping the fake tone.
Good converters and oversampling plugins push the filter point well clear of audible range.
Real examples
A cheap USB interface adds a thin metallic whistle on cymbals and hi-hats that no mic actually picked up.
A bargain pitch-shifter or guitar-amp plugin smears harsh gritty overtones onto high notes because it does not oversample.
Recording at 44.1 kHz with a broken anti-alias filter folds ultrasonic synth content down into an audible buzz.
Sample-rate converting 96 kHz down to 44.1 kHz with a poor algorithm dumps fake high-end artefacts into the mix.
How it helps in live sound
Run digital consoles and stageboxes at 48 kHz; it is the live standard and gives a 24 kHz Nyquist ceiling.
Buy reputable converters (the ADC quality decides whether highs stay clean); skip no-name interfaces for FOH recording.
On distortion, saturation or pitch plugins, switch on oversampling (2x-4x) so internal harmonics do not fold back as grit.
Match sample rate across ALL gear in the chain; mismatched rates force conversion that can add artefacts.
If cymbals sound gritty/whistly and not just bright, suspect aliasing in a plugin before you reach for more EQ.
Keep one project sample rate from capture to export; avoid repeated up/down conversions.
Everyday analogy
Like filming a spinning car wheel: the camera frames are too slow, so the wheel appears to crawl or spin backwards even though it is racing forwards.
Watch out
Myth: a higher sample rate alone removes aliasing. Truth: the anti-aliasing filter does the work; without it, any rate still folds too-high content down into fake tones.
Fun fact
The backwards-spinning wagon wheel in old Westerns is visual aliasing: 24 frames per second is too slow to track the spokes, the same maths that makes a 30 kHz tone fake-out a 44.1 kHz recorder.
Key takeaways
Sample faster than 2x the highest frequency or it aliases (Nyquist).
Nyquist frequency = sample rate / 2 = the clean ceiling.
Aliasing makes NEW fake tones that nobody played, gritty and inharmonic.
An anti-aliasing low-pass filter before the ADC is the real cure.
Live = 48 kHz; quality converters and oversampling plugins keep highs clean.