Nyquist Frequency

The Nyquist-Shannon sampling theorem, formulated independently by Harry Nyquist (1928) and Claude Shannon (1949), states that a continuous signal can be perfectly reconstructed from its samples if the sampling rate is at least twice the highest frequency present in the signal. This critical threshold is the Nyquist frequency. For audio, the standard sample rates are chosen with Nyquist in mind. CD audio at 44.1 kHz has a Nyquist frequency of 22.05 kHz, just above the nominal 20 kHz upper limit of human hearing. Professional audio at 48 kHz gives a Nyquist frequency of 24 kHz, providing more margin. Higher rates like 96 kHz (Nyquist 48 kHz) are used in recording for additional headroom in processing. Aliasing occurs when the input signal contains frequencies above Nyquist. These frequencies are not simply lost; they are reflected back into the audible band as phantom frequencies that bear no harmonic relationship to the original signal, creating harsh and unpleasant distortion. Anti-aliasing filters in ADCs remove content above the Nyquist frequency before sampling to prevent this. In SonaVyx's FFT-based analysis, the Nyquist frequency determines the upper bound of the frequency display. With 48 kHz sampling and a 16384-point FFT, the analysis provides 8192 frequency bins from 0 Hz to 24 kHz, with a resolution of approximately 2.93 Hz per bin. This granularity is sufficient for detailed acoustic analysis including narrow-band feedback detection and room mode identification. Higher FFT sizes do not extend the frequency range; they only improve frequency resolution within the same Nyquist-bounded range.