White noise contains all audible frequencies at equal amplitude, making it an ideal filter source

White noise is a random signal whose spectrum is essentially flat — it contains all audible frequencies with approximately equal energy. The name comes from the optical analogy: white light combines all visible colors. Because white noise has energy at every frequency, any filter applied to it will produce a result — there are no missing frequencies to prevent the filter from having an effect. This makes white noise the ideal source signal for subtractive synthesis: the filter reveals its own frequency response by acting on white noise. In digital systems it is produced by (pseudo-)random number generators running at the sampling rate.

Examples

Patching noise~ → lores~ (lowpass filter at 800 Hz) in Max produces a warm hiss. Modulating the filter cutoff with an LFO creates a classic sweeping noise effect used in techno and ambient music.

Assessment

Explain why a highpass filter applied to a soprano voice recording might have no audible effect at 50 Hz, whereas the same filter applied to white noise would always have an effect.