Modulating a signal at audio rates generates new sideband frequencies in the spectrum

Amplitude modulation (AM) and frequency modulation (FM) at sub-audio rates (below ~20 Hz) produce tremolo and vibrato respectively—time-domain variations the ear tracks as volume or pitch fluctuation. When the modulation rate enters the audio range (above ~20 Hz), the ear can no longer track the fluctuations individually. Instead, the interaction between carrier and modulator generates new spectral components called sidebands, located above and below the carrier frequency at intervals equal to the modulation frequency and its multiples. These sidebands add entirely new frequencies to the spectrum—a qualitatively different timbral effect from slow modulation.

Examples

AM at 3 Hz on a 440 Hz carrier: tremolo only. AM at 100 Hz: sidebands at 340 Hz and 540 Hz appear. FM at 50 Hz on a 440 Hz carrier: vibrato transitions to sidebands at 390 and 490 Hz, with additional harmonics from FM’s nonlinearity.

Assessment

A 500 Hz carrier is amplitude-modulated at 80 Hz. List the frequencies of the sidebands that appear. Then explain how the audible effect differs from the same modulator running at 4 Hz.