An amplitude-modulated tone is a carrier plus two side lines

Evidence

The panel “AM tone” depicts both the time function and the spectrum of a sinusoidally amplitude-modulated 2-kHz tone. The time function shows the sinusoidal oscillation, the envelope of which varies with the modulation frequency. The corresponding spectrum illustrates that an AM tone is described by three lines. … The period of the envelope fluctuation shown, 6 ms, corresponds to a modulation frequency of 167 Hz, which in the spectral domain represents the frequency difference between the centre line, called the carrier, and the upper and lower side lines, respectively. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 74

Categorization

• XREF: Connects to modulation-theory treatments of amplitude modulation used later in the book for fluctuation strength and roughness.