How can a radio wave carry the changing shape of a voice?

A steady radio-frequency wave by itself carries no changing voice message. A transmitter makes one of the wave's properties vary according to the information it wants to send. This process is modulation. The receiver detects those controlled variations and uses them to recover the message.

In amplitude modulation, the height of a rapidly oscillating carrier changes with a slower sound signal. The carrier still oscillates quickly, but its outer envelope rises and falls with the message. A suitable receiver can follow that envelope and recover the slower variation. The drawing shows only a few carrier cycles so the relationship is visible; a real carrier usually repeats much faster than the sounds it represents.

In frequency modulation, the carrier's instantaneous frequency varies instead. Cycles become closer together or farther apart while their ideal amplitude stays nearly constant. The amount and direction of the frequency shift represent the message. The receiver must therefore measure the changing timing pattern rather than merely follow the signal's height. AM and FM encode information in different measurable properties of a wave.

The transmitter obtains energy from its power supply and uses an antenna to radiate electromagnetic energy. A microphone can first convert sound-pressure changes into an electrical signal that controls modulation. At the receiver, demodulation reverses the encoding process sufficiently to recover the intended electrical message. Amplification and a speaker can then produce sound. The original sound wave does not fly all the way from the microphone to the radio as a packet of air.

A modulated signal occupies a range of frequencies, not just one carrier frequency. Noise and interference can alter the variations the receiver is trying to measure. Digital communication also uses modulation, with symbols represented by selected changes in amplitude, frequency, phase or combinations of them. The central idea remains simple: a predictable fast-changing wave provides a pattern on which slower or coded changes can carry information.