Amplitude
The extent of a vibration; it determines the loudness of a sound.
What it means
Amplitude denotes the maximum displacement of a vibrating quantity from its resting position, or, for a sound wave, the greatest change in air pressure or displacement relative to ambient pressure. The larger a vibration’s amplitude, the louder the resulting sound is perceived to be.
Amplitude should be distinguished from frequency: frequency determines how high or low a note sounds, while amplitude determines how loud it sounds. The two quantities can be changed independently of each other.
Musical use
In acoustics and electronic sound generation, amplitude is one of the basic quantities used to describe a sound signal, alongside frequency, timbre, and duration. In synthesizers and digital audio processing, the time course of amplitude is shaped by an envelope, which, for instance in an ADSR envelope, defines the attack, decay, sustain, and release stages.
When playing acoustic instruments, too, the amplitude of the resulting vibration is directly linked to the force of a keystroke, bow pressure, or breath support: the stronger the impulse, the greater the amplitude, and hence the loudness.
Performance practice
If you work with electronic sound generation, pay close attention to how changes in envelope parameters affect amplitude and thus a sound’s loudness curve, so you can shape dynamic contours deliberately rather than leaving them to chance.