The stage that a particle in vibration has reached in its CYCLE. Particles are in phase when they are at the same stage in the cycle at the same time, and out of phase when they are at opposite stages. Phase is measured by the angle of rotation, in degrees or radians.
The rate of change of phase is termed FREQUENCY. See: LAW OF UNCERTAINTY, PERIOD, SOUND PRESSURE, STANDING WAVES.
The auditory system is often said to be 'phase deaf' in that constant PHASE DIFFERENCEs are not audible, and therefore phase information is often ignored, as in FOURIER SYNTHESIS. However, small time delays which are the equivalent of phase differences play an important role in BINAURAL HEARING. Similarly, constantly changing phase differences are audible in the phenomenon of secondary BEATS.
See also: FOURIER ANALYSIS, FOURIER THEOREM, PHASE-SHIFT, PHASING, SIMPLE HARMONIC MOTION, TIMBRE.