(1) An opening in the front surface or baffle of a loudspeaker enclosure, called a ported enclosure.
(2) One or more openings on the body of a unidirectional microphone, either rear-entry or side-entry. Passing through the port(s), sound waves reach the rear side of the diaphragm and, through phase cancellation or reinforcement, contribute to its directional response pick-up pattern.
See acoustic labyrinth.