Some types of SONAR send out sound, other types (e.g. [0]) passively listen for sounds and use beamforming to localise the source of the noise. [0] https://en.

Some types of SONAR send out sound, other types (e.g. [0]) passively listen for sounds and use beamforming to localise the source of the noise. [0] https://en.wikipedia.org/wiki/Towed_array_sonar https://en.wikipedia.org/wiki/Towed_array_sonar