Hydrographic echo sounders are used to measure the depth to the seafloor by using the properties of acoustic waves. The principle of echo-sounders is basic - by measuring the two-way travel time between the acoustic waves transmitted on sea surface and those reflected at seafloor.

Echo-sounders are classified into two types; Single-Beam Echo Sounder (SBES) and Multi-Beam Echo Sounder (MBES). The names of 'single' and 'multi' stem from the number of depth points measurements collected at the same time.