Hydrophone systems in which the direction of acoustic waves impinging on an array of hydrophones is determined from measurements of the periods between successive zero-crossings of waves detected by the two hydrophones of each of several different pairs of the hydrophones, allowance being made for the fact that the start and finish of such a period may be due to zero-crossings of different waves. In one embodiment the hydrophones are arranged in two linear groups disposed at right angles to one another, with unequal spacing between adjacent pairs of hydrophones in each group. In another embodiment the hydrophone array consists of five hydrophones disposed at the corners of a rectangular pentagon.