The concept of electronic beam steering. (A) Seven elements of a phased-array transducer firing simultaneously. A short distance from the transducer the individual wavelets from each of the elements merge to produce a compound wavefront, which creates a sound beam in the direction perpendicular to the transducer face. (B) The elements are now fired in sequence but are all used to create a single sound beam. When the individual wavelets merge to form a compound wavefront, it is not perpendicular and the sound beam travels away at an angle. Varying the excitation sequence allows rapid steering of a sound beam in any direction through a sector. (C) Electronic beam focusing is realized by exciting the peripheral elements first and the centre element last (cylindrical time-gated excitation). In addition to focusing the transmitted sound beam, it is also possible to focus the returning signals so that at any one instant the transducer array is selectively receiving only those echoes coming from a specified beam direction and depth (dynamic receive focusing). This requires very complicated electronics. (D) The principle of cylindrical time-gated excitation can be used to steer and focus sound beams in any direction during both transmission and reception.