A phase plug comprises a body having an input side for receiving acoustic waves and an output side for transmitting acoustic waves, the body including a plurality of channels extending from the input side to the output side for propagating acoustic waves through the body. The input side comprises an input surface which includes a plurality of slots constituting entrances for the channels, each slot being arranged in a substantially radial orientation on the input surface about a central axis extending through the input surface. Substantially the entire input surface situated between the slots is concave and substantially part of a sphere or an ellipsoid in shape.