(57) A number of sonic transducers (11) are mounted, and located as an array, in a block
(22) of closed-cell foamed plastics material whose cells are filled with gas to be
acoustically opaque. The active faces, or heads of the transducers are located in
recesses (10J) in a mould (10) before introduction of the foamed material, so that
the active faces will be exposed and protrude from the moulded block. The array in
its block is mounted in an outer casing (30,32) sealed (36) against entry of water.
A space (34) within the casing and in communication with the active faces is filled
with an acoustic coupling medium (35).
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