Diaphone

A powerful compressed-air sound signal that produces a distinctive low-pitched blast.

A diaphone uses a slotted reciprocating piston to interrupt the air flow. Many installations produce a characteristic descending or grunting note at the end of the blast, and paired units can create a two-tone signal.

Diaphones were widely installed at lighthouses and fog-signal stations because their sound carried over long distances. They have largely been replaced by electric or electronic signals, but their distinctive characteristics remain part of maritime signalling history.