an expendable device which transmits acoustic information to the helicopter

Buoy containing sonar, dropped by ships or aircraft to detect submarines

a free-floating sea buoy containing radio equipment to relay data received by one or more detectors to recording equipment typically aboard the source vessel.

In sonar systems, a device that is used to detect acoustic waves, such as those produced by ships and submarines, that, when activated, relays information by radio, that may be active or passive, and that may be directional or nondirectional.

A sonobuoy (a portmanteau of sonar and buoy) is a relatively small (typically 4 7/8 inches, or ~124 mm, in diameter and 36 inches, or ~914 mm, long) expendable sonar system that is dropped/ejected from aircraft or ships conducting anti-submarine warfare or underwater acoustic research.