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A sonobuoy is an air-dropped sensor used in anti-submarine warfare. Occasionally, they may be launched over the side of ships. It activates on contact with water, and operates for a period of hours before automatically disabling itself and sinking; the devices are not reusable. U.S. Navy sonobuoys have a form factor of 36 inches in length and 4 7/8 inches in diameter; this is generally used by NATO and U.S. allies.

Most either passively listen for sound, or are full sonar devices that actively send and receive acoustical energy. There are a few special-purpose devices in a standard form factor, which include the bathythermograph function, sensing water temperature by depth.

  • Passive acoustic:
    • AN/SSQ-53B, AN/SSQ53D, and AN/SSQ-53E directional frequency analysis and recording (DIFAR) sonobuoys.
    • AN/SSQ-57B and AN/SSQ-36M calibrated low-frequency analysis and recording (LOFAR) sonobuoys.
    • AN/SSQ-77A (CZ) and AN/SSQ-77B vertical line array DIFAR (VLAD) sonobuoys. These have higher noise rejection, as needed, for example, in littoral warfar, using a multi-element omnidirectional hydrophone array and a beamforming filter assembly to enhance acoustic performance. The former is designed for convergence zone and the latter for bottom-bounce sonar beams.
  • Active acoustic
    • AN/SSQ-62B, AN/SSQ-62C, AN/SSQ62D, AN/SSQ-62E directional command activated sonobuoy system (DICASS)
    • AN/SSQ-110 and AN/SSQ-1lOA extended echo-ranging (EER) buoys. These carry small explosive charges used as additional sources of acoustic energy.
  • Bathythermograph
    • AN/SSQ-36 and AN/SSQ-36B air-expendable bathythermograph (AXBT) buoys
  • Data Link Communications Sonobuoy: The DLC sonobuoy is an expendable, acoustic communication device designed to transmit a preprogrammed message to a submerged submarine.