OVERVIEW: A passive Non-directional sonobuoy is a naval acoustic sensor deployed from aircraft or ships to detect underwater sounds, primarily from submarines and other vessels, by passively listening to acoustic signatures without emitting any signals.

DETAILS: Equipped with hydrophones the sonobuoy is dropped into the water where it floats and monitors ambient noise. Non-directional: It detects sound but doesn’t provide bearing information. Since it does not emit any signals, the sonobuoy remains covert, reducing the risk of detection by adversaries. Data is transmitted via radio link back to the deploying platform for real-time analysis. Passive sonobuoys are a key component in anti-submarine warfare (ASW) for wide-area surveillance and tracking.

ROLE SUMMARY:
Category: Naval Acoustic Sensor
Primary Use: Passive detection of underwater targets
Function: Listening of acoustic emissions from submarines and vessels
Strong Against: Submerged submarines, quiet surface vessels
Weak Against: Non-acoustic stealth techniques, ambient noise interference
Platform: Maritime patrol aircraft, ASW helicopters, surface ships
Integration: Linked to ASW mission systems and command centers

SOURCE:
Janes Naval Sensors and Systems
https://en.wikipedia.org/wiki/Sonobuoy