Sonar works two ways, active and passive.
Active sonar is a lot like radar, but with a microphone and speaker instead of a radio antenna. It sends out a blast of noise at a particular frequency, and then by measuring the delay in the echo you can tell how far away an object is.
Passive sonar is essentially an array of very sensitive microphones. You listen for particular sounds, and by listening to how the sound changes in pitch or volume over time or between different mics, you can judge how far away the source is or how it’s moving.
Sonobuoys are compact sonars housed in a deployable package. Usually airdropped by sub-hunting aircraft or floated by ships. One sonobuoy isn’t worth much more than just “THERE’S SOMETHING NEAR ME.” Multiple sonobuoys arranged in a pattern, on the other hand, can be used to triangulate the exact position of a source.
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