> Most sonar is active (sends out pings) and has only one input (one microphone)
While I believe the US Navy doesn't prefer the term "Passive Sonar", submarines and some surface ships are equipped with hydrophone arrays (either on the hull, or towed arrays, or both) as well as active sonar; this isn't an uncommon thing. (I was under the impression that the active sonar didn't use a single hydrophone, but instead used the same hydrophone array used for passive detection.)
While I believe the US Navy doesn't prefer the term "Passive Sonar", submarines and some surface ships are equipped with hydrophone arrays (either on the hull, or towed arrays, or both) as well as active sonar; this isn't an uncommon thing. (I was under the impression that the active sonar didn't use a single hydrophone, but instead used the same hydrophone array used for passive detection.)