In college our underwater acoustics prof used an old copy of the Navy sonar manual to teach some fundamental concepts early on in the course. One of my favourite passages was when the Navy manual definitively answers the question of "if a tree falls in the forest and nobody's around to hear it, does it make a sound?" The answer, according to the Navy is that no - in order for a sound to exist there must be both a transmitter and a receiver component, and if both are not present then the energy emitted by the lone transmitter does not constitute "sound."

So, according to that - you're both right in that it's a two party process. However, I believe the term is appropriate in underwater acoustics for anything which dumbly sends out a relatively predictable signal on a regular interval.