It seems to me that only the sender of the pulse would hear it's return.
ie: ASW SONAR guy up top hits 'send sound pulse' and hears that ping.... if nothing is down there, he gets no return ping. He can then send more pings at will... ping... ping... ping.... (individual events.)
... now he sends a ping and hears the return/rebound ping... a double event if you will... ping-PING...
Now from the hunted submarine crew perspective, they only ever hear abient individual pings that may be distant, or perhaps individual louder pings with increasing rapidity as the sender fixes the target. The sub crew never hears a ping-PING because the rebound of the pulse happens the instant it strikes the hull...
... but in a quick moment, that rebound is heard by the sender as above.
If my post looks funny, it is because Major Wifey is pinging me now...
