Hi-frequ audio directionality
Hi frequency audio at 30K+ is very high, and highly directional. I wonder of the towed array lost the pings as it moved into the "shadow" of a submerged mountain range. To experience the effect you only need a hi-fi. Put on some music with a lot of top and put your hand between the woofer and your ear- should be little effect on the bass because low frequencies are less directional. Now do the same between tweeter and ear- top loss, right? Now imagine the effect of a mountain between pinger and array, when the ping is over 10KH higher than most hi-fi set-ups go. the cut off "shadow" would be very abrupt.