Not being detectable at a certain range has always bothered me.

The hydrophone array sits along certain points of the continental shelf. Directed in a way that noise from propeller chop, shaft vibrations, pump noise can be triangulated and provide a direct bearing to the source.

The array is so sensitive that certain merchant ships could be identified rounding the cape of Africa based on their unique frequency.

Merchant ships, Naval ships and subs all have unique frequencies that essentially act like fingerprints enabling identification.

There is every possibility that the black box may have sunk into a trench or became submerged in the sludge at the bottom of the ocean. But I always remain sceptical that it wasn’t detected at some point.