AmirPatriot,
I admit I'm not expert when it comes to sonar/acoustics but I'll take a stab at it.
As several other members pointed out, since the front of the torpedo is not longer actually "in water", an acoustic-based seeker likely wouldn't function proper (likely having your ear just above the water and trying to hear a sound coming from the water, you may hear it but not as clearly/distinctly). This problem would apply to an active seeker (aka one that "pings" the target itself) and a passive-seeker.
Now, there is at least one other way to guide a torpedo these days: Wire-guidance.
Just as with wire-guided anti-tank missiles, this type of guidance is something of a double-edged sword. While they do provide excellent control of the weapon (and allow the submarine's fire control systems to do the work instead of having to design a compact onboard guidance kit), it also puts the launch vessel at risk of retaliation.
Think of torpedo guidance methods like this:
-Wire-guidance= Semi-active radar homing (SARH) A2A missiles
-Acoustic homing= "fire and forget" A2A missiles (whether radar guided or EO/IR guided)
With a torpedo like the VA-111/Hoot, the time to target and limited range could make wire-guidance somewhat more feasible, as its speed puts the launch vessel at risk of retaliatory attack for much less time, but also at a less safe distance from the enemy.
A "sprint, search, sprint" method using an acoustic-homing seeker suggested by Navweapons sounds like a feasible idea but would require modifications to allow the rocket motor to be capable of being throttled back (which given that it's a liquid-fuel rocket motor, shouldn't be that difficult to design).
But my question with this method would be this:
How well would an acoustic seeker work with the cavitator at the tip of the torpedo?
EDIT: Scratch that underlined bit, the VA-111 apparently uses a solid-fuel rocket motor, which are much more difficult to "throttle down". So I'm not how exactly that "sprint, search, sprint" method would actually work...
