Must be pretty high tech since modern torpedoes are more like a missile then the torpedoes of WWII era.
Man, you ain't lyin. Hyper-sonic torpedoes travel at infinitely higher velocity (hundreds of KPH) than conventional ones. They work by producing a bubble around themselves which works a little like the mythical "warp-field" of a Star Trek starship. The torpedoes have a cavitation mechanism at the front which produces a bubble around it reducing contact (and therefore friction) with the water. It essentially travels within its own self-generated bubble.
The challenge for the Navy will surely be to detect the incoming target and deploy the countermeasures before any hypersonic torpedo hits the hull or gets close enough to detonate a high-explosive and damage/destroy the target by a shock-wave or shrapnel.
When a torpedo is travelling at hundreds of meters per second, the time available between detection and reaction is cut dramatically.
High-power sonar that can detect small objects at long distances is one feature needed for these counter-measure systems and there have already been several successful legal challenges to Navy testing / use of these systems because of evidence that their use can disrupt or damage the hearing of marine life.
No doubt the details are top secret, but one may extrapolate that some of the counter-measures would involve sending exploding torpedoes into the trajectory of the incoming enemy torpedo and to detonate it in the path of the target in order to divert it away from the ship or destroy it.
There are also devices in use on military ground vehicles (like Humvees) which deploy a downward-directed stream of water along the sides, using precision sensors and explosive propellants. These systems effectively put up the equivalent of a "force field" in front of incoming RPG rounds, destroying them a few inches away from the surface of the vehicle which prevents the RPG round from detonating. It's a very high tech-fly swatter.
Even though water is '"soft", when accelerated to high speed it takes on the quality of a solid. A downward-directed charge driving a shock-wave through the water along the side of a vessel would destroy any object passing through it before it could explode - even (theoretically), a hyper-sonic torpedo.
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