Re: question about freezing point Changes of state produce changes in velocity and attenuation. Both are easily measured and in the case of water to ice very dramatic.
The simplest setup is a pulse echo immersion transducer in a water tank with either the amplitude of the back wall of the water tank monitored. Or the time of flight monitored. Or both.
The influence of biological material in the water, other than lowering the freezing point, is uncertain but I would still expect a dramatic change in both of these ultrasonic properties when the fluid freezes.
: How to define by a ultrasonic method a beginning crystallization and point of freezing of complex biological liquids, for example, milk