Ultrasound velocity is the probably the most sensitive probe to phase transitions. We use it to detect phase transition as a function of temperature or magnetic field. A transducer (a piezoelectric crystal) is glued on the surface of the sample to generate an acoustic pulse which travels back and forth in the sample, generating echos measured with another transducer. When travelling through the sample the acoustic pulse energy is absorbed by available degrees of freedom in the sample, causing a decay of the echo amplitude with time. This decay is related to the ultrasound attenuation which can be measured simultaneously with ultrasound velocity.
