How are crystals used to detect ultrasonic waves and sound absorption?

Crystals are essential tools in the study of relaxation phenomena because they can effectively detect ultrasonic waves and measure sound absorption through the piezoelectric effect.1

Detection of Ultrasonic Waves

Piezoelectric crystals, such as quartz, are used as transducers to convert energy between electrical and mechanical forms. In the context of ultrasound:23
* Generation: When an alternating electric field is applied to a crystal, the converse piezoelectric effect causes the crystal to vibrate at a high frequency, generating ultrasonic waves.23
* Detection: When these waves reflect off an object and return to the crystal, the direct piezoelectric effect converts the mechanical pressure of the returning sound back into an electrical signal. This process is the foundation for technologies like medical imaging and sonar.23

Measurement of Sound Absorption

To measure sound absorption, researchers often use an acoustic interferometer, which employs a vibrating crystal to radiate sound waves into a fluid medium. By reflecting these waves back to the source and analyzing the resulting standing wave pattern, scientists can determine the medium's absorption characteristics. This is critical for investigating molecular relaxation processes, where the time-dependent transfer of energy between molecular degrees of freedom—such as vibrational or rotational modes—causes measurable sound attenuation.45