Sonomicrometry

Sonomicrometry is a technique of measuring the distance between piezoelectric crystals based on the speed of acoustic signals through the medium they are embedded in. Typically, the crystals will be coated with an epoxy 'lens' and placed into the material facing each other. An electrical signal sent to either crystal will be transformed into sound, which passes through the medium, eventually reaching the other crystal, which converts the sound into electricity, detected by a receiver. From the time taken for sound to move between the crystals and the speed of sound in the medium, the distance between the crystals can be calculated.

Sonomicrometry was originally applied in the study of cardiac (heart) function in research animals (and limited investigational studies in humans) by Dean Franklin in 1956, and was quickly adopted by biologists working in biomechanics as well as other physiological organ systems and structures (gastro-intestinal, uro-genital and musculo-skeletal). Medical device companies also use sonomicrometry to assess the physical performance, durability and longevity of devices during R&D phase of development. Sonomicrometry is currently the most prevalent method for determining muscle length changes during animal locomotion, feeding, and other biomechanical functions.

When originally developed decades ago, care was taken to orient the crystals correctly to ensure satisfactory signal detection between the crystals, but more modern versions of sonomicrometer hardware (typically dating from 1995 to the present) do not require such attention to crystal orientation.

The primary criteria for making sonomicrometer measurements is that the intervening space or material between the crystals must be capable of ultrasonic sound conduction in the frequency range of 100 kHz to at least several MHz. This typically means that the crystals must operate in or on a medium composed of fluids, gels, soft solids and some hard solids, and this includes biological tissues (blood, muscle, fat, etc.). The measurement of distance between crystals becomes problematic or ceases completely if air gaps or solid objects intervene between the crystals.

Sonomicrometer measurements are only accurate when the medium surrounding the crystals or the object the crystals are measuring has a uniform speed of sound. In biological settings, almost all tissues have a speed of sound in the range of 1550 to 1600 meters per second, which is satisfactorily uniform to give accurate measurements to within 3% to 4%, and more typically to within 1%.

...
Wikipedia