Every object has its own characteristic acoustic impedance. Acoustic impedance is expressed as a numeric value indicating the ease of sound propagation. It can be calculated as density of medium × speed of sound traveling through medium.
Ultrasonic waves are reflected when the difference in acoustic impedance between media is large, and they are transmitted when the difference is small.
Murata ultrasonic sensors propagate ultrasonic waves through the air. Since there is a very large difference with the acoustic impedance of water ( 106 ), reflection occurs at the boundary surface.
Speed of sound and acoustic impedance within various media
Zo: Acoustic impedance
c: sound speed