Impedance conversion of matching layer for air ultrasonic transducers

This paper proposes a new simple equation of approximate impedance conversion relation of quarter wavelength thick matching layer for air ultrasonic transducers. The equation tells the impedance seen from inside of the matching layer is mainly determined by internal mechanical loss, not by air impedance. This means the impedance should be roughly the same even if it is in vacuum. Experiments were done is air and vacuum, and the result agreed with theoretical result.