The field of science and technology that is concerned with the diffraction of visible or infrared light (usually from a laser) by high-frequency sound in the frequency range of 50–2000 MHz. The term “acousto” is a historical misnomer; sound in this frequency range should properly be called ultrasonic. Such sound cannot be supported by air, but propagates as a mechanical wave disturbance in amorphous or crystalline solids, with a sound velocity ranging from 0.6 to 6 km/s (0.4 to 4 mi/s) and a wavelength from 3 to 100 μm.