a device used to modify the convergence of (that is, to focus) sound beams.

Like optical lenses, acoustic lenses are bounded by two working surfaces. Acoustic lenses are made of a material for which the speed of sound differs from that in the surrounding medium, so that the refractive index n is a number other than unity. In order to attain maximum transmittance, the wave drag of the material should approximate that of the ambient and the viscous losses should be minimal.

Acoustic lenses may be solid, liquid, or gas. In a liquid or gas lens, the lens’s solid casing should exhibit maximum transmittance. For work in liquid media, the material for the lens may be plastic (n = 0.5–0.8), chloroform, or carbon tetrachloride (n = 1.3–1.4). Lenses filled with hydrogen or carbon dioxide are used along with nonhomogenous acoustic lenses filled with pellets or mesh for work in gases (for example, air). Nonhomogeneous diverging air lenses are used to improve the directivity characteristics of loudspeakers. Solid and liquid lenses are used for acoustic images, flaw detection, medical diagnosis, and the concentration of ultrasound for a variety of technological and biological purposes.