Focusing, Sound

the conversion of plane or of diverging spherical or cylindrical sound waves into converging waves. Like optical and radio waves, sound waves are focused by reflection and refraction methods. The natural focusing of sound is observed, for example, in caverns with arched roofs. Partial focusing of sound occurs in an underwater sound channel in seas and oceans. When sound is focused, the energy of the sound waves is concentrated, achieving a maximum value at a focus that coincides ordinarily with the center of curvature of the converging wave front.

The systems used to focus sound are classified as active or passive. The former are represented by a concave acoustic radiator that produces a converging wave front directly; in the latter, the acoustic path length kL, where k is the wave number and L is the geometric path length, is altered such that a plane or diverging wave front is converted into a converging wave front. Examples of active systems are acoustic intensifiers, and of passive systems, acoustic lenses.

The principal characteristics for the quality of sound focusing are the shape and size of the focal region and the amplification factor for the sound pressure, which is the ratio of the sound pressure at the center (or on the axis) of the focal region to the pressure at the input of the system. For the simplest case of an axially symmetrical round beam converging at a small angle, the principal energy is concentrated within a focal spot, consisting in this case of a circle having a radius r₀ = 0.61λF/R and an amplification factor, without taking losses into account, equal to K = πR²/λF, where λ is the wavelength, F is the focal length, and R is the radius of the aperture for the focusing system.

Sound focusing is used to obtain an acoustic image in ultrasonic viewers, in the ultrasonic microscope, and in systems for acoustic holography. It is used to form a specified directional pattern for acoustic radiators and receivers. It is also used in scanning systems with an ultrasonic beam for medical diagnostic instruments. In addition, it is used to concentrate ultrasonic energy to be utilized in various technological processes and in ultrasonic surgery.