Foote, Kenneth G. Woods Hole Oceanographic Institution, Woods Hole, Massachusetts.
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A virtual array of sources of sound in a fluid (more fully termed a parametric acoustic array) that is formed by two collinear acoustic waves propagating in the same direction, which interact because of the nonlinearity (parameter) of the fluid medium to generate new acoustic fields. The array is virtual because it is formed in the fluid medium away from the devices responsible for the primary field components. If the primary interacting field is composed of two essentially monochromatic waves, with frequencies ν1 and ν2, then the new fields will form at the sum and difference frequencies, ν1 ± ν2. If ν1 and ν2 are slightly different, then the difference frequency, ν1 − ν2, will be quite small. Since absorption of sound in fluids generally increases rapidly with frequency, this difference-frequency wave will outlast the primary- and sum-frequency waves. In addition to suffering less absorption and propagating further, the difference-frequency wave possesses two remarkable properties: most of its energy is concentrated in a narrow angular sector in the direction of propagation of the primary waves, and it has no sidelobes. Parametric arrays can be used for both directional transmission and directional reception of sound. See also: Directivity; Sound; Sound absorption
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