A STUDY OF CORRELATION-PROPERTIES OF THERMAL ACOUSTIC RADIATION

A formula for the cross-correlation function of pressure fluctuations of thermal acoustic radiation in a sound-absorbing liquid is derived o n the basis of hydrodynamic fluctuation theory. A model of a two-eleme nt interferometer consisting of two piezoelectric transducers is used in the derivation. The resulting formula is an acoustical variant of t he van Cittert-Zernike theorem of optics. It relates the degree of coh erence of pressure fluctuations to the depth of penetration of the ove rlap region of the transducer directivity patterns into the liquid und er study.