Parametric Acoustic Arrays Formed by Diverging Sound Beams
Parametric acoustic arrays formed by two sound beams radiated by a circular piston source are considered, particularly with respect to arrays where the interactions in the farfield of the primary beams give the main contribution to the difference-frequency sound field. The influence
of the divergence of the primary beams on the amplitude and the directivity of the generated sound in the farfield, together with the properties of the difference-frequency sound in the transition region between the nearfield and the farfield of the parametric array are of main concern. To
discuss the farfield properties of parametric arrays a set of parameters have been used. A comparison of two theoretical models for the generated sound pressure on the axis in the farfield reveals the influence of the divergence of the primary beams. This is found to be described, to a good
approximation, by a single parameter. The comparison also shows that contributions from the side-lobe region of the array are important, in spite of the low source density which is present there. Experiments were performed on arrays in which the influence from the primary beam divergence could
be varied. Measurements taken between 0.5 and 3.7 array lengths from the primary source are presented. When the influence of the primary beam divergence was insignificant the observed difference frequency beamwidth was considerably less than that predicted by the Westervelt model, but when
the influence was large the beamwidth exceeded that of the Westervelt model. The pressure amplitude on the axis seems to become inversely proportional to the distance from the primary source at about two array lengths. The directivity pattern changes insignificantly in form while the half-power
angle increases slightly with distance in the region where measurements were taken. The relation between the observed results and the properties of the difference-frequency sound in the farfield of the array is discussed.
Document Type: Research Article
Publication date: 01 March 1977
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