Acoustic transmitting/receiving elliptic ring arrays

A typical sidescan sonar system uses a linear array of rectangular elements in order to generate a fan-like acoustic beam which is narrow (1-5 degrees) in the fore-and-aft directions but is broad (20-30 degrees) athwart. The array functions both as a transmitter and as a receiver. Inherent to such an array is the existence of relatively large sidelobes in its radiation pattern (on the order of 13 dB down from the main lobe maximum). Sidelobes cause deterioration of system performance by admitting signals from undesired directions. It is possible to reduce the sidelobes by using an array of elliptic rings. Fabrication of elliptic arrays from ceramic elements, however, is complicated and expensive. Instead, the authors propose to separate transmitting array from receiving array. The transmitting array can be a conventional ceramic liner array, while the receiving-only elliptic array can be fabricated using piezo-electric films such as polyvinylidene fluoride (PVDF). The authors employ optimization techniques to optimize the combined receiving/transmitting pattern. As an example it is shown that a 5°×32° fan-like beam with 60 dB sidelobe suppression in the two-way pattern can be obtained using only 15 elliptic rings