Cray Benjamin A
Newport Division, Naval Undersea Warfare Center, 1176 Howell Street, Newport, Rhode Island 02841-1708, USA.
J Acoust Soc Am. 2015 Dec;138(6):3765-72. doi: 10.1121/1.4938019.
Directivity is essentially a measure of a sonar array's beamwidth that can be obtained in a spherically isotropic ambient noise field; narrow array mainbeam widths are more directive than broader mainbeam widths. For common sonar systems, the directivity factor (or directivity index) is directly proportional to the ratio of an incident acoustic trace wavelength to the sonar array's physical length (which is always constrained). Increasing this ratio, by creating additional trace wavelengths for a fixed array length, will increase array directivity. Embedding periodic structures within an array generates Bragg scattering of the incident acoustic plane wave along the array's surface. The Bragg scattered propagating waves are shifted in a precise manner and create shorter wavelength replicas of the original acoustic trace wavelength. These replicated trace wavelengths (which contain identical signal arrival information) increase an array's wavelength to length ratio and thus directivity. Therefore, a smaller array, in theory, can have the equivalent directivity of a much larger array. Measurements completed in January 2015 at the Naval Undersea Warfare Center's Acoustic Test Facility, in Newport, RI, verified, near perfectly, these replicated, shorter, trace wavelengths.
指向性本质上是声纳阵列波束宽度的一种度量，它可在球对称各向同性的环境噪声场中获得；较窄的阵列主波束宽度比更宽的主波束宽度具有更高的指向性。对于常见的声纳系统，指向性因子（或指向性指数）与入射声迹波长与声纳阵列物理长度（总是受限）的比值成正比。通过为固定的阵列长度创造额外的声迹波长来增加这个比值，将提高阵列指向性。在阵列中嵌入周期性结构会使入射声平面波沿阵列表面产生布拉格散射。布拉格散射传播波以精确的方式发生频移，并产生原始声迹波长的更短波长复制品。这些复制的声迹波长（包含相同的信号到达信息）增加了阵列的波长与长度比，从而提高了指向性。因此，理论上一个较小的阵列可以具有与大得多的阵列相当的指向性。2015年1月在罗德岛纽波特海军水下作战中心声学测试设施完成的测量几乎完美地验证了这些复制的、更短的声迹波长。
