Sabra Karim G, Dowling David R
Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2133, USA.
J Acoust Soc Am. 2003 Dec;114(6 Pt 1):3125-35. doi: 10.1121/1.1625929.
Active acoustic time reversal may be accomplished by recording sounds with an array of transducers--a time-reversing array (TRA) or time-reversal mirror (TRM)--and then replaying the recorded and time-reversed sounds from the same array to produce back-propagating waves that converge at the location(s) of the remote sound source(s). Future active sonar and underwater communication systems suitable for use in unknown shallow ocean waters may be developed from the automatic spatial and temporal focusing properties of TRAs. However, ocean currents affect time reversal because they alter acoustic reciprocity in the environment. This paper presents a theoretical and computational investigation into how ocean currents influence TRA retrofocusing in shallow ocean environments for various array orientations. The case of TRA retrofocusing in a three-dimensional range-independent sound channel with a steady horizontal ocean current is covered here, based on a normal-mode propagation model valid for low Mach number currents. The main finding is that in the presence of ocean currents (typically <1 m/s), a TRA performs well (the associated retrofocus amplitude decay is less than 1 dB) except that a retrofocus shift (up to a few wavelengths at 500 Hz at a range of 2.5 km) may occur due to the differing interaction between the ocean current profile and each acoustic normal mode. In addition, TRA performance is predicted to depend on the array orientation relative to the ocean current direction, especially for horizontal arrays.
有源声学时间反转可以通过用换能器阵列（时间反转阵列（TRA）或时间反转镜（TRM））记录声音，然后从同一阵列重放记录并经过时间反转的声音来实现，以产生反向传播的波，这些波在远程声源的位置处汇聚。未来适用于未知浅海海域的有源声纳和水下通信系统可能会基于TRA的自动空间和时间聚焦特性而开发。然而，洋流会影响时间反转，因为它们会改变环境中的声学互易性。本文对洋流如何在各种阵列方向的浅海环境中影响TRA反向聚焦进行了理论和计算研究。这里基于对低马赫数洋流有效的简正波传播模型，涵盖了在具有稳定水平洋流的三维与距离无关的声信道中TRA反向聚焦的情况。主要发现是，在存在洋流（通常<1米/秒）的情况下，TRA性能良好（相关的反向聚焦幅度衰减小于1分贝），只是由于洋流剖面与每个声学简正波之间不同的相互作用，可能会出现反向聚焦偏移（在2.5千米的距离处，500赫兹时可达几个波长）。此外，预计TRA性能取决于阵列相对于洋流方向的取向，特别是对于水平阵列。
