Sabra Karim G, Dowling David R
Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2121, USA.
J Acoust Soc Am. 2003 Sep;114(3):1395-405. doi: 10.1121/1.1604124.
Acoustic time reversal exploits reciprocity between sources and receivers to generate backward propagating waves that automatically focus at their point of origin. In underwater acoustics, an array of transducers that can both transmit and receive, referred to as a time reversing array (TRA) or time reversal mirror (TRM), generates the back-propagating waves. Such arrays have been shown to spatially and temporally focus sound in unknown complicated multipath environments, and are therefore of interest for active sonar and underwater communication applications. Although stationary vertical linear TRAs have been favored in prior studies, practical applications of acoustic time reversal in underwater environments are likely to involve towed, tilted, horizontal, or bottom-mounted arrays. In particular, array motion introduces Doppler effects and eliminates source-receiver reciprocity, two factors that potentially impact the automatic focusing capability of TRAs. This paper presents the results from a theoretical and computational investigation into how array motion and orientation influence TRA retrofocusing in the shallow ocean. Here, the TRA tow speed is assumed constant, and the array is assumed to be straight and linear (vertical, horizontal, or tilted). And, for simplicity, the TRA is assumed to respond to a stationary point source emitting a broadband pulse. When a TRA moves, the retrofocus is predicted to shift in the direction of array motion due to the translation of the array between its reception and broadcast times. In addition, the performance of a towed horizontal TRA is predicted to degrade more rapidly with towing speed than that of an equivalent (but clearly idealized) towed vertical array because of range-dependent Doppler phase differences that do not influence the vertical array. However, short tilted arrays may approach vertical array performance and appear to be a potentially versatile compromise for implementing TRA concepts in active sonar or underwater communication systems.
声学时间反转利用源与接收器之间的互易性来产生向后传播的波，这些波会自动聚焦于其原点。在水下声学中，由既可以发射又可以接收的换能器阵列（称为时间反转阵列（TRA）或时间反转镜（TRM））来产生向后传播的波。已证明此类阵列能够在未知的复杂多径环境中在空间和时间上聚焦声音，因此在有源声纳和水下通信应用中备受关注。尽管在先前的研究中固定的垂直线性TRA受到青睐，但声学时间反转在水下环境中的实际应用可能会涉及拖曳式、倾斜式、水平式或底部安装的阵列。特别是，阵列运动会引入多普勒效应并消除源 - 接收器互易性，这两个因素可能会影响TRA的自动聚焦能力。本文展示了一项理论和计算研究的结果，该研究探讨了阵列运动和方向如何影响浅海中TRA的反向聚焦。在此，假设TRA的拖曳速度恒定，并且阵列是直的且为线性（垂直、水平或倾斜）。并且，为了简单起见，假设TRA对发射宽带脉冲的固定点源做出响应。当TRA移动时，由于阵列在接收和广播时间之间的平移，预计反向焦点会在阵列运动方向上发生偏移。此外，由于与距离相关的多普勒相位差异不会影响垂直阵列，预计拖曳式水平TRA的性能随拖曳速度下降的速度比等效的（但明显理想化的）拖曳式垂直阵列更快。然而，短倾斜阵列可能接近垂直阵列的性能，并且似乎是在有源声纳或水下通信系统中实现TRA概念的一种潜在通用折衷方案。
