水声无源材料回声降低测量时反聚焦方法

Measurement of the echo reduction for underwater acoustic passive materials with a time reversal technique

提出了利用时间反转（时反）聚焦技术的水声无源材料回声降低测量方法。首先实现有界水声环境下有、无试样接收信号的时反聚焦，然后利用聚焦信号测量试样反射系数，最后通过标准试样对反射系数进行修正，从而得到试样的回声降低测量值。在小型水池中进行了铝板试样和钢板试样回声降低的测量，铝板试样的尺寸为1．1m×1．0×0．005m，测量频率范围为3—20kHz，钢板试样尺寸与铝板试样相同，测量频率范围为0．5—20kHz，测量结果经修正后与平面波理论计算值基本一致，和理论计算值的相对误差小于10％，扩展不确定度小于1．5dB。本方法采用时反原理实现了接收信号的空时聚焦，提高了测量信混比，因此适用于非自由场环境下无源材料回声降低的测量试验，尤其适用于低频的回声降低测量。

A method of measuring the echo reduction of passive materials with the time reversal （TR） technique is presented. To measure the echo reduction with this approach, the received signals are firstly focused according to the TR theory with a sample. Then, the sample is removed and the TR processing is again employed to realize the focus of the received signal. Finally, the echo reduction of the sample is evaluated with these focusing signMs. Besides, to calibrate the echo reduction measured via the TR technique, a standard sample is employed to measure a constant coefficient that only depends on the measurement environment. An aluminum plate sample and a steel plate sample with the same size of 1.1 m×l.0 m×0.005 m are tested in a wave guide tank. The experimental results show that the calibrated values are well consistent with theoretical results of the free field for the measured frequency range of 0.5-20 kHz. The relative errors of all the measured values are less than 10% and the values of the expanded uncertainty are less than 1.5 dE. The TR processing focuses the energy in spatial domain and temporal domain, so it can be used to measure the echo reduction of passive materials in the environments with reflections induced by boundaries or low frequency sources.