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在水中进行回声形成实验时，目标常可分为软、硬两类，它们呈现某些显著不同的特性［１］［２］。软目标可用含有大量闭合小泡的多孔材料制成，其声阻抗接近于空气，与水的ＰＣ值相比往往很小，因而它的表面常可近似地当作软界面来处理。硬目标常用金属制成，它们的ＰＣ值比水的ＰＣ值大３０倍左右。从水中入射到金属表面的声波有一部分会透入金属内部，因而把这种材料当成绝对硬介质处理时，会产生较大的误差。若金属目标吴扁平状，其回声特性与用绝对硬模型所导出的理论结果相差更大。本文介绍在利用扁平金属目标进行回声形成实验所观察到的异常现象并对此作出解释。
In echo-forming experiments in water, the targets can often be divided into two types, soft and hard, and they present some significantly different characteristics [1] [2]. Soft targets are made of a porous material containing a large number of closed vesicles whose acoustic impedance is close to that of air and is often small compared to the PC value of water so that its surface can often be treated approximately as a soft interface. Hard targets made of metal, their PC value than water, about 30 times the PC value. Some of the sound waves incident on the metal surface from the water penetrate into the interior of the metal, which results in large errors when treating this material as an absolute hard medium. If the metal target Wu flat, its echo characteristics and absolute hard model derived theoretical results differ more. This article presents and explains the anomalies observed in echogenic experiments using flat metal targets.