direct transmitted echo 直达波
Echo Sounder is a kind of acoustic measuring depth equipment, which can measure depth with the time between transmitted wave and echo on condition in which the speed of ultrasonic is fixed.
回声测深仪是用来测量水深的一种声学测量设备，其以水中超声波的传播速度为确定条件，利用发射波与反射回波时间差来测量水深。
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