echo signal

例句：

1. 1. The echo signal from the mountain was so faint that it was almost impossible to hear.
   山的回声信号非常微弱，几乎无法听见。
2. 2. The police used a special device to create an echo signal for the search and rescue team.
   警方使用一种特殊设备为搜救队创造了回声信号。
3. 3. The scientist was able to detect the echo signal of the distant earthquake through his equipment.
   科学家通过他的设备能够探测到远处地震的回声信号。