电话中稍加一点背景噪音，我们的耳朵就听不到电话中的回声。
A bit of background noise on the phone enables our ears to filter out echoes.
在船体外壳上敲打发出的声音会从海底反射。通过测量敲打与接受回声的间隔时间，这一点的海底深度就可计算出来。
A sound made by tapping on the hull of a shop will be reflected from the sea bottom, and depth of the sea at that point can be calculated.
类似球形的囊状回声，囊内充满细小光点。
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