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Echo

An echo is a reflection of sound. A sound hits an object and is reflected back to its source.

If you stand inside a large empty room with smooth walls and begin talking you can produce an echo with your voice.

Echoes are used for measuring distances.

Ships use echoes to measure the depth of the ocean. The ship sends a sound wave down towards the sea bed. It is reflected back towards the ship and detected.

The time difference between sending the sound and receiving the echo is recorded.

The speed of sound in water is already known so the depth of the ocean is calculated using $$$\text{depth of ocean} = \frac{1}{2} \times \text{time difference} \times \text{speed of sound} $$$

A ship uses echo to calculate the depth of the ocean.
A ship uses echo to calculate the depth of the ocean.