What is reflection of sound?
When a sound wave travelling through the air meets a hard, smooth surface such as a wall or a cliff, it bounces back. This bouncing back is called the reflection of sound. A reflected sound that you hear a moment after the original sound is called an echo.
Hard, smooth and flat surfaces reflect sound well. Soft, rough surfaces such as curtains, carpets and foam absorb sound instead of reflecting it, which is why recording studios are lined with soft material to reduce echoes.
Key idea
The distance to a reflecting surface can be found from the echo using:
distance = (speed of sound × time) ÷ 2
We divide by 2 because the sound travels to the surface and back again.
Using echoes
Echoes are very useful. Ships use sonar to find the depth of the sea: they send a sound pulse downward and measure the time for the echo to return from the sea bed. Bats and dolphins use reflected sound (echolocation) to find food and avoid obstacles in the dark.
Example
A ship sends a sound pulse to the sea bed and the echo returns after 4 s. If the speed of sound in water is 1500 m/s, the depth = (1500 × 4) ÷ 2 = 3000 m.
Reducing unwanted echoes
In large halls, too many echoes make speech unclear. Engineers reduce this by using soft seats, curtains and special panels on the walls and ceiling that absorb sound. This is part of the study of acoustics.
Remember
- An echo is a reflected sound.
- Hard, smooth surfaces reflect sound; soft, rough surfaces absorb it.
- distance = (speed × time) ÷ 2.