# Echo

Show/Hide Sub-topics (Sound | O Level)

Sound waves can be reflected by large, hard surfaces like buildings, walls and cliffs. Reflection of sound occurs just like the reflection of light.

Echo is a distinct, reflected sound wave from a surface.

• A reflected sound can be heard separately from the original sound if the sound source is closer to the receiver while the reflecting hard surface is sufficiently far from receiver. Such reflected sound is called an echo.
• Generally the reflected sound is not distinctly heard, as it follows so closely behind the original sound and prolongs the sensation of the original sound. This effect is called reverberation.
• If the surface is rough, the incident sound waves are broken up and the original waveform is lost, thus no reflected sounds are heard. To reduce the effects of echo, walls can be roughened or “softened” (with padding) or covered with curtains and floors covered with carpets.
• Principle of echo is used in echo sounder to find the depth of a sea or the location of shoals of fish. Echoes can be used to measure the speed of sound.

Note: Remember that the distance travelled by the sound is doubled for echo. (The sound “go there and come back”) For instance, if a sound wave takes 10 seconds to travel to the bottom of the sea and back, the total distance travelled is 2d, where d is the depth of the sea.

Hence, the velocity of the sound for echoes can be calculated by:

\begin{aligned} v &= \frac{\text{Total distance travelled by sound}}{\text{Time taken}} \\ &= \frac{2d}{t} \end{aligned}