# Sound

## 06 Echo

• Echo is the repetition of sound caused by the reflection of sound waves reflection from the surface of a large obstacle.
• An echo is simply a reflected sound.
• Echo creates a persistence of sound even after the source of sound has stopped vibrating.
• Applications of Echo:
• Echo is used by bats and dolphins to detect obstacles or navigate.
• SONAR and hearing aids also use this principle.

#### CALCULATION OF MINIMUM DISTANCE TO HEAR AN ECHO

• We can hear the original sound and the reflected sound (echo) separately only if there is a time-interval (or time gap) of at least
$\frac{1}{10}$

th of a second.

• The minimum distance of the obstacle from the source of sound to a sound reflecting surface (wall, etc.) to hear an echo is 17.2 metres (at 20oC).

#### HEARING AID

The sound waves are received by the hearing aid and are reflected into a narrower area leading to the ear.

#### SONAR (Sound Navigation And Ranging)

• Sonar is a device used in oceanographic studies, that uses ultrasonic waves to measure the distance, direction and speed of underwater objects like shoals of fish, shipwrecks, and enemy submarines.
• The sonar device measures the time taken by the ultrasonic sound pulse to travel from the ship to the bottom of the sea, and back to the ship.
• Half of this time gives the time taken by the ultrasonic sound to travel from the ship to the bottom of the sea.
• A sonar apparatus consists of two parts:
• (i) A transmitter (for emitting ultrasonic waves), and
• (ii) A receiver (for detecting ultrasonic waves).