• The repetition of sound caused by the reflection of sound waves is called an echo.

→ We can hear echo when there is a time gap of 0.1 second in original sound and echo (reflection of


→ Echo is produced when sound reflected from a hard surface (i.e. brick wall, mountain etc.) as

surface tends to absorb sound.

Minimum distance to hear an echo

Speed = Distance/Time

Here, Speed of sound in air = 344 ms -1 at 22ºC

Time = 0.1 second

344 = Distance/0.1 sec

⇒ Distance = 344 × 0.1 = 34.4 m

So, distance between reflecting surface and audience = 34.4/2 = 17.2 (at 22ºC).


• The persistence of sound in a big hall due to repeated reflection of sound from the walls, ceiling

and floor of the hall is called reverberation.

→ If reverberation is too long, sound becomes blurred, distorted and confusing due to overlapping

of different sound.

Methods to reduce reverberation in big halls or auditoriums

→ Panels made of felt or compressed fibre board are put on walls and ceiling to absorb sound

→ Heavy curtains are put on doors and windows.

→ Carpets are put on the floor.

→ Seats are made of material having sound absorbing properties.