- Books Name
- Science Made Easy Science Book
- Science Made Easy
- CBSE Class 9
• 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.
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