• 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.


→ The word ‘SONAR’ stands for ‘Sound Navigation And Ranging’.

→ SONAR is a device which is used to find distance, direction and speed of underwater objects

→ SONAR consists of a transmitter and a receptor or detector and installed at the bottom of a sea

→ The transmitter produces and transmits ultrasonic waves.

→ These waves travel through water and after striking the objects on the bottom of sea, are

reflected back and received by detector.

→ These reflected waves are converted into electric signals by detector.

→ The sonar device measures the time taken by ultrasound waves to travel from ship to bottom of

sea and back to ship.

→ Half of this time gives the time taken by the ultrasound waves from ship to bottom.

• Let the time interval between transmission and reception of ultrasound signal is t.

Speed of sound through sea water is v

Total distance travelled by waves = 2d.

→ The sonar is used to find the depth of sea, to locate underwater hills, valleys, submarines, ice

and sunken ships etc.

→ Bats fly in the dark night by emitting high pitched ultrasound waves which are reflected from

obstacle or prey and returned to bats ear.

→ The nature of reflection tells the bat where the obstacle or prey is and what it is like.


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