- 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.
→ 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.