NCERT Class 9 Science Solutions: Chapter 12 – Sound Part 6 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 11:

Q. When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflected surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?


An echo is heard when the time interval between the original sound and the reflected sound is at least 0.1 s. The speed of sound in a medium increase with an increase in temperature. Hence, on a hotter day, the time interval between the original sound and the reflected sound will decrease. Therefore, an echo can be heard only if the time interval between the original sound and the reflected sound is greater than 0.1 s.

Question 12:

Q. Give two practical applications of reflection of sound waves.



  • Reflection of sound is used to measure the distance and speed of underwater objects. This method is known as SONAR.
  • Working of a stethoscope is also based on reflection of sound. In a stethoscope, the sound of the patient՚s heartbeat reaches the doctor՚s ear by multiple reflection of sound.

Question 13:

Q. A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, and speed of sound .


Height of the tower,

Velocity of sound

Acceleration due to gravity,

Initial velocity of the stone, (since the stone is initially at rest)

Time taken by the stone to fall to the base of the tower,

According to the second equation of motion:

Now, time taken by the sound to reach the top from the base of the tower,

Therefore, the splash is heard at the top after time, t


Question 14:

Q. A sound wave travels at a speed of . If its wavelength is what is the frequency of the wave? Will it be audible?


Speed of sound,

Wavelength of sound,

Speed of sound waveklength frequency

The frequency range of audible for humans lies between . Since the frequency of the given sound is more than 20,000 Hz it is not audible.

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