Loading Solutions...
Sound
Physics IX : Complete NCERT Exercise Solutions
Get complete NCERT Solutions for Class 9 Science Chapter 12: Sound. Learn about production & propagation of sound, sound waves, reflection, echo, ultrasound, range of hearing, and structure of the human ear with diagrams and solved exercises.
Question 12.1
How does the sound produced by a vibrating object reach your ear?
Answer & Explanation:
A vibrating object sets the surrounding medium (air) into vibrations, producing sound waves.
These waves travel through the medium as compressions and rarefactions and reach the ear, where they cause the eardrum to vibrate, producing the sensation of sound.
These waves travel through the medium as compressions and rarefactions and reach the ear, where they cause the eardrum to vibrate, producing the sensation of sound.
Question 12.2
Explain how sound is produced by your school bell.
Answer & Explanation:
When the school bell is struck, it begins to vibrate.
These vibrations set the surrounding air particles into motion, producing sound waves that travel to our ears.
These vibrations set the surrounding air particles into motion, producing sound waves that travel to our ears.
Question 12.3
Why are sound waves called mechanical waves?
Answer & Explanation:
Sound waves are called mechanical waves because they require a material medium for their propagation and cannot travel through vacuum.
Question 12.4
Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend? Give reason.
Answer & Explanation:
No, sound cannot be heard on the moon because there is no atmosphere.
Since sound waves need a material medium to travel, they cannot propagate in vacuum.
Since sound waves need a material medium to travel, they cannot propagate in vacuum.
Question 12.5
Which wave property determines (a) loudness, and (b) pitch of sound?
Answer & Explanation:
(a) Loudness depends on the amplitude of the sound wave.
(b) Pitch depends on the frequency of the sound wave.
(b) Pitch depends on the frequency of the sound wave.
Question 12.6
What are wavelength, frequency, time period, and amplitude of a sound wave?
Answer & Explanation:
• Wavelength (λ): Distance between two consecutive compressions or rarefactions.
• Frequency (f): Number of vibrations per second.
• Time period (T): Time taken for one complete vibration.
• Amplitude: Maximum displacement of particles from their mean position.
• Frequency (f): Number of vibrations per second.
• Time period (T): Time taken for one complete vibration.
• Amplitude: Maximum displacement of particles from their mean position.
Question 12.7
Write the formula to find the speed of sound. What is its SI unit?
Answer & Explanation:
The speed of sound is given by:
v = λf
SI unit of speed of sound is m s−1.
v = λf
SI unit of speed of sound is m s−1.
Question 12.8
What is the audible range of hearing for human beings?
Answer & Explanation:
The audible range of hearing for human beings is from 20 Hz to 20,000 Hz.
Question 12.9
What are infrasonic and ultrasonic sounds?
Answer & Explanation:
Infrasonic sounds: Sounds having frequency less than 20 Hz.
Ultrasonic sounds: Sounds having frequency greater than 20,000 Hz.
Ultrasonic sounds: Sounds having frequency greater than 20,000 Hz.
Question 12.10
Explain how bats use ultrasound to catch their prey.
Answer & Explanation:
Bats emit ultrasonic sound waves which get reflected from obstacles or prey.
By interpreting the reflected echoes, bats can determine the distance, direction, and size of their prey, a process known as echolocation.
By interpreting the reflected echoes, bats can determine the distance, direction, and size of their prey, a process known as echolocation.
📘 Exam Preparation Tip:
Understand the difference between transverse and longitudinal waves (sound is longitudinal). Learn the formulas for speed, frequency, wavelength, and time period of a wave. Focus on applications of ultrasound and the concept of echo.