Study Materials

# NCERT Solutions for Class 9th Science

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## Chapter 12. Sound

### LESSON – 12

Que. – How is sound Produced?
Ans. – Sound is produced by Vibrating objects.
Que. – What is Medium?
Ans. – The matter or substance through which sound is transmitted is called a medium?
Que. – In Which Medium Sound transmits fast?
(a)   solid   (b)   liquid      (c)   gas
Ans. – Solid.
Que. – Explain how sound is produced by your school bell?
Ans. – school bell is made of metal. When a wooden hammer is stroked on it the metal produces vibrations, which causes sound.
Que. – Why are sound waves called the mechanical waves?
Ans. – for propagation of sound it need a material medium like air, water and steel etc. Sound can be visualized as a wave. It is characterized by physical motion of particles in the medium. When a sound passes through a medium, the medium shows its physical motion and vibrates. So the sound waves are called the mechanical waves.
Que. – Write the characteristics of a sound wave.
Ans. – sound wave is described by its
1. Frequency – The number of oscillations completed by sound wave in per unit time is known as frequency of the sound wave. S.I Unit of frequency is Hertz (Hz)
2. Amplitude – The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. It is usually denoted by the letter A.
3. Speed – The speed of sound is defined as the distance which a point on a wave, such as a compression or rarefaction, travels per unit time.
Que. – How does the sound produced by a vibrating object in a medium reach to our ears?

Ans. - When an object vibrates, it sets the particles of the medium around it vibrating. The particles do not travel all the way from the vibrating object to the ear. The particles in the medium in contact with the vibrating object displace from its equilibrium position. It then exerts force on the adjacent particles. After displacing the adjacent particle the first particle of medium comes back in its original position. This process continues in the medium till the sound reaches your ear.
Que. – What is compression?
Ans. – When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure, this region is called a compression.
Que. – What is rarefaction?
Ans. – When a vibrating object moves backwards. It creates a region of low pressure is called rarefaction.
Que. – What are called the regions of high pressure and low pressure in sound wave?
Ans. - The region of high pressure is called compression and the region of low pressure is called rarefaction.
Que. – In which form sound propagates in any medium?
Ans. – Sound propagates in the medium as a series of compressions and rarefactions.
Que. – Sound waves are longitudinal waves. Why?
Ans. – The individual particles of the medium moves in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another but they simply oscillate back and forth their position of rest. This is exactly how a sound wave propagates; hence sound waves are longitudinal waves.
Que. – What is transverse wave?
Ans. – The wave particles do not oscillate along the line of wave propagation but oscillate up and down about their mean position as the wave travels, such the wave is called transverse wave.
Que. – Which wave property determines (a) loudness (b) pitch?
Ans. – The loudness or softness of a sound is determined basically by its amplitude. The amplitude of the sound wave depends upon the force with which an object is made to vibrate.
While, the pitch of sound is determined by its frequency. If the frequency is increased the pitch also raises.
Que. – Write the name of most common medium through which sound travels.
Ans. – Air.
Que. – Write the difference between soft and louder sound.
Ans. – Soft sound has small amplitude and louder sound has large amplitude. Soft sound is produced by less energy while louder sound is produced by more energy.
Que. – What is called a sound of single frequency?
Ans. – Tone
Que. – What is called a sound of a mixture of several frequencies?
Ans. – Note
Que. – Define time period.
Ans – The time taken by a pendulum to complete one oscillation is called its time period.
Que. – What is the wavelength?
Ans. – The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength.
The wavelength is denoted by Greece word (λ)
Speed of sound = wavelength × frequency
Que. – What is the pitch of sound.
Ans. – How the brain interprets the frequency of an emitted sound is called the pitch.
Que. – Write the different between transverse wave and longitudinal wave.
Ans. – The different between transverse wave and longitudinal wave.
transverse wave
1. Particles of the medium moves in a direction parallel to the direction of propagation of the disturbance.
2. It forms compression and rarefaction.
3. Example – light wave.
longitudinal wave
1. Particles do not oscillate along the line of wave propagation but oscillate up and down about their mean position as the wave travels
2. It forms crest and trough.
3. Example – sound wave.

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