NCERT Solutions for Class 9 Science Chapter 12 Sound

Ans : When an object vibrates, it forces the neighbouring particles of the medium to vibrate. These vibrating particles then force the particles adjacent to them to vibrate.
In this way, vibrations produced by an object are transferred from one particle to another till it reaches the ear. 

Ans : When the school bell vibrates, it forces the adjacent particles in air to vibrate. This disturbance gives rise to a wave and when the bell moves forward, it pushes the air in front of it. This creates a region of high pressures known as compression. When the bell moves backwards, it creates a region of low pressure know as rarefaction. As the bell continues to move forward and backward, it produces a series of compressions and rarefactions. This makes the sound of a bell propagate through air. 

Ans : Sound waves force the medium particles to vibrate. Hence, these waves are known as mechanical waves. Sound waves propagate through a medium because of the interaction of the particles present in that medium. 

Ans : Sound needs a medium to propagate. Since the moon is devoid of any atmosphere, you cannot hear any sound on the moon. 

Ans : (a) Amplitude (b) Frequency
(a) The loudness of a sound depends on its amplitude. If the amplitude of a sound is large, then the sound produced will also be loud.
(b) The pitch of a sound depends on its frequency. A sound will be considered a high pitched sound, if its frequency is high.