NCERT Book Class 9 Science Sound

SOUND

Everyday we hear sounds from various sources like humans, birds, bells, machines, vehicles, televisions, radios etc. Sound is a form of energy which produces a sensation of hearing in our ears. There are also other forms of energy like mechanical energy, heat energy, light energy etc. We have talked about mechanical energy in the previous chapters.

You have been taught about conservation of energy, which states that we can neither create nor destroy energy. We can just change it from one form to another. When you clap, a sound is produced. Can you produce sound without utilising your energy? Which form of energy did you use to produce sound? In this chapter we are going to learn how sound is produced and how it is transmitted through a medium and received by our ear.

12.1 Production of Sound

Activity _____________12.1

• Take a tuning fork and set it vibrating by striking its prong on a rubber pad. Bring it near your ear.

• Do you hear any sound?

• Touch one of the prongs of the vibrating tuning fork with your finger and share your experience with your friends. 

• Now, suspend a table tennis ball or a small plastic ball by a thread from a support [Take a big needle and a thread, put a knot at one end of the thread, and then with the help of the needle pass the thread through the.Touch the ball gently with the prong of a vibrating tuning fork (Fig. 12.1).

• Observe what happens and discuss with your friends.

Activity _____________ 12.2

• Fill water in a beaker or a glass up to the brim. Gently touch the water surface with one of the prongs of the vibrating tuning fork, as shown in Fig. 12.2.

• Next dip the prongs of the vibrating tuning fork in water, as shown in Fig. 12.3.

• Observe what happens in both the cases.

• Discuss with your friends why this happens.

Fig. 12.1: Vibrating tuning fork just touching the From the above activities what do you conclude? Can you produce sound without a vibrating object? In the above activities we have produced sound by striking the tuning fork. We can also produce sound by plucking, scratching, rubbing, blowing or shaking different objects. As per the above activities what do we do to the objects? We set the objects vibrating and produce sound. Vibration means a kind of rapid to and fro motion of an object. The sound of the human voice is produced due to vibrations in the vocal cords. When a bird flaps its wings, do you hear any sound? Think how the buzzing sound accompanying a bee is produced. A stretched rubber band when plucked vibrates and produces sound. If you have never done this, then do it and observe the vibration of the stretched rubber band

12.2 Propagation of Sound

Sound is produced by vibrating objects. The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas. Sound moves through a medium from the point of generation to the listener. 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. A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position. It then exerts a force on the adjacent particle. As a result of which the adjacent particle gets displaced from its position of rest. After displacing the adjacent particle the first particle comes back to its originalposition. This process continues in the medium till the sound reaches your ear. The disturbance created by a source of sound in Fig. 12.3: Both the prongs of the vibrating tuning fork dipped in water.

the medium travels through the medium and not the particles of the medium. A wave is a disturbance that moves through a medium when the particles of the medium set neighbouring particles into motion. They in turn produce similar motion in others. The particles of the medium do not move forward themselves, but the disturbance is carried forward. This is what happens during propagation of sound in a medium, hence sound can be visualised as a wave. Sound waves are characterised by the motion of particles in the medium and are called mechanical waves.

Please refer to attached file for NCERT Class 9 Science Sound