NCERT Solutions for Class 9th Science Chapter 10 Chapter introduction — In-text Questions

Mechanical energy — specifically the kinetic energy of a vibrating object — is converted into sound energy.

• When you pluck a rubber band or a sitar string, your muscles do work on it. That work becomes the kinetic energy of the vibrating string, which is then handed on to the air as sound.
• In a loudspeaker the chain is different: electrical energy → mechanical (vibration of the diaphragm) → sound.
• In a firecracker it is chemical energy → heat → rapid expansion of gas → sound.

Sound is produced by a vibrating object and reaches the ear as a travelling disturbance in the density of a medium.

1. Production. A source (vocal cords, a tuning fork, a drum membrane, an air column in a bansuri) vibrates — moves to and fro about a mean position.
2. Propagation. Each forward push of the source squeezes the nearby air particles together and makes a compression (density above average). Each backward movement leaves a rarefaction (density below average). Particles collide with their neighbours and pass the squeeze forward, so the compression travels on while each particle only oscillates about its own mean position.
3. Detection. The arriving compressions and rarefactions push and release the eardrum, making it vibrate. Tiny bones amplify the vibration, the cochlea turns it into electrical signals, and the brain reads them as sound.