Class 10 > Unit # 11: Sound > Introduction to Sound and Bell Jar Experiment

Q.1 Define Sound. How is Sound Produced?

Definition of Sound: Sound is a form of energy that produces a sensation of hearing in our ears. Physically, sound waves are mechanical, longitudinal waves comprising alternating series of compressions and rarefactions.

Production of Sound by Vibrating Sources

Sound is produced by vibrating sources placed in a material medium. When an object vibrates (moves rapidly to and fro), it continuously disturbs the surrounding molecules of the medium:

• Compressions: As a vibrating body moves forward, it pushes and squeezes the air molecules in front of it, creating a high-pressure region of high density.
• Rarefactions: As the body moves backward, it stretches the air layer, creating a low-pressure region of low density.

These continuous squeezes and stretches propagate outward as longitudinal waves, where the direction of particle vibration is parallel to the direction of wave propagation (similar to longitudinal waves produced in a slinky spring).

Everyday Examples:

• Drum: Striking the stretched membrane (skin) of a drum causes it to vibrate up and down rapidly, generating sound waves in the air.
• Guitar: Plucking a guitar string sets it into rapid vibration, creating musical notes.
• Human Speech: Air expelled from the lungs forces our vocal cords to vibrate, producing human voice.

Q.2 Describe how sound cannot travel through a vacuum using the Electric Bell Jar Experiment.

Because sound waves are mechanical in nature, they rely on the vibratory motion of medium particles (solids, liquids, or gases) to transport energy from one point to another. Sound cannot propagate through a vacuum.

Electric Bell Jar Experiment

Apparatus:

An electric bell, an airtight glass bell jar, a rubber cork, a battery/power supply, and a vacuum pump connected via the base pipe.

Procedure & Observations:

1. Suspend the electric bell inside the airtight glass bell jar and seal it tightly with a cork.
2. Connect the bell to a battery circuit and switch it on. The hammer strikes the gong, and a loud, clear sound is heard outside through the air and glass.
3. Start the vacuum pump to gradually evacuate air from inside the jar.
4. Observation: As the quantity of air inside decreases, the sound becomes fainter and fainter, even though the hammer continues striking the gong with equal force.
5. When almost all the air is completely pumped out (creating a partial vacuum), the bell's sound becomes virtually inaudible to human ears outside.

Conclusion: When air is removed from the jar, there are no air molecules left to vibrate and transmit mechanical wave energy. This proves that sound waves require a material medium to propagate and cannot travel through a vacuum.