MP Board · Class 9 · Science · SoundWhy are sound waves called mechanical waves? Describe the Bell Jar experiment to show that sound requires a material medium for its propagation and cannot travel through a vacuum.

Sound waves are called mechanical waves because they require a material medium (such as a solid, liquid, or gas) for their propagation. Sound energy travels by vibrating the particles of the medium, creating regions of compression and rarefaction. In a vacuum, there are no particles present to vibrate and transmit these energy variations; hence, sound cannot travel through a vacuum.

Bell Jar Experiment:\nTo demonstrate this, an electric bell is suspended inside an airtight glass bell jar connected to a vacuum pump. Initially, when the jar is full of air and the switch is pressed, the sound of the bell is clearly audible. As the vacuum pump gradually extracts air from the jar, the sound of the ringing bell becomes fainter and fainter. Eventually, when almost all the air is pumped out, no sound can be heard, even though the hammer can still be seen striking the gong. This experiment conclusively proves that sound requires a material medium to propagate and cannot travel through a vacuum.