Question

With the help of a diagram describe how compression and rarefaction pulses are produced in air near a source of sound.

Answer 1

• Consider a vibrating membrane of a musical instrument like a drum or tabla. 
• As it moves back and forth, it produces a sound. 
• The figure shows the membrane at different instants and the condition of the air near it at those instants.
• As the membrane moves forward, it pushes the particles of air in the layer in front of it.
• So, the particle of air in the layer get closer to each other, hence the density increases.
• This layer of air pushes and compresses the layer next to it and so on. 
• We call this disturbance as compression pulse. 
• When the membrane moves backward, it drags back the layer of air near it. Hence the density decreases. 
• The particles of air in the next layer on the right move into fill this less dense area. 
• This is a rarefaction pulse moves to right. As the membrane moves back and forth repeatedly, compression and rarefaction pulses are produced, one after the other. 
• These two pulses travel one behind the other, carrying the disturbance with it.