$(a)$ How does sound propagate through a medium?
$(b)$ Show a sound wave in graphic form and mention the crest,trough,wavelength,and amplitude of the wave in it.
$(c)$ Why are sound waves called longitudinal waves?

(N/A) When a sound wave is produced in a medium,it causes the particles of the medium to vibrate. Consider,for example,the vibration of the diaphragm of a speaker. As the diaphragm vibrates outwards,it creates a region of high pressure called a compression,which travels through the medium. When the diaphragm moves inwards,it creates a region of low pressure called a rarefaction. Thus,sound travels through the medium in the form of a series of compressions and rarefactions.
$(b)$ The graphical representation of a sound wave is shown below:
(See the provided image for the graph showing density/pressure vs distance,with crest,trough,wavelength $\lambda$,and amplitude $A$ marked.)
$(c)$ Sound waves are called longitudinal waves because the particles of the medium vibrate parallel to the direction of the propagation of the wave.