Recall that in a longitudinal wave, the individual particles in the medium oscillate in the direction parallel to wave propagation. These “parallel” oscillations change the spacing between the medium particles, causing regions of higher density (compression) and lower density (rarefaction) in the medium to propagate through the medium.
A longitudinal wave that is familiar to all of us, is sound. When we hear silence, it is because the air pressure outside our ear drum is constant, at 1 atm.
When the diaphragm of a loudspeaker vibrates, it causes the air pressure in the immediate vicinity to alternate between high and low pressure, above and below 1 atm. This disturbance is then propagated away from the loudspeaker. How come?
It may not look like it, but the air around us, consisting of air molecules, is actually “springy” and acts like an elastic medium. The pressure fluctuations at the loudspeaker will be relayed by the air molecules in the air, eventually arriving at our ear drum.
Our ear drum reacts to the pressure fluctuations, and our brain interprets that as sound. The amplitude of the pressure fluctuations determines the loudness, and the frequency the pitch.