Sound waves in air differ from the transverse waves described in the previous chapter in that the wave particles oscillate in the same direction as the wave instead of at right angles to it. Waves of this type are called longitudinal waves. Owing to the longitudinal motion of the wave particles, sound waves consist of a series of compressions followed by rarefactions. Fig. 27.3 shows how a vibrating tuning-fork sends out a sound wave. When the prong moves to the right it compresses the air particles together. This disturbance is then transmitted from particle to particle through the air, with the result that a pulse of compression moves outwards. Similarly, a reverse movement of the prong gives rise to a pulse of rarefaction of the air.
It is important to note that the particle at the centre of a compression is moving through its rest position in the same direction as the wave, while the particle at the centre of a rarefaction is moving through its rest position in the opposite direction to the wave.
As in the case of a transverse wave, the distance between two successive particles in the same phase is called the wavelength, and the same wave equation applies, namely.