The wavelength of a wave is the length of one complete cycle of a wave. For a transverse wave, the wavelength is determined by measuring from crest to crest. A longitudinal wave does not have crest. In this case the wavelength can be determined by measuring the distance between any two corresponding points on adjacent waves. In the case of a longitudinal wave, a wavelength measurement is made by measuring the distance from

Apeak to peak.
Btrough to trough.
Cwavelength to wavelength.
Dcompression to compression.

To exemplify this type of wave, let us now consider a person speaking or a speaker emitting a certain sound. The sound (mechanical wave) of the person's voice propagates in space in all directions (for this reason, the sound wave can also be characterized as a three-dimensional wave), moving away from the source. The sound, being transmitted in the air, produces compressions and rarefactions. According to the sound sequence emitted by the person, we can have layers of air more compressed or less compressed, as it is represented in the figure as light regions and dark regions. The distance between a light and a dark region represents half a wavelength (λ / 2).

For a transverse wave, the wavelength is determined by measuring from crest to crest, however a longitudinal wave has no crest and its length is made by measuring the distance from compression to compression.