Final answer:
In a longitudinal wave, the regions where the medium's particles are less crowded and further apart are called rarefactions. The wavelength is the distance between two consecutive rarefactions or compressions. Compressions are analogous to peaks and rarefactions to troughs of transverse waves.
Step-by-step explanation:
The part of a longitudinal wave where particles of the medium are further apart (less crowded) is known as a rarefaction. In contrast, areas where the particles are closest together are called compressions. The wavelength of a longitudinal wave is the distance between two consecutive compressions or two consecutive rarefactions. It is important to note that in a longitudinal wave, particles move parallel to the wave's direction of propagation. A good analogy would be to compare compressions and rarefactions in longitudinal waves to peaks and troughs in transverse waves. As sound waves move away from their source, their amplitude decreases, which is a result of both the spreading out of the energy and the absorption of some of the energy by objects and its conversion to thermal energy.