What is the term for a part of a longitudinal wave where the particle density is less than normal density?

The term that describes a part of a longitudinal wave where the particle density is less than normal density is "rarefaction." In the context of longitudinal waves, such as sound waves, the wave consists of alternating regions of compression and rarefaction. The compression is where particles are pressed together, leading to a higher density, while rarefaction corresponds to the regions where particles are spread apart, resulting in a lower density than the average.

Understanding rarefaction is crucial as it directly relates to how sound waves propagate through different mediums. When a sound wave travels, it creates areas of varying pressure, with rarefaction being essential for the transmission of the wave energy. This occurs because the energy from a vibrating source causes particles in the medium to oscillate, forming these alternating high and low-pressure regions that travel through the medium.