Question

what is the most dense part of a longitudinal wave

Answer 1

In a longitudinal wave, the most dense part is referred to as a "compression." Longitudinal waves consist of regions where particles of the medium are close together (compressions) and regions where the particles are spread apart (rarefactions). In the compression areas, the density of the medium is higher because the particles are compressed closer together. Conversely, in the rarefaction areas, the density is lower because the particles are more spread out.