The amplitude of a compressional wave, like a sound wave, is  determined by the concentration of molecules in each compression.  The higher the amplitude, or energy that a w … ave carries, the more  compact the molecules are in a compression.The lower the  amplitude, the more spread out those particles are. the amplitude  of a compressional wave,like a sound wave,is determined by the  contretration of waves in a compression.The higher the amplitude, or energy that a w … ave carries, the more  compact the molecules are in a compression. The lower the  amplitude, the more spread out those particles are. the amplitude  of a compressional wave,like a sound wave,is determined by the  contretration of waves in a compression.