What part of the compressional wave has the lowest density

a compressional wave move with sequence of compression and rarefaction. In compression, the particles of the wave come closer to each other in a particular region while in rarefaction, the particles move away from each other. density depends on the space between the particles in a fixed volume of region. Since rarefaction, number of particles are farther apart as compared to compression region, wave has lowest density in compression region

The downstream side of the wave has the lowest density.