Compression

If the gas is compressed, the same molecules occupy less space. They strike the walls more frequently, increasing the force per unit area and hence the pressure. This explains why pressure rises during compression, provided the temperature is kept constant.

Rock particles oscillate parallel to the wave’s direction, producing compressions and rarefactions. P waves can travel through solids and liquids.

Compression or expansion depends on the pressures acting on the boundaries of the gas. If the pressure pushing inwards is greater than the pressure pushing outwards, the boundary moves inwards and the gas is compressed. If the outward pressure is greater, the boundary moves outwards and the gas expands.

It can be measured from crest to crest, trough to trough or compression to compression.