If you press on a gas it will compress,
but gas pressure will fight you and eventually reverse the compression.
It will overshoot
its original size and rarefy
the medium until pressure from the surrounding gas reverses
the motion again. The result is an oscillating
sequence of compressions and rarefactions.
By analogy to the process in air where a travelling
compressional wave is perceived as sound, we call these oscillations in
the photon-baryon fluid sound waves
or acoustic oscillations.
