Effect of sound pressure on refractive index/density

Hello fellow Comsol users,

Most of my experience comes from structural mechanics and fracture and would like to ask a little help regarding pressure acoustics.

The presence of high and low pressure in air will change it's refractive index due to a change in the density. I would like to model this change in refractive index, but am a little lost.

Could anyone tell me how I could go about modelling/solving for a change in material property?

I am using a time dependant pressure model in air. Within the materials section I can see that the density is a function of temperature and pA [which I assume to be absolute pressure], but when I plot the change in density it says that there is no change in density even if there is a change in pressure.

I'm sorry if this question sounds a little dumb, I'm sure that I am just being stupid today.

Also, I have had a look at the forum and a little in the documentation but could not find anything to help.

Anyone have some ideas out there? I would greatly appreciate your help.

Rob