Periodic Sound Waves

[Gear300]

Apparently, by what I read, the pressure intervals are 90 degrees out of phase with the displacement intervals for a sound wave. How is that so? When the medium is compressed, shouldn't the pressure be at a maximum and when at rarefactions, shouldn't the pressure be at a minimum?

[Gear300]

bump...
but seriously...if a particle of the wave was displaced inward and outward...its maximum displacement should be where the pressure is highest and lowest, shouldn't it?...why are the displacement and pressure equations 90 degrees out of phase?