Wave Frequency in Piston: Pressure Variation & Adiabatic Compression

[WiFO215]

When you have a wave in sound inside a piston of length L, A.P. French says that the fundamenal frequency $$\nu$$ of an oscillation is given by

$$\nu$$ = 1/4L $$\sqrt{(\gamma$$ p / $$\rho)}$$

Where p is the pressure, $$\gamma$$ is the factor that accounts for adiabatic compression of the gas, and $$\rho$$ is it's density.

My question is this : doesn't the pressure p vary as you compress the gas? How can you assume it to be constant?

 

[WiFO215]

Hold up. I think I got it. That's not ANY pressure that you plug into that formula, it's the pressure when the gas is at equilibrium.