Longitudinal Wave Equation meaning and derivation

Hi,

I have recently been studying waves, and I understand the transversal wave formula

y=Asin(w(t-x/v))

which gives the y coordinate of a point at x along the x-axis in the instant t.

However, Wikipedia (http://en.wikipedia.org/wiki/Longitudinal_wave) gives this as the equation for longitudinal waves too.

If this is true, what does y represent? The displacement of a point relative to what? Also, does anyone know how to derive this formula specifically for longitudinal waves?

Thanks very much!