Your 'amplitude' of a longitudinal wave is basically the maximum
displacement of a particle from its equilibrium position.
In a longitudinal wave, if you measure the displacement of a
particle from its equilibrum as a function of time and you plot a
graph of its displacement versus time, you will get a transverse
graph of displacement vs time.
Hence the amplitude is thus the maximum displacement of a particle
in a longitudinal wave from its equilibrium position.
A bit hard to visualise, but these are the principles.
Cheers!
