  Sound waves are excited on lipid monolayers using a set of planar electrodes
aligned in parallel with the excitable medium. By measuring the frequency
dependent change in the lateral pressure we are able to extract the sound
velocity for the entire monolayer phase diagram. We demonstrate that this
velocity can also be directly derived from the lipid monolayer compressibility
and consequently displays a minimum in the phase transition regime. This
minimum decreases from v0=170m/s for one component lipid monolayers down to
vm=50m/s for lipid mixtures. No significant attenuation can be detected
confirming an adiabatic phenomenon. Finally our data propose a relative lateral
density oscillation of \Delta\rho/\rho ~ 2% implying a change in all area
dependent physical properties. Order of magnitude estimates from static
couplings therefore predict propagating changes in surface potential of 1-50mV,
1 unit in pH (electrochemical potential) and 0.01{\deg}K in temperature and
fall within the same order of magnitude as physical changes measured during
nerve pulse propagation. These results therefore strongly support the idea of
propagating adiabatic sound waves along nerves as first thoroughly described by
Kaufmann in 1989 and recently by Heimburg and Jackson, but claimed by Wilke
already in 1912.
