  The existence and propagation of acoustic pressure pulses on lipid monolayers
at the air/water-interfaces are directly observed by simple mechanical
detection. The pulses are excited by small amounts of solvents added to the
monolayer from the air phase. Employing a deliberate control of the lipid
interface compressibility k, we can show that the pulses propagate at
velocities, which are precisely reflecting the nonlinear behavior of the
interface. This is manifested by a pronounced minimum of the sound velocity in
the monolayer phase transition regime, while ranging up to 1.5 m/s at high
lateral pressures. Motivated by the ubiquitous presence of lipid interfaces in
biology, we propose the demonstrated sound propagation as an efficient and fast
way of communication and protein modulation along nerves, between cells and
biological units being controlled by the physical state of the interfaces.
