The existence and propagation of acoustic pressure pulses on lipid monolayers at the air-water interface are directly observed by simple mechanical detection. The pulses are excited by small amounts of solvents added to the monolayer. Controlling the state of the lipid interface, we show that the pulses propagate at velocities c following the lateral compressibility κ. This is manifested by a pronounced minimum in c (∼0.3m/s) within the transition regime. The role of interface density pulses in biology is discussed, in particular, in the context of communicating localized alterations in protein function (signaling) and nerve pulse propagation.