The nonlinear evolution of wavepackets in a laminar boundary layer has been observed experimentally. The packets originated from disturbances generated by a loudspeaker coupled to the boundary layer by a small hole in the plate. In a preliminary experiment two types of short-duration acoustic pulses were used, one with a positive excitation and another with a negative excitation. The experiments indicated that the packet that originated from a positive pulse displayed nonlinear behaviour at considerably lower amplitudes than that from a negative pulse. However, the preliminary experiments suggested that at some distance from the source the packets were identical in shape with a relative phase shift of 180°. Using complex-amplitude pulses it was possible to extend the experiments to include packets with other phases. This more comprehensive experiment not only showed a strong influence of the phase on the evolution of the packet, but also demonstrated that this nonlinear behaviour is not determined by the local effects of the excitation process. The observations suggested that the important parameter is the phase of the packet relative to the modulation envelope.