The bypass transition scenario was first observed experimentally by P. S. Klebanoff, during his experiments in elevated free-stream turbulence flow. Klebanoff identified an important aspect of the bypass transition. In an experiment using hot wires, he studied flow over a flat plate that was subjected to a 0.3% free-stream turbulence level. At this moderate free-stream turbulence level, he observed a velocity perturbation signal with a frequency under 12Hz, much smaller than the usual Tollmien-Schlichting wave frequency. He also observed fluctuations in boundary layer thickness, which does not occur in low-turbulence free-stream flow.