In a series of experiments over flat plate honeycomb seal surfaces with deep cavities, Ha and Childs  have reported, under certain conditions, sudden jumps in the friction factor, which possibly could seriously impair honeycomb seal rotordynamic performance. This phenomenon was never fully explained before. In this study, the results have been investigated both analytically and computationally in the light of the Strouhal number dependency, and it was found that the friction factor jump phenomenon is nearly uniquely correlated with the Strouhal number of the flow. This lends strong credence to the hypothesis that the friction factor jump is due to a flow-acoustic interaction. Indeed, the measured friction factors were found to jump at the exact Strouhal Numbers where the energy transfer rate from the flow mode to the acoustic mode were at a maximum. CFD direct numerical simulations at the configurations of friction factor jump also demonstrate a lock-in between the acoustic and vortex shedding frequencies.