1. A method of determining a slowness of a propagating wave in an earth formation; the method comprising: (a) using a logging tool in a borehole to generate the wave and acquiring a plurality of signals indicative of the propagating wave; (b) determining a semblance-frequency coherence of the plurality of signals; (c) determining a histogram of a slowness distribution from the semblance-frequency coherence; (d) using an edge of the histogram to determine the slowness of the propagating wave; and (e) recording the determined slowness on a suitable medium.