A stationary source S generates circular outgoing waves on a lake. The wave speed is $6\mathrm{.}0$ m$/$s and the crest$-$to$-$crest distance is $2\mathrm{.}0$ m$.$ A person in a motorboat heads directly toward S at $4\mathrm{.}0$ m$/$s$.$ To this person, the frequency of these waves is