If one derives the transfer characteristics for an exponential horn with Websters Equation then it predicts that below a certain frequency - based on flare rate - there is no transmission of sound, i.e. its "cutoff frequency". But experience, and logic, tells us that this is not the case. Again, in theory, Websters EQ does not predict a high frequency limit, although we all know that one exists.There is a lot to waveguide design, more than simply predicting (inacurately) its axial response.