In ferrimagnets, under a dc magnetic field, magnon-phonon coupling can result in such effects as acoustic birefringence and Faraday rotation. From our work on acoustic Faraday rotation in nonellipsoidal samples of YIG, we found three regions of rotation. The low field region has an internal magnetic field insufficient for effective phonon-magnon conversion. Fitting the rotation observed to the theory we find values for the second magnetoelastic constant and the average demagnetizing field. Between the low and middle field regions a decrease in the amplitude of the transmitted elastic wave was observed and attributed to spin-wave losses. The middle field region was characterized by conversion of elastic energy into spin waves. Regions of inefficient conversion were observed. Faraday rotation was again observed in the high field region.