Anomalies observed in electromagnetic generation of ultrasound in aluminum are explained on the basis of known Fermi-surface geometry. At large values of ${\ensuremath{\omega}}_{c} \ensuremath{\tau}$ the topology of the second-zone hole sheet permits only holelike cyclotron orbits, while for small values of ${\ensuremath{\omega}}_{c} \ensuremath{\tau}$ the local curvature of the hole sheet is mostly electronlike. If follows that the effective number of electronlike and holelike carriers must be allowed to vary with magnetic field. These features, together with the Bragg-reflection force (caused by accelerating holes), are incorporated into the theory of electromagnetic generation. The observed anomalies are explained without using any adjustable parameter.