An increment of the x-ray flux in crystal Bragg–Fresnel lens (BFL) focus in backscattering geometry obtained by means of acoustic excitation of the BFL crystal substrate has been investigated. The dependence of the x ray’s total reflected power versus ultrasound parameters has been studied in a low frequency range (10–50 MHz). The proposed technique allows an increase in the flux in a BFL focus by a factor of 2 which almost achieves the kinematic limit. ©1997 American Institute of Physics.