A remote-control unit (10) is adapted for sending a control signal with instructions as an acoustic signal for controlling a hearing assistive device (20). A calibration method comprises outputting tone frequencies applied by the control signal, receiving the output tone frequencies and compare tone frequency levels relatively to a reference, adjusting relative volumes of the tone frequencies applied by the control signal in dependence of the comparison, and storing the adjusted relative volumes of the tone frequencies for use when sending a control signal with instructions as acoustic signals.