A hearing aid is provided that avoids disturbing acoustic effects caused by on, off, or switchover events. A magnetic recording medium which comprises a lower layer comprising a nonmagnetic powder and a binder and a magnetic layer comprising a ferromagnetic powder, an abrasive and a binder provided in this order on a nonmagnetic flexible support. When the second gain is less than a predesignated threshold, a match-corrected source signal is acquired and used with the first gain and the second gain to determine the distortion of the DUT.