An apparatus for confirming a frequency measurement of a received signal includes a discrete Fourier transform module for receiving a sampled digital signal. The present invention involves using bandpass sampling to track high-frequency tissue displacements. The piston has a striking face at an end thereof proximate the bit assembly, and the piston is operable to impact on a contact surface of the bit assembly in an impact region of the casing. Methods and apparatus are presented herein to enhance the acoustic quality of a wideband-to-narrowband converted signal.