Abstract: An acoustical logging apparatus which has a means for interfering with those unwanted signals which inherently travel in the mud between the transmitter and the closest receiver of said apparatus during a logging operation. The signal interference means comprises a rotating sleeve having signal attenuation elements mounted thereon which are adapted to extend to the wall of the borehole to act as a centralizer when the apparatus is in the borehole. These elements are vertically stacked in an overlapped relationship so as to present an obstructed path through the mud for said signals but so as to offer only nominal resistance to displacement of mud as the apparatus is moved therethrough.