A plurality of heavy mass irregularities attached to an inner wall of the drill
collar attenuate waves traveling through the collar. The plurality of heavy mass
irregularities are spaced and sized for the maximum attenuation of acoustic pulses
in a predetermined frequency range. The mass irregularities may be rings firmly
coupled to the outer surface of the collar. Alternatively, the mass irregularities
may be rings firmly coupled to the outer collar surface by neck pieces, extending
inwardly from the inner circumference of the ring. The mass irregularities may
be made of steel or tungsten. In another preferred embodiment, the mass irregularities
are asymmetrically coupled to an outer collar wall for providing preferential directional attenuation.
