  We have made precision measurements of the frequency of a collective mode of
the superfluid 3He-B order parameter, the J=2- imaginary squashing mode.
Measurements were performed at multiple pressures using interference of
transverse sound in an acoustic cavity. Transverse waves propagate in the
vicinity of this order parameter mode owing to off-resonant coupling. At the
crossing of the sound mode and the order parameter mode, the sound wave is
strongly attenuated. We use both velocity and attenuation measurements to
determine precise values of the mode frequency with a resolution between 0.1%
and 0.25%.
