  3aPA14. Greenspan acoustic viscometer: Recent results.  Keith
A. Gillis  Michael R. Moldover  NIST, Thermophys. Div.,
Gaithersburg, MD 20899  James B. Mehl  Univ. of Delaware,
Newark, DE 19716-2570  Acoustic resonators consisting of a duct
coupling two cavities (Greenspan viscometers) have a low-
frequency mode whose response is a sensitive function of the
viscous diffusivity of the enclosed gas. The response function
is determined experimentally by measuring the acoustic pressure
in one cavity as a function of the frequency of a source in the
other cavity. A theoretical response function, based on a model
which includes viscous and thermal boundary layer phenomena in
both the duct and the cavities, as well as inertial and
resistive duct-end corrections, is fit to the data to determine
the viscous diffusivity. Recent measurements in argon, helium,
and propane near room temperature span a range of viscous
diffusivity from 7x10[sup -7] to 2x10[sup -4] m/s[sup 2].
Experimental values determined with three diffe