  Several experimental candidates for quantum spin liquids have been discovered
in the past few years which appear to support gapless fermionic $S = {1\over
2}$ excitations called spinons. The spinons may form a Fermi sea coupled to a
$U(1)$ gauge field, and may undergo a pairing instability. We show that despite
being charge neutral, the spinons couple to phonons in exactly the same way
that electrons do in the long wavelength limit. Therefore we can use sound
attenuation to measure the spinon mass and lifetime. Furthermore, transverse
ultrasonic attenuation is a direct probe of the onset of pairing because the
Meissner effect of the gauge field causes a "rapid fall" of the attenuation at
$T_c$ in addition to the reduction due to the opening of the energy gap. This
phenomenon, well known in clean superconductors, may reveal the existence of
the U(1) gauge field.
