  We report on a new type of localized structure, an ultrasonic cavity soliton,
supported by large aspect-ratio acoustic resonators containing viscous media.
The spatio-temporal dynamics of this system is analyzed on the basis of a
generalized Swift-Hohenberg equation, derived from the microscopic equations
under conditions close to nascent bistability. These states of the acoustic and
thermal fields are robust structures, existing whenever a spatially uniform
solution and a periodic pattern coexist. An analytical solution for the
ultrasonic cavity soliton is also presented.
