  We develop a theory of ultrasound decay rate $\alpha$ in a model of a
superconductor with a large pseudogap $\Delta_P$, and show that at low
temperatures ($T \ll T_c$) the magnitude of the decay rate $\alpha$ is
controlled by the ratio of $T/\Delta$, where $\Delta \ll \Delta_P$ is a
superconducting collective gap. Thus we propose new method to measure the
collective gap $\Delta$ in a situation when strong pseudogap is present.
