Ultrasonic attenuation in a pseudogapped superconductor

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