  The results of the comprehensive ultrasonic research of high quality single
crystals of FeSe are presented. Absolute values of sound velocities and their
temperature dependences were measured; elastic constants and Debye temperature
were calculated. The elastic C11-C12 and C11 constants undergo significant
softening under the structural tetra-ortho transformation. The significant
influence of the superconducting transition on the velocity and attenuation of
sound was revealed and the value of the superconducting energy gap was
estimated.
