Ultrasound Studies of the High T sub c Superconductors (La,Sr) CuO sub 4, YBa sub 2 CuO sub 7 and (Ba,Pb) BiO

We report measurements of the sound velocity in a wide variety of the high T sub c superconductors. They all show a pronounced hardening of the sound velocity below T sub c. The hardening is proportional to the square of the gap and is a result of (DELTA/E sub F) being much larger in these materials than in ordinary superconductors. Some of the samples show clearly the sharp drop in the sound velocity at T sub c which is required by thermodynamics and related to the pressure dependence of T sub c.