A new apparatus for the measurement of velocity changes due to applied
mechanical uniaxial stress working from room temperature down to liqu
id nitrogen temperatures is described. The experimental data. obtained
with this apparatus allow to calculate nonlinear elastic stiffnesses.
Results on Potassium Dihydrogen Phosphate (KDP) are presented. No ano
maly of the nonlinear coefficient c(456) near the phase transition cou
ld be found contrary to the Landau theory but in agreement with former
measurements of the nonlinear piezomodul e(345).