It is well known that a pressure or stress derivative measurement of the sound velocity gives information about the nonlinear elastic properties of the material under investigation. Over the last several decades numerous experiments on various solids using uniaxial stress apparatus or oil-filled hydrostatic pressure chambers have been reported. In most cases the pressure or stress must be increased to several thousand kilobars to obtain reasonable measurements. The necessity of using such high pressure usually results from the low sensitivity of the devices or techniques used to measure the change in the sound velocity.