Studies of the elastic and non-elastic properties of uniaxially pre-stressed solid bodies can provide much very useful information about the internal physical properties. These properties have been under investigation by many scientists. Most of these studied the static modulus, though dynamic moduli can be measured much more accurately and can provide much more information. Attempts at using accurate ultrasonic methods (especially in the kilohertz frequency range, where it is necessary to use the resonance technique) under high stress usually are not successful, mostly because of two conflicting demands, both of which must be satisfied. The sample must be “acoustically free” ( that means, that it can vibrate freely, without any significant losses of energy through the suspension or supporting system ). At the same time the supporting system must be strong enough to transfer very high mechanical forces. These limitations are not so severe in the MHz-frequency range, where the pulse-interference method can be used, although still with many specific difficulties. The method and apparatus used in this frequency range, as well as the results obtained are not in the scope of this paper. One can find some of the results in reference , and more recent results are the subject of a later paper. As for frequency range 30-100 kHz, as far as the author knows, one of the first attempts of such measurements , was made by Whymark and Witting . They used specially designed specimens mounted to measure the displacement at the resonance, the mechanical Q-factor and the relative coupling factor of some piezomagnetic materials under high uniaxial pre-stress. A significant influence was found. Obviously, the high uniaxial stress somehow affect the internal structure of the material. Since the ultrasonic method is very accurate, it is excellent for studying this influence.