Abstract: The apparatus comprises a base and two movable opposite centers, one of which is fastened to a rotatable axle, while the other is fastened to a screw by means of a bearing, and mounted in two opposite parallel plates. Between the plates there is disposed a tank filled with an acoustic medium, and underneath it there is mounted a sensor which is connected to a measuring system. Two parallel guides are transversely fastened to the plates. The base is suspended on the parallel guides with a collar fastened to it. The base is provided with a driving screw and a handle. A spring, within the collar and attached to it, is fastened rigidly to a guiding sleeve. To this sleeve there is fastened the tank, on which there are seated parallel rotating rollers. A limiting sleeve of the sensor is mounted inside a guide by means of a locking nut, which is pressed against a second nut fastened to the guiding sleeve.