An ultrasonic probe and a method for the nondestructive testing of a test specimen are described, which are activatable individually or in groups as phased array for the emission or reception of ultrasonic plate waves in a predefinable propagation direction in the test specimen wall to be tested. The invention uses a conditioning cell with an adjustable DC potential which decreases towards the exit aperture to compress the phase volume of the ions by damping their kinetic energies, collecting the ions after thermalization in the spatial potential minimum thus created and letting them drain away relatively slowly through a central potential minimum in the exit aperture system. The proppant-loaded container is placed onto a tilting mechanism and then tilted so that the proppant is discharged through a flow gate of a container into a hopper. The shutter lever is provided at a position coming into contact with the lock lever and the dustproof shutter when the cover is closed. In detecting watermark, it is possible to effectively detect the watermark, by using relation to a generated watermark in case where the peak is detected.