An apparatus provides for the non-destructive testing of samples. The samples can in this respect also be formed from critical materials which change their properties during manufacture, for example on a hardening or solidification. In an apparatus in accordance with the invention, at least one ultrasonic pressure wave transducer is arranged at a forward cell. The forward cell is filled with a liquid and is placed with an open side at one side onto a surface (A) of a conversion prism, in the form of a solid body, so that the liquid and the surface (A) are in touching contact. The conversion prism is placed with a further surface (B) onto a surface of a sample to be tested. At least one ultrasonic pressure wave transducer can be positioned or is positioned at the forward cell at at least two positions (T1, T2) so that the positions are arranged at an equal spacing from the center of the surface (A) of the conversion prism, said surface being placed onto the sample.