This paper describes an impedance measuring assembly consisting of a piston and pressure microphone arranged in such a manner that the ratio of the pressure to the particle velocity may be measured in both magnitude and phase at the surface of the piston. The assembly measures the driving point impedance at the surface of the piston. The sample to be measured is confined in a tube of diameter equal to that of the piston and is placed almost in contact with the vibrating surface of the piston. Thus, the pressure and particle velocity at the surface of the sample are prescribed by the velocity of the piston and the specific acoustic impedance of the sample. Constructional details and performance characteristics of the assembly are given.