The acoustical damping system described in this paper uses a diaphragm, coupling chamber, and a resistive element in the form of a plug of porous sintered metal. A major feature of the development is the use of pressurization to match the acoustic impedance of the coupling chamber to that of the plug. Critical requirements which must be satisfied by the diaphragm, the chamber, the plug, and its acoustical termination are presented. The theoretical frequency response is presented in dimensionless form, and expressions for the dimensionless parameters are given in terms of physical constants of the instrument. Experience with instruments designed according to these principles and manufactured in quantity by Statham Instruments, Inc., has confirmed the theory.