IT IS proposed to describe a method of measuring the acoustic impedance 1 of solids or gases in the form of discs, or cylinders, to the passage of plane waves of sound. The method is based upin the reaction of the acoustic conductor upon the vibrations of a telephone receiver diaphragm which is used as the source of plane waves in a tube (Figs. 2 and 8). This reaction in the receiver is measured electrically, by connecting its coils in the fourth arm of a Rayleigh bridge (Fig. 3). The method also depends upon certain analogies between uniform electric-line conductors carrying alternatin, v currents, and acoustic-tube conductors carrying sound. Only a few measurements of acoustic impedance measured by this method are at present available, and the theory may have to undergo modification, as numerical data are accumulated, but the method itself is definite, and should give interesting results’ in comparison with other methods of acoustic measurement. 1t may first be advantageous to outline the analogies between an electric-line conductor and an acoustic-tube conductor. The experimental facts concerning the electric line are well established. The corresponding experimental facts concerning the acoustic tube are still only imperfectly known, and the acoustic results can therefore only be depended upon, at present, to a first approximation.