Measurement Of Acoustic Impedance Research Articles

The Measurement of Acoustic Impedance and the Absorption Coefficient of Porous Materials

Various ways of determining the acoustic impedance and the absorption coefficient of porous materials from measurements on the standing waves in tubes are discussed. In all cases the material under investigation is placed at one end of the tube and the sound is introduced at the other end. Values of the coefficient of absorption of a number of commonly used damping materials as obtained by one of the methods are given. Several types of built-up structures are shown to have a greater absorption coefficient for low frequency sound waves than is conveniently obtainable by a single layer of material.

The measurement of acoustic impedance with the aid of the telephone receiver

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.