A Method of Measuring Acoustic Impedance

in acoustics during past few years has caused acoustic impedance same relative THE progressimpedancemeasurementthetoin have the work has had importance that measurement electrical for many years. The concept of acoustic impedance is derived from the analogy that exists between electrical and acoustic devices, as shown by the analogous differential equations describing their action. Acoustic impedance is usually defined as the complex ratio of pressure to volume velocity (or flux) but it is sometimes more convenient to deal with ratios of pressure to linear velocity or force to linear velocity. The magnitudes of these are interrelated, of course, by powers of the area involved. The earliest efforts to measure acoustic impedance seem to have been made by Kennelly and Kurokawa. In their method, electrical measurements were made of the motional impedance of a telephone receiver, with and without an attached acoustic impedance. Except for frequencies near resonance, the method was inaccurate because the acoustic impedance was associated with a relatively large mechanical impedance. 1 2 * Presented before Acous. Soc. Amer., New York City, May 3, 1932. This analogy was first pointed out by A. G. Webster in Nat. Acad, of Science, 5, 275 (1919). Proc. Am. Ac. Arts and Sc., 56, 1 (1921). 402 1 2 A METHOD OF MEASURING ACOUSTIC IMPEDANCE 3 403 Later, a direct method was described by Stewart who measured the change in acoustic transmission through a long uniform tube when the unknown impedance was inserted as a branch.