Development of a New Method of Measuring the Characteristic Impedance and Complex Wave Number of a Porous Acoustic Material

摘要

The specific DC flow resistance, characteristic impedance and complex wave number of air-filled fiberglass have been extracted from measurements of the peak and half-power frequencies of the two lowest-frequency normal modes of a rectangular plexiglass cavity partially filled with fiberglass. Using Biot theory Ref's. 8, 9 and 10 to describe the sound field within the fiberglass, a dispersion relation for the allowed modes in the cavity was derived, from which the above properties were extracted. The values determined by this new method compared favorably (within 25%) with the results of measurements using classical methods. The new method differs from the classical resonance tube method Ref. 4 by 1) the use of a wave theory to describe the sound field within the fiberglass 2) the use of a mode with particle velocity parallel to the air-fiberglass interface. The differences make it possible to extract the acoustic properties of bulk fiberglass using only a thin sample.