Abstract
A brief survey is made of the limitations of existing methods for the measurement of the normal acoustic impedance of sound absorbing materials. A more exact method has been developed which utilizes a cylindrical tube terminated at one end by a rigid wall at whose center the sound is admitted and at the other end by a movable piston on which the absorbing sample may be mounted. The acoustic impedance is computed from the widths and positions of pressure-length resonance curves, the pressure being measured at the tube wall at the source end. An exact mathematical analysis is provided which includes correction for the effects of dissipation in the tube and distortion of the sound field in the vicinity of the source. In the frequency range of 100 to 8000 cycles, using two tubes of different diameters, measurements are made to show the various types of impedance curves available for practical applications, and to show variations between different samples of the same material. The effect of sample mounting conditions is discussed in connection with the correlation of small tube and full scale reverberation chamber measurements.
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