Complex impedance measurements in guided wave tubes

Abstract

A pulsed guided wave tube operating over 2 decades of frequency was constructed for the measurement of the acoustic properties of materials. The impedance (Z) is calculated from the coefficient of reflection and the difference in phase shift between waves reflected from a steel reference plate and waves reflected from the material. The phase shift is obtained from the angle formed by the real and imaginary components of the Fourier transform of the reflected waveforms. Additional points for plotting in the Z plane are found by interpolation between the data points defined by the Fourier transform zeros in the reflection coefficient‐phase angle space. Well‐formed trajectories in the Z plane can now be plotted. Using the FFT has the advantage that (1) only a few accurately known data waveform points are needed as compared to traditional techniques that require a more complete waveform description of the waveforms to be compared and (2) the storage of data for calibration is facilitated. The entire system has been completely automated. Data will be presented showing the change of impedance with frequency for silicone rubber.A pulsed guided wave tube operating over 2 decades of frequency was constructed for the measurement of the acoustic properties of materials. The impedance (Z) is calculated from the coefficient of reflection and the difference in phase shift between waves reflected from a steel reference plate and waves reflected from the material. The phase shift is obtained from the angle formed by the real and imaginary components of the Fourier transform of the reflected waveforms. Additional points for plotting in the Z plane are found by interpolation between the data points defined by the Fourier transform zeros in the reflection coefficient‐phase angle space. Well‐formed trajectories in the Z plane can now be plotted. Using the FFT has the advantage that (1) only a few accurately known data waveform points are needed as compared to traditional techniques that require a more complete waveform description of the waveforms to be compared and (2) the storage of data for calibration is facilitated. The entire system ha...