Logarithmic Computer-Plotter for Mechanical and Acoustical Impedances

Abstract

An electrical-analog device to simplify measurement of mechanical and acoustical impedances has been built. It is intended for use with several of the types of two-channel vibration-and-sound transducers generically known as impedance heads. This equipment facilitates continuous measuring and plotting of mechanical or acoustical impedances or their reciprocals, mobility and admittance. The computer features externally adjustable compensation for mass or inertance inherent in many impedance heads, independent logarithmic compression of both impedance-head signals for wide dynamic range (approximately 60 dB over the frequency range 20–2000 cps), and relative phase measurement after logarithmic compression to insure that phase-meter input signals from both information channels are of acceptable level. Computations are effected by a summing network that produces a linear combination of dc voltages proportional to the logarithms of the two impedance-head signals, the logarithm of frequency, and a constant representing combined-scale factors. By appropriate choice of impedance head and algebraic signs of the summed voltages, a dc voltage proportional to magnitude of the measured quantity is produced. Both this magnitude and a phase-angle signal can be recorded vs the logarithm of frequency by an x-y plotter with two sweeps of the frequency interval. This paper describes the computer, discusses its capability, and compares automatically-plotted impedance curves with ideal curves and point-by-point plotted measurements for certain mechanical structures and acoustical systems. [This work was sponsored by the U. S. Navy Bureau of Ships.]