A simplified absolute method for point impedance measurement

Abstract

A previous work has indicated that an electromechanical transducer, i.e., an electrodynamic‐type or piezoelectric‐type vibration exciter, can be used for determining the point impedance by the reciprocity principle, and a general formula was given [L.‐F. Ge, J. Acoust. Soc. Am. Suppl. 1 87, S208 (1990)]. The present study develops a simplified and practical method. It is only needed to measure four complex electrical impedances of the exciter, W1, W2, W3, and Wx, corresponding to three known different mass loads, M1, M2, M3, and an unknown structure or material coupled successively on the exciter table while it is vibrated at a frequency of interest. The driving point impedance of the structure or material at the frequency f can be found from the equation: Zx = jω(M2M31W2Wx1 − M1M32W31Wx2)/(M31W32Wx1 − M32W31Wx2), where j2 = −1, ω = 2πf, double subscripts mean differences, such as M31 = M3 − M1, Wx1 = Wx − W1. The equation would be further simplified by the selection of M1 = 0, i.e., no‐load, to: Zx = jωM...