The nonlinear impedance of a Helmholtz resonator is an important parameter when predicting noise reduction in the environment of high sound-pressure level and grazing flow in fan ducts of aircraft engines. In recent experiments [G. D. Garrison, A. C. Schnell, C. D. Baldwin, and P. R. Russell, “Suppression of Combustion Oscillations with Mechanical Damping Devices,” Pratt & Whitney Aircraft Rep. PWA-FR-3299 (1969)], the impedance was found to be affected by the exciting sound spectrum distribution. An integration method was presented by Rice [E. J. Rice, “A Model for the Acoustic Impedance of a Perforated Plate Liner with Multiple Frequency Excitation,” NASA TMX-67950 (1971)[ to compute the impedance under such an environment. However, the integration requires substantial computation time. Since the impedance is determined by solving the multiple-frequency velocity components of the nonlinear resonator, a direct method to compute the velocity components of the resonator is desirable. A simple iteration method to determine the impedance is presented by the author. In this method, the lengthy integration is not used; instead, an algebraic equation converted from the nonlinear differential equation has to be solved. The physical modeling includes both the effects of multiple-frequency excitation and grazing flow, using similar relationships to those presented in Rice's paper. The impedance computation of this approach requires much less computer time and the results are in good agreement with those from the integration method.