Survey of impedance methods and a new piezo‐disk‐driven impedance head for air columns

The six major techniques used heretofore for the swept‐sinusoid measurement of air column input impedances are first reviewed, after which the basic theory of a new, impedance head design that requires no servo mechanism is outlined. In this simple device, a low‐cost piezoelectric disk is used as the primary flow excitation source, and the resulting response pressure is detected by a similarly inexpensive miniature electret microphone. Detailed experimental tests of the device are presented. Analysis of these tests shows that, up to about 5000 Hz, essentially no calibration corrections are needed; furthermore, the driver impedance is so high that its perturbations of the air column are experimentally negligible. A detailed mathematical analysis of the major finite‐impedance, displaced‐microphone, and higher‐mode perturbations associated with the driving disk itself are set forth, and an outline of general procedures for obtaining the absolute calibration of any air column impedance head is given.