The input impedance Zin as normally defined is the air column pressure response to a unit plane‐wave flow excitation. The experimental measurement of Zin for an arbitrary air column is affected by the nearfield interactions between an excitory source (capillary, annulus, membrane, etc.) and microphone. In addition to the plane‐wave component, the source produces a local disturbance (due to the evanescent modes) at the microphone. The net microphone pressure pm can be written in terms of the source strength us, the wave impedance R0 and radius a of the air column entryway, and wavenumber k: pm = usR0[(Zin/ R0)+jkaℰ]. The evanescent mode factor ℰ is real and independent of frequency for ka<1. Its value depends on the source and microphone geometry. Measured values agree with detailed calculation. Placement of one transducer along the center axis of the air column, with the other at R = 0.6a from the center eliminates all azimuthal mode coupling (Jm Bessel functions, m≳0), and minimizes the coupling to the lowest order J0 evanescent mode. If neither transducer can be placed at the center, 90° azimuthal separation will eliminate all J1 modes. Placing one or both at R = 0.6a from the center then minimizes J0 effects. These configurations make the ℰ contribution to pm negligible for ‖zin/R0‖≳2, but experimental study near Zin minima (i.e., input admittance maxima) still poses special problems whose reduction is discussed.

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