### Abstract

The input impedance Z_{in}as normally defined is the air column pressure response to a unit plane-wave flow excitation. The experimental measurement of Z_{in}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 p_{m}can be written in terms of the source strength u_{s}the wave impedance R_{0}and radius a of the air column entryway, and wavenumber k:p_{m}= u_{s}R_{0}[(Z_{in}/ R_{0}) + jka&]. The evanescent mode factor & 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 (J_{m}Bessel functions,m > 0), and minimizes the coupling to the lowest order J_{0}evanescent mode. If neither transducer can be placed at the center, 90° azimuthal separation will eliminate all J_{1}modes. Placing one or both at R = 0.6a from the center then minimizes J_{0}effects. These configurations make the & contribution to p_{m}negligible for |z_{in}/R_{0}| > 2, but experimental study near Z_{in}minima (i.e., input admittance maxima) still poses special problems whose reduction is discussed.

Original language | English (US) |
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Pages (from-to) | 1489-1495 |

Number of pages | 7 |

Journal | Journal of the Acoustical Society of America |

Volume | 69 |

Issue number | 5 |

DOIs | |

State | Published - May 1981 |

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### ASJC Scopus subject areas

- Arts and Humanities (miscellaneous)
- Acoustics and Ultrasonics

### Cite this

*Journal of the Acoustical Society of America*,

*69*(5), 1489-1495. https://doi.org/10.1121/1.385783