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Journal of the Acoustical Society of America / Volume 120 / Issue 6 / ACOUSTICAL MEASUREMENTS AND INSTRUMENTATION [58]

Modal decomposition method for acoustic impedance testing in square ducts a

a Portions of this work were presented in “Modal decomposition method for acoustic impedance testing in rectangular ducts” at the 44th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, 9–12 January 2006, paper number AIAA 2006-2695, and in “Modal Decomposition Method for High Frequency Acoustic Impedance Testing,” ASA Fall Meeting, Minneapolis, MN, 17–21 October 2005.

J. Acoust. Soc. Am. Volume 120, Issue 6, pp. 3750-3758 (2006); (9 pages)

Todd Schultz, Louis N. Cattafesta, III, and Mark Sheplak

Interdisciplinary Microsystems Group, Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611-6250

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Accurate duct acoustic propagation models are required to predict and reduce aircraft engine noise. These models ultimately rely on measurements of the acoustic impedance to characterize candidate engine nacelle liners. This research effort increases the frequency range of normal-incidence acoustic impedance testing in square ducts by extending the standard two-microphone method (TMM), which is limited to plane wave propagation, to include higher-order modes. The modal decomposition method (MDM) presented includes four normal modes in the model of the sound field, thus increasing the bandwidth from 6.7 to 13.5 kHz for a 25.4 mm square waveguide. The MDM characterizes the test specimen for normal- and oblique-incident acoustic impedance and mode scattering coefficients. The MDM is first formulated and then applied to the measurement of the reflection coefficient matrix for a ceramic tubular specimen. The experimental results are consistent with results from the TMM for the same specimen to within the 95% confidence intervals for the TMM. The MDM results show a series of resonances for the ceramic tubular material exhibiting a monotonic decrease in the resonant peaks of the acoustic resistance with increasing frequency, resembling a rigidly-terminated viscous tube, and also evidence of mode scattering is visible at the higher frequencies.

© 2006 Acoustical Society of America

ACKNOWLEDGMENTS

Financial support for the research project was provided by a NASA-Langley Research Center Grant (Grant No. NAG-1-2261) monitored by Mr. Michael Jones, who also provided material samples. The first author would like to thank the NASA Graduate Student Research Program Fellowship and the National Defense Science and Engineering Graduate Fellowship administered by the American Society for Engineering Education for their financial support.

Article Outline

  1. INTRODUCTION
  2. DATA REDUCTION ALGORITHM
    1. Complex modal amplitudes
    2. Reflection coefficient matrix
    3. Acoustic impedance
    4. Acoustic power
  3. EXPERIMENTAL METHODOLOGY
    1. Waveguide
    2. Equipment description
    3. Signal processing
    4. Numerical study of uncertainties
  4. EXPERIMENTAL RESULTS
  5. DISCUSSION OF RESULTS
  6. CONCLUSIONS

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KEYWORDS and PACS

Keywords

acoustic wave propagation, jet engines, noise abatement, acoustic field, acoustic impedance, acoustic wave scattering, acoustic wave reflection, acoustic resonance, modal analysis, acoustic waveguides

PACS

  • 43.58.Bh

    Acoustic impedance measurement

  • 43.20.Ye

    Measurement methods and instrumentation

  • 43.20.Mv

    Waveguides, wave propagation in tubes and ducts

ARTICLE DATA

History
Received 17 Feb 2006
Accepted 15 Sep 2006
Revised 13 Sep 2006
Digital Object Identifier
http://dx.doi.org/10.1121/1.2360423

PUBLICATION DATA

ISSN

0001-4966 (print)  

Publisher

$abstract.society.value is a Member of CrossRef Acoustical Society of America

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