Improvements to the Two-Thickness Method for Deriving Acoustic Properties of Materials

This paper describes how the two-thickness method is one of several methodologies that are available for measuring a material’s acoustic properties in an impedance tube. In order to obtain the characteristic impedance, two specific impedance measurements are made of two distinct thicknesses of material samples. To simplify derivation of the characteristic impedance, one sample thickness is constrained to be a factor of two of the other. This can be a problem in that it is often difficult to obtain samples that meet the thickness constraint exactly, thus introducing an undetermined error into the analysis. The analysis is further hampered by a need to ‘unwrap’ the phase information contained in the solution before the propagation constant can be properly formed. Many of the measurement/analysis techniques described in the paper would have a similar requirement. In general, the unwrapping phase is difficult to automate and can obscure relevant data under certain conditions. The two solution techniques described in the paper can be used to analyze data taken from material samples of any two thicknesses as long as the thickness difference is sufficient to produce numerically meaningful data. In addition, the solutions produce the propagation constant directly so that phase unwrapping of intermediate terms is not necessary. Finally, the quality of the data can judged before the solution is extracted by viewing the wave number spectrum produced by the analyses. One technique applies the Newton-Raphson method to approximate the roots of the solution. The second technique produces a wave number/frequency space diagram from which the solution is easily extracted. One benefit realized by the relaxation of the sample thickness constraint is that several samples of various thicknesses can be measured and cross analyzed. This capability provides a means of verifying results and better understanding the limitations of the impedance tube data. Before these results are presented, the applicable theory and experimental setup are described.


  • English

Media Info

  • Media Type: CD-ROM
  • Features: Figures; Tables;
  • Pagination: pp 632-640
  • Monograph Title: Noise-Con 04. The 2004 National Conference on Noise Control Engineering

Subject/Index Terms

Filing Info

  • Accession Number: 01054311
  • Record Type: Publication
  • Files: TRIS, TRB
  • Created Date: Jul 25 2007 2:52PM