A Finite Element Propagation Model for Extracting Normal Incidence Impedance in Nonprogressive Acoustic Wave Fields
✍ Scribed by Willie R. Watson; Michael G. Jones; Sharon E. Tanner; Tony L. Parrott
- Publisher
- Elsevier Science
- Year
- 1996
- Tongue
- English
- Weight
- 366 KB
- Volume
- 125
- Category
- Article
- ISSN
- 0021-9991
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✦ Synopsis
tion model method. The T-tube and in situ methods have several drawbacks that are discussed at length in Ref. [4].
A propagation model method for extracting the normal incidence impedance of an acoustic material installed as a finite length seg-These two measurement methods do, however, serve as ment in a wall of a duct carrying a nonprogressive wave field is useful complements to the propagation model method, presented. The method recasts the determination of the unknown which is the subject of this paper.
impedance as the minimization of the normalized wall pressure Propagation model methods for evaluating the acoustic error function. A finite element propagation model is combined with a coarse/fine grid impedance plane search technique to extract impedance of a material are popular because of their conthe impedance of the material. Results are presented for three differvenience. The conventional method involves measuring ent materials for which the impedance is known. For each material, the sound attenuation properties in a waveguide lined with the input data required for the prediction scheme were computed the acoustic material over a length sufficient to be effecfrom modal theory and then contaminated by random error. The tively infinite. These data are then used with the solution finite element method reproduces the known impedance of each material almost exactly for random errors typical of those found in to the wave equation in an infinite waveguide to establish many measurement environments. Thus, the method developed the impedance of the material. The evolution of waveguide here provides a means for determining the impedance of materials models for this purpose began over 20 years ago with a in a nonprogressive wave environment such as that usually encoununiform mean flow model (Ref. [4]). For this case, an tered in a commercial aircraft engine and in most laboratory settings.