TECHNICAL NOTES AND RESEARCH BRIEFS

Paul B. Ostergaard 10 Glenwood Way, West Caldwell, NJ 07006

Editor's Note: Original contributions to the Technical Notes and Research Briefs section are always welcome. Manu- scripts should be double-spaced, and ordinarily not longer than about 1500 words. There are no publication charges, and consequently, no free reprints; however, reprints may be purchased at the usual prices.

Advanced-degree dissertations in acoustics Editorb note: Abstracts of Doctoral and Master's theses will be wel-

comed at all times. Please note that they must be double spaced, limited to 200 words, must include the appropriate PACS classification numbers, and formatted as shown below (don't make the editor retype them, please!). The address for obtaining a copy of the thesis is helpful. Please submit two copies.

Active noise control of low-frequency sound inside enclosures [43.55.Ka, 43.50.Ki]mJeffrey M. Orzechowski, Graduate Program in Acoustics, The Pennsylvania State University, University Park, PA 16802, December 1991 {M. S. }. Several aspects of the application of active control to the sound field inside a lightly damped rectangular enclosure at low fre- quencies are addressed. In particular, the placement of the secondary sound source and error microphone, relative to the modal composition of the sound field inside the enclosure, is examined at a modal frequency and between modal frequencies. Both single-channel and dual-channel active noise control (ANC) systems, based on the least-mean-squares algorithm, are used to perform both system identification and control in real time. The effects of source and microphone placement are studied by placing them in different locations and observing the changes in the resulting sound field. It is found that the application of the ANC system often increases the acoustic potential energy inside the enclosure and therefore the sound reduction is localized near the error microphone(s). Finally, the subject of achieving a greater area of localized cancellation near the error microphone(s) is stud- ied by implementing a single-channel ANC system, which uses the sum of the signals from two error microphones, and a dual-channel ANC system. It is found that the dual-channel system provides significantly better perfor- mance than the single-channel system which sums the two error micro- phone signals.

Thesis advisor: Jiri Tichy.

Acoustic scattering from a rigid prolate spheroid [43.20.Bi, 43.20.Fn, 43.30.Gv]mRobert Dean Kollars, Graduate Program in Acous- tics, The Pennsylvania State University, University Park, PA 16802, Decem- ber 199•{M.S.}. This thesis documents an investigation of the acoustic scattering from a rigid prolate spheroid. A model of a prolate spheroid based on the modal solution of the wave equation in prolate spheroidal coordinates is developed and validated. This model will ultimately be used as a building block for modeling a full-scale submarine to obtain a clear scientific and physical understanding of the scattering mechanism associat- ed with the illumination of and scattering from a submarine. Paramount to the development of this model is the implementation of the analytic equa- tions on a computer. An existing computer model developed in the 1970's was rewritten using current numerical accounting procedures, quadruple precision coding, and high-powered grapics. The background development, analysis, and verification of this computer implementation are the keystone of this thesis. The applicability of this model is tested by producing bistatic scattering patterns for a rigid prolate spheroid under, end-on, broadside, and oblique illumination by a plane monochromatic acoustic wave. For the most commonly documented case of end-on illumination, it is shown that as the aspect ratio (length/diameter) of the prolate spheroid increases, an off- axis forward scattered lobe appears and moves away from the axis. For broadside illumination, it is shown that the amount of forward scatter de- creases with increasing aspect ratio. Oblique illumination has rarely been documented due to the complexity and volume of the computations. Here, the bistatic scattering pattern for oblique illumination is shown where both a specularly reflected lobe and a forward scattered lobe are present. Thesis advisor: Alan Stuart.

The intelligibility of words, sentences, and continuous discourse using the articulation index [43.71 .Gv, 43.71 .An, 43.71 .Es]mRory A. DePaolis, Graduate Program in Acoustics, The Pennsylvania State Universi- ty, University Park, PA 16802, December 1991 {Ph.D. }. The effect of message redundancy upon intelligibility was investigated using the original method- ology for the articulation index (AI) [French and Steinberg, J. Acoust. Soc. Am. 19, 90-119 (1947) ]. One primary consideration was to derive the relations between words, sentences, and continuous discourse (CD) with tightly controlled, highly repeatable experimental conditions such that any differences between them could be attributed solely to inherent contextual differences. One male speaker recorded 616 monosyllabic words, 176 mean- ingful speech perception in noise (SPIN) sentences and 44 seventh-grade reading level CD passages. Twenty-four normal hearing subjects made in- telligibi!ity estimates of the CD and sentences and identified words at each of 44 conditions of filtering and signal-to-noise ratio. A plot of the impor- tance of the frequency bands used toward understanding speech revealed that the peak importance was centered around 2000 Hz for all types of speech. As message redundancy increased (words to sentences to CD) the importance function spread to include lower and higher frequencies. The differences between the frequency importance functions for the bands used in this study demonstrated the assumption that one frequency importance function can be used to compute the AI for all types of speech is not valid.

Thesis advisor: Claus P. Janota.

Acoustic radiation from a point excited infinite bilaminar plate [43.40.Dx, 43.30.Jx]mSi Hwan Yum, Graduate Program in Acoustics, The Pennsylvania State University, University Park, PA 16802, May 1992 {Ph.D. }. Presented in this thesis is a refined model for plate deformation that allows for new types of thickness and extensional displacements not cur- rently feasible in the classical or Timoshenko-Mindlin plate theories. An infinite bilaminar plate is fabricated in such a way that the two plates, made of different, isotropic, homogeneous materials, are perfectly bonded. A new mathematical model of the vibration of an infinite bilaminar plate has been obtained by using energy methods. The displacement field is based on sym- metric and antisymmetric displacement functions in both the thickness stretch and the thickness shear of each layer. From this model, the Timo- shenko-Mindlin thick plate theory and the Bernoulli-Euler classic plate theory for a single plate can be recovered as special limiting cases. For this model, six-branched frequency-wave-number spectrums of the single and the bilaminar plates are computed. This allows the introduction of suitable correction coefficients to correct the resulting frequency spectra so that they correspond to those of the exact elasticity theory. In addition, the relationship between frequency and critical angle spectra has been exam- ined. The acoustic pressure directivity of a point force excited plate has been obtained, and the influence of structural damping on the directivity func- tion of the plate is investigated.

Thesis advisor: Sabih Hayek.

Dispersion of elastic waves in a bilayer: An experimental and theo- retical study [43.35.Cg, 43.35.Zc]mJan Laperre, Katholieke Universiteit Leuven Campus Kortrijk, Interdisciplinar Research Center, Universitaire Campus, 8500 Kortrijk, Belgium, November 1991 {Doctoraat}. First it is shown that the dispersion relation, which describes propagation of a Lamb wave in a multilayer, can be deduced within a ray model. This model is well known in optics and has the advantage that each term in the dispersion relation has a physical meaning. This ray model and the obtained dispersion relation are experimentally and theoretically verified for a bilayer composed of an aluminum plate and a water layer of variable thickness. It is shown

3584 J. Acoust. Soc. Am. 91 (6), June 1992 0001-4966/92/063584-02500.80 ¸ 1992 Acoustical Society of America 3584

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