mined by the properties of the respiratory tract and that the decreased amplitude of transmission at higher frequencies can be largely attributed to the absorption of sound in the surrounding lung parenchyma. The model provides a theoretical framework for further investigation into the effects of structural changes on sound transmission in both health and disease.

Thesis advisors: Daniel C. Shannon, Kenneth N. Stevens.

Sound propagation under the Arctic ice canopy [43.30.Ma]--David Glen Wegmann, Naval Postgraduate School, Monterey, C•4 93943, M.S., Engrg. •4coustics, M. $., Systems Technology, March 1989. Propagation in a shallow-water waveguide, covered by a layer of"ice," was studied in a lO- rn-long, 19-cm-deep laboratory facility that models both the scale and the

physical properties of the Arctic. Smooth ice measurements were compared with predictions of the Naval Research Laboratory computer model KEN, which includes the effects of elastic layers. Although the ice canopy thickness is only a small fraction of a wavelength, the sound field is distin- guishable from the usual Pekeris waveguide (no ice cover) behavior by the evidence of seismic-type plate modes. The observed effects are related to the stimulation of ice head waves, which has been reported previously for deep water regions [J. Acoust. $oc. Am. 83, 1794 (1988) and $uppl. 1 82, $31 (1987) ]. The autoregressive and Prony spectral estimation techniques were demonstrated to be effective in identifying the acoustic modes, but they were not as suitable as Fourier techniques for describing the mode coupling caused by a rough ice cover.

Thesis advisor: H. Medwin.

845 J. Acoust. Soc. Am., Vol. 86, No. 2, August 1989 Notes and Briefs 845

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 128.114.34.22 On: Fri, 28 Nov 2014 16:14:49