885095996

WEDNESDAY AFTERNOON, 1 MAY 1991

LIBERTY A, 1.00 TO 3:05 P.M.

Session 5EA

Engineering Acoustics: Signal Processing and Waves in Elastic Media

Roger T. Richards, Cochair

Naual Underwater Systems Center\ New London, Connecticut 06320-5594

Thomas R. Howarth, Cochair

HVS Technologies, Inc., 820 North Uniuersity Drive, State College, Pennsyluania 16803

Chair'9 Introduction—1K)0

Contńhuted Papers

1:05

5EA1. Passive fetal heart ratę monitor using piezopolymer pressure sensors. Allan J. Zuckerwar (NASA Langley Res. Ctr., M.S. 238, Hampton, VA 23665), John W. Stoughton, Robert A. Pretlow (Old Dominion University, Norfolk, VA 23529-0246), and Donald A. Baker (Holy Family Medical Clinic)

Fetal heart ratę monitoring is performed routinely by pulsed Doppler ultrasound, but is currently confined to a clinical setting because of the hulkiness and high cost of the equipmenl. A passivc monitor, using piezopolymer pressure sensors located on a belt wom by the mother, has been developed to detect the fetal heart tonę and to determine the fclal heart ratę through appropriate signal processing. The sensors are con-structed to fulfill the following functions: detecting pressure pulses on the abdominal surface. shielding against 60-Hz interference, isolating the patient electrically, insulating against ambient sound, and canceling matemal rigid-body motion. The signal processing employs linear pre-diction techniqucs to identify the fetal heart tonę from arnong compet-ing background signals and yields a real-time evaluation of the fetal heart ratc. The monitor has been applied to conduct clinical fetal non-stress tests simultancously with ultrasound, and the results are com-pared. The monitor is not only inexpensive but lends itself to an ambu-latory modę of operation, whereby the mother can conduct the fetal nonstress test in her home.

1:20

5EA2. Signal processing for an acoustically based fetal heart ratę monitor. Robert A. Pretlow and John W. Stoughton (Dept. of Electr. and Computer Eng., Old Dominion University, Norfolk, VA 23529-0246)

A least-mean-square (Ims) linear prediction algorithm has been de-veloped to accomplish detection of fetal heart tonca and thereby dcrivc heart ratę from a raw signal generated by a previously described passivc acoustical sensor array. The desired heart tonę signal has a character-istic signature but is of extremely Iow amplitudę and conlaminated with noise consisting of large-amplitude maternal heart tones, abdominal sounds, body motion, and environmental sounds, and also mild 60 Hz. The predictor cocfficients were derived by adaptively “training** on ideał fetal heart tones recorded from several palients. The Ims algorithm detects a heart tonę event when the predictor mean-square error falls below an adaptively updated threshold lcvel. The algorithm contains logie for correction of spurious and missed heart tones. A real-time working system was fabricated consisting of a sensor belt, front end electronics, a TMS320C25 digital signal processing board, an 80386 PC,

1922 J. Acoust Soc. Am., Vol. 89, No. 4, Pt. 2, April 1991 and a strip chart recorder. The apparatas allows performance of the fetal nonstress test (NST) in a manner similar to that conventionally accomplished via ultrasound. The acoustical system was implemented in parallel with a commercial ultrasound unit on a series of patients undergoing NSTs. The heart ratę records are corupared.

1:35

5EA3. The acoustic materia! signature for a cracked surface using a point focus acoustic microscope. D. A. Rcbinsky and J. G. Harris (Dept. of Theoretical Appl. Mech., UIUC, 216 Talbot Lab., 104 S. Wright St., Urbana. IL 61801)

Using an asymptołic model of a point focus scanning acoustic microscope, the acoustic materiał signature for a cracked surface is calcu-latcd. The scattered wave fields in the coupling fluid are relatcd to the output voltage by an electromechanical reciprocity relation. This rcla-tion may be calculated using either asymptotically approximated wave fields or their asymptotically approximated spectra. Because this relation is linear, the component of the acoustic signature produced by the crack may be separated from that produced by a defect-free surface. The component arising from the scattering from the crack is calculated from approximations to the wave fields rather than their spectra. It is as-sumed that the crack opening docs not perturb the geometrie wave field, but that it does strongly scatter the leaky Rayleigh wave, excited by the microscope. It is also assumed that scattering of the Rayleigh waves from the crack can be characterized by reflection and transmission co-efficients. The wave fields scattered from the crack are thus calculated geomelrically and the corresponding contribution to the acoustic signature is estimated. [Work supported by the MRC at UIUC.]

1:50

5EA4. Structural acoustic analysts using the fast Fourier transform. Peter K. Kasper (CASDE Corp., Suitę 600, 2800 Shirlington Rd., Arlington. VA 22206)

Fourier analysis has long been an important tool in acoustical and structural dynamics technologies. Many of the classical problems in acoustics and dynamics can, for example, be solved in closed form using Fourier transforms. The fast Fourier transform (FFT), itself, is an efficient method for calculating the Fourier transform of discretized or sampled variables. The current paper describes a generał approach using the FFT for obtaining numerical Solutions of fundamental acoustics and structural dynamics diflerential equations. Specific examplcs treated in-clude harmonie response of finite strings and beams. The generał ap-

121 st Meeting: Acoustical Society of America 1922