885096010

9:50

10*J5

6EA5. Full-duplex speakerphone with acoustic and linę echo cancelers. Sen M. Kuo and Jier Chen (Depr. of Elect. Eng., Northern Illinois Univ., Dekalb, IL 60115)

In this paper, a complete speakerphone is implemented with an AT&T WE DSP16A microprocessor. This single chip implementation has the advantage of Iow cost and hence can be applied to the speakerphone inslalled in a smali conference room or a hands-free cellular phone. A robust speech detector is developed to classify four diflferent operation modes; a corresponding gain regulation algorithm is then applied to guarantee the stabiłity of local acoustic and electronic loop. Morę specifically, it reduces signal levels on both channels when in idlc modę, freczes acoustic (or linę) echo canceler during the transmit (or receive) modę, and freczes both echo cancelers when double talk modc occurs. The performance of the system is evaluated by Computer simu-lation and teslcd by real timc experiment.

I(h05

6EA6. Choked-jet edge-tone experiments. Alan Powell and Dan Lin

(Dept. of Mech. Eng., Univ. of Houston, Houston, TX 77204-4792)

Some experimental results for a choked jet from a 4.86:6 rectangular orifice impinging on a long 8* (w)edge are presented. For nozzle-edge distance A>three celi lengths, < 156 and in the pressure ratio rangę 2.09 to 3.31, violent edge-tone oscillations occur that may be above, approximately equal to and merge with. or below the frequency of the simultaneous single scrcech modę of the frec jet. The pressure field (2=evanescent waves) of the growing sinuous jet instability are clearly visible in Schlicren photographs, as is the associated periodic asymmet-rical sound field. Preliminary estimates indicate consistency with the proposed fecdback formula for the fundamcntal acoustic wavelength A [Powell, Acoustica 3. 233-243 (1953)], h'/A = (N + p)X(M_con)/ (I +    with: instability convection Mach number ^20.5-0.7;

nozzle to acoustic source distance h'zzh + (sinuous instability wavc-length); integral number of cycles in the feedback loop N = 4, 5, or 6 with/>;s0.4±0.2. Normal hysteretic jumps appear negligible, but other liysteretic eflects are suspected. [Partial support from the Texas Ad-vanced Research Program.]

10:20

6EA7. Screech: Edge-tone-like bchavior induced by a smali piąte. Alan Powell (Dept. of Mech. Eng., Univ. of Houston, Houston, TX 77204-4792), Y. Umeda. and R. Ishii (Dept. of Aeronautical Eng., Kyoto Univ., Kyoto 606, Japan)

The dominant screech frequency of round choked jets falls steadily with inereasing pressure ratio, interrupted by jumps as the jet instability modę—Ai and A₂ (toroidal), B (sinuous), C (helical), and D (sinuous again~#)—changes. A smali normal piąte (diam/nozzle diam =0.35) on the axis radically changes the behavior. Preliminary results indicate that oscillations in the screech frequency rangę take place only in four “preferred” frequency bands, three bcing narrow of constant frequcncy, separated by prominent “dead’* bands of no activity. With variable nozzle-plate distance, the compasite plot for all pressure ratios shows each composite band to be reminiscent of classical sawtooth edge tones, with the number of cycles N in the apparent fecdback loops from 2 to at least 8, with continuity of each N from band to band. But each band may contain morę than one screech-like modę: Just toroidal for the band of highest frequency, then sinuous and helical, then sinuous, helical and sinuous again and finally just sinuous for the lowest frequency band. [Partial support from the Texas Advanced Research Program.] 6EA8. Reflcction coefficient of a ttuid-ioaded anisotropic piąte. D. E.

Chimenti (Dept. of Mater. Sci. and Eng., Johns Hopkins Univ._t Baltimore, MD 21218) and S. I. Rokhlin (Ohio State Univ., Columbus, OH 43210)

The acoustic reflcction and guided wave behavior of an orthotropic piąte immersed in an ideał fluid is examined as a function of fluid loading. Coincidence of zeros and real parts of rcflection coefficient poles occurs only in the limit of vanishing or infinite fluid density. The modification of the pole and zero piąte spectra in the presence of a variable fluid density will be discusscd in the context of Lamb wave measurcments on Iow density solid plates, such as graphite-reinforced plastics (GRP). Numerical results are presented showing functional dependencc of pole and zero branches on fluid density and piąte elastic properties at constant phase velocity or constant frequency-thickness product. It will be shown that propagating and evancscent wave modes exchange portions of their branches, and that reflection coefficient zeros also undergo extcnsivc, but diflering, transformations. Even very Iow fluid density ( <0.1 g/cm ’) values can causc significant changes in the wave behavior of the fluid-loaded GRP piąte. Fluid loading transforms the wave spectrum from Lamb waves in a traction-free piąte to “constrained-slip” waves in a piąte satisfying mixed boundary condi-tions.

10*30

6EA9. Measuremcnt of uasteady pressure and shear stress using surface acoustic wave devices. Michael Macrorie, Vasundara V. Varadan, and Wayne R. Pauley (Dept. of Aerospace Eng., 233 Hammond Bldg., Pcnn State Univ., University Park, PA 16802)

A new techniquc is being invcstigated for the direct measurement of surface stresses (both pressure and shear) in steady and unsteady wali bounded flows. The techniquc utilizes surface acoustic wave (SAW) devices. A first generation sensor has becn used to measure mean wali shear stress beneath a turbulent boundary layer at speeds from 10 to 20 m/s in air. Good agreement with a standard technique was found. To demonstrate the dynamie response, an artificially generated vortex structure was propagated over the SAW sensor. The time dependent pressure and shear stress oscillations due to the passage of the unsteady vortex were measured by the SAW sensor and co m pa red with the response of a hot wire probe. Further reduction in the size of the sensors, multiple sensing elements, and refinement of the calibration technique will produce sensors capable of resolving the fuli stress vector due to near wali flow structures in transitional and turbulent boundary layers. (Work supported by ONR under Grant No. N00014-89-J-3102.]

11.*05

6EA10. Scattcr of subsonic waves on a fluid-loaded cylindrical shell from an interna! obstać le. P. W. Smith, Jr. (Bolt Be ranek and Newman Inc., 10 Moulton St., Cambridge, MA 02138)

An axisymmetric subsonic wave on a thin, fluid-loaded cylindrical shell of infinite extent impinges on a “shear diaphragm,” which con-strains the normal velocity to vanish locally by the action of a ring force. The energy scattcr coefficients (wave transmission, reflection, and sound radiation) have bcen cvaluated, as wełl as the directional radia-tion source strength for a ring-force source. Results are presented for parameters appropriate to Steel and water in the frequency rangę around the in-uacuo ring resonance frequency. The wave passes the obstacle ncarly undisturbed at Iow frequcncies (ko < 1.5, where a is the radius and A: is the sound wave number). At high frequencies (ka> 2.5) the wave is largely reflected, behaving asymptotically like a flexural wave on a flat piąte of the same thickness as the shell, with fluid on one side.

1944

J. Acoust. Soc. Am., Vol. 89, No. 4, Pt. 2, April 1991

121st Meeting: Acoustical Society of America

1944