84TH MEETING ß ACOUSTICAL SOCIETY OF AMERICA

an average error of 6.98 ø . After 240 training trials in which the subject was allowed to visually correct his auditory localiza- tion, the average error in 360 ø was 13.1 ø . Reversals were reduced to 7.1% of the total responses.

X13. Subliminal Underwater Sound Localization. H. HOLLIEN, J. KINNEY,* S. FEINSTEIN, AND P. HOLLIEN, Communication Sciences Laboratory, University of Florida, Gainesville, Florida 32601.--Recently Hollien [-"Underwater Sound Localization in Humans," J. Acoust. Soc. Amer. 52 (1972)• suggested that individuals attempting to localize sound underwater may utilize kinesthetic cues for this purpose. In an effort to obtain data on this issue, two experiments were conducted; in both cases the divers were dressed only in bathing suits and

life support equipment. In the first experiment, signals were provided at a pressure level that was approximately 10 dB below the divers' threshold at each of the respective fre- quencies tested. In the second study, signal strength was at 105 dB SPL (re 0.0002 dyn/cm•'), but it was masked out by a sawtooth signal delivered to the divers' auditory mechanism via a set of underwater earphones held against their heads by a neoprene hood. Data from both investigations suggest that underwater sound localization is an auditory function, as there was no evidence that any information was obtained by tactile sensation.

* Also with Naval Submarine Medical Center, New London, Conn.

THURSDAY, 30 NOVEMBER 1972 ROYAL ROOM, 9:00 A.M.

Session Y. Underwater Acoustics IV: Signal Processing

A. J. VAN WOERKOM, Chairman

New London Laboratory, Naval Underwater Systems Center, New London, Connecticut 06320

Contributed Papers (12 minutes)

Y1. Small Signal Suppression in a Class of Optimal Adaptive Beamformers. R. S. VARGA, Bell Laboratories, Whippany, New Jersey 07981.--Optimal adaptive beamformers are often designed to minimize the beam noise response while maintain- ing a fixed response to an incoming signal. In the real world, improvements in detection performance that could result from noise rejection may not be realized because of a signal suppression effect. In this paper, analysis of a particular adaptive beamformer (ABF) shows the signal suppression effect to be related to the reception of signals whose wave- front departs from an assumed nominal wavefront (usually planar). Experimental results demonstrate that slight dis- tortions in the wavefront of weak signals can cause significant degradation. Moreover, the degradation in signal response for the ABF is uniformly larger than that of the delay and sum beamformer (DSBF), for a given amount of wavefront dis- tortion. In addition, ABF signal degradation is found to be not only dependent upon the amount of wavefront distortion, but upon the background noise field as well. This result is specific to the ABF and not the DSBF. In particular, ABF signal suppression is shown to increase as the noise field anisotropy increases. Thus when the conditions for ABF noise improvement are best, the potential for ABF signal degrada- tion is greatest.

Y2. Influence of Input Quantizer Step Sizes on the Processing Gain of Digital Beamformers. H. S.C. WANG, Edo Corpora- tion, College Point, New York 11356.--Utilizing a formulation of output power of a digital beamformer with input quantizers of unequal input and output step sizes reported elsewhere [H. S.C. Wang, Quantizer Functions and Their Use in the Analyses of Digital Beamformer Performance (to be pub- lished); also see J. Acoust. Soc. Amer. 51, 135(A) (1972)-], the optimum quantizer step sizes that yield maximum proc- essing gain are searched by the method of "steepest ascent" with a digital computer. For the case of 2 bits/channel quanti- zation with small input S/N (<--10 dB), the array gain surface as a function of two independent step variables ex- hibits a maximum that is somewhat greater than the maxi-

mum array gain achievable with uniform input quantizers. For larger input S/N (> --l0 dB), no maximum can be found, and, as the input S/N exceeds -4 dB, the array gain in- creases monotonically with respect to both input and output step variables. These phenomena were clarified by a numerical study of a "3-level quantizer" as a limiting case. The large theoretical processing gain at large step sizes is attributed to a threshold effect of the input quantizers.

Y3. Curvature Effects on Side and Aliasing Lobes of Trans- parent Arrays. P. W. SMITH, JR., AND E. F. BERKMAN, Bolt Beranek and Newman Inc., Cambridge, Massachusetts 02138.-- A transparent array, either continuous or discrete, is located on a spherical surface. Its sensitivity is spatially constant in magnitude and phase, compensated for maximum signal in the direction of the main lobe. A stationary-phase analysis shows that a sufficiently large continuous array has large sidelobes in a direction dependent on array size. A discrete array has aliasing lobes (grating plateaux) at a lower fre- quency than does a similar flat array.

Y4. Information Derivable from the Doppler Polar Profile Spectrum. RONALD L. SPOONF. R AND JUDE R. NITSCHE, Bolt Beranek and Newman Inc., Arlington, Virginia 22209.-- There has been considerable interest recently in the use of Doppler shifted signal information as a technique for passive acoustic localization. An investigation of this phenomenon using conventional techniques, however, rapidly becomes complicated by the large number of parameters and geometri- cal configurations that must be considered. In particular, it is especially difficult to determine the sensitivity of the tech- nique to measurement errors. As an alternative, this paper treats the Doppler profile in terms of its inherent information content and indicates how such information may be utilized to obtain both qualitative and quantitative results. By in- voking Fourier transform and sampling techniques, the limitations of Doppler localization are viewed in terms of conventional filter bandwidth and aliasing problems. Further- more, measurement sensitivity is treated as a generalized S/N ratio enhancement problem. Utilizing the methods of

336 Volume 53 Number 1 1973

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