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A novel piezoelectric material technology is used to design high-frequency (>10 kHz) air-deployed sonobuoys that exploit ambient noise anisotropy to enhance their passive performance. High frequencies have the advantages of smaller arrays and reduced clutter from distant shipping noise. PVDF wires are used to design vector sensors in azimuth with vertical directionality. Such hydrophones are used to design vertical and volumetric array configurations. Performance predictions of developed array designs are presented for selected environmental conditions. Results clearly indicate that vertical aperture is necessary to increase array gain against wind-driven noise by exploiting its anisotropy while azimuthal discrimination is required to enhance gain against nearby shipping interference. [This work is sponsored by the Office of Naval Research (ONR).]

Mid to high-frequency (1-20 kHz) noise is generally dominated by wind-driven wave activity but under certain conditions potentially exploitable ambient noise fields can be severely degraded by nearby ships, especially in the lower end of the band. The use of directive elements and adaptive methods are shown as possible ways to mitigate this problem. For a submerged receiver in a downward-refracting environment without nearby ships, a vertical noise notch that can offer increased array gain over the directivity index can be filled in by scattering from volume inhomogeneities. Just outside the notch, the ambient noise vertical directivity is sensitive to the assumed surface source directionality. A ray-based model is used to assess the sensitivity of the performance of a vertical line array and a volumetric array to these mechanisms in a tactically relevant environment.

High-frequency sonobuoy arrays with vertical aperture were developed and tested to exploit the noise anisotropy for enhanced array gain [Ferat and Arvelo, J. Acoust. Soc. Am. 114, 2462–2463 (2003)]. Afterwards, an inexpensive and conformal piezoelectric wire material was employed in the design and construction of a volumetric array [Arvelo et al., J. Acoust. Soc. Am. 116, 2650 (2004)]. However, detection performance of air-deployed sonobuoys is severely affected by noise interference through the beamformers sidelobes. An improved sonobuoy system design with in-buoy processing for sidelobe suppression is introduced and its detection performance in anisotropic wind noise with nearby shipping interference is evaluated and compared against previous designs in selected environmental conditions. The high-frequency array gain for increased vertical aperture will be compared against the gain for increased horizontal aperture to explore coherence limitations. The effects of array tilt, as well as interelement amplitude, phase, and topology errors, on the performance deterioration of the new design will also be addressed. [This effort is supported by the Maritime Sensing (MS) Program of the Office of Naval Research (ONR Code 321).]

Proper knowledge of geoacoustic bottom properties can be critical for accurate acoustic propagation predictions in the ocean, particularly in shallow environments. Unfortunately, these properties are unknown in much of the ocean. This knowledge gap presents a need for measurement techniques that enable the inference of critical bottom properties in a timely, cost effective manner. One such approach, proposed by Harrison and Simons, exploits the spatial structure of surface-generated, mid-frequency ambient ocean noise to infer bottom loss in a region as a function of bottom grazing angle. This method, known as the Up-Down Ratio, is fully passive and necessitates only a vertical line array with sufficient resolution for the frequencies of interest. A second approach, introduced by Buckingham, utilizes passive noise correlation between two vertically-separated hydrophones for similar purposes. In this presentation, these passive methods will be employed to estimate bottom parameters from mid-frequency (2–4 kHz) MACOS glider measurements collected within the New England Shelf Break Area as part of the Seabed 2021 joint experiment. In addition to comparisons of the inferred bottom parameters, discussion will include the ease of implementation of the methods as well as robustness against real data-collection challenges such as array tilt and strong surface interferers.