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Passive cylindrical sonar arrays are operated over the widest possible frequency range to exploit all available acoustic energy. But it is a challenge to obtain acceptable performance at low frequencies due to high levels of ambient and platform-generated noises and poor bearing resolution.¹ Adaptive beamformers (ABFs) are important for this purpose. In addition to providing improved bearing resolution and side-lobe suppression, ABFs can potentially provide a higher array gain than conventional beamformers if the noise has a high degree of spatial correlation.² This is the case for ambient noise at low frequencies.¹ Typically, the beamformer processes an arc of sensors mounted on a cylindrical metal baffle. An arc smaller than 180° is typically used because sensors in the acoustic shadow of the baffle have low signal-to-noise ratio (SNR) and make little contribution to the output over most of the frequency range. At low frequencies, however, significant signal energy is diffracted around the baffle, and detection may be improved by using a larger arc.

Because ABF is sensitive to steering vector errors,² the ability to exploit these additional gains depends on accurate modeling of the signal in the acoustic shadow region. The usual approach is to treat the baffle as a rigid scatterer.^3,4 Meyer,⁵ Teutsch and Kellermann,⁶ Teutsch,⁷ and Bertilone et al.⁸ analyzed beamforming, detection, localization, and array gain for arrays mounted on rigid baffles. However, the rigid model is inadequate at low frequencies where the elastic properties of the materials are important.⁴ Unfortunately it is difficult to develop a more accurate model, as it requires detailed analysis of the baffle, its mounting to the platform, and scattering from other parts of the platform. Despite the limitations of the infinite rigid cylinder model, it is of interest to examine the gains achievable in practice when it is incorporated into an ABF with ultra-wide arc. This letter presents results obtained using data recorded at sea from a platform-mounted array, as the arc is extended into the acoustic shadow by increasing the number of processed sensors while keeping sensor spacing fixed.