Beamforming applications of the frequency-difference acoustic autoproduct

Frequency-difference beamforming (Abadi et al., 2012, JASA, 132, 3018–3029) is an array signal processing technique that overcomes the limitations of the spatial Nyquist criterion by utilizing the acoustic autoproduct to shift the processing to below-band frequencies. This is accomplished using a quadratic product of complex signal amplitudes at different frequencies, resulting in wave propagation information at the out-of-band difference-frequency. The resulting field is capable of mitigating many of the in-band challenges often associated with high frequency acoustic signal processing, including sparse receiver arrays and features of the physical environment that are on a scale significant for the in-band wavelength. This presentation uses both laboratory and ocean-based experiments to demonstrate capabilities of the method. The mitigation of sparse array aliasing effects on beamforming (caused by elements that are spaced by many wavelengths) in both a laboratory water tank and an ocean environment utilizing data collected during the KAM11 experiment are considered. Additionally, the method is used to localize a high frequency source in the presence of strong, random scatterers in a water tank experiment. Work sponsored by NAVSEA and ONR.