Stereo to five-channels bombyx sonobuoys : from four years cetacean monitoring to real-time whale-ship anti-collision system

The BOMBYX stereo sono buoy is a non expensive device that can derive azimuth and radial velocities with acoustic pressure. This information enables computation of azimuths of low-frequency and high acoustic sources from a compact sensor array. The standard approach for estimating azimuth from these antenna could be by conventional beamforming, but it is computationally expensive, and vulnerable to directional noise contamination for weak signals. We recall here the alternative multiplicative processing scheme that computes the ?active intensity? of an acoustic signal to obtain the dominant directionality of a noise field as a function of time and frequency. This information is conveniently displayed as an ?azigram,? which is analogous to a spectrogram, but uses color to indicate azimuth instead of intensity. Data from Bombyx over 4 years demonstrate this approach. We illustrate how azigrams improves detectability of low signal-to- noise ratio signals and may also enhance the detection and potential classification of signals embedded in directional noise fields.