Various attempts have been made to provide signal communications through ship hulls without the need to penetrate the hull for the passage of cables, or other feed-throughs. Techniques used have included transmission of electrical signals or acoustic signals. In one example of an acoustic technique, described in Hobart et al., “Acoustic Modem Unit”, Oceans 2000 MTS/IEEE Conference and Exhibition, pp. 769-772, vol. 2, September 2000, piezoelectric transducers were used to generate acoustic signals for carrying low bit rate data through a ship's hull, suitable for conveying environmental, e.g., sea temperature and salinity data from outside the hull without the need or the inconvenience of modifying the vessel by drilling holes in the hull. These data are typically transmitted at a low data rate, in a burst mode.