Maintaining quality of acoustic data: Calibration methods for active and passive devices, with extended sampling volume

A key technology that continues to evolve to meet special requirements of underwater sampling and observation is that of acoustics. This technology is used both actively to ensonify fish, zooplankton, other marine organisms, and the environment, and passively to listen to and record sounds produced by marine organisms and other sources, e.g., shipping and environmental noise. When calibrated, acoustic devices offer the potential for quantification. The essential case for calibration is made, and principal methods for the calibration of active and passive devices are reviewed. These include the standard-target method for the calibration of active devices, e.g., sonars, and the three-transducer spherical-wave reciprocity method for the calibration of passive devices, e.g., electroacoustic transducers and hydrophones. Recent advances in understanding the spatial structure of the transducer nearfield may safely extend the range at which such calibrations can be performed, as well as extending the range of measurements themselves. This extension can be quantified through the acoustic sampling volume. Reference is also made to the IEEE Oceanic Engineering Society (OES) Standards Initiative, with website at http://www.oceanicengineering.org/page.cfm/cat/105/OES-Standards-Initiative/, which is providing a forum for dissemination of information on standards, protocols, quality assurance procedures, and best practices that are important in ocean engineering. This includes information on current calibration methods for acoustic instruments.