A digital recording system with several months’ endurance was designed to monitor biological sounds. The system combined off-the-shelf components (microcontrollers, notebook hard drives) with an IDE interface designed for this purpose. The present storage capacity of the system is 25 Gbytes, with a maximum sampling rate of 50 kHz at a resolution of 12 bits. Power consumption during typical long-term continuous recording averages 250–500 milliwatts, and its physical dimensions are approximately 3×7×10 cm, including the hard drive. Hardware and software support for GPS clock synchronization enables correction of clock drift in the system, as well as precise synchronization of multiple units in an array. This unit has been extensively utilized in bottom-moored recording buoys, called ‘‘popups.’’ Popups include an acoustic communication system that receives and acknowledges commands from a surface support vessel, and an electronically controlled galvanic release. Everything is housed in a single 17-in. glass pressure housing. More recently, these systems have been used to monitor low-frequency terrestrial signals in studies of forest and savannah elephant populations. An upcoming project will drift these systems over inaccessible habitats using helium balloons.
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Calupca et al. (2000) studied this question.