Abstract
The idea behind the Drifting Ears design is to maximize the advantages of recording ambient sound from a drifting platform and minimize the disadvantages of being associated with a boat or other surface vessel. The central feature of the design is that the hydrophone, rather than any other component, is fixed relative to the body of moving water and that other constituents do not impart artificial noise into the recordings.
To do this, the hydrophone is placed inside the bounds of a large underwater drogue (Fig. 1). The boat is dispensed with and replaced by a buoy and small floating case containing the ancillary electronics (batteries, recorder etc). The drogue is weighted to drift at depth and held there by the opposition of the weight and a small surface buoy. Wide diameter hawser and shock chord is used to negate strum and add physical separation between the sensing and surface parts of the unit. The entire unit is self sufficient and dropped into the water upstream of the area of interest. The unit then drifts passively in the current over the area of interest, recording as it goes and is retrieved further downstream. The floating case includes a logging GPS so that, on download, the precise path of the drifter is known and can be tied to the acoustic recording using the time code.
This upstream-drop and downstream-retrieve approach has the dual benefit of providing a solution to the ambient noise problem (above) and recording from a swath of locations. This latter feature provides the potential to map the sound intensity over the area of interest. Furthermore, a single support boat can simultaneously deploy multiple acoustic drifters to increase the spatial resolution of acoustic data recorded and make optimal use of brief weather or operational windows.