This type of device uses one single focused transducer element with a conical or dish-shaped sound reflector to focus the signals, not unlike a reflector telescope. These types of hydrophone can be made from low-cost, omnidirectional types, but can only be used when stationary as the reflector restricts its movement when in water. A modern way of directing it is using a spherical body around the hydrophone. The advantages of these ‘directivity spheres’ is that the hydrophone can be moved within the water, causing less interference than a conical element.
Directional hydrophone beam pattern:
‘Array hydrophone bioacoustics’ (multiple transducers daisy-chained together) are each wired collectively to enable reception of a large sound signal. These type transducers are packed together typically in a tube containing oil or resin which aids in the collection of pressure waves. With the use of multiple hydrophones towed behind a research vessel, these arrays allow triangulation of signals that can be used to pinpoint locations or help to filter out ambient noises.
To further aid in the signal quality, several hydrophones can be arranged in an array which will combine the signals from the selected echolocation while deducting signals from the other directions’ beam pattern.
(the included example blasts a column of air onto the sea bed to highlight the sub-terrain or anything between).
The array may be guided using a ‘beam former’. Commonly hydrophones are arranged in an array line, but may be in 2 or 3 dimension arrangements, thus giving directional hydrophone capability.
The Navy hydrophone was used for early detection and spying. The sosus hydrophone – positioned strategically on the seabed and connected by underwater cables – was used in the early 1950s by the U.S.A. Navy to track movement of Russian submarines in the Cold War, tracing a line from Greenland, Iceland, and Great Britain, known as the GIUK gap, abbreviated from the first letters of each country.