A sonobuoy is a device which is dropped into the ocean and used to gather acoustic data. There are a number of different types of sonobuoys, designed for a variety of applications from anti-submarine warfare to whale research. All sonobuoys are characterized by being very rugged, built to withstand severe weather and extreme temperature and pressure, and many are also designed to be essentially disposable, as loss of a sonobuoy is quite common.
The sonobuoy owes its development to the Allied need to monitor submarine traffic in the First World War. With the development and deployment of the German U-Boat, the Allies realized that they would be powerless against the Germans unless they had a way to identify and track the German U-Boats. The result was the development of early sonar systems, which used sound waves in a variety of ways to identify objects moving through the ocean. Planes started dropping sonobuoys into the Atlantic to track the course of U-Boats, and ever since then, these devices have been refined and retooled for an assortment of purposes.
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There are two main parts to a sonobuoy: the buoy itself, and a radio transmitter. When a sonobuoy is dropped into the water, the buoy detaches from the transmitter, allowing the transmitter to float on the surface of the water while the sonobuoy sinks below. As the sonobuoy gathers data, it passes the information on to the transmitter, which in turn transmits the data to an aircraft or ship. When it is possible to do so, the sonobuoy will be retrieved after use.
A basic sonobuoy is simply passive, recording the sounds of the water it is immersed in. These sounds can sometimes be quite interesting, as in addition to revealing passing ship traffic, a sonobuoy will also record the sounds of ocean life. Militaries use sonobuoys to watch out for submarines and other hazards, while scientific researchers utilize the data to find out more about the life in the ocean. A scientific sonobuoy also often collects data about currents, temperatures, and pressure.
An active sonobuoy, on the other hand, emits sound waves and then listens, waiting for their reflection and using this data to gather information about objects in the water. Active sonobuoys can identify a submarine even when it is on “silent running,” meaning that the engines are off and everyone on board is expected to be silent, as the sound waves will reflect from the sub's hull in a characteristic fashion.