When you think of underwater acoustics, you may think of SONAR technology, which is used in nature and in submarines to determine location and terrain. Before SONAR, the only real way to find out the location of a submarine was to wait until it surfaced for visual confirmation; not exactly ideal in times of war. Therefore, SONAR was developed in World War II and subsequently installed on many ships to aid in the detection of enemy ships. As technology inevitably does, SONAR has evolved and been put to use for many other reasons outside of the military realm, in the form of sonobuoys.
The word itself, “sonobuoy”, gives you a little clue as to its function. The first half of the word comes from SONAR, and the second half indicates that it works similar to ocean buoys, which float in place as a reference marker to some point. They are used as radio transmitters, being ejected from an aircraft to be deployed upon impact with the surface of the body of water, in order to communicate information to a distant vessel. A hydrophone sensor, or multiple hydrophone sensors, are then activated to descend to a specific depth to transmit acoustic information. Sonobuoys are used in underwater acoustic research as well as anti-submarine warfare.
When sonobuoys were first employed in underwater acoustics, they had very limited range, short battery life, and often dropped out or data became inconsistent due to background noise caused by the surrounding ocean. Early models also had to be around six feet tall, with accompanying sensors roughly two feet in diameter. Since then they have become much more advanced and have been greatly reduced to a compact tube that is easily parachuted and extremely reliable when deployed.
There are three categories of sonobuoys, and each serves different purposes. The first type are passive sonobuoys, which do not deploy any equipment, such as acoustic sensors or hydrophones, deep in the water, but instead passively collect acoustic information from waves generated by a propeller, or even doors being closed on ships or submarines. The second type are active sonobuoys, which you can guess actually deploy equipment in the water, then “ping” sound into the water and wait for the echo back, just like traditional SONAR technology. These active sonobuoys typically transmit the echo range and bearing information to a ship or aircraft, and will continue to ping and retransmit the information for a set period of time.
The last type of sonobuoy is called a special purpose sonobuoy. They are the only type not used for submarine detection, although the other two types are not used exclusively for submarine detection either. Special purpose sonobuoys are different in that they are specifically designed to transmit data related to the geography of the ocean floor. They can transmit data to ships, planes, and even satellites. This is also extremely useful in search and rescue missions, as these sonobuoys can mark the locations of plane crashes and sunk ships, and locate crash survivors and transmit location data to search and rescue crews.