Use of Sonobuoys in Underwater Acoustics

When you think of underwater acoustics, you may think of SONAR technology, which is used in nature as well as by submarines in order 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 war time. Therefore, SONAR was developed in World War II and was subsequently installed in many ships to aid in enemy ship detection. As technology inevitably does, SONAR has evolved and been put to use for many other reasons outside of the military, in the form of sonobuoys.

The word itself, “sonobuoy”, gives you a bit of a clue as to its function. The first half of the word comes from SONAR, and the second half indicates that it operates in a similar fashion to ocean buoys, which float in place as a marker for reference to some point. They are used as a radio transmitter, 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 several hydrophone sensors, are then activated to descend to a specific depth to relay acoustical information. Sonobuoys are used in acoustic research underwater, as well as in anti-submarine warfare.

This article provides a brief introduction on the topic of underwater acoustics. For a deeper dive (sorry…) on the subject, we’d like to refer you to these two books from Amazon:
Underwater Acoustics: Analysis, Design and Performance of Sonar

Transducers and Arrays for Underwater Sound (Modern Acoustics and Signal Processing)

When sonobuoys were first being employed in underwater acoustics, they had a very limited range, a short battery life, and were often interrupted or the data made inconsistent due to background noise caused by the surrounding ocean. The early models also had to be around six feet tall, with accompanying sensors that were about two feet in diameter. They have since become much more advanced, and slimmed down considerably to a compact tube that parachutes easily and is extremely reliable upon deployment.

There are three categories of sonobuoys, and each serve different purposes. The first type is passive sonobuoys, which do not deploy any equipment such as acoustic sensors or hydrophones into the watery depths, but instead passively collect acoustic information from waves made by a propeller, or even doors closing on ships or submarines. The second type is active sonobuoys, which you may guess do in fact deploy equipment into the water, then emit sound “pings” into the water and wait to receive the returning echo, much like traditional SONAR technology. These active sonobuoys usually transmit the information of the range and bearing of the echoing ping to a ship or an aircraft, and will continue to ping and relay the information for a set period of time.

The last type of sonobuoy is called a special purpose sonobuoy. They are the only type that is not used for submarine detection, although the other two types are not exclusively used for submarine detection either. Special purpose sonobuoys are different in that they are specifically designed to relay data that pertains to the geography of the ocean floor. They can transmit data to ships, aircraft, and even satellites. This is also extremely useful in search and rescue missions, as these sonobuoys can mark the location of crash sites for planes, ships that have sunk, and locate crash survivors and transmit the location data to search and rescue crews.

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