acoustics is the branch of physics responsible for studying the production and propagation of sound waves. Within it, we can find a great multitude of fields, such as electroacoustics, architectural acoustics or underwater acoustics. The latter refers to the propagation of sound within the water and the study of noise sources that are located in it. Sounds in water travel at 1,500 meters per second, much faster than the speed of sound propagation through air, which is 350 meters per second. For this reason, there are so-called transmission channels, which allow low frequencies or deep tones to be perceived at great distances. underwater acoustics was born as a discipline at the beginning of the 20th century, and its development was closely linked to the development of electronics and the military tradition, since its stage of great development was gave from World War II. His study combines three disciplines: marine biology, geology and geophysics.

Hydrophones, the allies of underwater acoustics

Advances in technology have made it possible to make recordings at the bottom of the oceans and seas in a completely faithful way, being able to store them in flash memories for later treatment and analysis. The possibility of capturing and analyzing the sound in the water has opened various fields of research on the fauna of the seabed and has helped to understand the causes of different natural phenomena. For the success of underwater acoustic research, hydrophones or underwater microphones are used. They are devices designed to capture sound sources in a liquid, especially in water. There are models that emit sound waves by themselves, but they are less frequent. Sound waves are present in water in the form of pressure. The hydrophones are responsible for converting this pressure into electrical energy, that is, into an audible sound spectrum for the human being, its understanding and study. Therefore, they have to adapt to different pressures. When sound signals are received by different hydrophones, the emitting source can be located by triangulation. After that, it is represented on a monitor and its position and direction are observed through algorithms. However, before analyzing the sound recorded by the hydrophone or sonar, it must be isolated from the noises it contains, such as water movement, man-made or thermal noise. Hydrophones are very useful when detecting a hydroacoustic event. This concept refers to noise under water, which can range from the sound caused by an animal to the noise of a boat engine. It is very important to analyze the frequency, amplitude and permanence of the noise to determine if it is a hydroacoustic anomaly.