No, we are not talking about a musical instrument but a high-sensitivity underwater listening device. Although hydrophones are mainly used in military activities, there are some that are used as acoustic transmitters in marine research. Very few companies manufacture this type because there is little demand and it costs much more.
What is it exactly? Well, let’s start by saying that a hydrophone is first and foremost an electroacoustic transducer: a device that converts sound vibrations (in this case, underwater) into electrical energy. Hydrophones pick up different vibrations and adapt them to the human audible spectrum. The first hydrophones were built by British, American, and French scientists during World War I to detect enemy submarines.
Many transducers used in hydrophones are made primarily of ceramic and housed in steel walls, but more recently thin quartz crystal transducers bonded with steel plates have been used and have reached resonant frequencies in excess of 150 kilohertz. It must be accompanied by a submersible housing which, depending on depth and pressure, may be made of a lightweight, stainless metal such as aluminum, or of plastic or glass.
The main difference between microphones and hydrophones is that the former is designed to pick up sound vibrations in the air at pressures rarely exceeding one atmosphere, while the latter must adapt to varying pressures or risk affecting the sound and causing damage to the equipment.
Why are they so important? Simple: it is no easy task to analyze the sounds produced at sea. The activity has even been compared to space exploration — even though there are more companies and institutions doing research in space while our oceans remain mysterious. Underwater listening efforts are currently focused on three disciplines: marine biology, geology (volcanology, oil exploration, seismology, etc.), and geophysics, in addition to national defense activities, carried out by the armies of countries having access to the sea.
Thanks to its properties, the hydrophone has become a very important tool in the study of seismic energy and movement, since it is capable of reporting unusual geological activity on the seabed, thus alerting different regions of the possibility of a tsunami or even an earthquake. Likewise, studying the behavior of marine animals and alerting them to possible changes affecting ecosystems is a task that is beginning to be associated with hydrophones. Here, mainly portable devices are used. The objective is clear: to listen to what is happening on the seabed, capture the different sounds of the fauna, and determine both their environment and their presence and location, which includes protected species that are almost always sent to reserves for further research.
Depending on the purpose, hydrophones vary in size: from 11 inches to 6 feet. Some even allow you to listen directly to what is happening underwater without transduction, and the most advanced designs can withstand pressures to a depth of 65,000 feet (to submerge them, it is recommended to do so at a maximum speed of 16 feet per second to prevent the hydrophones from bursting from pressure changes). The listening capability of each hydrophone depends on the design, but the most powerful hydrophones can pick up sounds up to 1.8 miles away. It can be designed to be omnidirectional, primarily for marine biology, or directional, where it not only picks up sound but also uses triangulation with other hydrophones to determine the location and distance of the sound source.
The most difficult process when using a hydrophone is the actual extraction of the sounds. One of the closest solutions is the use of flash memory, but it is only suitable for projects in which the equipment remains underwater for no more than 30 days because if we are faced with the creation of a large network of hydrophones, the most appropriate solution is to go through all the equipment and reach a fiber optic hub that stores all the information. Of course, this is the most expensive solution and is mainly used for government institutions and military purposes — after all, hydrophones are essential for the operation of radars used in submarines and warships, which also results in extensive and powerful surveillance networks, allowing not only the monitoring of borders but of critical infrastructures, such as submarine cables, oil, and gas pipelines.
Another point that requires special attention is the installation of a power module for each hydrophone. If it is a separate project, a battery will be integrated, which provides the necessary autonomy during the project; when it comes to submarine eavesdropping networks, they must be supported by modules with emergency batteries, since the power supply comes from the information center, which uses the same cables for the optical fiber.
If you are interested in learning more about this type of equipment, their technical specifications, as well as what you can do with them, please contact us.