PIEZOLOGY – Underwater acoustics

The history of underwater acoustics

We can find the very first traces of underwater acoustics in the writings of Leonardo da Vinci. He wrote in 1490: “If you cause your ship to stop and place the head of a long tube in the water and place the other extremity to your ear, you will hear ships at a great distance from you.”. Centuries later, in 1826, Colladon and Sturm‘s experiment on Lake Geneva also helped the development of underwater acoustics. By measuring the difference in arrival times of a flash of light and the sound of a ship’s bell underwater, Daniel Colladon and Charles Strum measured the speed of sound in water for the first time.

Modern underwater acoustics was truly born at the beginning of the 20th century. Several events particularly motivated the development of underwater acoustics, such as the sinking of the Titanic in 1912 and the main world conflicts. During the First World War, German U-boats and their torpedoes wreaked havoc on the Allied fleets. It was therefore necessary to quickly find a solution to detect these submarines. The French Paul Langevin and Constantin Chilowski developed the first active sonar during the First World War. World War II then highlighted the importance of underwater acoustics for military applications.

Since then, civil and military applications have multiplied, such as hydrography or minesweeping for example, and have enabled great technical advances.

Before explaining its importance today, let’s go back to the term underwater acoustics to understand what it means.

What is underwater acoustics?

Underwater acoustics is the study and use of sound in water; from the production to the reception of sound and ultrasonic waves, through their propagation. These acoustic waves are mechanical vibrations created by the disturbance of a fluid and which can propagate through it. They spread very well in water.

Underwater acoustics makes it possible to study different environments such as oceans, seas, lakes, or even reservoirs.

We can therefore say that underwater acoustics is the equivalent in water of radar and radio waves in air.

The ocean is an extremely complex environment for the propagation of acoustic waves and it behaves, on a large scale, like a waveguide, its surface and the seabed acting as reflectors. The speed of sound varies depending on localization and the time of year and generates effects of refraction of sonar waves. Another very important point is that the ocean, no offense to Louis Malle and Commander Cousteau, is not the world of silence. Seismic waves, rain, waves, maritime traffic, cetaceans, or shrimp are all sources of noise that add complexity to the underwater acoustic environment.

This is what makes the beauty of this discipline, but also the complexity of sonar systems.

Now let’s focus on sonars, essential tools for underwater perception.

How do sonars work?

A sonar (Sound Navigation and Ranging) can detect and locate a target in water. There are two types of sonar, passive sonars and active sonars.

• Passive sonars receive and listen to noise emitted by the target
• Active sonars emit a signal which will be reflected or backscattered on the target and then recovered by the sonar which will study it. An active sonar is composed of an acoustic signal transmission system and a reception system.

All sonars are composed of transducers (projectors or hydrophones), transmission and reception electronics, mechanical components, signal processing (extracting information from signals) and software to configure the sonar and visualize information.

Sonars are complex systems that should respond to various mechanical, acoustic, and electronic challenges. For example, there is a large difference in levels between the excitation voltages of the transducers and the signals received (a factor of 1 billion!), and the sonar must be able to manage both.

Also, at the mechanical level, the sonar must work in severe environments: corrosion, pressure, moisture stability…

There are a multitude of different sonars to meet the needs of various applications, we can classify them into three main families:

• Sonars for the perception of the marine environment, which represents their vast majority: for defense (hull mounted sonar or variable-depth sonars, passive sonars, sonobuoys, helicopter-borne sonars), hydrography (multibeam echosounders, sediment sounders, side-scan sonars) or even fishing (split-beam sounders, omni sonars) .
• Sonar-aided navigation for surface or underwater vehicles: Doppler log, acoustic positioning systems (LBL, USBL).
• Modems for acoustic communication

We will detail these different sonars in a future article, but all of them must meet the challenges of understanding the underwater environment and controlling human activities underwater.

Why explore the marine environment?

Despite modern technologies, only 10% of the seabed has been imaged at a resolution lower than a decameter, the rest is still to be explored. It is, therefore, necessary to improve the perception of the marine environment, i.e. to develop solutions for detecting, measuring and imaging underwater, to meet various challenges:

• Security issues: The oceans and seas are environments to secure. Today, more than 50% of the world’s population lives less than 100km from the coast and this figure could rise to 75% by 2035 due to climate change. Also, 90% of traded goods are carried overseas. It is therefore extremely important to know the maritime environment to protect populations and organize world trade in complete safety.
• Territorial issues: Imaging the seas and oceans makes it possible to delimit spaces, in particular the EEZ (Exclusive Economic Zone). These maritime spaces between territorial waters and international waters allow the owner States to exploit them. These areas are therefore often disputed by States and underwater acoustics makes it possible to delimit these zones and to decide between states.
• Strategic and military issues: The ocean can be a conflict zone and underwater acoustics can provide solutions for minesweeping, underwater warfare or protection of coastal areas.
• Economic issues: The seas and oceans are full of resources to locate, identify and quantify, mainly halieutic resources and sea sand. Acoustics provides solutions for their management and exploitation. For fishing, underwater acoustics can both assess stocks and select the species to be caught. Fish selection is a way to fight against overfishing, while in 2020, 34% of the various fish stocks were overexploited. Regarding marine sand, resource management is also important because this sand is essential for manufacturing concrete, and therefore for construction. From an economic point of view, underwater acoustics offer another interesting solution: underwater monitoring. It makes it possible to monitor the condition of submarine cables that allow data transfer between the different continents and to monitor the pipelines used for the transport of hydrocarbons. It is also possible to monitor underwater installations producing renewable or fossil marine energy to secure them and control their condition.
• Scientific challenges: 90% of the seabed remains to be explored, and it is just as many spaces unknown to scientists. Underwater acoustics makes it possible to observe and study underwater volcanism, marine ecosystems (from plankton to cetaceans), underwater archeology or even astrophysics (eg: Laboratoire Sous-marin Provence Mediterranean)…

In addition to improving the perception of the marine environment by making it possible to detect, measure and image underwater, underwater acoustics also makes it possible to communicate and navigate in the marine environment.

Underwater acoustics is a real field of innovation because it must constantly meet new challenges.

What are the new challenges for underwater acoustics?

We observe two major trends in the underwater acoustics environment.

On the one hand, we are seeing an increase in the number of autonomous platforms at sea and a robotization of marine activities (AUV, ROV, towfish, etc.). This trend requires to improve the performance of current solutions: developing smarter, more compact, more energy-efficient, more autonomous systems, etc.

On the other hand, the stakes of marine activities are increasingly important, whether for energy supply (gas pipelines, pipelines) or for communication (submarine cables). The ocean also contains very important resources for the future, such as marine energy and aquaculture.

And what about PYTHEAS Technology?

We decided to dedicate an article to underwater acoustics because, in addition to being an extremely rich and diverse field, it is a sector in constant search of innovation.

Although its use began more than a century ago, underwater acoustics remains essential to maritime activities and new needs are constantly emerging.

From electroactive materials to the design of electronics, including the design of transducers, PYTHEAS Technology has mastered the whole electroacoustic value chain and can support its partners in their sonar’s development, from technical specification to serial production.

If you have a specific need underwater, just contact us!