Interview: Ute Kehse
Mr Camprubí, you investigated the history of underwater acoustics. How did this history begin?
Before the First World War, very few attempts were made to understand underwater sound. The main purpose was initially communications. The telegraph was discovered in the 19th century and it was thought that it might even be possible to communicate under water. It was already known that the speed of sound was higher under water than in air. The hydrophone, an underwater microphone making it possible to receive sounds under water, was invented in 1889. When submarines were used for the first time during the First World War, the importance of this research increased. Here, it became apparent that there were many noises in the sea. The question arose as to why the oceans are not quiet, as had been believed until then, but instead were very, very loud.
What does one hear, when one puts a hydrophone underwater?
The most common noises are wind, waves, storms, rain - but also ice cracking. That’s a very impressive sound. You can also hear sounds related to civilization, such as ships’ engines. And, of course, marine life. For example, when the tide goes out you can hear pistol shrimps making very loud, clicking noises with their claws, sounding almost like a castanet. Whales and dolphins also produce very interesting sounds.
What kind of sounds?
Marine mammals produce a very broad spectrum of sounds, from the famous songs of the humpback whale to the less beautiful clicking noises that whales and dolphins use for echolocation.
What can we learn about animals by listening to these noises?
First, we can discover where the animals are. For example, the fishing industry uses sonar equipment to localize fish – the trawlers emit sound and capture the echo in order to find fish. In addition, marine biologists can learn a lot about animal behaviour. Some animals stun their prey using sound, such as the pistol shrimps. And marine mammals communicate with each other using sound.
You say that underwater acoustics opens up a new view of the world.
Exactly. Prior to the 20th century, the deep oceans were only regarded as a place where fish live. Access to the deep sea was very limited. However, thanks to acoustics, humans have added the oceans to their environment. There are good sides and bad sides to this: fishing can be much more targeted and the fish are easier to find. We can produce oil offshore and navigate the world in submarines. But, of course, there are numerous disadvantages: by acoustically exploring the oceans we have exposed them to enormous danger. The largest problem is overfishing, and acoustics plays a major role here. Another example is military sonar, which interferes with whale orientation. Overall, there are many more noises in the seas now than in the past.
What role has military research played in the evolution of marine acoustics?
In my opinion, the history of underwater acoustics, including marine biology, cannot be separated from military history. During the First and Second World Wars and the Cold War, the primary purpose of listening underwater was to detect submarines. And the better the hydrophones became, the more background noises were recorded. The clicking noises of sperm whales, for example, sound like an engine, because they are strongly rhythmic. The US Navy has trained its personnel to differentiate between engine noises and biological sounds. Marine bioacoustics as a discipline, therefore, emerged in part from the efforts to classify different underwater sounds. This had never been systematically done before.
So, initially, scientific research was more a by-product of military research?
Yes. Early investigations of plankton and its movement in the ocean were also carried out by oceanographers occupied with submarine surveillance. Plankton is very difficult to see – but the sound waves emitted by sonar cannot penetrate large masses of plankton, they are reflected. Oceanographers investigating sound propagation in water at the end of the 1940s discovered a scattering layer consisting of plankton, which was very deep during the day and rose to the surface at night. From this, they concluded that it must be a layer of biological material.
What else have biologists been able to learn from military data?
In 1960 the US Navy installed the Sound Surveillance System, SOSUS for short, to globally monitor Soviet submarines. In principle, it consisted of hydrophone arrays installed on the sea floor. They were primarily located in sea straights, namely natural narrow points where submarines had to transit. This enormous surveillance system was top secret. It was not until after the end of the Cold War in 1990 that civilian scientists could exploit the data, for example the marine biologist Christopher Clark of Cornell University. When he saw the data, he recognized that he could track whale migration.
How important are the military data for underwater acoustics today?
During the last two decades, civil oceanography has become much more independent. But there are still fields in which military data may be used for environmental research in the future. In the South China Sea, for example, China is currently building an “acoustic wall” - in principle, a large array of acoustic sensors. In the coming decades, the armed forces there will collect a wealth of information, which scientists may one day profit from - who knows.