One day I was out on some forest trails on my Mountain Bike and I came across a person lying with his ear pressed against the ground. He was mumbling away. “White 4WD, bull bar, driving lights, NSW registration plates.” I was impressed. “You can tell all that from just listening to the ground?” I asked. “No, that is the car that just ran over me!”
Listening to the ground to describe a car seems like a silly idea but it was as early as 1490 that Leonardo da Vinci inserted a tube into water to detect ships that were approaching over the horizon. Sound travels in water approximately 4.3 times faster than it travels through air because water particles are packed in more densely. This method of detecting a vessel in the water was not entirely accurate and it was not until World War I that active sonar was developed to detect submarines.
The concept is one that most people are familiar with in bats. Bats have good eyesight but to fly and catch prey at night, they use echolocation to ‘see’ their surroundings. They contract their larynx muscles to make sounds and their hearing is finely tuned to the returning echoes. They can determine the direction and distance of objects. As they approach prey, they increase the frequency of their sounds to increase their accuracy.
Ships use an array of projectors to transmit signals in the 100-500Hz frequency range and then listen for any return sounds which are reflected from an object such as a submarine. At these frequencies, a submarine can be detected hundreds of kilometres away. Sonar is also used at higher frequencies of 2-10kHz to find and track underwater objects that are tens of kilometres away. The higher frequency delivers higher accuracy at the expense of range.
The detection technology is constantly improving but researchers are working on methods to try and make a submarine invisible in the same way that a B-2 Stealth Bomber minimises its radar image.
The Stealth Bomber uses a variety of curves and non-reflective surfaces to try and reduce the radar image. Submarines have typically used some form of anechoic rubber tiles since the second World War to try and minimise the sonar reflection but researchers at Xi’an Jiaotong University in China have used a machine learning algorithm to develop a new rubber coating to make a submarine almost invisible to active sonar. The ultimate aim of a surface on a submarine is to absorb the emitted sonar signals rather than redirect them.
The new rubber coating is only 32mm thick but in tests so far is capable of absorbing 95 per cent of the sonar signal in the 1-10kHz range.
The material is made from three layers of rubber strips with some strips having a rectangular piece of lead running through them. Others have pyramid-shaped air cavities. The algorithm determined that this was the best configuration for maximising the absorption of many different sound wave frequencies.
Compared to some of the existing anechoic tiles which are over 100mm thick, the new surface is thinner whilst being more absorbent.
While it may seem that this has a very specific application in coating submarines to avoid detection, sound absorption is increasingly important in our modern society where we see more videoconference meetings than ever imagined. Once people finally manage to correct their lighting mistakes, the next issue is the reverberating echo you hear in many video calls. If better sound absorption materials are developed for sonar, those same techniques may possibly be applied to surfaces in the board rooms of tomorrow.