In partnership with a number of universities around the world, DTU is leading the way in an area of research, which—for once—should ideally not make (sound) waves.
Silent motorways and submarines that are undetectable to sonar. These are dreams that could actually come true thanks to a new material that absorbs sound rather than reflecting it. DTU is responsible for the theoretical modelling of the literally ‘unheard of’ material that is also known as ‘acoustic metamaterial’.
Acoustic metamaterial is a material with the capacity to change the way sound waves are reflected, such that the sound directed at the material does not create an echo and therefore ‘disappears’, so to speak. This means that it is possible to absorb the sound rather than reflecting it, thus making objects ‘invisible’ to sound.
Research into acoustic metamaterial is being carried out at DTU Fotonik, and PhD student Johan Christensen has recently described the subject in an article published by the respected journal Nature.
“What we’re actually doing is creating a kind of Harry Potter-like cloak of invisibility for sound. We’re changing the macrostructure of the material, thus creating altered properties that result in the sound wave interacting with the structure. However, there are some limits to what the materials can do as they only work within specific frequency ranges. This is one of the aspects we’re currently trying to change,” relates Johan Christensen.
It may sound rather abstract, but if it proves possible to create a material that can eliminate a broad spectrum of sound, the invention may have huge potential in a variety of industries.
“The military is usually the first sector to show an interest in research of this kind,” says Johan Christensen, who goes on to explain that a metamaterial coating could, for example, make submarines exceptionally hard to detect because most detection systems use sonar and other acoustic methods to ‘see’ under water.
Quiet residential area—at 130 km/h
However, there is plenty of potential in more everyday industries, too. For example, an enquiry from a residents’ association from the western part of Copenhagen has already landed on Johan Christensen’s desk. The residents are keen to look into the opportunities for using acoustic metamaterial to ‘soundproof’ motorways so as to reduce the level of disruption to those who live close to the road.
“It’s not actually unrealistic to envisage coming up with super-absorbers one day with the capacity to cover a broad sound spectrum—materials that are commonly referred to as ‘deaf material’,” says Johan Christensen.
Johan is also looking at the world of medicine, because the material could well be used in ultrasound scanners, enabling them to measure much smaller irregularities than they can today.
Development of the acoustic metamaterial is taking place in partnership with a range of educational institutions including UC Berkeley, the Karlsruhe Institute of Technology and the Hong Kong University of Science and Technology (HKUST). DTU Fotonik is responsible for the theoretical modelling, while the other parties are running practical tests in various parts of the world.
“Broadband response (i.e. the capacity of the material to obviate sound waves in a broad spectrum, ed.) currently demands a thick layer of material, but we want to produce a thin solution. It should be possible to make a super-absorber in which the metamaterial is as thin as a sheet of paper—that’s when things can start to get really interesting,” concludes Johan Christensen.