Photo credit: Theodor S. Becker, ETH Zürich.
Sound waves do not always hit our ears directly, they will also bounce-off other objects & the walls of the space that we are in, which is why listening a band play in a cavernous cathedral is a different experience to listening in a small music club.
Now, scientists developed a technique for ‘cloaking’ the impact that objects have on acoustic fields, so sound waves do not appear to hit or reflect back from them. In effect, these objects can be made invisible as far as acoustics concerned.
It works using an outer ring of microphones (used as audio sensors) & an inner ring of loudspeakers (used as audio sources). By analyzing the sound waves picked-up by the mics, a computer directs the speakers to instantly adjust the acoustic field so it behaves as if the object being concealed was not there.
“This opens previously inaccessible research directions & facilitates practical applications, including architectural acoustics, education & stealth,” researchers explain in their paper.
The idea of hiding objects acoustically is not itself new, it is also been tried with what are referred to as metamaterials, designed to soak-up all the sound waves as they arrive a surface. However, this is a passive, fairly-inflexible approach that only works across a limited range of frequencies.
With this new real-time approach, there is a lot more versatility in making objects disappear and it can even work the other way around also, to make it sound as if a non-existent object is taking-up space in the room (holography).
A primary source emits an initial wavefield, which is scattered by an object in the case of cloaking (A) or propagates unobstructed in the case of holography (C). Active control sources augment the incident wavefield to either cloak the scattering object (B) or create a hologram of an object that is not physically present (D). The input to the control sources is obtained by real-time forward extrapolation using measurements of the control sensors.
Robertsson et al., Science Advances, 2021
What called Field Programmable Gate Arrays (FPGAs), integrated circuits which can be custom-coded, to ensure that audio source outputs are able to-respond-to the audio speaker outputs with virtually no delay at all.
So far, researchers managed to urge their system working for 2D objects upto 12 centimeters (4.7 inches) in size. With further study, team expects to be able to scale-up the techniques to work with 3D objects which can be much larger in size. What is more, it is already functioning across a wide frequency range.
“Our facility allows us to manipulate the acoustic field over a frequency range of more than three & a half octaves,” says geophysicist Johan Robertsson from ETH Zurich in Switzerland.
The technology could potentially be put to good use in any field, where sound waves logged & analyzed, which covers a whole range of scientific applications, like the study of underground structures.
Further down the line, researchers hope to urge a system like this working underwater too, where acoustics are significantly different. Again, any type of sound wave scanning process, where existing objects got to be hidden or virtual objects got to be placed could benefit.
This new research is another demonstration of the incredible patience of many scientists too, with initial groundwork for the acoustic cloak developed a several years ago, as mathematical geoscientist Andrew Curtis from University of Edinburgh in UK explains.
“This collaboration started 15 years ago when the underlying theory was developed which illustrates the long-term nature of scientific projects,” Curtis said.
This research has been published in Science Advances.
