Researchers Devise New Method of Making Objects Disappear with Sound

Researchers Devise New Method of Making Objects Disappear with Sound
The object that is to be made invisible acoustically is placed at the centre of the experiment. The initial acoustic field is produced by the loudspeakers in the outer ring. The 228 control sensors in the central ring record this field and transmit the data to a computer. Subsequently, 36 control sources in the centre emit a secondary signal that augments the initial field in real-​time. (Photograph: ETH Zurich / Astrid Robertsson)
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When we listen to music, we are immersed in its echoes from our surroundings as well as the notes produced by the instruments. Sound waves reverberate off the walls and objects around us, creating a distinct sound effect – an acoustic field. This explains why the same piece of music sounds so different when performed in an old church versus a modern concrete structure.

Architects have long used this fact to their advantage when designing structures such as concert halls. The principle, however, can be applied to other applications: for example, objects hidden underground can be visualized by measuring how sound waves from a known source are reflected.

Active and passive manipulation

Some scientists want to go a step further and systematically manipulate the acoustic field to achieve an effect that, given the real-life situation, should not exist. For example, they’re attempting to create an illusory audio experience that fools the listener into thinking they’re in a concrete building or an old church. Alternatively, objects can be rendered inaudible by manipulating the acoustic field so that the listener no longer perceives them.

Typically, the desired illusion is achieved through passive methods such as surface structuring with the help of what are known as metamaterials. One method of acoustically concealing an object is to coat its surface and prevent it from reflecting any sound waves. However, this approach is rigid and typically only works within a narrow frequency range, making it unsuitable for many applications.

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Active methods attempt to create the illusion by superimposing another layer of sound waves on top of the first. In other words, a second signal is added to the initial acoustic field. However, until now, the application of this approach has been limited because it only works if the initial field can be predicted with some certainty.

Real-​time illusion

Johan Robertsson, Professor of Applied Geophysics at ETH Zurich, has now collaborated with scientists from the University of Edinburgh to develop a new concept that significantly improves the active illusion. The researchers, led by Theodor Becker, a postdoc in Robertsson’s group, and Dirk- Jan van Manen, a senior scientist who was instrumental in designing the experiments, have managed to augment the initial field in real time, as they report in the journal Science Advances. As a result, they can make objects vanish and imitate non-existent ones.

The researchers installed a large test facility for the project in the Centre for Immersive Wave Experimentation at the Switzerland Innovation Park Zurich in Dübendorf to achieve the special acoustic effects. This capability, in particular, enables them to conceal the existence of an object measuring approximately 12 centimetres in size or to simulate an imaginary object of equal size.

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An outer ring of microphones serves as a control sensor, while an inner ring of loudspeakers serves as a control source, enclosing the target object. The control sensors record which external acoustic signals from the initial field reach the object. A computer then calculates which secondary sounds the control sources must produce based on these measurements in order to achieve the desired augmentation of the initial field.

The test facility simulates two situations. While cloaking (on the left), the control sensors and control sources around the physical scatterer ensure that the sound waves of the primary source are not reflected. During holography (on the right), they produce an acoustic illusion as if a physical scatterer were at their centre. (Visualisations: from Becker TS, Sci.Adv., 2021)

Cutting-edge technology

The control sources emit a signal that completely obliterates the sound waves reflected off the object to mask it. To simulate an object (also known as holography), the control sources augment the initial acoustic field as if sound waves were bouncing off an object located in the center of the two rings.

To make this augmentation work, the data measured by the control sensors must be instantly converted into instructions for the control sources. The researchers use field-programmable gate arrays (FPGAs) with extremely short response times to control the system.

“Our facility allows us to manipulate the acoustic field over more than three and a half octaves,” Robertsson says. The maximum frequency for cloaking is 8,700 Hz, and the maximum frequency for simulating is 5,900 Hz. To date, the researchers have been able to manipulate the acoustic field on a two-dimensional surface. They intend to expand the process to three dimensions and expand its functional range as a next step. Currently, the system augments airborne sound waves. However, according to Robertsson, the new process could also create acoustic illusions under water. He sees a wide range of potential applications in fields such as sensor technology, architecture, and communications, as well as in education.

The new technology is also extremely important in earth sciences. “We use ultrasound waves with a frequency of more than 100 kHz in the lab to determine the acoustic properties of minerals. In the field, however, we study underground structures using seismic waves with frequencies less than 100 Hz,” Robertsson explains. “The new process will allow us to assist in bridging this ‘dead zone.’”

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“This collaboration began 15 years ago when the underlying theory was developed, which illustrates the long-term nature of scientific projects,” said co-author Andrew Curtis, Chair of Mathematical Geoscience at the University of Edinburgh in the United Kingdom. The European Research Council’s funding, which brings together European scientists, has been critical.”

You can find the original study here.

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