Do you know acoustic illusions?

When listening to music, we not only hear the tones produced by the instruments but we are also immersed in its echoes from the surroundings. Sound waves bounce off the walls and objects around us and create a characteristic sound effect – a specific acoustic field. This explains why the same music sounds very different when played in an old church or a modern concrete building.

Architects have been taking advantage of this fact for a long time in the construction of e.g. concert halls. However, this principle can be applied elsewhere: underground objects can be visualized by measuring how sound waves are reflected from a known source.

Active and passive manipulation of the acoustic field

Some scientists want to go a step further and systematically manipulate the acoustic field to achieve an effect that should not exist by itself in real life. For example, they try to create an illusory sound experience that tricks the listener into believing they are inside a concrete building or an old church. Alternatively, objects can be made invisible by manipulating the acoustic field so that the listener stops perceiving them.

The desired illusion usually relies on the use of passive methods, which involve structuring surfaces using what we know as metamaterials. One way to acoustically hide an object is to coat its surface to prevent it from reflecting any sound waves. However, this approach is inflexible and usually works only in a limited frequency range, making it unsuitable.

Active methods try to achieve the illusion by superimposing another layer of sound waves. In other words, by adding a second signal to the initial acoustic field. However, so far this approach has been limited in scope because it only works if the initial field can be predicted with some certainty.

Illusion in real-time

A group led by Johan Robertsson, professor of applied geophysics at ETH Zurich, collaborated with scientists from the University of Edinburgh to develop a new concept that significantly improves the active illusion. Led by Theodor Becker, a postdoctoral fellow in Robertsson's group, and Dirk-Jan van Manen, a scientist who helped design the experiments, the researchers were able to expand the initial field in real-time, reports the journal Science Advances. As a result, objects can disappear and "pretend" not to exist.

To achieve the special acoustic effects, the researchers installed a large test facility for the project at the Center for Submersible Wave Experimentation at the Swiss Innovation Park Zurich in Dübendorf. Specifically, this device allows them to mask the existence of an object approximately 12 centimeters in size or to simulate an imaginary object of the same size.

The target object is enclosed in the outer ring of microphones as control sensors and the inner ring of loudspeakers as control sources. Control sensors register which external acoustic signals reach the object from the initial field. Based on these measurements, the computer then calculates what secondary sounds the control sources must produce to achieve the desired magnification of the initial array.

Sophisticated technology

To mask an object, control sources emit a signal that completely erases the sound waves reflected from the object. In contrast, to simulate an object (known as holography), the control sources magnify the initial acoustic field as if the sound waves were reflected from the object in the center of the two rings.

For this extension to work, the data measured by the control sensors must be immediately transformed into instructions for the control resources. To control the system, the researchers, therefore, use field-programmable gate arrays (FPGAs) with extremely short response times.

"Our device allows us to manipulate the acoustic field over a frequency range of more than three and a half octaves," says Robertson. The maximum frequency for masking is 8700 Hz and 5900 Hz for simulation. Until now, scientists have been able to manipulate the acoustic field on a surface in two dimensions. As a next step, they want to increase the process to three dimensions and expand its functional range. The system currently amplifies sound waves transmitted through the air. However, Robertson explains that the new process could also create acoustic illusions underwater. It foresees a wide range of potential applications in various fields such as sensor technology, architecture, and communication, as well as in the education sector.

New technology is also very important for earth sciences. "In the laboratory, we use ultrasound waves with a frequency above 100 kHz to determine the acoustic properties of minerals. In contrast, in the field we study underground structures with seismic waves of less than 100 Hz," says Robertson. "The new process will allow us to help bridge this 'dead zone'."

Source:

ETH Zurich. “Acoustic Illusions.” ScienceDaily. ScienceDaily, 10 September 2021.www.sciencedaily.com/releases/2021/09/210910172804.htm.