Depending on how sound travels, the human brain can infer the presence of an obstacle and its proximity. Each material, object, returns sound waves in a different way, according to its texture, its size … That is why the same music will resonate much more in a church than in a study covered with a carpet, the change of environment modifies the propagation of sound . its. But sound waves are also used for more scientific purposes. Depending on how the sound bounces, geologists can deduce what the medium in which the sound has been propagated is made of. It is the principle of topography, which studies land or marine relief.
But recently, a team of scientists led by the University of Zurich (ETH) created a device to simulate the absence of a present object or the presence of an absent object. Based on a modification of the sound waves, it creates an acoustic illusion that deceives the perception of the environment.
Led by Johan Robertsson, professor of applied geophysics at ETH Zurich, the team succeeded in modifying the acoustic field in real time, thanks to an experiment installed at the Immersive Wave Experimentation Center in the Innovation Park in Zurich. The created device allows to imitate the presence of an imaginary disk-shaped object with a maximum diameter of 12 centimeters, or on the contrary to simulate its absence. Their results were published September 10 in the journal Science Advances.
Sound illusions manipulate the sound field.
Unlike light, formed by electromagnetic waves that can propagate in a vacuum, sound waves are said to be “mechanical”: they only evolve in a material environment, in which the disturbance is transmitted by successions of compressions / decompressions of matter .
The manipulation of sound waves can be done in two ways: the most used is the so-called “passive” method, as explained in the press release: “in general, the illusion sought is based on the use of passive methods that they consist of structuring sound waves. Surfaces that use what are called metamaterials. One way to acoustically mask an object is to coat its surface and prevent it from reflecting sound waves. ” But such a method, as explained in the article, only works for certain frequencies, which limits the range of devices and their application.
The other way to create acoustic illusions is the active method, favored by the team of researchers: the illusion is obtained by superimposing a second signal on the initial acoustic field. But, as the press release recalls, “so far, the scope of using this approach has also been limited, as it only works if the initial field can be predicted with certainty.”
Simulate the absence or presence of an object
It was thanks to a microphone and speaker system that the team achieved its goals. For this they created an innovative device, although a bit cumbersome. The target object that will be invisible by acoustic waves is placed inside two rings. On the outside there are a series of microphones that will serve as control sensors and will record the incident signal, while on the inner ring the speakers will return another signal, calculated by computer, that will produce the desired illusion.
The device simulates two situations: the first, on the left, consists of obscuring the presence of an object. The second, holography, consists on the contrary in simulating its presence. The diagrams to the left of each situation represent the natural propagation (blue and pink circles) of the sound waves, while to the right of each situation the propagation with intervention of the device is represented. Microphones are indicated by purple triangles and speakers by red stars. Credits: by Becker TS, Sci.Adv., 2021
To mask the object, the control sources emit a signal that completely erases the sound waves reflected by the object. On the other hand, to give the illusion of their presence, they modify the acoustic field as if the sound waves were bouncing off an object in the center of the two rings. In this way, the researchers were able to cover a wide range of frequencies. “Our installation allows us to manipulate the sound field in a frequency range of more than three and a half octaves,” explains J. Robertsson.
Potential applications in many areas
By scanning a sound frequency range of three and a half octaves, this device can be used in various fields of science that use sound waves, especially in earth sciences, which use the reflection of sound waves for mapping in relief. “In a laboratory we use ultrasonic waves with a frequency higher than 100 kHz to determine the acoustic properties of minerals. On the other hand, in the field, we are studying underground structures with seismic waves at a frequency lower than 100 Hz”, explains J. Robertsson. “The new process will allow us to help save this ‘dead zone’.”
Applications in different propagation media can also be envisaged. “The methodology is not limited to experiments in the air. For example, it can also be used to create acoustic layers and underwater illusions,” explains J. Robertsson. To date, the simulated or hidden object is a two-dimensional object – a disk. But in the future, the team plans to expand the process carried out to three dimensions, in addition to improving its frequency range.