Sound projector with “acoustic lenses” - we understand how the technology works

We discuss a device for transmitting directional sound. It uses special “acoustic lenses”, and the principle of its operation resembles the optical system of the camera.

About the variety of acoustic metamaterials

With various metamaterials , the acoustic properties of which depend on the internal structure, engineers and scientists have been working for a long time. For example, in 2015, physicists managed to print an “acoustic diode” on a 3D printer - it is a cylindrical channel that lets air through, but completely reflects sound coming from only one direction.

Also this year, American engineers developed a special ring that blocks up to 94% of noise. The principle of its operation is based on the Fano resonance , when the energy of two interfering waves is distributed asymmetrically. We talked more about this device in one of our posts .

In early August, it became known about another audio development. Engineers from the University of Sussex presented a prototype device that, with the help of two metamaterials ("acoustic lenses") and a video camera, allows you to focus sound on a specific person. The device was called a "sound projector."

How it works

There are two “acoustic lenses” in front of the sound source (audio speaker). These lenses are a 3D printed plastic plate with a large number of holes. How these “lenses” look can be seen in the whitepaper from the developers on the first page (you need to open the full text of the document).

Each hole of the “audio lens” has a unique shape - for example, irregularities on the inner walls. When sound passes through these holes, it changes its phase. Since the distance between two “acoustic lenses” can be varied with the help of electric motors, it becomes possible to direct the sound to one point. The process resembles the focusing of camera optics.

Focusing is done automatically. For this, a video camera (costing approximately $ 12) and a special software algorithm are used. It remembers the person’s face in the video and tracks its movement in the frame. Further, the system calculates the relative distance and accordingly changes the focal length of the projector.

Where will they use

Developers note that in the future, the system may replace headphones - devices will broadcast sound from a distance directly into the ears of users. Another potential area of ​​applicability is museums and exhibitions. Visitors will be able to listen to lectures by electronic guides and not disturb others. Of course, one cannot but note the advertising sphere - it will be possible to communicate the conditions of personal promotions to visitors to stores.

But engineers have yet to solve a number of problems - so far the audio projector is able to work only in a limited frequency range. In particular, he plays only notes from G (salt) to D (pe) in the third and seventh octaves.

Hacker News residents also see potential legal issues. In particular, it will be necessary to regulate who and under what conditions will be able to receive personalized advertising messages. Otherwise, chaos will begin in the premises of shopping centers. As the developers of the "audio projector" say, this issue will partially help solve the face recognition system. It will determine whether the person gave their consent to receive such advertisements or not.

In any case, there is no talk of the practical implementation of the technology “in the field”.

Other methods of transmitting directional sound

At the beginning of the year, engineers from MIT developed a technology for transmitting directional sound using a laser with a wavelength of 1900 nm. It is harmless to the human retina. Sound is transmitted using the so-called photoacoustic effect , when water vapor in the atmosphere absorbs light energy. As a result, a local increase in pressure occurs at a point in space. A person is able to perceive the fluctuations in air with the “naked ear”.

Similar technology is being developed by specialists from the US Department of Defense. Using a femtosecond laser, they create a plasma ball in the air and cause sound vibrations in it with the help of another nanolaser. True, in this way it is only possible to generate a roar and an unpleasant noise, similar to a howling siren.

So far, these technologies have not gone beyond the boundaries of laboratories, but their analogues begin to "penetrate" into user devices. Last year, Noveto already introduced an audio speaker that creates “virtual headphones” on a person’s head using ultrasonic waves. Therefore, the massive spread of directional sound technology is only a matter of time.

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