Sound customization: “lenses” from metamaterial to control the sound field

When going to the cinema, the first thing we pay attention to is the picture. Bright colors, a clear image without any blemishes are of great importance for our perception of the film that we are watching. But do not forget about the sound. If its quality is lame, then whatever the picture, the viewing experience will be spoiled. Image quality is given much more attention: new screens, glasses for 3D video, cameras, lenses and much, much more are being developed. Today we’ll talk with you about a study in which a group of scientists decided to correct this injustice. They devoted all their attention, time and intelligence to the sound, or rather the development of a new device that can work with sound, like with light. A telescope, a magnifying glass, a collimator and even a varifocal lens, and all this with the prefix “acoustic”. How exactly did the scientists manage to achieve control over the sound waves, what is their device like, how difficult is it to create it, and what results did it show during the tests? We learn about this from the report of the research group. Go.

Study basis

Scientists note that the formation and management of sound fields is the most important component of modern technologies related to sound reproduction. As a rule, this is achieved by controlling the intensity or phase of the sound source-generator using phased arrays. This method allows you to control sound in real time, however, devices of this kind are often bulky and expensive.

In turn, light requires a different approach if we want to gain control over it. Perception can be improved through related details (filters, lenses, etc.). Changing the parameters of these elements allows you to get a certain type of device with its own unique properties (cameras with autofocus, LCD displays, VR headsets, etc.). Such manipulations with sound are not yet possible. If we want the best sound, we need large and powerful speakers, exaggeratedly speaking.


Image No. 1: (a) - comparison of previous developments (left) and described in this work (right); (b) - conversion of a standard column to a directional one; (c) - installation of a focusing acoustic lens.

Metamaterials can help solve this problem. A distinctive feature of such materials is that their properties practically do not depend on the characteristics of the substances from which they are made. It is much more important how exactly they are made, that is, what structure, architecture, topology, form, etc. they have. Unfortunately, the use of metamaterials in work with sound is not yet very common due to some difficulties: thickness that does not correspond to wavelengths; device static and limited frequency range.

For scientists, these limitations are a challenge that they boldly accept. They developed a new method for designing metamaterials that resemble lenses, but not for light, but for sound. At the same time, we managed to circumvent the above limitations. How exactly we will analyze in more detail.

Metamaterial Design

Researchers identify four main steps in the process of creating a metamaterial:

• the choice of its functions (what it should do with sound);
• converting this information into a similar phase / intensity distribution ( 2a ) on the surface of the metamaterial (hereinafter metasurface);
• selection of working cells ( 2a );
• creation of a metasurface, taking into account the limitations in terms of space and amplitude-frequency characteristics ( 2b ).

Image No. 2: (a) - comparison of two phase profiles; (b) - COMSOL transmission simulation of cell # 15, scaled so that its base is 10.4 mm; (c) - the principle of operation of a cell of type B.

It must be understood that the distribution of acoustic pressure passing through the device will depend on the future function of the metasurface. Accordingly, the geometry of the metasurface and the intensity distribution play an important role.

Scientists, obviously, know what exactly they go from their creation - to act like a lens, but for sound. In this case, the lens will be characterized by two parameters: focal length and physical size (in the case of a metasurface, how many cells the lens occupies).

Once the desired focal length ( f ) is set along the axis of the lens (ˆz), the phase distribution φ (x, y) on the metasurface (it is assumed that it is in the z = 0 plane) is obtained by asserting the fact that all contributions from the cells enter into phase on (0, 0, f ). For this specific work, scientists used a parabolic profile:

φ ( r ) = φ ₀ - А ² (x ² + y ² )

where φ (x, y) is the local phase related to the cell, A is the constant associated with the local phase curvature profile, λ ₀ is the calculated wavelength, and φ ₀ is an arbitrary constant.

The parabolic phase profile in optics allows one to obtain more compact lenses, therefore, the designed metasurface will also be small. In addition, such a profile connects the parameter A with the “curvature” of the lens, that is, the larger A, the more the focusing lens is obtained ( 2a ).

After establishing φ (x, y), it is necessary to choose which cells on the metasurface will be involved. It is also necessary to take into account the fact that the lower the frequency, the larger the cell should be.

A 16-cell metasurface model was used in the study: rectangular cuboids ~ 4.3 x 4.3 x 8.6 mm in size, designed for maximum transmission (~ 97% of the input sound) at f ₀ ± Δ f _2dB= 40 ± 1 kHz. The easiest way to apply such a model at a different frequency ( f ) is to scale: resize each cuboid until its thickness is equal to the new wavelength λ = c ₀ / f (where c ₀ ~ 343 m / s is the speed of sound in air) .

At the new frequency, each of the cells applies the same phase delay in the range 0 ... 2π, while all of them have the same throughput as with f ₀ .

Scientists note that a cuboid designed under f ₀ has the same transmission at different frequencies ( 2b ). These frequencies are defined as follows:

f _j = f ₀ - j ⋅ c ₀/ L _eff

where j = 0, 1, 2 ... N is an integer, L _eff is the calculated parameter of a particular cell, N = round (L _eff / λ ₀ ) is an (integer) number of times when L _eff contains a wavelength .

From this it follows that it is possible to work with cells at one of the frequencies f _j ( 2s ), supporting a transmission comparable to that which is at f ₀ .

During the tests, the frequency f ₀ was used.= 5.600 Hz. This frequency corresponds to a wavelength of 6 cm. It was chosen solely due to technical limitations (a 3D printer could not print larger cells). But, according to scientists, given the scalability of their model, this limitation during tests does not affect the conclusions.

Two types of lenses were used:

• Type A is obtained by scaling the cells, so that their first resonance (j = 0) is 5.6 kHz, and the thickness is equivalent to λ ₀ (i.e. 60 mm). Each of the lenses of this type consists of an array of 8x8 cells, and the total size is 240x240x60 mm ( 1a , left). The lens bandwidth is 2 ⋅ Δf _2dB ∼ 0. 05 ⋅ f ₀ .
• Type B is obtained by scaling the cells, so that their second resonance is 5.600 Hz. Each lens of this type consists of an array of 10x10 cells, and the total size is 104x104 mm with a thickness of 20.8 mm ( 1a , right). The throughput of type B is also quite large. Calculations showed that it is 2 ⋅ Δf _2dB ∼ 0.28 ⋅ f ₀ . The main disadvantage of type B lenses is the following: given that the 16-cell model covers only part of the phase space, only a limited number of focal lengths can be realized with a fixed-size lens.

In the graph above, we can see the simulation results, which show that in the case of using a 10x10 lens, the maximum focal length will be 57 mm. That is, to increase the focal length it is necessary to increase the lens.

The main points in the design of the metasurface have become clear to us. Now we will go on to describe how all this was put into practice in the form of prototypes.

Acoustic collimator

Researchers, given the above achievements, were able to create an acoustic collimator - a system that corrects the geometric divergence of the source, as a result of which the sound is spatially represented in the form of a beam at the output. Simply put, the sound does not spread wherever he wants, but forms a focused beam.



The picture above shows how sound propagates without metamaterial (blue field) and with metamaterial (red field).

In optics, collimators are used both in beacons for projecting light over long distances, and in the production of spotlights. In such devices, the lens is located at a distance from the light source equal to the focal length of the device, due to which the incident wave turns into a parallel beam.

In the case of an acoustic collimator, a type A metamaterial lens was located at a distance of 150 ± 2 mm from the sound source.


Image No. 3: acoustic collimator performance and installation diagram.

Graph 3a in the image above shows that the acoustic pressure measured at different distances from the sound source is much greater with a lens than without it. The angular radiation, measured at a distance of 4.24 m, shows that the divergence angle of the speaker (sound source) due to the lens decreased from 60 ° ± 1 ° to 27 ° ± 1 ° (3b).

Scientists also note that a metamaterial lens has changed the sound quality of cheap dynamics used in experiments. At the same time, tests in the open air showed a significant increase in the distance of sound perception: without an acoustic collimator - 10 m, with a collimator - 40 m.

Scientists suggest that the divergence angle can be made even smaller by more accurately adjusting the distance between the speaker and the acoustic lens (collimator) .

How can an acoustic collimator be used in life? The developers of this device have several options:

• Personalization of sound - projecting sound exclusively into certain areas of the cinema ( 3s ); different acoustic signals depending on the position in space (VR-headset); the creation of different sound zones (for example, 3 people are sitting on the couch and everyone is listening to his own, without disturbing the others).
• Increasing the performance of speakers - at concerts and in cinemas, they always try to optimize the sound so that everyone can hear everything, but there is a part of the audience where the sound is “inferior”. The 3d image shows 2 speakers symmetrically directed in different directions. In this position, there is a gap where the sound will be bad, roughly speaking. Using the acoustic collimator installed in this gap can fix this.
• Improving the spatial sensitivity of acoustic sensors.

Acoustic magnifier

Image No. 4: scheme and photo of the installation of an acoustic magnifier.

We are all familiar with the inherent attribute of the image of a detective - a magnifying glass or a magnifying glass. We look through a magnifying glass at something and see this object in an enlarged form. The same thing happens with sound if you use an acoustic magnifier. In the test setup ( 4a ), scientists placed metamaterial (magnifier) ​​between the microphone and the speaker. The position of the magnifying glass was adjusted until the maximum signal received by the microphone was reached. Due to this, a weak sound is amplified.

The scope of the acoustic magnifier is also not limited to one option:

• Changing the position of the source - an example is shown in diagram 4b : a man sits on a sofa in front of a TV with a built-in speaker. If you use an acoustic magnifier, you get the feeling that the speaker is right in front of it.
• Enhanced capabilities of tactile devices (feeling of touch in the air, video below). Such technologies are directly related to sound, but are limited in the maximum distance between the "virtual" object and its generator. An acoustic magnifier can increase this distance.

Tactile technology creates a sense of touch through sound.

• Improving sound reception - an acoustic lens can change the spatial characteristics of a microphone. Figure 4d shows the use of an acoustic magnifier to focus on a specific object, surrounded by many others. Simply put, such a magnifier will allow you to listen only to what you need, eliminating all associated and background noises.
• Sound leveling from different sources. Imagine that you are talking with two people in a large room. One interlocutor is standing nearby, the second is far away. An acoustic magnifier would allow you to hear both interlocutors the same way, as if they were both standing at the same distance from you (visual example in image 4e ).

Acoustic telescope

Telescopes are needed to study what is very far away. A banal and exaggerated statement, but from that it does not lose its veracity. Telescopes work due to two lenses located at a certain distance from each other. An acoustic telescope also uses a similar principle.



Above is a photo of the installation of an acoustic telescope: two lenses from metamaterial, the distance between which can be changed with an accuracy of 1 mm, and a speaker.

The main advantage of the telescope is that it can circumvent the limitation of the focal length of one lens, because two are used, and the ability to change the distance between them allows you to change the focal length.


Image No. 5: installation of an acoustic telescope and application example.

In practice, an acoustic telescope allows you to hear sound coming from a long distance, and isolate it from many other sounds. Figure 5b shows that the acoustic telescope allows you to hear a person in a crowd at a great distance. We could observe similar things in spy films.

For a more detailed acquaintance with the nuances of the study, I strongly recommend that you look into the report of scientists available at this link or this .

Epilogue

To summarize the above, the researchers were able to create a simple and effective device that allows you to manipulate sound. Focus the sound at one point, equalize the sound level from two sources, isolate a certain sound by eliminating noise, amplify the sound - all this can be done using a lens made of metamaterials, more like a ventilation plug or a waffle baking dish.

This work demonstrates that an accurate understanding of the nature of the phenomenon, physical, chemical or biological, allows you to gain control over it and change its properties as required by the situation. So far, just guessing exactly how acoustic lenses will be used. Scientists themselves are not going to stop there and will continue to research in order to improve their brainchild.




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