Acoustic Metamaterials


or how can we control the sound

It is a well known fact that acoustic waves, like all waves, interact with the medium through which they pass. However, it was not known until recently that we learned how to control this interaction by constructing acoustic media with effective properties of our choice. Such media, called metamaterials, have been known for over a decade in electromagnetics, where, through particular arrangements of elementary structures called unit cells, unconventional behavior of the medium with respect to the propagation of electromagnetic waves can be obtained. The interesting thing is that such media derive their effective properties through the structure and arrangement of the unit cells rather than their chemical composition. What is even more interesting is that through formal analogies between various types of waves, a range of phenomena well studied in electromagnetics can be transferred to other domains such as mechanics or acoustics.

 

Near-Zero Acoustic Metamaterials

 

In case of fluids, the properties determining the velocity of acoustic waves are the compressibility and the density of the fluid. By employing particular geometry of unit cells, we can manipulate either of them, make them close to zero or even negative at certain frequency ranges. For example, in a near-zero acoustic metamaterial, at frequency ranges where either of the two effective parameters is close to zero, acoustic waves propagate with virtually infinite velocity, accumulating extremely small phase variations over physically large distances. This gives rise to a number of exciting new applications, including acoustic energy tunneling and tailoring radiation phase patterns.

 

BioSense researchers have proposed acoustic metamaterials which support previously unachievable tunneling of acoustic waves through the medium, thus opening a way for the development of various concepts, in both audible and ultrasound ranges, such as advanced acoustic beam forming and acoustic super¬lensing. Ultimately, by extending the proposed concept from one to two or three dimensions, full control of interference of acoustic waves may be obtained, leading to various applications in the fields of acoustic cloaking or focusing, noise control, novel miniaturized sonars or medical ultrasound scanners with enhanced resolution.

Within the research at BioSense Institute, materials based on two new types of resonant acoustic unit cells have been theoretically developed and experimentally demonstrated. Such unit cells exhibit negative effective density and compressibility, respectively, at certain frequency ranges, but also provide a narrow frequency range where the effective property of interest is very close to zero, which means that an acoustic wave at a particular frequency can tunnel through the medium. We have shown that, using these two types of unit cells in various setups, we can achieve more sophisticated behavior of the material, including band-pass or band-stop near-zero acoustic filtering, as well as double-negative propagation.

Control of acoustic surface waves

It is also a well known fact that electromagnetic waves can propagate not only through space but also along surfaces. An important topic of recent research at BioSense has been the identification of the acoustic analogy of surface electromagnetic waves, particularly surface plasmon polaritons. The understanding of this analogy opened up new ways of controlling the behavior of surface acoustic waves. The fact that the wave velocity depends on the temperature of the surrounding fluid was exploited to design new ways of controlling acoustic surface waves by applying different temperature profiles to the fluid. By using our approach, acoustic wave can be bent or focused, as well as slowed down and even stopped at a particular point along the medium. Furthermore, since the propagation also depends on frequency, waves of different frequencies stop at different points, and the surface behaves as an acoustic spectral analyzer. The described phenomenon can also be exploited the other way round – instead of controlling wave propagation by manipulating the temperature, a temperature change can be sensed through the change in the wavelength of an acoustic surface wave, which gives rise to new applications in the domain of temperature sensing.

Bending (a) and focusing (b) of an acoustic wave achieved by BioSense researchers using a gradient index (GRIN) acoustic lens opens up new perspectives for a number of new applications, ranging from interferometry to temperature mapping