Enhancement of sound in chirped sonic crystals

Abstract

We propose and experimentally demonstrate a novel mechanism of sound wave concentration based on soft reflections in chirped sonic crystals. The reported controlled field enhancement occurs at around particular (bright) planes in the crystal, and is related to a progressive slowing down of the sound wave as it propagates along the material. At these bright planes, a substantial concentration of the energy (with a local increase up to 20 times) was obtained for a linear chirp and for frequencies around the first band gap. A simple couple mode theory is proposed, that interprets and estimates the observed effects. The results are obtained for the case of sound waves and sonic crystals, however they are extendable to other type of waves in modulated host matter.PACS numbers: 43.20.Fn, 43.20.Gp, 43.20.Mv, Manipulation and control of wave propagation, a problem of fundamental interest, is at root of many applications in different branches of science and technology. One important issue of wave manipulation is the localization and concentration (or local enhancement) of the wave energy. Artificial materials, and among them, artificial crystals are emerging as promising tools for manipulating wave propagation. In the case of sound waves considered here, such artificial periodic materials are called sonic crystals, structurally similar to photonic crystals in the field of optics. They are synthetic materials formed by a periodic distribution of elements or scatterers, whose properties (i.e., elasticity and density) differ from those of the host medium. This results in a periodic modulation of the acoustic properties of the medium at the scale of wavelength. The strong interest in these materials comes from their ability of manipulating the propagation of sound waves, due to their peculiar dispersive properties. A number of exotic and useful effects such as the formation of band-gaps, We present here a novel wave propagation effect, consisting in specifically the wave energy concentration due to progressive decrease of the group velocity in chirped sonic crystals, in which the lattice constant, i.e. the distance between scatterers in longitudinal (the wave propagation) direction, gradually changes along the propagation direction. We propose and demonstrate here a substantial increase of the wave intensity in controlled zones inside the crystal. Chirped (sometimes called graded) crystals have been introduced in optics 12 and acoustics 13-15 for different purposes, such as opening wide full band gaps or waveguiding of beams. An intriguing phenomenon shown in chirped crystals is the smooth deflection of a light beam from the straight trajectory as it propagates through the crystal, the so-called mirage effect 16 . Another interesting effect reported recently is the socalled rainbow trapping effect, the dependence of the turning point position on the color of radiation. It has been predicted for one-dimensionally modulated chirped photonic structures 17 and tapered optical and plasmonic waveguides 18,19 . Rainbow trapping and...

Enhancement of sound by soft reflections in exponentially chirped crystals

“…20 Such a progressive slowing-down of the waves was predicted and demonstrated in a twodimensional chirped (also known as graded or adiabatic tapered) sonic crystal, a structure made of rigid scatterers embedded in air in which the lattice constant along the wave propagation direction gradually changes with a profile, a = a(x), depending on the position, x. Sound enhancement and slowing down are two related phenomena that occur as the wave approaches to the bandgap; at this position, the waves reaches a zero group velocity and starts propagating backwards, in a proccess that we call a soft reflection.…”

Elastic Wave Control Beyond Band-Gaps: Shaping the Flow of Waves in Plates and Half-Spaces with Subwavelength Resonant Rods

“…As already anticipated in Figure 1B, the first snapshot shows the band-gap (here the field is filtered between 0.35 and 0.4 MHz) produced by an array of resonators of constant height. In Figure 2E, we show the well-known phenomena of rainbow trapping (Tsakmakidis et al, 2007;Romero-Garcia et al, 2013; Zhu et al, 2013) for elastic waves (Colombi et al, 2016a). The combined graded and resonant structure allows the incoming Rayleigh waves to be slowed down selectively at different propagation distances inside the metasurface, and eventually to be trapped in a subwavelength area.…”

“…Initially, attention focused on the existence of subwavelength band-gaps generated by the resonators (Pendry et al, 1998;Movchan and Guenneau, 2004;Achaoui et al, 2011;Lemoult et al, 2011;Colombi et al, 2014), and resulting frequency-dependent effective material parameters for negative refraction and focusing effects (Pendry, 2000;Smith et al, 2000;Yang et al, 2002;Li and Chan, 2004), and now consideration is transitioning to methods for achieving more complete forms of wave control by encompassing tailored graded designs to obtain spatially varying refraction index , wide band-gaps and mode conversion. In the fields of photonics and acoustics, Elastic Wave Control Beyond Band-Gaps this transition has already taken place and new graded designs allow for the tailored control of the propagation of light (Kadic et al, 2011;Maradudin, 2011), micro-waves , water waves (Farhat et al, 2008), and sound (Cummer and Schurig, 2007;Zhang et al, 2011;Romero-Garcia et al, 2013;Chen et al, 2014). Elastodynamic media have, in contrast to acoustic and electromagnetic systems, additional complexity such as supporting both compressional and shear wave speeds that differ and which mode converts at interfaces (Craster and Guenneau, 2012).…”

Broadband quasi perfect absorption using chirped multi-layer porous materials

“…Recently, some of us presented a system with acoustic wave enhancement due to the progressive decrease of the group velocity along the propagation direction. 11,16 Chirped structures have also been used as efficiently absorbers. In the electromagnetic counterpart, an omnidirectional absorber has received a significant attention recently.…”

“…These artificial materials are emerging as promising tools for potential applications in several branches of research and technology. 7 Several applications for focusing, 8,9 trapping, [10][11][12] bending waves, 13 opening of wide full band gaps 14 and controlling the spatial dispersion beams in reflection 15 have been developed. Recently, some of us presented a system with acoustic wave enhancement due to the progressive decrease of the group velocity along the propagation direction.…”