Acoustic Metamaterials


Limitations set by men, are often broken by the same set. To know more than what we have been provided with is what we need and work upon. Our world of science has taken us to a place far enough to engineer materials have properties different from its default nature. Metamaterials are materials engineered to satisfy the statement above. They derive their properties from their newly designed structures, such as their precise shape, geometry, size and orientation gives them their smart properties capable of manipulating electromagnetic waves; by blocking, absorbing, enhancing or bending waves, to attain profit over a large scale which the conventional materials fail to provide. Metamaterials have opened doors to many ideologies and theories that involve headline hitting discoveries. They consist of composite materials with small inclusions and/or intricate structuring that can average out over a wavelength. There are various types of metamaterials over changing their primary properties such as; negative index, single index, band gap, chiral, elastic, acoustic, structural, non-linear, tunable, plasmonic etc. Changes in wavelength affects fundamentals of acoustic properties of materials in general. Acoustic metamaterials are materials designed to control, direct and manipulate sound waves as these occur in gases, liquids and solids. Involves propagation of sound in materials dictated by factors such as bulk modulus and mass density, which are analogies of electromagnetic parameters, permittivity and permeability. Research over negative-index metamaterials have led to acoustic metamaterials, materials engineered to replace our current requisites. Negative-index metamaterials are materials engineered for the sole purpose of having refractive index, for an electromagnetic wave, as a negative value, over some frequency range. Artificial characteristics of acoustic metamaterials are often achieved through collective excitation of sub-wavelength (localised resonators). This article specifically gives an overview on metamaterials and its main topic of interest being acoustic metamaterials. On-going researchers have produces various applications of these; eg: construction of thin acoustic barrier in air and now being tested underwater, Sonardyne International concerns over development of metamaterials to improve underwater imaging facilities, DSTL advances this research over collecting spatial transformation technique (in acoustic domain), ‘designer’ surface plasmons (Science Vol 208,p670,2005) to understand transportation and absorption of energy through excitation of acoustic surface waves on structured substrates (work presented at Meta 2012), etc. Other work beyond treatment over acoustic involves its exploitation, for Eg: dark acoustic metamaterials as super absorbers for low-frequency sound (absorption of low frequency airborne sound at resonant frequencies ranging from 100-1000 Hz). A distributed acoustic system can be effectively described by the analogous lumped circuit model in which the behaviour of the current resembles the motion of the fluid. Based on this analogy, an acoustic metamaterial can be implemented by a two-dimensional transmission line network to realize negative refractive index and further inhomogeneous anisotropic density and compressibility. In one application, a PI/NI interface is constructed by a two-dimensional array of acoustic circuit network. An ‘eye-catching’ interest among these involve, acoustic cloaking, hypothetical device that would make objects immune to sound waves (sound-proof materials). Mainly absorbs ultrasound waves of wavelengths 40-80 Hz. Other applications of acoustic metamaterials involve; super lens, diode, double C resonators, sonic crystals, etc. Applications of metamaterials involve; antennas, absorber, seismic protection, sound filtering, etc. Researches are done over these since over a long-term by MURI (The Multidisciplinary University of Research Institute) along with other group of universities and Metamorphose, The Virtual Institute for Artificial Electromagnetic Materials and Metamaterials “Metamorphose VI AISBL” is an international association to promote artificial electromagnetic materials and metamaterials. Also, there is much more ground to investigate for the future that include various other factors, which has been left to us for discovery.