Lucian Zigoneanu

Three-dimensional broadband omnidirectional acoustic ground cloak

The control of sound propagation and reflection has always been the goal of engineers involved in the design of acoustic systems. A recent design approach based on coordinate transformations, which is applicable to many physical systems, together with the development of a new class of engineered materials called metamaterials, has opened the road to the unconstrained control of sound. However, the ideal material parameters prescribed by this methodology are complex and challenging to obtain experimentally, even using metamaterial design approaches. Not surprisingly, experimental demonstration of devices obtained using transformation acoustics is difficult, and has been implemented only in two-dimensional configurations. Here, we demonstrate the design and experimental characterization of an almost perfect three-dimensional, broadband, and, most importantly, omnidirectional acoustic device that renders a region of space three wavelengths in diameter invisible to sound.

Measurement of a broadband negative index with space-coiling acoustic metamaterials

We report the experimental demonstration of a broadband negative refractive index obtained in a labyrinthine acoustic metamaterial structure. Two different approaches were employed to prove the metamaterial negative index nature: one-dimensional extractions of effective parameters from reflection and transmission measurements and two-dimensional prism-based measurements that convincingly show the transmission angle corresponding to negative refraction. The transmission angles observed in the latter case also agree very well with the refractive index obtained in the one-dimensional measurements and numerical simulations. We expect this labyrinthine metamaterial to become the unit cell of choice for practical acoustic metamaterial devices that require broadband and significantly negative indices of refraction.

Design and measurements of a broadband two-dimensional acoustic metamaterial with anisotropic effective mass density

We present the experimental realization and characterization of a broadband acoustic metamaterial with strongly anisotropic effective mass density. The metamaterial is composed of arrays of solid inclusions in an air background, and the anisotropy is controlled by the rotational asymmetry of these inclusions. Transmission and reflection measurements inside a one-dimensional waveguide are used to extract the relevant components of the effective density tensor together with the effective bulk modulus of the metamaterial. Its broadband anisotropy is demonstrated by measurements that span 500–3000 Hz. Excellent agreement between these measurements and numerical simulations confirms the accuracy of the design approach.

Design of an acoustic metamaterial lens using genetic algorithms

The present work demonstrates a genetic algorithm approach to optimizing the effective material parameters of an acoustic metamaterial. The target device is an acoustic gradient index (GRIN) lens in air, which ideally possesses a maximized index of refraction, minimized frequency dependence of the material properties, and minimized acoustic impedance mismatch. Applying this algorithm results in complex designs with certain common features, and effective material properties that are better than those present in previous designs. After modifying the optimized unit cell designs to make them suitable for fabrication, a two-dimensional lens was built and experimentally tested. Its performance was in good agreement with simulations. Overall, the optimization approach was able to improve the refractive index but at the cost of increased frequency dependence. The optimal solutions found by the algorithm provide a numerical description of how the material parameters compete with one another and thus describes the level of performance achievable in the GRIN lens.

A low‐frequency near‐field interferometric‐TOA 3‐D Lightning Mapping Array

We report on the development of an easily deployable LF near-field interferometric-time of arrival (TOA) 3-D Lightning Mapping Array applied to imaging of entire lightning flashes. An interferometric cross-correlation technique is applied in our system to compute windowed two-sensor time differences with submicrosecond time resolution before TOA is used for source location. Compared to previously reported LF lightning location systems, our system captures many more LF sources. This is due mainly to the improved mapping of continuous lightning processes by using this type of hybrid interferometry/TOA processing method. We show with five station measurements that the array detects and maps different lightning processes, such as stepped and dart leaders, during both in-cloud and cloud-to-ground flashes. Lightning images mapped by our LF system are remarkably similar to those created by VHF mapping systems, which may suggest some special links between LF and VHF emission during lightning processes.

Sound Manipulation With Acoustic Metamaterials

Acoustic metamaterials are engineered materials with properties hard or impossible to find in natural materials (e.g. negative effective density and/or negative bulk modulus). Therefore, a myriad of novel applications could be imagined and some of them have already been theoretically and/or experimentally demonstrated. Gradient index acoustic lenses, acoustic cloaks or acoustic absorbing panels are some common examples. Here, we review the coordinate transformation approach (transformation acoustics) which provides the material parameters needed to precisely control the acoustic wave propagation. Then, we use this technique to design an acoustic black hole and a 3D acoustic ground cloak. We use numerical simulations to explore the practical feasibility of the material parameters required by these applications and design non-resonant, highly sub-wavelength unit cells that will implement them in practice.Copyright