Browsing by Author "Cummer, Steven A"

Audio systems have become an integral part of our daily lives, transforming the way we hear sound in a myriad of applications, including TV, cinema, laptops, mobile phones, and even AR/VR sets. However, although there have been significant technological advancements in recent years, nearly all of these applications still rely on the same century-old electrodynamic transducer technology. This technology operates based on the fundamental principle of an AC motor, where the electrical signal generates a magnetic field that interacts with a permanent magnet. This interaction produces a force that moves the attached diaphragm back and forth, creating sound waves that propagate through the air. Over the years, the electrodynamic transducer has proven to be an effective technology, and its implementation in loudspeakers has become a ubiquitous component of modern audio systems.

Despite the electrodynamic loudspeakers' ability to reproduce high-fidelity sound at a relatively low cost, the physical design of audio systems has remained largely unchanged since the 1970s, leading to many unresolved problems. Although electrodynamic loudspeakers are commonly used in modern audio systems, their dimensions and directional characteristics are not satisfactory. This can result in poor sound quality, uneven distribution of sound, and the inability to deliver sound to certain areas effectively. As a result, listeners may not be able to fully appreciate the intended audio experience.

Acoustic metamaterials offer a promising solution to the growing need to improve the physical design of audio systems. These complex physical structures are intentionally formulated to engineer the propagation of sound, and over the past two decades, they have demonstrated remarkable capabilities to steer and shape sound fields into various patterns, introducing exotic physical phenomena to an otherwise ordinary system. Compared to traditional methods like digital signal processing (DSP) and multi-element arrays, acoustic metamaterials offer several advantages, including passivity, compactness, and cost-effectiveness. Furthermore, with the advent of 3D printing technology, producing acoustic metamaterial structures that work with airborne audible sound has become much easier, as they can be made of essentially rigid plastic that divides air into different compartments. This facilitates the rapid prototyping of novel metamaterial designs for audio systems, accelerating the pace of progress.

In this dissertation, we explore the use of innovative acoustic metamaterial design principles to address the persistent issues associated with electrodynamic loudspeaker-based audio systems and to elevate the user experience. Specifically, we examine how passive metamaterial structures can be used to modulate the frequency response and provide broadband directivity control of these systems. To achieve our objectives, we use a modeling approach that incorporates the entire sound path, balancing accuracy with computational cost. Additionally, we utilize a computerized algorithm to generate inverse designs that help us achieve our desired outcomes. By leveraging these techniques, we aim to design audio systems that provide users with high-quality sound and an optimal listening experience.

The field of metamaterials has gained much attention within the scientific community over the past decade. With continuing advances and discoveries leading the way to practical applications, metamaterials have earned the attention of technology based corporations and defense agencies interested in their use for next generation devices. With the fundamental physics developed and well understood, current research efforts are driven by the demand for practical applications, with a famous example being the well-known microwave "invisibility cloak." Gaining exotic electromagnetic properties from their structure as opposed to their

intrinsic material composition, metamaterials can be engineered to

achieve tailored responses not available using natural materials. With typical designs incorporating resonant and dispersive elements much smaller than the operating wavelength, a homogenization scheme is possible, which leads to the meaningful interpretation of effective refractive index, and hence electric permittivity and magnetic permeability. The typical metamaterial is composed of arrays of scattering elements embedded in a host matrix. The scattering elements are typically identical, and the electromagnetic properties of the medium can be inferred from the properties of the unit cell. This convenience allows the designer to engineer the effective electromagnetic parameters of the medium by modifying the size, shape, and composition of the unit cell.

This dissertation summarizes several key projects related to my research efforts in metamaterials. The main focus of this dissertation is to develop practical approaches to frequency tunable and reconfigurable metamaterials. Chapter one serves as a background and introduction to the field of metamaterials. The purpose of chapters two, three and four is to develop different methods to realize tunable metamaterials - a broad class of controllable artificially engineered metamaterials. The second chapter develops an approach to characterizing metamaterials loaded with RF MEMS switches. The third chapter examines the effects of loading

metamaterial elements with varactor diodes and tunable ferroelectric

thin film capacitors (BST) for external tuning of the effective medium parameters, and chapter four develops a more advanced method to control the response of metamaterials using a digitally addressable control network. The content of these chapters leads up to an interesting application featured in chapter five - a reconfigurable frequency selective surface utilizing tunable and digitally addressable tunable metamaterials. The sixth and final chapter summarizes the dissertation and offers suggestions for future work in tunable and reconfigurable metamaterials. It is my hope that this dissertation will provide the foundation and motivation for new researchers in the field of metamaterials. I am confident that the reader will gain encouragement from this work with the understanding that very interesting and novel practical devices can be created using metamaterials. May this work be of aid and motivation to their research pursuits.

Acoustic metamaterials are engineered materials that were extensively investigated over the last years mainly because they promise properties otherwise hard or impossible to find in nature. Consequently, they open the door for improved or completely new applications (e.g. acoustic superlens that can exceed the diffraction limit in imaging or acoustic absorbing panels with higher transmission loss and smaller thickness than regular absorbers). Our objective is to surpass the limited frequency

operating range imposed by the resonant mechanism that s1ome of these materials have. In addition, we want acoustic metamaterials that could be experimentally demonstrated and used to build devices with overall performances better than the previous ones reported in the literature.

Here, we start by focusing on the need of engineered metamaterials in general and acoustic metamaterials in particular. Also, the similarities between electromagnetic metamaterials and acoustic metamaterials and possible ways to realize broadband acoustic metamaterials are briefly discussed. Then, we present the experimental realization

and characterization of a two-dimensional (2D) broadband acoustic metamaterial with strongly anisotropic effective mass density. We use this metamaterial to realize a 2D broadband gradient index acoustic lens in air. Furthermore, we optimize the lens design by improving each unit cell's performance and we also realize a 2D acoustic ground cloak in air. In addition, we explore the performance of some novel applications (a 2D acoustic black hole and a three-dimensional acoustic cloak) using the currently available acoustic metamaterials. In order to overcome the limitations of our designs, we approach the active acoustic metamaterials path, which offers a broader range for the material parameters values and a better control over them. We propose two structures which contain a sensing element (microphone) and an acoustic driver (piezoelectric membrane or speaker). The material properties are controlled by tuning the response of the unit cell to the incident wave. Several samples with interesting effective mass density and bulk modulus are presented. We conclude by suggesting few natural directions that could be followed for the future research based on the theoretical and experimental results presented in this work.

Recent advances in electromagnetics introduced tools that enable the creation of arti-

cial electromagnetic structures with exotic properties such as negative material pa-

rameters. The ability to express these parameters has experimentally demonstrated

using passive metamaterial structures. These structures, based on their passivity and

resonant properties, are typically associated with high loss and signicant bandwidth

limitations.

Enhancing and further exploring novel electromagnetic properties can be done

through embedding active circuits in the constitutive unit cells. Active elements

are able to supplement the passive inclusions to mitigate and overcome loss and

bandwidth limitations. The inclusion of these circuits also signcantly expands the

design space for the development of functional metamaterials and their potential

applications.

Due to the relative diculty of designing active circuits compared with passive

circuits, using active circuits in the construction of metamaterials is still an under-

developed area of research. By combining the two elds of active circuit design and

metamaterial design, we aim ll the functional active metamaterial design space.

This document provides the basis for understanding the design and synthesis of

functional active metamaterials.

To provide necessary background matter, chapter 1 will function as an introduc-

tion chapter, discussing how active electromagnetic metamaterials are created and characterized. There are also several required design techniques necessary to suc-

cessfully engineer a functional active metamaterial. The introduction will emphasize

on linking metamaterial unit cell response with RF/analog circuit design with a brief

introduction to the semiconductor physics important to aid in the understanding of

the full active metamaterial design and fabrication process.

The subsequent chapters detail our specic contributions to the eld of func-

tional active RF metamaterials. Chapter 2 introduces and characterizes a meta-

material designed to have a tunable quality factor (tunable resonant bandwidth).

This metamaterial is essentially passive but demonstrates the transistor's versatility

as a combination of tunable elements, motivating the use of embedding transistors

in metamaterials. After establishing a simple application of a transistor in a pas-

sive metamaterial, chapter 3 outlines the design and characterization of an active

metamaterial exhibiting the properties of loss cancellation and gain. Chapter 4 in-

troduces another active metamaterial with the ability to self-adapt to an incident

signal. Within the self-adapting system, several complex RF circuit systems are

simulatenously developed and implemented such as a self-oscillating mixer and a

phase locked loop. Conclusions and additional suggested future research directions

are discussed in chapter 5.

There are also several appendices attached at the end of this document that are

meant to assist future graduate students and other readers. The additional topics

include the experimental verication of a passive magnetic metamaterial acting as a

near eld parasitic, the stabilization and measurement of a tunnel diode, a discussion

on the challenges of realizing active inductors from discrete components, and a basic

strategy for creating a non-volatile metamaterial. It is my aim for these appendices

to help provide additional inspiration for future studies within the eld.

Metamaterials have gained extensive attention in recent years due to their ability to exhibit material properties otherwise difficult or impossible to obtain using natural materials. Nonlinear and active metamaterials in particular exhibit great promise for exploring new effects and applications, from tunability to mixing. However, nonlinear and active metamaterials have been explored significantly less than linear metamaterials to this point and much work has focused on the fundamental physics of nonlinear metamaterials. Our aim is to further extend the knowledge of practical nonlinear metamaterials and to demonstrate how they can be transformed to real-world applications through the use of embedded devices. In this dissertation, we demonstrate a variety of ways that devices can be embedded within metamaterial unit cells to provide nonlinear and active effects.

Chapter 1 introduces the basic theory of metamaterials, background of existing work, and the current limitations of nonlinear and active metamaterial design. In Chapter 2, we present the design, simulation, fabrication, and verification of an RF limiter metamaterial. We show how a metamaterial can be designed using RF engineering principles to act as an effective limiter in a new topology, relying on nonlinear devices embedded within a metamaterial. Chapter 3 shows our design and demonstration of a power harvesting metamaterial. We design a nonlinear metamaterial towards a potential application, discussing how the selection of an appropriate embedded device provides our desired functionality. In Chapter 4 we show how nonlinear and active metamaterials can be used to realize phase conjugation, including demonstration of negative refraction and imaging through the use of these metamaterials. We also discuss design approaches to moving these metamaterials towards real-world applications. Chapter 5 discusses our work concerning metamaterials based on transistors. First we show that appropriate design of a transistor circuit allows us to tune the quality factor and resonant frequency of a metamaterial. We use this metamaterial for time-varying mixing, as well, demonstrating a mixing metamaterial that remains linear. We then illustrate how using transistors as nonlinear devices provides much greater design freedom for use with metamaterials. We show that the nonlinearity of a metamaterial can be dramatically enhanced through the use of transistors and even dynamically tuned, applying these nonlinear metamaterials to applications including phase conjugation and acoustoelectromagnetic modulation. In Chapter 6 we summarize the achievements of the presented research and directions for future work that build on the work described in this thesis.

Metamaterials are artificially engineered materials or structures that exhibit exotic properties that are not found in nature. They have been serving as a primary approach to fully control the behavior of electromagnetic waves, acoustic waves and elastic waves in recent years, and is at present a highly active research area. Metasurfaces, as the 2D version of metamaterials, have opened up unprecedented possibilities for controlling waves at will, offering a solution of molding wave propagation within a thin sheet of structures. Most metasurface designs are based on the so-called generalized Snell's Law (GSL) which achieves their functionalities by engineering the local phase shift in the unit cells. However, the efficiency of phase-gradient metasurfaces is fundamentally limited by the impedance mismatch and local porer flow mismatch between incident field and reflected/transmitted field, so that part of the energy is scattered into unwanted higher-order diffracted modes, which hinders the applicability in various scenarios. In this work, we approach these issues by exploiting acoustic bianisotropy (Willis coupling for acoustics) as an additional degree of freedom to control waves. We have explored highly efficient wavefront engineering in airborne acoustics, from manipulating simple plane waves and cylindrical harmonics to more complicated fields and finally, arbitrary wavefronts. Then we extended the application of bianisotropic metasurfaces to general impedance matching problems and demonstrated wavefront engineering in underwater acoustics with two examples: an aberration-layer penetration metasurface and a 3D acoustic tweezer.

This dissertation provides a summary of the work undertaken to achieve highly efficient and functional wavefront engineering devices, and briefly outlines some objectives for future work. Firstly, we designed an acoustic bianisotropic unit cell with full control over its scattering properties and demonstrated bianisotropic metasurfaces that overcome the fundamental limits of phase-gradient based metasurfaces. Second, we mapped the approach from Cartesian coordinates into cylindrical coordinates and demonstrated the generation of a pure field with high angular momentum. Third, we introduced surface waves to help power redistribution along the metasurface and achieved highly-efficient beam splitting and reflection. Forth, we further introduced the power-flow conformal metasurface to meet the power balance requirements for an arbitrary perfect wavefront transformation. Then we extended the application of bianisotropic metasurfaces and proposed a general impedance matching strategy, and demonstrated the idea with a case of aberration-layer penetration in water. Last but not least, by shaping the wavefront of underwater ultrasound, a 3D acoustic tweezer is demonstrated for manipulating a wide range of particles in a contact-less manner.

The high power, broadband very low frequency (VLF, 3--30 kHz) and extremely low frequency (ELF, 3--3000 Hz) electromagnetic waves generated by lightning discharges and propagating in the Earth-ionosphere waveguide can be used to measure the average electron density profile of the lower ionosphere (D region) across the wave propagation path due to several reflections by the upper boundary (lower ionosphere) of the waveguide. This capability makes it possible to frequently and even continuously monitor the D region electron density profile variations over geographically large regions, which are measurements that are essentially impossible by other means. These guided waves, usually called atmospherics (or sferics for short), are recorded by our sensors located near Duke University. The purpose of this work is to develop and implement algorithms to derive the variations of D region electron density profile which is modeled by two parameters (one is height and another is sharpness), by comparing the recorded sferic spectra to a series of model simulated sferic spectra from using a finite difference time domain (FDTD) code.

In order to understand the time scales, magnitudes and sources for the midlatitude nighttime D region variations, we analyzed the sferic data of July and August 2005, and extracted both the height and sharpness of the D region electron density profile. The heights show large temporal variations of several kilometers on some nights and the relatively stable behavior on others. Statistical calculations indicate that the hourly average heights during the two months range between 82.0 km and 87.2 km with a mean value of 84.9 km and a standard deviation of 1.1 km. We also observed spatial variations of height as large as 2.0 km over 5 degrees latitudes on some nights, and no spatial variation on others. In addition, the measured height variations exhibited close correlations with local lightning occurrence rate on some nights but no correlation with local lightning or displaced lightning on others. The nighttime profile sharpness during 2.5 hours in two different nights was calculated, and the results were compared to the equivalent sharpness derived from International Reference Ionosphere (IRI) models. Both the absolute values and variation trends in IRI models are different from those in broadband measurements.

Based on sferic data similar to those for nighttime, we also measured the daytime D region electron density profile variations in July and August 2005 near Duke University. As expected, the solar radiation is the dominant but not the only determinant source for the daytime D region profile height temporal variations. The observed quiet time heights showed close correlations with solar zenith angle changes but unexpected spatial variations not linked to the solar zenith angle were also observed on some days, with 15% of days exhibiting regional differences larger than 0.5 km. During the solar flare, the induced height change was approximately proportional to the logarithm of the X-ray fluxes. During the rising and decaying phases of the solar flare, the height changes correlated more consistently with the short (wavelength 0.5-4 Å), rather than the long (wavelength 1-8 Å) X-ray flux changes. The daytime profile sharpness during morning, noontime and afternoon periods in three different days and for the solar zenith angle range 20 to 75 degrees was calculated. These broadband measured results were compared to narrowband VLF measurements, IRI models and Faraday rotation base IRI models (called FIRI). The estimated sharpness from all these sources was more consistent when the solar zenith angle was small than when it was large.

By applying the nighttime and daytime measurement techniques, we also derived the D region variations during sunrise and sunset periods. The measurements showed that both the electron density profile height and sharpness decrease during the sunrise period while increase during the sunset period.

Lightning is one of the most familiar, impressive, but catastrophic natural phenomena that occur commonly on Earth. It produces perhaps the loudest sound, the most broadband radio emission, and the brightest light in the atmosphere. However, lightning remains relatively poorly understood since it is so transient (usually < 1 second) and so unpredictable that hinders direct measurements inside thunderstorms. For these reasons, radio remote sensing has been widely used for lightning studies. With recent advances in instrumentation and remote sensing technique, some basic problems like how lightning initiates inside the thundercloud begin to be addressed, and new challenging scientific problems are being discovered, such as the Terrestrial Gamma-ray Flashes (TGFs, energy > 20 MeV) associated with lightning, photonuclear reactions triggered by lightning, and needle-like plasma structures on the positive lightning leader, connecting lightning as part of atmospheric physics to high-energy physics, plasma physics, etc.

This dissertation aims to address fundamental questions like how lightning initiates and propagates, and how are TGFs related to lightning processes, by applying state-of-the-art radio remote sensing and imaging techniques. We measure and analyze electromagnetic signals produced by lightning from the vicinity to more than a thousand miles away, at radio frequencies from VLF, LF, to VHF and UHF. First, we investigated LF/VLF lightning sferics at the time of TGFs and found a statistically consistent connection between a slow LF pulse (~80 $\mu s$ duration) and TGFs, suggesting that the radio pulse is produced directly by the TGF production process. Second, in light of the slow pulse-TGF connection, we discovered a new type of downward CG-TGF with a reverse positron beam detected by Fermi GBM on the orbit, which could constitute 5--10 % of the previously known TGF population. Third, we employed supervised and unsupervised machine learning approaches to classify energetic lightning radio pulses for unprecedented ground detection of TGFs as well as understanding lightning sferics and ionospheric effects. In the meanwhile, we developed a short-baseline VHF interferometer with 200 MHz bandwidth to image lightning channels in high spatiotemporal resolution, shedding new insights into needles and lightning leader dynamics. Last but not least, we demonstrated and applied a new approach to indirectly measuring electric fields in the discharge region during lightning initiation and positive leader propagation using VHF-UHF radio spectrum, enabling an entirely new and useful capability for probing the ambient condition during lightning discharge processes. Implications of the estimated electric fields for lightning physics and high-energy physics are discussed.

Electromagnetic waves in the very low frequency (VLF, 3-30 kHz) and extremely low frequency (ELF, 3-3000 Hz) bands propagate extremely well in the cavity between the earth and the ionosphere with low attenuation. Because of this, radio waves emitted in this frequency range can be measured at extremely large distances (thousands of kilometers) from the sources of such emissions. Two main sources of signals at these frequencies are lightning events and VLF transmitters designed for communicating with submarines and other naval vessels. Measurement of the signals from both of these sources can be used to discover information about the source, in the case of lightning, or to measure the factors affecting propagation and other signal properties, as with VLF transmitter signals. This document provides a summary of the work undertaken to measure both of these signal sources and to outline goals and briefly outlines some objectives for future work.

A brief background on the atmospheric ELF and VLF environments is given in chapter 1, including a description of the conditions that allow for excellent propagation. A brief introduction to the lightning processes, as well as classification and measurement techniques is included as background information. Details describing current VLF transmitters examined in this work and basics of minimum-shift keying are also described.

Chapter 2 describes the design process and operating characteristics of a sensor designed for measuring magnetic fields in the ELF and VLF frequency ranges of interest in this work. This sensor system is robust and suitable for long-term deployment in thunderstorm environments. Chapter 3 details a method of measuring faint average signals generated by some lightning processes at large distances. Such an averaging process allows for the extraction of extremely small-magnitude processes that are otherwise not visible and enables the comparison of lightning on a larger scale. Averaged waveforms for four separate thunderstorms are compared and post-first stroke flash parameters are analyzed. Chapter 4 applies the averaging procedure to a specific type of lightning known as narrow bipolar events (NBEs). NBEs play an important role in the initiation of other types of lightning but not all NBEs initiate other lightning. This work divides positive NBEs according to whether they initiate other lightning events and examines the differences between them, helping to investigate the processes and conditions that give rise to lightning. Chapter 5 describes a method of unambiguously determining the position of a receiver through the measurement of terrestrial MSK-encoded VLF transmitters. Such a system has many advantages over other methods of navigation and simulated and field-tested capabilities and limitations are discussed, as well as factors affecting system accuracy. Finally, proposals and suggestions for future work are given in chapter 6.

Lightning, a phenomenon occurring within thunderclouds, has spurred numerousstudies seeking to understand its origin and behavior. The initiation of lightning remains elusive, particularly due to difficulties in conducting direct observations within thunderstorms. Subsequent evolutions of intracloud (IC) ashes characterize bi-level IC phenomena and have been a focal point of recent research. Advancements in observation techniques have significantly enhanced our understanding of these lightning processes within thunderclouds. Broadband interferometry, a prevalent method for radio imaging of lightning, has consistently been re ned across various studies to adeptly capture the nuanced progression of lightning breakdown. Furthermore, the recent launch of the space-based instrument, the Atmosphere-Space Interactions Monitor (ASIM) aboard on the International Space Station, provides a more detailed insight into the optical radiance of IC lightning activities compared to ground-based observations. ASIM particularly highlights the 337.0 nm and 777.4 nm bands, which are associated with non-thermal discharges and swift conductive channel formation, respectively. LF magnetic  eld measurements capture a substantial amount of energy from lightning's sudden current discharges. Due to their long-range reach, they are also key in detecting and tracking extensive areas of lightning activity. By leveraging comprehensive observational data, this dissertation focuses on the intricacies of IC lightning. First, the high bandwidth and fast time resolution VHF interferometry enables us to examine the preliminary stages of fast breakdown (FB) in lightning initiation. Through observing thousands of FBs, a consistent pattern showed they start as either a positive polarity streamer or a mix of both polarities. We also identified a new FB variant, mixed FB, which further suggests positive and negative streamers are both likely propagating simultaneously from the initiation point. Our simulations on VHF emissions from these streamers revealed how slight changes in emission intensities affect streamer paths, enhancing our understanding of dielectric discharge and setting the stage for future research on streamer behaviors. Shifting from streamer dynamics, we studied the evolution of IC leaders in thunderclouds. By combining the LF magnetic  eld measurements, ASIM optical observations, and VHF interferometry data, we charted IC lightning's growth from an upward path to a branching horizontal spread. We identified three key IC stages: Ascending, Transition, and Horizontal, revealing IC's transition from streamer activities to leader-like behaviors. This dissertation also delved into uncommon pulse trains associated with the late rapid discharge phase of IC, focusing on chaotic pulse trains (CPTs) and regular pulse trains (RPTs). Over four years of observations, we captured a full negative IC ash event in both the ASIM optical system and Duke observation network. This comprehensive data highlighted the distinct nature of CPTs from RPTs and provided a fresh understanding of their origins. In conclusion, this dissertation, enriched by a wide collection of observational techniques, offers a holistic view of IC lightning in thunderclouds.

Universal software radio peripheral (USRP) was utilized to receive the radiation produced by lightning flashes in VHF and UHF bands, with the bandwidth ranging from 2MHz to 8MHz. The software radio was programmed to record this radiation by integrating GPS clock and absolute timing. Moreover, two USRP N210 were employed to simultaneously record data at VHF and UHF bands with different programmable gain settings. This data was compared with the data from National Lightning Detection Network (available as location, type and peak current of lightning) and the magnetic sensor operating at LF (30 to 300 kHz). The output of USRP is the antenna displacement current ∂E/∂t (uncalibrated) and of LF magnetic sensor is the induced voltage ∂B/∂t. From comparison, the following results were obtained. K processes or regular pulse bursts in both cloud and cloud to ground discharges were clearly visible at UHF-VHF-LF. These processes were even visible at VHF with 0 dB gain, if superimposed on high magnitude slow (electric field change) processes such as J process probably. Distant Narrow bipolar pulses were observed with significant magnitude at VHF. Initial breakdown in cloud discharge was strong at LF and VHF but not significant at UHF. Instead the short pulses, probably stepped leaders, with 1 to 2.5 µs of time duration produced high magnitudes at UHF (while LF pulses remained small yet visible). Moreover, in few cloud discharges some processes occurring during final stage produced strong VHF-UHF radiation.

Acoustic metamaterials are a family of engineered materials that can be designed to possess flexible acoustic properties. They are composed of subwavelength periodic structures that can be homogenized as effective materials within the designed frequency bands. Acoustic wave controlling devices with spatially inhomogeneous or/and anisotropic acoustic properties can be designed with metamaterials. The early versions of acoustic metamaterials generally share several drawbacks that limit their applications: relatively high loss, narrow bandwidth, as well as difficulty in fabricating multiple samples with uniform properties. In this work, we approach these issues with a family of geometry-based acoustic metamaterials and demonstrate several devices based on these building blocks with various wave manipulation functionalities. The presented acoustic metamaterial-based devices are categorized into two kinds. The first kind of devices, including negative refraction prism, planar acoustic lenses, beam-steering metasurfaces and phase acoustic holograms, control the propagation or the states of existence of acoustic waves. The second kind focuses on a reciprocal process—instead of controlling the forward propagation, the sensing signals are modulated with randomized resonant metamaterials to realize computation sensing.

Our research approach is summarized as follows: firstly, we designed various metamaterial unit cells as the building blocks, adding to the existing unit cell library. Particularly, a family of labyrinthine or space-coiling unit cells provide access to a broader materials parameters space previously inaccessible by conventional spring-mass model-based unit cell designs. Second, with the extended unit cell library, we designed thin planar wave modulation devices, including acoustic lenses and metasurfaces that can bend the acoustic beam as predicted by the Generalized Snell’s Law. Third, we extend the spatially inhomogeneous modulation from 1D to 2D by designing computer generated phase holograms. Last but not least, a metamaterial-based compressive sensor is designed and demonstrated for the localization of multiple audio sources and the separation of overlapping audio signals.