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Airy beams have offered great opportunities for ultrasound beam manipulation. However, one critical barrier that limits the broad applications of Airy beams in ultrasound is the lack of simply built device to generate Airy beams in water. This work presents a family of Airy beam-enabled binary acoustic metasurfaces (AB-BAMs) for underwater ultrasound beam manipulation. AB-BAMs are designed and fabricated by 3D printing with two coding bits: a polylactic acid unit acting as a bit “1” and a water unit acting as a bit “0.” The distribution of the binary units is determined by the pattern of Airy beam. We demonstrate the capability of AB-BAMs in flexibly tuning the focal region size and beam focusing in 3D space. The focal depth of AB-BAMs can be continuous and electronical tuned by adjusting the operating frequency of the planar transducer without replacing the AB-BAMs. The superimposing method is leveraged to enable the generation of complex acoustic fields. Furthermore, the proposed 3D-printed AB-BAMs are simple to design, easy to fabricate, and low-cost to produce with the capabilities to achieve tunable focal size, flexible 3D beam focusing, arbitrary multipoint focusing, and continuous steerability, which creates unprecedented potential for ultrasound beam manipulation.

Ultrasound imaging can be used in time sensitive and dynamic environments like trauma care due to its advantages, such as real-time, affordable, portable, noninvasive, and nonionizing. Despite these advantages, transcranial ultrasound is hindered for brain imaging by the severe phase aberration of the human skull. To study phase aberration, it is crucial to have a fundamental understanding of the relationship between the macro-/micro-structure of the skull and the speed of sound distribution in the skull. This is proven to be challenging with ex vivo skulls, because the properties of these skulls are not controllable. To address this issue, we used binder jetting (BJT) 3D printing, high fidelity additive manufacturing, to print skull phantoms using calcium hydroxyapatite (HA) powder because HA is one of the major constituent materials of the human bone. After printing phantoms, they were sintered to get to the final form. The phantoms were validated via ultrasound measurements.

Passive cavitation mapping (PCM), which generates images using bubble acoustic emission signals, has been increasingly used for monitoring and guiding focused ultrasound surgery. This study investigates a transient angular spectrum (AS) approach for PCM. The working principle of this approach is to backpropagate the received signal to the domain of interest and reconstruct the spatial–temporal wavefield encoded with the bubble location and collapse time. The transient AS approach is validated using an in silico model, water bath, and in vivo experiments. It is found that the transient AS approach yields similar results to delay and sum, but is considerably faster. The results obtained by this study suggest that the transient AS approach is promising for fast and accurate PCM.

The parametric array loudspeaker (PAL) is known for its ability to generate highly directional audio beams. It has been widely used in a variety of audio applications, such as the sound field reproduction and active noise control. Fast and accurate computations of the audio sound field generated by a PAL are challenging due to the complexity of nonlinear interactions between airborne ultrasonic waves. This paper presents an overview of the commonly used computational methods in the frequency domain under the quasilinear assumption, which include the direct integration method, convolution directivity model, Gaussian beam expansions, cylindrical expansions, spherical expansions, and finite element methods. The computational efficiency and the accuracy of these methods in the near field, Westervelt far field, and the inverse-law far field are compared. Future challenges and perspectives are discussed.

Photoacoustic imaging is a promising hybrid imaging modality that can visualize both microscopic and macroscopic structures and oxygenation changes in brain function. Phase aberration caused by the skull is a major barrier for high-quality photoacoustic imaging of the brain. Time-reversal methods have been used to address this issue, though they rely on solving the full-wave equation, and is therefore computationally heavy and slow. Herein, a near real-time model-based image reconstruction algorithm is proposed. The acoustic forward model is mathematically described as a model matrix and image reconstructions are performed by employing the pseudoinverse of the model matrix and calculating the initial pressure distribution. The pseudoinverse matrix only needs to be computed once for the same experimental setup and acoustic medium and is obtained offline prior to imaging. The proposed algorithm shows equivalent image quality but considerably faster reconstruction time (>40 times faster), compared to the time-reversal method and therefore could potentially enable near real-time photoacoustic imaging.

Wave fields with acoustic orbital angular momentum have drawn greatattention because of its wide range of applications such as acoustic communication and particle manipulation. We designed, fabricated, and experimentally validated a type of lossy metasurface which enables an extremely selective transmission of acoustic wave filed depending on the chirality of its orbital angular momentum. The metasurface is composed of a phase gradient metagrating (PGM) and microperforated panel (MPP). By harnessing the multiple internal reflection of PGM and the carefully engineered loss of MPP, the left-handed incident vortex has a near-zero power transmission while its right-handed counterpart has a 60% power transmission. Such an extremely selective transmission was experimentally validated by retrieving the full scattering matrix using a generalized twelve-microphone method, which shows a good agreement with numerical simulations.

Shock wave lithotripsy (SWL) and laser lithotripsy (LL) have been widely used for clinical treatment of kidney stones. Cavitation plays an important role in stone fragmentation in both SWL and LL, yet it may also contribute to renal tissue injury. It is therefore crucial to determine the spatiotemporal distributions of cavitation activities to maximize stone fragmentation while minimizing tissue injury. Passive cavitation mapping (PCM) has most practical applications in deep biological tissues and is most promising for clinical translation. We have developed a set of technologies for 2D/3D PCM that can be seamlessly integrated with ultrasound imaging and photoacoustic imaging. Our 2D/3D PCM has achieved a spatial resolution of hundreds of micrometers and a temporal resolution of several microseconds. We also developed a transient angular spectrum approach for PCM reconstruction, which is ten times faster than the traditional delay-and-sum method. Using the 2D/3D PCM system, we imaged shockwave- and laser-induced single cavitation bubbles in both free field and constricted space, as well as on large animal models. Collectively, our results have demonstrated the high reliability and spatial-temporal accuracy of the 2D/3D PCM approach, which paves the way for future in vivo applications and human studies during SWL and LL.

Phase aberration caused by skulls is a main challenge in transcranial ultrasound imaging for adults. Aberration could be corrected if the skull profile (i.e., thickness distribution) and speed of sound (SOS) are known. We previously designed a deep learning (DL) model to estimate the skull profile and SOS using pulse-echo ultrasound signals. This study’s objective is to develop strategies to improve the estimation and evaluate the effectiveness of aberration correction in transcranial ultrasound imaging. Acoustic simulations were performed using k-Wave in this numerical study. The following strategies were used to improve estimation: (1) A phased array was used instead of a single-element transducer; (2) Channel radiofrequency data were used instead of beamformed data as the DL model input; (3) A DL model was developed to incorporate physics into architecture design and model training. Compared with previously reported results, these strategies improved the correlation coefficient between the estimated and ground-truth values from 0.82 to 0.94 for SOS, and from 0.98 to 0.99 for thickness. Simulated transcranial images of point targets with phase correction using the estimated SOS and thickness values showed significantly reduced artifacts than those without correction. The results demonstrate feasibility of the proposed approach for transcranial ultrasound imaging.

We designed, fabricated, and experimentally validated an architected mechanical metamaterial vibration shielding while ensuring high effective stiffness with low density. The structure is made of a periodic succession of octet truss units with alternating circular strut diameters. The octet truss endows the meta-structure with a high effective stiffness and a low effective density, while alternating the units with different struts radii enables the opening of a wide band-gap for elastic waves. Samples were 3D printed and experimentally validated using a shaker and accelerometer. Unidirectional compression test was also performed to characterize the mechanical properties of the metamaterial. The results are in good agreement with the numerical prediction.

Over the past few years, the rapid development in the fields of condensed matter physics, electronic, and photonic systems have inspired the design and experimental demonstration of various acoustic topological insulators (TIs). Among these, the topologically protected one-way propagation is a phenomenon that is gaining increased attention. Pseudospin states, which is the analogue of the quantum spin Hall effect from electronic systems, has been proven to enable topological edge states in acoustics. Similarly, the valley Hall (VH) effect is also observed in acoustic systems and provides a pair of valley vortex states with opposite chirality. These valley vortex states can similarly form topologically protected edge states and, in turn, realize robust one-way propagation. However, the differences in the physics behind these acoustic systems give rise to distinct features such as different angle selections and immunization levels to various types of defects. This article conducts a comparison study between topological states in valley Hall phononic crystals and TIs that reveals the differences and similarities in several aspects. Both of them have topologically protected edge states and thus the robust one-way propagation. For the maximum transmission incident angle and defect immunization, however, VH topological waveguides and TI waveguides show different characteristics.

Inspired by twisted bilayer graphene, we present an elastic bilayer consisting of a plate with two honeycomb lattices of mechanical resonators on both sides of the plate. Surface acoustic waves (SAW), which can interact depending on the thickness of the plate, are supported by each side of the plate. Beyond twisting a lattice with respect to the other, the coupling strengths between the two sides can be controlled via the plate thickness, which strongly affects the dispersion of elastic waves in the bilayer structure. The dispersion relationship of the SAW is theoretically characterized by calculating the band structure for the cases of AA and AB stacking configurations, as well as for special cases of twisting angles that produce sublattices with even and odd symmetries. Furthermore, we explore the topological characteristics of the bands and uncover a possible mechanism of creating topological elastic Valley states. The proposed bilayer plate could constitute a promising platform for manipulating mechanical waves and exploring quantum analog phenomena which could open routes toward innovative mechanical and optomechanical devices at the microscale, for instance.

There is a trade-off between the sparseness of an absorber array and its sound absorption imposed by wave physics. Here, near-perfect absorption (99% absorption) is demonstrated when the spatial period of monopole-dipole resonators is close to one working wavelength (95% of the wavelength). The condition for perfect absorption is to render degenerate monopole-dipole resonators critically coupled. Frequency domain simulations, eigenfrequency simulations, and the coupled mode theory are utilized to demonstrate the acoustic performances and the underlying physics. The sparse-resonator-based sound absorber could greatly benefit noise control with air flow and this study could also have implications for electromagnetic wave absorbers.

Numerical modeling is an important tool for the study of acoustic metasurfaces as it can shed light on the physical principles of these engineered acoustic structures and be used for optimization purposes. In this talk, I will discuss challenges that we have encountered in modeling acoustic metasurfaces using commerical software and semi-analytical models. I will focus on sound-absorbing acoustic metasurfaces and diffractive acoustic metasurfaces. I will also discuss the possibility of using open-source wave solvers (such as k-Wave and mSOUND) to model acoustic metasurfaces, which has been largely unexplored.

The effective densities of plate- and membrane-type acoustic metamaterials (AMMs) without mass attached are studied theoretically and numerically. Three models, including the analytic model (based on the plate flexural wave equation and the membrane wave equation), approximate model (under the low frequency approximation), and the finite element method (FEM) model, are first used to calculate the acoustic impedance of square and clamped plates or membranes. The effective density is then obtained using the resulting acoustic impedance and a lumped model. Pressure transmission coefficients of the AMMs are computed using the obtained densities. The effect of the loss from the plate is also taken into account. Results from different models are compared and good agreement is found, particularly between the analytic model and the FEM model. The approximate model is less accurate when the frequency of interest is above the first resonance frequency of the plate or membrane. The approximate model, however, provides simple formulae to predict the effective densities of plate- or membrane-type AMMs and is accurate for the negative density frequency region. The methods presented in this paper are useful in designing AMMs for manipulating acoustic waves.

Over the past decade there has been a great amount of research effort devoted to the topic of acoustic metamaterials (AMMs). The recent development of AMMs has enlightened the way of manipulating sound waves. Several potential applications such as low-frequency noise reduction, cloaking, angular filtering, subwavelength imaging, and energy tunneling have been proposed and implemented by the so-called membrane- or plate-type AMMs. This paper aims to offer a thorough overview on the recent development of membrane- or plate-type AMMs. The underlying mechanism of these types of AMMs for tuning the effective density will be examined first. Four different groups of membrane- or plate-type AMMs (membranes with masses attached, plates with masses attached, membranes or plates without masses attached, and active AMMs) will be reviewed. The opportunities, limitations, and challenges of membrane- or plate-type AMMs will be also discussed.

Over the past decade, metamaterials have straddled traditional subject boundaries and emerged as futuristic materials with unconventional functionalities. Architecting material microstructure has been shown to facilitate a myriad of applications for sound and vibration manipulation. The progress of this field has thus been mapped by 3-D printing techniques and the precision that such manufacturing techniques can offer. Recently, large area projection micro-stereolithography, a novel additive manufacturing technique has been shown to be capable of fabricating samples with a high structural complexity and smallest feature sizes ranging from a few microns to over tens of centimeters. From the perspective of mechanical wave propagation, this implies a precise control over micro-structure that can be engineered for a variety of applications. In this work, we develop a unique class of elastic metamaterials that are architected for low frequency, wide-band vibration attenuation. The material is made up of three-dimensional micro-lattices that are shown to possess local resonance band gaps which can be precisely tuned via unit cell geometry and the intrinsic material employed for its fabrication. The working of this material is experimentally verified and the associated functionalities that it can facilitate are discussed.

An optimization scheme based on the genetic algorithm is proposed to achieve a desired sound absorption spectrum using coupling of multiple resonators at a minimum thickness. The proposed optimization strategy considers the area of supercell, the number of unit cells in a supercell and the shape of each unit cell. Besides the optimization based on the mechanism of coupled resonances, topology optimization with thermoviscous losses being taken into account is further proposed to explore the absorption performance of metasurface-enabled sound absorbing panels. A simplified set of equations and a relatively simple interpolation for the distribution of design variables make the topology optimization an viable option.

Over the past few years, the rapid development in the fields of condensed matter physics, electronic and photonic systems have inspired the design and experimental demonstration of various acoustic topological insulators. Among these, the topologically protected one-way propagation is a phenomenon that is gaining increased attention. Pseudospin states, which is the analogue of the Quantum Spin Hall Effect from electronic systems, has been proven to enable topological edge states in acoustics. Similarly, Valley Hall (VH) effect is also observed in acoustic systems and provides a pair of valley vortex states with opposite chirality. These valley vortex states can similarly form topologically protected edge states and in turn realize robust one-way propagation. However, the differences in the physics behind these acoustic systems give rise to distinct features such as angle selection and immunization level to various types of defects. In this article, the comparison between topological insulators (TI) and valley hall topological insulators (VHTI) address the difference and similarities in several aspects. Both of them have topologically protected bandgaps and thus the robust one-way propagation. For the maximum transmission incident angle and defect immunization, however, VH topological waveguide and TI waveguide show different characteristics.

Over the past decade, gradient index metasurfaces(GIMs) have been voraciously studied for the numerous wave control capabilities that they facilitate. In this regard, a hybrid structure consisting of shunted Helmholtz resonators and a straight channel is often chosen as building blocks of the metasurface. Prior research, however, has primarily focused on GIMs that operate in the audible frequency range, due to the difficulties in fabricating such intricate structures at the millimeter and sub-millimeter scales, for ultrasonic applications. In this paper, we design, fabricate and experimentally realize a gradient index metasurface for airborne ultrasound at 40kHz. The fabrication of such a GIM is made possible by projection micro-stereolithography, an emerging additive manufacturing technique capable of micro-scale, high aspect-ratio features over a wide area. Simulations were first conducted to verify the metasurface design. Experiments were subsequently performed to corroborate the simulations and theory. The challenges faced by the thermoviscous effects, their usefulness in certain applications and optimal designs for minimal dissipation are discussed.

A mixed-domain method (MDM) dubbed frequency-specific MDM (FSMDM) is introduced for the simulation of the second-harmonic ultrasound field in weakly heterogeneous media. The governing equation for the second harmonics is derived based on the quasi-linear theory. The speed of sound, nonlinear coefficient, and attenuation coefficient are all spatially varying functions in the equation. The fundamental frequency pressure field is first solved by the FSMDM and it is subsequently used as the source term for the second-harmonics equation. This equation can be again solved by the FSMDM to rapidly obtain the second-harmonic pressure field. Five 2-D cases, including one with a realistic human tissue map, are studied to systematically verify the proposed method. Results from the previously developed transient MDM are used as the benchmark solutions. Comparisons show that the two methods give similar results for all cases. More importantly, the FSMDM has a crucial advantage over the transient MDM in that it can be two orders of magnitude faster.

A metasurface-based sound absorber panel is proposed to achieve high sound absorption with a deep sub-wavelength thickness and a relatively broad bandwidth. The panel composed of different periodically-arranged Helmholtz resonators has been designed by a radiation impedance model which takes the mutual radiation impedance into account. Numerical simulations based on COMSOL Multiphysics show that the radiation impedance model yields more accurate results compared with the traditional equivalent surface impedance method. Broad bandwidth of sound absorption is achieved under proper coupling by carefully tuning the geometry and distance between Helmholtz resonators.

The development of acoustic metasurfaces has enabled numerous wave control abilities. The effect of losses in acoustic metasurfaces for sound transmission manipulation, however, is largely unexplored. Here, we show that robust asymmetric transmission can be achieved by harnessing judiciously tailored losses. Theoretical investigations show that the asymmetric behavior stems from loss-induced suppression of high order diffractions. Multiple reflections occur inside the individual slits for the negative incident direction, which lead to different orders of diffraction and asymmetric responses for opposite directions. Numerical simulations based on effective medium are performed and are in good agreement with theoretical analysis. Real structures based on unit cells with designed internal viscous loss are fabricated and measured in a 2D waveguide. The peak energy contrast is about 10 times in a certain range of incident angles and frequencies. This study may open up new possibilities in lossy acoustic metamaterials and metasurfaces. The theory can also be readily extended to electromagnetic waves.

We are surrounded by noise. Planes, trains, cars, crowds, cooling fans — just about everything is a potential source of noise. Traditional noise abatement methods have a number of limitations and they are particularly ineffective for low frequency noise. As we are becoming increasingly aware of the noise pollution issue, there is a pressing need to develop novel materials for more effective noise reduction. In this talk, I will give an overview of recent development on noise reduction using metamaterials or metasurfaces. I will cover structures such as decorated membrane resonators, hybrid resonators, and coiled Helmholtz resonators.

Honeycomb sandwich panels have long been preferred for their good mechanical properties. Here, we demonstrate a design of honeycomb metasurface panel that can achieve 90% sound absorption from 600 Hz to 1000 Hz with a thickness of less than 30 mm (approximately λ/20) by applying minor modifications to commercialized honeycomb sandwich panels. The panel was made up of periodically and horizontally arranged honeycomb “supercells” which consist of different types of honeycomb unit cells. Each unit cell could be considered a Helmholtz resonator (HR) with hexagonal prism-shaped cavity and cylindrical neck. The absorption performance of designed honeycomb metasurface panel was studied both with surface impedance theory and the complex frequency plane theory, and was then validated by numerical simulations.

This study investigates a flow-free, pseudospin-based acoustic topological insulator. Zone folding, a strategy originated from photonic crystal, is used to form double Dirac cones in phononic crystal. The lattice symmetry of the phononic crystal is broken by tuning the size of the center “atom” of the unit cell in order to open the nontrivial topological gap. Robust sound one-way propagation is demonstrated both numerically and experimentally. This study provides a flexible approach for realizing acoustic topological insulators, which are promising for applications such as noise control and waveguide design.

Omnidirectional sound barriers are useful for various applications in noise reduction. Conventional sound insulating structures like micro-perforated plates or porous materials prevent the exchange of airflow. Here, we propose the design of an acoustic metacage which can shield acoustic waves from all directions and have the ability of allowing air pass through freely. The mechanism is that the strong parallel momentum along the surface rejects sound regardless of the directions of the incident wave. Structures based on open channels and Helmholtz resonators are designed at an operation frequency of 2.49 kHz with thickness less than half of the wavelength. A prototype is fabricated using 3D printing and further verified experimentally in a waveguide. Simulation and measurement results clearly show that the proposed metacage can shield acoustic waves when the sources are placed either interior or exterior. An average energy decay of more than 10 dB is achieved when a loudspeaker is placed inside the metaca...

Acoustic metamaterials offer large degree of design freedom and precise control over amplitude and phase at subwavelength scales. In the past, we have demonstrated a family of labyrinthine metamaterial unit cells as premium building blocks for phase modulation devices, as well as several 1D wavefront shaping devices based on these unit cells. Here we extend the complex modulations to 2D by demonstrating a computer generated metamaterial-based phase hologram. Through spatially modulating the phase of the wavefront, the hologram projects the incident wave to a designed three-dimensional amplitude pattern. The hologram is designed with a two-step process: first, an iterative holographic reconstruction algorithm is used to obtain the optimal phase pattern of the hologram; second, unit cells with desired phase modulations are designed and fabricated. Ray tracing and full-wave simulations have been performed to verify the design. The measurements of reconstruction in an anechoic chamber will be taken for a hologram designed to operate around 4 kHz. The metamaterial-based hologram creates a three-dimensional acoustic illusion with only passive structures. The designing process can also be extended to devices such as multi-focal lenses and wave-based analog processing/computing interfaces.

Acoustic holograms in analogy with optical holograms are useful for a variety of applications, such as multi-focal lensing, multiplexed sensing and synthesizing three-dimensional complex sound fields. We previously presented the designs and simulation results of several metamaterial-based acoustic holograms (J. Acoust. Soc. Am. 138, 1751 (2015)). Here, we demonstrate the experimental realization of two of them: one projects a complex pattern on the image plane, while the other focus energy onto multiple circular spots of different sizes. We showcase the holographic reconstruction measurements carried out in an anechoic chamber. The demonstrated passive holograms, without phase-shifting circuitry and transducer arrays, are advantageous for applications where simplicity, robustness, and small footprint are preferred over adaptive control. We also discuss the higher frequency versions of such holograms and their applications in robotic sensing and wireless power transfer. Such metamaterial-based holograms ca...

Gradient-index metasurfaces have shown a great potential for wavefront modulation and transmission/reflection control. However, the effect of loss is rarely discussed in acoustic metasurfaces. Here, we study the effect of loss in gradient-index metasurfaces and suggest its application in asymmetric transmission. For a gradient-index metasurface composed of six types of unit cells, theoretical analysis reveals that, when loss is considered, the transmission coefficients for different diffraction modes will have different responses for acoustic waves with oblique incidence. The gradient-index metasurface thus can be utilized to achieve asymmetric transmission. Numerical simulations based on effective medium and real structure are performed to validate the theoretical findings using a single layer of gradient-index metasurface. The transmission contrast can be greater than 20 times in simulations within a certain range of incident angles. This design may be useful in ultrasonic and noise control applications.

We present a design of acoustic metasurfaces yielding highly asymmetric transmission within a certain frequency band. The design consists of a bottom layer of gradient-index metasurface and a top layer of low refractive index metasurface. When the incident waves transmit from the bottom side, the wave-front will be steered, and the total reflection occurs on the boundary of the top layer, leading to extremely low transmission. When the incident waves transmit from the top side, the transmission is high as the impedance of the low refractive index metasurface is matched with the background medium. Numerical simulations show that the transmission contrast between the two incident directions is high within a certain frequency band. Compared to previous designs, the proposed one can be more compact. This design may have potential applications in ultrasound imaging and noise reduction.

In this talk, we will present our recent studies on the use of fast marching methods for transcranial beam focusing. Three topics will be included: beam focusing for transcranial B-mode imaging, beam focusing for transcranial photoacoustic tomography, and beam focusing of a spherical array for therapy. To correct for the phase aberration from the skull, two critical steps are needed prior to brain imaging or treatment. In the first step, the skull shape and speed of sound are acquired by either CT scans or ultrasound scans. In the second step, fast marching methods are used to compute the phase delay based on the known skull shape and sound speed from the first step, and the computation can be completed in seconds even for 3D problems. The computed phase delays are then used in combination with the conventional delay-and-sum algorithm for generating images. They can also be readily used for transcranial beam focusing for therapeutic purposes. Numerical simulation results will be presented to show the robu...

This paper presents our recent work on transient nonlinear acoustical holography. A higher order stepping algorithm is first introduced, which is shown to be significantly more accurate and efficient than the original one (Evaluation of a wave-vector-frequency-domain method for nonlinear wave propagation, Jing et al., J. Acoust. Soc. Am. 129, 32) through systematic numerical study. Underwater experimental results from a highly focused transducer will be presented here to show the validity of the model. Both linear and nonlinear, forward and backward projection of the acoustic field are conducted. While linear acoustical holography is shown to produce erroneous results, good agreement is found between our nonlinear model and the experiment.

In this paper, a recently developed wave-vector-frequency-domain method for nonlinear wave propagation is verified by underwater experiments. A specially designed focused transducer was used to generate short high intensity pulses. 2D scans were conducted at a pre-focal distance, which were later used as the input to the numerical model to predict the acoustic field at focal and post-focal zones. Adaptive attenuation was introduced to reduce the Gibbs effect. Graphic processing units (GPU) were also employed to speed up the computation. Good agreement was observed between the simulation and experiment.

Coupled-volume rooms remain as one of the most exciting and challenging research lines in room acoustics. Their benefits lie on their multiple-slope energy decay profiles, being of interest in many current concert halls. However, so far there is no consistent predictive model being able to help architects and acousticians in selecting appropriate design parameters. This work is devoted to studying effects of aperture-size and source/receiver positions on the energy decay characteristics. For this purpose, a diffusion equation model is used to model a coupled-volume system, providing an effective tool for analysis. The diffusion equation model is first validated by experimental investigations using scale models. Bayesian energy decay analysis is applied to the results of both the acoustical scale model and the diffusion-equation model to provide deeper insight in the energy decay characteristics and their dependence on the aperture sizes and the sound source/receiver positions.

In this invited talk, our preliminary study is presented on a virtual hole and a broadband acoustic collimator, by combination of the concept of complimentary media with transformational acoustics. Such effect is exemplified by a segmental defect in the original cloak, which appears as if a dipole scatterer was under the acoustic imager. A set of spatially varying effective parameter was derived from coordinate transformation. These parameters can be readily implemented using non-resonant acoustic elements. The numerical study confirmed the collimation of acoustic beam from a small hydrophone behind the metamaterial device. The potential application of such novel device concept in underwater communication and medical ultrasound will be also discussed.

The skull has been a barrier to transcranial focused ultrasound therapy, because of its strong phase aberration. Previous methods for phase correction are based on numerically solving the wave equation, which outputs the desired phase delay for each transducer element. These methods are typically quite time-consuming. The present method aims to achieve real-time phase correction. This method is based on the Eikonal equation, which is a high frequency approximation to the wave equation. It fully accounts for the refraction in the skull, which is the main contribution to the phase aberration in the skull. Fast marching method (FMM) is used to solve the Eikonal equation. Preliminary results show that, solving the Eikonal equation is over 100 times faster than solving the wave equation by the finite-difference time-domain method. More importantly, a relatively sharp and accurate focus can be achieved in the brain using the present method.

A k‐space method for nonlinear wave propagation in absorptive media is presented. The Westervelt equation is first transferred into k‐space via Fourier transformation and is solved by a wave‐vector time‐domain scheme. The present approach is not limited to forward propagation or parabolic approximation. One‐ and 2‐D problems are investigated to verify the method by comparing results to the finite element method. It is found that, in order to obtain accurate results in homogeneous media (errors of the fundamental and first two harmonics being less than 0.5 dB), the grid size can be as little as two points per wavelength, and for a moderately nonlinear problem, the Courant–Friedrichs–Lewy number can be as small as 0.4. As a result, the k‐space method for nonlinear wave propagation is shown here to be computationally more efficient than the conventional finite element method or finite‐difference time‐domain method for the conditions studied here. However, although the present method is highly accurate for weakly inhomogeneous media, it is found to be less accurate for strongly inhomogeneous media.

There has been an increasing need in analyzing sound energy decays consisting of more than one decay slope, so‐called non‐single‐exponential decays. It has been considered very challenging to estimate parameters associated with double‐slope decay characteristics, even more challenging when the coupled‐volume systems contain more than two decay processes. To meet the need of characterizing energy decays of multiple decay processes, this work reports investigations using both acoustical scale‐models and numerical models of three coupled volumes. Two levels of Bayesian inference: model selection and parameter estimation, have been applied to detection and characterization of sound energy decays. Experimental results measured in acoustical scale models and numerical results from diffusion‐equation‐based models are used to validate the Bayesian analysis methods. The analysis methods are then applied to geometric‐acoustics simulations of a conceptual concert hall. This work demonstrates that the analysis method...

This paper presents an experimental study on steady‐state sound pressure levels and reverberation times in a tenth long‐space scale model. The purpose of this work is to validate a recently developed one‐dimensional transport equation model for acoustic predictions in long spaces. A series of measurements has been conducted in the scale model to reveal receiver‐position dependence of measured and modeled parameters in the long spaces. Experimental results at different octave bands are obtained. This paper will compare the acoustically measured results with that of simulations.

This paper compares two numerical models: diffusion equation model and transport equation model, for room‐acoustic predications in long spaces. Mathematically, the diffusion equation is the asymptotic approximation to the transport equation, therefore is expected to be less accurate. In addition, the transport equation intrinsically includes the direct sound field while the diffusion equation does not. However, due to its simple form of the diffusion equation, it has certain computational advantages over the transport equation model, e.g., it is less computationally expensive. When comparing the two models with the ray‐tracing based method, the transport equation is shown to be more precise, especially when the walls are highly absorptive. Nevertheless, the diffusion equation model performs fairly well provided that the absorption on the walls is low. In this case, the diffusion equation model might be preferred over the transport equation model due to a low computational load.

In this work, a diffusion equation model [J. Xiang, J. Acoust. Soc. Am., 123, 145–153 (2008)] is applied to two coupled spaces to study the time‐dependent sound energy flows through the coupling area (aperture). The energy feedback is found when the primary room is less reverberant than the secondary room. The Bayesian framework is applied to quantify double‐slope characteristics of sound‐energy flow decays obtained from the diffusion equation numerical simulations. This work also reveals that the turning point on a double‐sloped energy decay is highly correlated to the time instant when the energy flow direction flips over.

Steady‐state and transient sound field modeling of open spaces, such as urban squares, courtyard‐type traditional Chinese theaters, is discussed in this paper. It is well known that the application of the statistical room‐acoustic theory is strongly limited in this case, because of the high non‐uniformity of the absorption coefficient. In this study, a diffusion equation along with a recently proposed modified boundary condition [Y. Jing and N. Xiang, J. Acoust. Soc. Am., 123, 2008, 145‐153] is used to predict the acoustic characteristics of open spaces, in terms of both sound pressure level distributions and reverberation times. Three different types of open spaces are presented, including urban squares, quasi‐cubic open rooms, and urban streets. This paper will compare the simulation results with the experimental results conducted in all these three cases (scale model), to verify the validity and efficiency of the diffusion equation model.

Acoustically coupled spaces have been studied and applied to concert halls due to a number of interesting phenomena inside the spaces, including nonexponential energy decays, which are believed to benefit both desired clarity and reverberance. A diffusion equation model has been recently applied to acoustically coupled spaces to predict both steady‐state and time‐dependent sound field [A. Billon, et. al., J. Acoust. Soc. Am., 120, 2006, pp. 2043‐2054], good agreements between simulations and experimental measurements have been found. In this paper, the diffusion equation along with a recently proposed modified boundary condition [Y. Jing and N. Xiang, J. Acoust. Soc. Am., 123, 145‐153 (2008)] is used, to reveal intriguing characteristics of coupled spaces, including the sound pressure level distribution along the aperture, energy flow in both rooms, and location dependence of the acoustic source on energy decay characteristics. Experimental results are employed to verify the model, and show the capability...