Enhancement of acousto-optic interaction using a phoxonic cavity with structural hierarchy.

We propose a phoxonic cavity with structural hierarchy to enhance acousto-optic interaction in acoustically dissipative media. In a conventional phoxonic cavity, interaction between infrared light and hypersound with the same wavelength scale became weak due to large acoustic attenuation whose coefficient is proportional to the square of the frequency. To alleviate the acoustic attenuation, it is necessary to use low-frequency sound with much longer wavelength than the infrared light, but the conventional phoxonic cavity is not suitable for confining such hypersound and infrared light simultaneously. In this study, we employ the concept of structural hierarchy into the phoxonic cavity to control infrared light and hypersound with different wavelength scales. A phoxonic cavity with two different scales achieves the acousto-optic interaction approximately 1.6 times that in the conventional one. To further enhance the interaction, we adjust geometrical constitution and material properties of the two-scale phoxonic cavity using quasi-static homogenization theory, leading to the interaction about 2.1 times that in the conventional cavity.

Near-perfect sound absorption using hybrid resonance between subwavelength Helmholtz resonators with non-uniformly partitioned cavities.

We present near-perfect sound absorption using a metasurface composed of meta-atoms (MAs) which are subwavelength Helmholtz resonators (HRs) with cavities non-uniformly partitioned by membranes. By embedding the membranes at different horizontal locations in the cavities, we break geometrical symmetry between the MAs so as to derive hybrid resonance between the MAs at our target frequency. The resonance frequency of each MA is determined by delicately adjusting the locations of the membranes, resulting in perfect absorption at the target frequency which is different from the resonance frequencies of MAs. The metasurface is designed to satisfy impedance matching conditions with air at one or more target frequencies with the aid of a theoretical model for frequency-dependent effective acoustic impedance. The theoretical model is established with physical reality by considering the higher-order eigenmodes of the membrane, the visco-thermal losses in narrow orifices, and the end corrections of the subwavelength HR. The designed metasurface is fabricated and its absorption performance is verified experimentally in an impedance tube. Near-perfect absorption of sound is achieved at the target frequency of 500 Hz, which is 12.3% lower than that of near-perfect absorption by previous metasurfaces inducing hybrid resonance between HRs without membranes.

Nonplanar metasurface for perfect absorption of sound waves.

We propose a sound-absorbing nonplanar metasurface by considering locally different incidence angles along the metasurface. Perfect sound absorption is realized with the aid of hybrid resonance between two different subwavelength Helmhwoltz resonators comprising a unit cell. We theoretically investigate the effect of incidence angles on the sound absorption of the unit cells, and present a design method of the nonplanar metasurface that achieves perfect absorption by considering locally different incidence angles along the metasurface. The perfect absorption of plane sound waves on nonplanar surfaces is numerically demonstrated at the target frequency of 1 kHz. The numerical results show that at least 99.8% of the incident wave energy is absorbed by the designed metasurfaces with a thickness of λ/24. A nonplanar metasurface is fabricated via three-dimensional printing, and perfect sound absorption is experimentally validated at the target frequency of 1 kHz. Furthermore, we design nonplanar metasurfaces that can perfectly absorb cylindrical sound waves when a line source is located near the metasurface. While previous sound-absorbing metasurfaces focused only on planar surfaces, the proposed method achieves perfect sound absorption on nonplanar surfaces, expanding the range of practical applications in various industrial areas.

Bandgap characteristics of phononic crystals in steady and unsteady flows.

Over the past 30 years, most phononic crystal research has been done for a stationary medium. As reported in a recent experimental study, phononic crystals cannot preserve their bandgaps in the presence of flow. In this study, the bandgap characteristics of a two-dimensional phononic crystal in steady and unsteady flows are investigated theoretically. To identify the effect of the flow on sound insulation in the bandgap frequency ranges, the acoustic reflectance spectra of phononic crystals for different types of background flows, including a uniform flow, a compressible potential flow, and a turbulent flow were calculated. For the steady flows, which include uniform and compressible potential flows, the reflectance spectra are shifted to a lower frequency by the factor 1-M2 due to convection when the flow is in the same direction as the incident wave. Moreover, the reflectance spectra of a phononic crystal in a turbulent flow were evaluated for various combinations of inflow speeds and geometric parameters, such as the filling ratio and the number of layers. Due to the aerodynamic noise and fluid convection, a phononic crystal cannot work as an acoustic barrier, rather it becomes an aeroacoustic source in a turbulent flow.

Hierarchical phononic crystals for filtering multiple target frequencies of ultrasound.

Hierarchically structured phononic crystals are proposed for filtering multiple frequency bands. The advantages of using structural hierarchy come from its multiscale periodicity and the increased number of controllable parameters, which contribute to open multiple bandgaps in broadband frequency ranges and adjust the positions of those bandgaps. By deriving a transfer-matrix-based theoretical formula, hierarchical phononic crystals are designed that filter the frequency bands for randomly selected frequencies in the ultrasonic range of 20 kHz to 10 MHz. Their wave-filtering capability is demonstrated by using numerical simulations with consideration of material loss. By comparing the transmittance spectra of the hierarchical phononic crystals with those of conventional ones, the structural hierarchy of the former is shown to be advantageous in filtering multiple frequency bands.

Author Correction: Effect of compressibility and non-uniformity in flow on the scattering pattern of acoustic cloak.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.

Effect of compressibility and non-uniformity in flow on the scattering pattern of acoustic cloak.

During the last decade, most of acoustic cloak research has been done within a theoretical framework in which the medium is at rest. However, such an acoustic cloak cannot preserve its unique properties or functions to make an object acoustically invisible in the presence of flow. In this study, we propose a theoretical framework to accurately investigate the effect of compressibility and non-uniformity in flow on the scattering pattern of acoustic cloak. In the formulation, the wave operator is coupled with the non-uniform velocity vector, and the equivalent source terms due to mean flow are divided into the compressibility effect and the non-uniformity effect with their own physical meanings. Numerical simulation shows the difference in far-field directivity between previous and present formulations. The polarity of the equivalent sources in the present formulation shows hexapole and skewed quadrupole patterns for non-uniformity and compressibility effects, respectively, and their magnitudes increase with power laws of Mach number as the Mach number increases. As an application, we make use of the present formulation for predicting the acoustic scattering from newly designed convective cloaks. The simulation results show better performance compared to the existing convective cloak.

Vibration damping using a spiral acoustic black hole.

This study starts with a simple question: can the vibration of plates or beams be efficiently reduced using a lightweight structure that occupies a small space? As an efficient technique to damp vibration, the concept of an acoustic black hole (ABH) is adopted with a simple modification of the geometry. The original shape of an ABH is a straight wedge-type profile with power-law thickness, with the reduction of vibration in beams or plates increasing as the length of the ABH increases. However, in real-world applications, there exists an upper bound of the length of an ABH due to space limitations. Therefore, in this study, the authors propose a curvilinear shaped ABH using the simple mathematical geometry of an Archimedean spiral, which allows a uniform gap distance between adjacent baselines of the spiral. In numerical simulations, the damping performance increases as the arc length of the Archimedean spiral increases, regardless of the curvature of the spiral in the mid- and high-frequency ranges. Adding damping material to an ABH can also strongly enhance the damping performance while not significantly increasing the weight. In addition, the radiated sound power of a spiral ABH is similar to that of a standard ABH.

Evaluation of rapid cell division in non-uniform cell cycles.

To better understand the mechanisms of development of harmful algal blooms (HABs), accurate estimates of species-specific in situ growth rates are needed. HABs are caused by rapid cell division by the causative microorganisms. To accurately estimate the in situ growth rates of harmful algae having non-uniform and/or irregular cell cycles, we modified a standard equation based on the cell cycle, and calculated the in situ growth rate to describe the process of bloom development in nature. Sampling of a developing bloom of Heterosigma akashiwo in Pohang Bay, Korea, was conducted every 3 h from 15:00 on August 2 to 07:00 on August 4, 2006. The amount of H. akashiwo DNA was measured using flow cytometry following tyramide signal amplification-fluorescence in situ hybridization. On August 2, the percentage of G1 phase cells decreased from 15:00 to 19:00 then increased until 22:00; it then decreased until 07:00 on August 3, followed by an increase to 10:00. This indicates the ability of the cells in nature to undergo more than one round of division per day. During the following night two rounds of division did not occur. The in situ growth rates estimated using the modified equation ranged from 0.31 to 0.53 d(-1) . We conclude that the use of this equation enables more accurate estimates of bloom formation by rapidly dividing cells.

Simulation study of territory size distributions in subterranean termites.

In this study, on the basis of empirical data, we have simulated the foraging tunnel patterns of two subterranean termites, Coptotermes formosanus Shiraki and Reticulitermes flavipes (Kollar), using a two-dimensional model. We have defined a territory as a convex polygon containing a tunnel pattern and explored the effects of competition among termite territory colonies on the territory size distribution in the steady state that was attained after a sufficient simulation time. In the model, territorial competition was characterized by a blocking probability P(block) that quantitatively describes the ease with which a tunnel stops its advancement when it meets another tunnel; higher P(block) values imply easier termination. In the beginning of the simulation run, N=10, 20,…,100 territory seeds, representing the founding pair, were randomly distributed on a square area. When the territory density was less (N=20), the differences in the territory size distributions for different P(block) values were small because the territories had sufficient space to grow without strong competitions. Further, when the territory density was higher (N>20), the territory sizes increased in accordance with the combinational effect of P(block) and N. In order to understand these effects better, we introduced an interference coefficient γ. We mathematically derived γ as a function of P(block) and N: γ(N,P(block))=a(N)P(block)/(P(block)+b(N)). a(N) and b(N) are functions of N/(N+c) and d/(N+c), respectively, and c and d are constants characterizing territorial competition. The γ function is applicable to characterize the territoriality of various species and increases with both the P(block) values and N; higher γ values imply higher limitations of the network growth. We used the γ function, fitted the simulation results, and determined the c and d values. In addition, we have briefly discussed the predictability of the present model by comparing it with our previous lattice model that had been used to explain the territory size distributions of mangrove termites on the Atlantic coast of Panama.

A constraint condition for foraging strategy in subterranean termites.

Previous studies have explored the relationship between termite branch tunnel geometry and foraging efficiency in a model simulation in which foraging efficiency, γ, for two termite species, Coptotermes formosanus Shiraki and Reticulitermes flavipes (Kollar) (Isoptera: Rhinotermitidae), was investigated in response to two variables, the probability of tunnel branching (P(branch)) and the probability of tunnel branch termination (Pterm). It was found that simulated tunnel patterns based on empirical data did not have maximum foraging efficiency. We hypothesized that termites could increase their foraging efficiency in response to landscape heterogeneity. The present study investigated how termites could control the two variables, P(branch) and P(term), in response to the external environment in terms of tunnel network connectivity. It was found that the best simulated strategy for C. formosanus and R. flavipes termites would occur if both P(branch) and P(term) were increased together. This study provides possible mechanisms for foraging strategies in subterranean termites and a baseline for future empirical work.