Diffraction of directional sources by rigid wedges or barriers.

The secondary-edge-source method (SESM) has found widespread application in predicting edge diffraction. The method, however, is limited to point sources. Based on the concept of weighted summation of diffractive contribution from each propagation path, this letter extends the SESM to directional sources with equivalent acoustic centers. The validity of the proposed method is demonstrated by comparing theoretical results to those obtained using the omnidirectional monopole superposition method.

A swap-based sparse array design method in personal sound zone.

Sparse array design (SAD) optimizes the geometrical configuration of the loudspeaker array. It provides possibilities for further improving the performance of personal sound zone (PSZ). In this paper, the SAD problem in PSZ is formulated as a sparse multi-objective optimization problem, which takes multiple performance metrics into consideration during the sparsity promotion procedure. A swap-based group search algorithm is designed and proposed for solving the formulated non-convex and combinatorial problem to near-optimality. An approximate form is also proposed for reducing computational burden when there exist multiple target sound fields in bright zone. Simulation and experimental results suggest that the proposed method is more suitable in PSZ for controlling over trade-offs and achieving the same good or even better performances, and the proposed approximate form is effective.

Multi-objective optimization for generation of personal sound zone.

Personal Sound Zone (PSZ) allows listeners to enjoy their individual sound without being disturbed by sound from other zones. Acoustic contrast, signal distortion, and array effort are the most frequently used metrics for measuring the performance of a PSZ system. However, usually, the three metrics cannot be optimized at the same time. A trade-off between the three metrics has to be made when designing a PSZ system. In this paper, two generalized methods based on multi-objective optimization are proposed for dealing with all possible trade-off problems between the three metrics in PSZ. Optimality analysis of the two proposed methods is taken, and the relationship between the two proposed methods is investigated. Numerical simulations are presented to validate the efficacy and flexibility of the two proposed methods.

Shape optimization of acoustic horns for improved directivity control and radiation efficiency based on the multimodal method.

The multimodal method is used to develop an approach for optimizing the shape of axisymmetric acoustic horns for both well-controlled directivity and high radiation efficiency over a wide frequency range. A horn with an arbitrary profile can be efficiently modeled with the multimodal method by projecting the wave field over transverse modes in connected short cylinders; the radii of the cylinders are used directly as design variables. Many design variables are employed in the optimization process to ensure design flexibility and computational accuracy. The relative weights for the design objectives of constant directivity, high radiation efficiency, and acceptable shape smoothness are adjusted by two coefficients in the objective function. The optimization problem is solved with a gradient-based algorithm, which takes advantage of algebraic gradient expressions. Numerical experiments demonstrate that the optimization procedure generates smooth horn contours that exhibit considerably improved performance over the target frequency band. Interestingly, a high-quality horn produced with moderate weight coefficients is similar in shape to constant-directivity horns invented earlier while having good low-frequency loading properties. The proposed method provides an attractive alternative to conventional horn design approaches.

Analysis of total harmonic distortion of miniature loudspeakers used in mobile phones considering nonlinear acoustic damping.

Total harmonic distortion (THD) plays an important role in the performance of miniature loudspeakers used in mobile phones. Traditional nonlinear models fail to predict THD in the higher frequency domain. Significant discrepancies can be found between the prediction and measurement. In this study, an extended model considering nonlinear acoustic damping is proposed to address this issue. Flow separation occurring near the orifices of the sound port is of particular interest. Two velocity-dependent parameters were derived to model the nonlinear damping effect. Based on the extended model, the dynamics of miniature loudspeakers are described using the state-space method. Experiments and calculations confirm the validity of the extended model. The influence of nonlinear acoustic damping on THD is also discussed.

Sound field synthesis of arbitrary moving sources using spectral division method.

The spectral division method (SDM) is one of several sound field synthesis techniques of spatial sound reproduction. The synthesis accuracy of SDM is usually higher than that of wave field synthesis (WFS). However, existing SDM-based sound field synthesis methods are developed for virtual sources moving uniformly along a straight trajectory. This Letter proposes a 2.5-dimensional SDM driving function for arbitrary moving sources. Numerical simulations have validated the driving function. The results indicate that the synthesis error of the SDM with the proposed driving function is lower than that of WFS, especially when the virtual source is close to the secondary sources.

Intensity evaluation of bottom backscattering between high-frequency underwater acoustic instruments and its experimental validation.

Running several high-frequency underwater acoustic instruments simultaneously on board a surface or underwater platform can cause interference between the instruments. By combining ray theory and the high-frequency bistatic scattering model for water bottoms, this work presents a practical method for evaluating the relative intensities of such interference signals corresponding to different instrument-to-instrument distances. To examine this method's effectiveness, a series of lake experiments were conducted. The relative intensity of the interference signal was first evaluated by using the proposed method, and then it was measured for comparison. The experimental results showed good agreement between the evaluated and the measured intensities.

Integrated calculation method of acoustic radiation and propagation for floating bodies in shallow water.

Based on the three-dimensional sono-elasticity theory for ship structures, the Green's function is incorporated and an integrated calculation method of acoustic radiation and propagation for floating bodies in shallow water considering the sound velocity profile is proposed. The near-field and arbitrary far-field acoustic radiation problem can be efficiently calculated. A numerical example of a rigid sphere is given and the results are compared with the finite element method solution to validate the reliability and demonstrate improvements in efficiency. The method is applied to an elastic capsular shell, verifying the applicability of the method for any elastic floating bodies.

Practical method for evaluating the sound field radiated from a waveguide.

This letter presents a simple and practical method for evaluating the sound field radiated from a waveguide. By using the proposed method, detailed information about the radiated sound field can be obtained by measuring the sound field in the mouth of the baffled waveguide. To examine this method's effectiveness, the radiated sound pressure distribution in space was first evaluated by using the proposed method, and then it was measured directly for comparison. Experiments using two different waveguides showed good agreement between the evaluated and the measured radiated sound pressure distributions.

Enhanced Mechanical Properties of Al2O3 Nanoceramics via Low Temperature Spark Plasma Sintering of Amorphous Powders.

In this work, Al2O3 nanoceramics were prepared by spark plasma sintering of amorphous powders and polycrystalline powders with similar particle sizes. Effective comparisons of sintering processes and ultimate products depending on starting powder conditions were explored. To ensure near-full density higher than 98% of the Al2O3 nanoceramics, the threshold temperature in SPS is 1450 °C for polycrystalline Al2O3 powders and 1300 °C for amorphous powders. The low SPS temperature for amorphous powders is attributed to the metastable state with high free energy of amorphous powders. The Al2O3 nanoceramics prepared by amorphous powders display a mean grain size of 170 nm, and superior mechanical properties, including high bending strength of 870 MPa, Vickers hardness of 20.5 GPa and fracture toughness of 4.3 MPa∙m1/2. Furthermore, the Al2O3 nanoceramics prepared by amorphous powders showed a larger dynamic strength and dynamic strain. The toughening mechanism with predominant transgranular fracture is explained based on the separation of quasi-boundaries.

An examination of the Northern Hemisphere mid-latitude storm track interannual variability simulated by climate models-sensitivity to model resolution and coupling.

The model fidelity in simulating the Northern Hemisphere storm track interannual variability and the connections of this variability to the low frequency atmospheric variations and oceanic variations are examined based on the atmospheric European Centre for Medium-Range Weather Forecasts (ECMWF) model and coupled NCAR Community Climate System Model (CCSM) systems at different horizontal resolutions. The atmospheric general circulation model (AGCM) runs are forced by observed sea surface temperatures (SST) with varying atmospheric resolutions, while the coupled general circulation model (CGCM) runs have a fixed atmospheric resolution but varying oceanic resolutions. The phases, between the North Pacific (NP) and North Atlantic (NA) sectors, of the simulated hemisphere-scale Empirical Orthogonal Function (EOF) modes of the storm track fluctuations change with the model resolution, suggesting the storm track variability in NP and NA basins are largely independent. The models can qualitatively reproduce the basin-scale EOFs of both NP and NA storm track variability. These EOFs are not sensitive to either atmospheric or oceanic model horizontal resolutions, but their magnitudes from the CGCM runs are substantially underestimated. The storm track variations over NP basin are hybrid of internal atmospheric variations and external forcing from the underlying conditions, but the fluctuations over the NA basin are merely atmospheric internal variability. The NP storm track variability from SST forcing accounts for 4.4% of the total variance in observations, while it only has less than 2% of the total in all AGCM simulations. The external forcing to the storm track variations is more realistically reproduced in the higher atmospheric resolution runs. The air-sea coupling makes the SST feedbacks to the atmospheric internal variability, absent in the atmospheric ECMWF model hindcasts, emerge in the coupled CCSM simulations.

Precipitation characteristic changes due to global warming in a high-resolution (16 km) ECMWF simulation.

Changes in precipitation amount, intensity and frequency in response to global warming are examined using global high-resolution (16 km) climate model simulations based on the European Centre for Medium-range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) conducted under Project Athena. Our study shows the increases of zonal-mean total precipitation in all latitudes except the northern subtropics (15°-30°N) and southern subtropics-to-midlatitudes (30°-40°S). The probability distribution function (PDF) changes in different latitudes suggest a higher occurrence of light precipitation (LP; ≤1 mm/day) and heavy precipitation (HP; ≥30 mm/day) at the expense of moderate precipitation reduction (MP; 1-30 mm/day) from Tropics to midlatitudes, but an increase in all categories of precipitation in polar regions. On the other hand, the PDF change with global warming in different precipitation climatological zones presents another image. For all regions and seasons examined, there is an HP increase at the cost of MP, but LP varies. The reduced MP in richer precipitation zones resides in the PDF peak intensities, which linearly increase with the precipitation climatology zones. In particular in the Tropics (20°S to 20°N), the precipitation PDF has a flatter distribution (i.e. HP and LP increases with MP reduction) except for the Sahara Desert. In the primary precipitation zones in the subtropics (20°-40°) of both hemispheres, precipitation over land switches toward higher intensity (HP increases, but MP and LP decrease) in both winter and summer, while precipitation over ocean in both seasons shows a flattening trend in the intensity distribution. For the major precipitation zones of the mid-to-high latitude belt (40°-70°), PDF of precipitation tends to be flatter over ocean in summer, but switches toward higher intensities over land in both summer and winter, as well as over ocean in winter.