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Since the Morfey-Howell Q/S was introduced as a single-point frequency-domain nonlinearity indicator for propagation of intense broadband noise [AIAA J. 19, 986–992 (1981)], there has been debate about its validity, utility, and interpretation. In this Letter, the generalized Burgers equation is recast in terms of specific acoustic impedance along with linear absorption and dispersion coefficients, normalized quadspectral density (Q/S), and newly proposed normalized cospectral density (C/S). The formulation leads to a rather straightforward interpretation in which Q/S and C/S, respectively, represent the additional absorption and dispersion at a locale, produced by the passage of a finite-amplitude wave. [ABSTRACT FROM AUTHOR]

Abstract: A multilayer ceramic capacitor (MLCC) contains layers of ceramics as the dielectric materials. It has been known that Class 2 MLCCs, made of ferroelectric ceramics such as barium titanate, tend to suffer from electromechanical coupling hence vibration, which leads to the generation of acoustic humming noise, a source of annoyance in many modern electronic devices. In this article, a repoling method to control the electromechanical properties and the resulting vibration of MLCCs is presented. The repoling protocol hinges on the understanding that two independent mechanisms are responsible for the electromechanical coupling in MLCCs: piezoelectricity and electrostriction of the ceramic layers. The vibration due to piezoelectricity is linearly proportional to the input voltage, whereas the vibration due to electrostriction shows a quadratic dependence. Given the DC bias and the AC input voltage under normal operating conditions, the vibration is composed of the fundamental component at the frequency of the AC input and the second harmonic component spawned by the quadratic nonlinearity of electrostriction. It is demonstrated that by engineering the coefficients of piezoelectricity and electrostriction of the ceramic layers through a carefully designed repoling treatment, vibration reduction can be achieved for both the fundamental and second harmonic components. Especially, the fundamental component of vibration can be reduced significantly, as the piezoelectric effect is made to offset the electrostrictive effect. [ABSTRACT FROM AUTHOR]

A method for fabricating disposable membrane hydrophones is presented. The disposable hydrophones are intended for onetime use in such damaging environments as chemically contaminating fluids and high-amplitude (peak amplitude ∼100 MPa) shock wave fields, where the use of commercial membrane hydrophones is not recommended. Fabrication of a hydrophone is done using only off-the-shelf components and hand tools, which translates into ease of fabrication and orders-of-magnitude reduction in unit cost. In particular, poling and sputtering, the two processes that are chiefly responsible for the cost and difficulty associated with the conventional fabrication method, are replaced with the use of pre-poled polyvinylidene fluoride (PVDF) films and polyethylene terephthalate (PET)-coated aluminum foils, respectively. Despite the seemingly crude construction, these disposable hydrophones can exhibit voltage sensitivity response that compares favorably with that of commercial hydrophones. For example, one prototype having a 2 mm × 2 mm active element shows the end-of-cable voltage sensitivity of −270 (±1.9) dB re 1 V/μPa over the frequency range of 1–10 MHz. [ABSTRACT FROM AUTHOR]

Virtual transmitting arrays have been demonstrated for acoustic/elastic wave focusing in water and solids, using a chaotic cavity and a small number of transducers in conjunction with time-reversal acoustics. However, virtual arrays for focusing acoustic waves in air have received relatively little attention. This paper presents a virtual transmitting array based on a time-reversal chaotic cavity for airborne acoustic wave focusing. The virtual transmitting array comprises a three-dimensional printed chaotic cavity and a baffled loudspeaker to focus a high-amplitude acoustic wave only at the desired location. The spatial-temporal focusing performance of the virtual array was evaluated via a two-dimensional raster scan depending on the cavity shape, which is a crucial factor in the performance of a virtual array. The virtual array presented in this paper can be applied to focusing acoustic waves in environments with low reverberation and high absorption, where the application of time-reversal acoustics is challenging. [ABSTRACT FROM AUTHOR]

The microcellular foaming process consists of the saturation process for dissolving gas molecules into plastic and the subsequent foaming process for cell formation. Foaming characteristics of microcellular plastics (MCPs) such as foaming ratio and cell morphology are largely determined by the saturation conditions, particularly by the saturation pressure. In this study, we investigate the effects of saturation pressure on the foaming characteristics of MCPs, when the quantity of dissolved gas (or the weight gain) is kept constant. Because the weight gain of a specimen is an increasing function of saturation pressure, different desorption times are used in order to maintain the same weight gain across specimens from different saturation pressures. Contrary to the common belief, for specimens with the same weight gain higher saturation pressures lead to lower foaming ratios. A hypothesis for the underlying mechanism and a practical ramification of the phenomenon are discussed. [ABSTRACT FROM AUTHOR]

Microcellular plastics (MCPs) are manufactured through a batch process comprised of saturation and foaming stages. In the saturation process, gas molecules are dissolved into plastic in a high-pressure vessel. Following the saturation process, micro-cells are formed inside the plastic as the gas-dissolved plastic sample undergoes the foaming process. In this paper, we investigate the effects of repeating the batch process on the formation of MCPs. Because the plastic sample after the first batch process has developed microcells, these pre-existing cells are expected to affect the second round of the batch process. Of particular interest is the effect of repeated saturation at different saturation pressures. Experiments show that repeating the batch process can lead to favorable outcomes in terms of foaming ratio and cell morphology, which are otherwise unattainable particularly with a single batch process. [ABSTRACT FROM AUTHOR]

Microcellular plastics (MCPs) are one of the most significant developments in the plastics industry over the last decade. Microcellular plastics keep mechanical strength loss and plastic weight to a minimum through microcells inside MCPs. The size and number of cells inside MCPs have a direct correlation with foaming ratio. The cell size has to be consistent to ensure higher quality of MCPs, and a great number of cells are needed for a high foaming ratio. This underscores the need to control the size and number of cells to meet products' required performance. In the microcellular batch process, saturation and foaming conditions are major factors that influence the size and number of cells. This article describes how desorption time, defined as the time lapse between saturation and foaming processes, affects cell formation. Our particular concern is the effect of varying desorption time on foaming ratio and cell morphology. The general rule is that the shorter the desorption time, the higher the foaming ratio (i.e., more cells of smaller sizes are formed because of more dissolved gas molecules remaining in the sample). In our study, however, an opposite trend was observed for desorption time less than 2 minutes. As such, different desorption times ranging from 1 to 5 minutes were used to systematically investigate the corresponding changes in foaming ratio and cell morphology. A possible mechanism of action for this anomalous phenomenon is suggested. [ABSTRACT FROM AUTHOR]

Abstract: At low frequencies (below 10Hz), performance of a conventional shaker is limited by small acceleration amplitudes and a high level of total harmonic distortion. The present article describes a low-frequency vibration generator that overcomes these limitations. The vibration generator consists of a cantilever beam excited by a conventional shaker. The cantilever beam is tuned to resonate at the desired excitation frequency, which leads to a relatively large vibratory motion at the beam tip with very small harmonic distortion. Analysis of the system is performed by means of model equations describing both the flexural and longitudinal components of vibration. A comprehensive measurement of the generator''s performance confirms that it can serve as an economically attractive alternative to existing low-frequency vibration generators used in vibration measurement and calibration. [Copyright &y& Elsevier]

Chaotic reverberation in a cavity, when coupled with time reversal acoustics, can be harnessed to build a perfect time-reversal mirror for transmitting and receiving highly focused sounds with a small number of transducers. In this article, a virtual receiving array, comprised of a single receiving transducer and a chaotic cavity, is developed based on time reversal processing of the reverberation inside the cavity. A prototype array, having 10 × 10 virtual receiving elements, is built and evaluated against a comparable physical array in terms of its localization and waveform reproduction capabilities. It turns out that the most crucial factor in the success of a virtual array is the ergodicity of its chaotic cavity, the exact mathematical expression for which is also derived. The virtual receiving array presented here may find some niche applications in reverberant environments, where a physical array turns out to be too costly or cumbersome to operate. [ABSTRACT FROM AUTHOR]

This Letter reports measurements of nonlinearity parameter B/A of water-saturated glass beads under uniaxial compression representative of the gravitational loading in seabed. The finite-amplitude method in conjunction with the multi-Gaussian beam model is used to determine B/A, in which the use of an accurate source function is found to be important in achieving the overall measurement accuracy. The estimated B/A is on the order of a hundred and decreases with increasing uniaxial compression up to 50 kPa, so that B/A could change as much as 20% over a depth of 5 m for a typical coarse sandy sediment. [ABSTRACT FROM AUTHOR]

The success of active noise control (ANC) is largely deTermined by the fidelity of the estimated secondary path, which encapsulates the "room acoustics" between the secondary sound source and the error sensor. In a time-invariant system the secondary path is usually measured and hard-coded in the controller prior to the ANC operation. When ANC is to be performed in a time-varying environment, however, the estimated secondary path should be updated accordingly, a task that poses many challenges in Terms of efficacy, cost, and user comfort. In this paper we present a deep learning-assisted secondary path update technique, in which deep neural networks are trained to estimate the secondary path in real time according to changing boundary conditions. The feasibility of the technique is tested in an airborne duct, where the error sensor is allowed to move along the duct to simulate changes in boundary conditions. Results have shown that even in the face of a dramatic change in boundary conditions, the ANC system equipped with the present update scheme is capable of reducing broadband noise by up to 10 dB. [ABSTRACT FROM AUTHOR]

An abstract of the article "Generation of Low-frequency vibration using a cantilever beam for calibration of accelerometers," by Won-Suk Ohm and colleagues is presented.

A smartphone contains electronic components that can inadvertently act as sources of unwanted vibration and noise. Also known as the "buzz," the noise is produced primarily by multilayer ceramic capacitors, the piezoelectric pulsation of which drives the circuit board into vibration, hence sound radiation. Given the close proximity of the smartphone to the ear, the audible noise, albeit low in amplitude, can be a nuisance and degrade the call quality. Thus accurate measurement and analysis of the noise are needed as the first step toward resolving the problem. The popular farfield-based techniques, however, are not applicable to smartphone-radiated noise because of its low sound pressure level (∼20 dB). This paper presents an alternative method based on nearfield acoustic holography (NAH), here tailored for a small mobile device radiating low-intensity noise. The NAH method is shown to be capable of visualizing sound pressure and intensity anywhere near the smartphone as well as the vibration of the circuit board, which could lead to an effective design strategy for a quieter smartphone. [ABSTRACT FROM AUTHOR]

The goal of this paper is to examine the computational approaches for predicting both of the overall sound pressure level (OASPL) at a few locations and acceleration power spectral density (APSD) of surrounding thin plates due to the aero-acoustic pressure generated by a cold jet with M = 1.8. First, computational fluid dynamics (CFD), particularly delayed detached eddy simulation, are applied to predict the OASPL at the near-field and compute the acoustic properties. Second, the linearized boundary element method (BEM), that is, the Helmholtz-Kirchhoff method is utilized to propagate the pressure and obtain the OASPL at the far-field. Finally, the finite element method is implemented to predict the APSD for a clamped thin plate based on the optimal triangle membrane element, discrete Kirchhoff triangle plate bending element, and Newmark- β time integration scheme. Using the present CFD and BEM, the OASPLs are compared with the experimental results measured by microphones at both the near- and far-fields, respectively. Moreover, APSDs are compared with the experimental results obtained by an accelerometer at a few different locations. Although OASPLs are overestimated because of the coarse meshes in the higher-angle area and low order scheme of the present CFD analysis, the present integrated aero-vibro-acoustic analysis is capable of predicting the OASPL and APSD generated by a cold jet with M = 1.8. [ABSTRACT FROM AUTHOR]

This paper describes an active silencer system for low-frequency noise in water-filled pipes, with implementation in marine vessels and structures in mind. Active noise control in underwater environment has its unique challenges arising from bubbles. In this light the effect of bubbles on active noise control in water-filled pipes is explored, and a design guideline for a robust active silencer is proposed. The silencer consists of an underwater sound source, an error hydrophone, and an electronic controller programmed with an adaptive filter. The adaptive filter is updated using the filtered-x least mean square algorithm. Estimation of the incoming noise is performed using an adaptive notch filter. The performance of the active silencer is tested for pure tone noises below 500 Hz in a water-filled pipe connected to a reservoir. Notable reduction of noise is achieved with varying degrees of success at different bubble fractions. [ABSTRACT FROM AUTHOR]

A fast computational method for modeling and simulation of large projector arrays is presented. The method is based on the array equations that account for the acoustic interaction among the projector elements as well as the individual characteristics of each projector. Unlike the existing solution method in which the acoustic interaction must be known a priori in the form of interaction impedance matrix Z, the present method seeks the solution of modified array equations through iterations without explicitly evaluating the Z matrix. This significantly speeds up the analysis of complex arrays with surrounding structures, where the evaluation of the Z matrix may require a large number of time-consuming finite element computations. The method is compared with the traditional Z-matrix method for the case of a cylindrical array of 72 × 8 Tonpilz transducers. For the same level of accuracy, the iterative method is shown to be up to 2 orders-of-magnitude faster than the Z-matrix method. The method can be used for rapid design and analysis of active sonar arrays and medical ultrasonic transducers, often made of hundreds and even thousands of elements. [ABSTRACT FROM AUTHOR]

A class of metasurfaces for underwater sound absorption, based on a design principle that maximizes thermoviscous loss, is presented. When a sound meets a solid surface, it leaves a footprint in the form of thermoviscous boundary layers in which energy loss takes place. Considered to be a nuisance, this acoustic to vorticity/entropy mode conversion and the subsequent loss are often ignored in the existing designs of acoustic metamaterials and metasurfaces. The metasurface created is made of a series of topographic meta-atoms, i.e., intaglios and reliefs engraved directly on the solid object to be concealed. The metasurface is acoustically sticky in that it rather facilitates the conversion of the incident sound to vorticity and entropy modes, hence the thermoviscous loss, leading to the desired anechoic property. A prototype metasurface machined on a brass object is tested for its anechoicity, and shows a multitude of absorption peaks as large as unity in the 2–5 MHz range. Computations also indicate that a topographic metasurface is robust to hydrostatic pressure variation, a quality much sought-after in underwater applications. [ABSTRACT FROM AUTHOR]

We report a method of fabricating backing blocks for ultrasonic imaging transducers, using alumina/epoxy composites. Backing blocks contain scatterers such as alumina particles interspersed in the epoxy matrix for the effective scattering and attenuation of ultrasound. Here, the surface integrity can be an issue, where the composite material may be damaged during machining because of differences in strength, hardness and brittleness of the hard alumina particles and the soft epoxy matrix. Poor surface integrity results in the formation of air cavities between the backing block and the piezoelectric element upon assembly, hence the increased reflection off the backing block and the eventual degradation in image quality. Furthermore, with an issue of poor surface integrity due to machining, it is difficult to increase alumina as scatterers more than a specific mass fraction ratio. In this study, we increased the portion of alumina within epoxy matrix by obtaining an enhanced surface integrity using a net shape fabrication method, and verified that this method could allow us to achieve higher ultrasonic attenuation. Backing blocks were net-shaped with various mass fractions of alumina to characterize the formability and the mechanical properties, including hardness, surface roughness and the internal micro-structure, which were compared with those of machined backing blocks. The ultrasonic attenuation property of the backing blocks was also measured. [ABSTRACT FROM AUTHOR]

Despite its introduction more than a decade ago, a two-dimensional ultrasonic array remains a luxury in nondestructive evaluation because of the complexity and cost associated with its fabrication and operation. This paper describes the construction and performance of a two-dimensional virtual array that solves these problems. The virtual array consists of only two transducers (one each for transmit and receive) and an aluminum chaotic cavity, augmented by a 10 × 10 matrix array of rectangular rods. Each rod, serving as an elastic waveguide, is calibrated to emit a collimated pulsed sound beam centered at 2.5 MHz using the reciprocal time reversal. The resulting virtual array is capable of pulse-echo interrogation of a solid sample in direct contact along 10 × 10 scan lines. Three-dimensional imaging of an aluminum test piece, the nominal thickness of which is in the order of 1 cm, is successfully carried out using the virtual array. [ABSTRACT FROM AUTHOR]