Effect of thermal aging on the transport and acoustic properties of partially reticulated polyurethane foams.

This study aims to investigate the acoustic effects of thermal aging on partially reticulated polyurethane (PU) foam. An accelerated test was performed under appropriate test conditions as determined by thermal analyses of the material. Measurements of the absorption coefficient showed that the performance of the partially reticulated PU foam can be significantly reduced by thermal aging. The transport parameters were evaluated to analyze the origin of this change in the absorption behavior. Sensitivity analyses revealed that a decrease in the static airflow resistivity had the greatest effect in terms of reducing the absorption coefficient owing to thermal aging. In addition, observation and characterization of the microstructure of the aged foam to determine the root cause of this acoustic degradation indicated that heat-induced damage to the membrane was the most important factor. To verify this assertion, a periodic unit cell model that mimicked the topology of the cellular structure was constructed, and the mechanism responsible for the change in the acoustic behavior was simulated. The results presented herein can be used as durability guidelines for maintaining the performances of partially reticulated PU foams that are employed in high-temperature environments.

Energy flow analysis of equivalent fluid models for porous media.

Based on a heat conduction analogy that enables the energy flow analysis (EFA) to describe the smoothed energy variations, the EFA has been applied to a variety of structures to predict vibrational responses at high-frequency regions. In this paper, energy equations in the form of heat conduction laws are derived to represent dilatational waves in rigid- and limp-frame porous media using equivalent fluid models. Homogeneous and inhomogeneous waves are considered. Within the EFA framework, the group velocity and loss factor included in the energy models for structures are replaced with the energy velocity and effective loss factor, respectively. The capabilities of the energy models are illustrated using configurations in which the porous layer backed by a rigid wall is in a normal and oblique incident sound field. The results of the numerical simulations confirm the feasibility of the energy model. From an energy perspective, the adequacy of using rigid- and limp-frame equivalent fluid models is also discussed. It is shown that the use of a rigid-frame equivalent model for predicting the energy distribution is more restrictive than predicting acoustic performance such as sound absorption.

Convolutional neural networks for estimating transport parameters of fibrous materials based on micro-computerized tomography images.

This study proposes a method for estimating the transport parameters of fibrous materials from x-ray micro-computed tomography (CT) images using convolutional neural networks (CNNs). Two-dimensional (2-D) micro-CT images and numerically obtained transport parameters were used to train the CNNs; Stokes flow and potential flow were used to numerically obtain the transport parameters using geometrical models extracted from the raw CT images. Then, analogously to constructing a three-dimensional image of the fibrous material by stacking the 2-D slice images, the volumetric transport parameters of the fibrous materials were calculated using the parameters of each 2-D image predicted by the trained CNN models. The transport parameters of the fibrous volume predicted by the CNN models showed good agreement with the measured values. In addition, the sound absorption coefficient was calculated by applying both the predicted and measured transport parameters to the semi-phenomenological sound propagation model and compared with the measured sound absorption coefficient. The results of the study confirm the feasibility of predicting transport parameters of fibrous materials using a neural network model based on raw micro-CT images.

Estimation and uncertainty analysis of fluid-acoustic parameters of porous materials using microstructural properties.

This study quantified the microstructure of polyurethane foams and elucidated its relationship to fluid-acoustic parameters. The complex morphology derived from the three-dimensional images obtained by micro-computed tomography was analyzed using digital image processing and represented by a pore network model (PNM) and a distance map model. The PNM describes the fluid phase of a porous medium with equivalent spherical pores and circular throats, whereas the distance map model describes the solid phase with the average frame thickness. The porous materials were then modeled by six representative microstructural parameters that describe the geometry and topology of the fluid and solid phases. These parameters were pore radius, throat radius, distance between adjacent pores, coordination number, pore inclination angle, and frame thickness. Semi-phenomenological and empirical approaches were proposed to relate the microstructural properties to the fluid-acoustic parameters. These models effectively described the acoustic parameters and sound absorption performance of six different polyurethane foams. Since the representative microstructural parameters were obtained from small sample volumes of a heterogeneous material, notable variations were observed across different regions of the sample. Hence, this study quantified the effect of the uncertainty in each microstructural parameter on the resulting acoustic parameters using global sensitivity analysis.

35 Hz shape memory alloy actuator with bending-twisting mode.

Shape Memory Alloy (SMA) materials are widely used as an actuating source for bending actuators due to their high power density. However, due to the slow actuation speed of SMAs, there are limitations in their range of possible applications. This paper proposes a smart soft composite (SSC) actuator capable of fast bending actuation with large deformations. To increase the actuation speed of SMA actuator, multiple thin SMA wires are used to increase the heat dissipation for faster cooling. The actuation characteristics of the actuator at different frequencies are measured with different actuator lengths and results show that resonance can be used to realize large deformations up to 35 Hz. The actuation characteristics of the actuator can be modified by changing the design of the layered reinforcement structure embedded in the actuator, thus the natural frequency and length of an actuator can be optimized for a specific actuation speed. A model is used to compare with the experimental results of actuators with different layered reinforcement structure designs. Also, a bend-twist coupled motion using an anisotropic layered reinforcement structure at a speed of 10 Hz is also realized. By increasing their range of actuation characteristics, the proposed actuator extends the range of application of SMA bending actuators.

Analysis of impact noise induced by hitting of titanium head golf driver.

The hitting of titanium head golf driver against golf ball creates a short duration, high frequency impact noise. We analyzed the spectra of these impact noises and evaluated the auditory hazards from exposure to the noises. Noises made by 10 titanium head golf drivers with five maximum hits were collected, and the spectra of the pure impact sounds were studied using a noise analysis program. The noise was measured at 1.7 m (position A) and 3.4 m (position B) from the hitting point in front of the hitter and at 3.4 m (position C) behind the hitting point. Average time duration was measured and auditory risk units (ARUs) at position A were calculated using the Auditory Hazard Assessment Algorithm for Humans. The average peak levels at position A were 119.9 dBA at the sound pressure level (SPL) peak and 100.0 dBA at the overall octave level. The average peak levels (SPL and overall octave level) at position B were 111.6 and 96.5 dBA, respectively, and at position C were 111.5 and 96.7 dBA, respectively. The average time duration and ARUs measured at position A were 120.6 ms and 194.9 units, respectively. Although impact noises made by titanium head golf drivers showed relatively low ARUs, individuals enjoying golf frequently may be susceptible to hearing loss due to the repeated exposure of this intense impact noise with short duration and high frequency. Unprotected exposure to impact noises should be limited to prevent cochleovestibular disorders.

Acoustic transmission analysis on cavity resonance sound in a cylindrical cavity system: application to a Korean bell.

This paper presents an analytical model for acoustic transmission characteristics of a cylindrical cavity system representing the acoustic resonance conditions of a Korean bell. The cylindrical cavity system consists of an internal cavity, a gap, an auxiliary cavity, and a rigid base. Since the internal cavity is connected to the external field through a gap, determination of the acoustic transmission characteristics becomes a coupling problem between the internal cavity and external field. The acoustic field of the internal cavity is considered by expanding the solution method of the mixed boundary problem, and the external field is addressed by modifying the radiation impedance model of a finite cylinder. The analytical model is validated by comparison with both experiment and a boundary element method. Using the analytical model, the resonance conditions are determined to maximize the resonance effect. Thus, the resonance frequencies of the bell cavity system are investigated according to the gap size and auxiliary cavity depth. By adjusting gap size or auxiliary cavity depth, the cavity resonance frequency is tuned to resonate partial tones of the bell sound. Finally, the optimal combination of gap size and auxiliary cavity depth is determined.

The effect of initial stress on the propagation behavior of SH waves in piezoelectric coupled plates.

This study analytically investigates the propagation of shear waves (SH waves) in a coupled plate consisting of a piezoelectric layer and an elastic layer with initial stress. The piezoelectric material is polarized in z-axis direction and perfectly bonded to an elastic layer. The mechanical displacement and electrical potential function are derived for the piezoelectric coupled plates by solving the electromechanical field equations. The effects of the thickness ratio and the initial stress on the dispersion relations and the phase and group velocities are obtained for electrically open and mechanically free situations. The numerical examples are provided to illustrate graphically the variations of the phase and group velocities versus the wave number for the different layers comparatively. It is seen that the phase velocity of SH waves decreases with the increase of the magnitude of the initial compression stress, while it increases with the increase of the magnitude of the initial tensile stress. The initial stress has a great effect on the propagation of SH waves with the decrease of the thickness ratio. This research is theoretically useful for the design of surface acoustic wave (SAW) devices with high performance.

Two-dimensional poroelastic acoustical foam shape design for absorption coefficient maximization by topology optimization method.

Optimal shape design of a two-dimensional poroelastic acoustical foam is formulated as a topology optimization problem. For a poroelastic acoustical system consisting of an air region and a poroelastic foam region, two different physical regions are continuously changed in an iterative design process. To automatically account for the moving interfaces between two regions, we propose a new unified model to analyze the whole poroelastic acoustical foam system with one set of governing equations; Biot's equations are modified with a material property interpolation from a topology optimization method. With the unified analysis model, we carry out two-dimensional optimal shape design of a poroelastic acoustical foam by a gradient-based topology optimization setting. The specific objective is the maximization of the absorption coefficient in low and middle ranges of frequencies with different amounts of a poroelastic material. The performances of the obtained shapes are compared with those of well-known wedge shapes, and the improvement of absorption is physically interpreted.

Dual mode tuning strategy of a slightly asymmetric ring.

Clear beats with proper periods in the first and the second vibration modes are very important factors for the sound of the Korean bell. In this study, the Korean bell is expressed as a circular ring with multiple point masses and a dual mode tuning strategy for clear beat with proper period. For the dual mode tuning, a dual mode equivalent ring model is composed with two point masses, which satisfies the mode pair conditions of the first mode and second mode simultaneously. By adding a suitable amount of additional masses to the dual mode equivalent ring model at appropriate positions, a clear beat with proper period in both of the two important modes is generated. The position and amount of the additional masses are determined analytically. Analytical tuning results are compared and verified with those of the finite element analysis.

Optimal poroelastic layer sequencing for sound transmission loss maximization by topology optimization method.

Optimal layer sequencing of a multilayered acoustical foam is solved to maximize its sound transmission loss. A foam consisting of air and poroelastic layers can be optimized when a limited amount of a poroelastic material is allowed. By formulating the sound transmission loss maximization problem as a one-dimensional topology optimization problem, optimal layer sequencing and thickness were systematically found for several single and ranges of frequencies. For optimization, the transmission losses of air and poroelastic layers were calculated by the transfer matrix derived from Biot's theory. By interpolating five intrinsic parameters among several poroelastic material parameters, distinct air-poroelastic layer distributions were obtained; no filtering or postprocessing was necessary. The optimized foam layouts by the proposed method were shown to differ depending on the frequency bands of interest.