Abstract Globally, the development of porous structured materials has been receiving incredible responses for various high-performance engineering applications. Piezoelectric cellular foams have recently attracted the attention of researchers to emerging applications of acoustic sensors, low-frequency hydrophones, and energy-harvesting devices. As pore morphology is closely related to the shape and the size of the pore-foaming agent, it is necessary to address the influence of particle size of the foaming agent on cell morphology to expand their application area. Hence, this research article establishes the impact of particle size of pore foaming agents on pore morphology, hydrophobicity, and acoustic characteristics of open-cell polyvinylidene fluoride (PVDF) based piezoelectric cellular composites. Open-cell PVDF cellular composites have been fabricated using the template removal method with sodium chloride (NaCl) as a sacrificial templating agent in three different particle sizes: larger, medium, and finer. Based on the experimental results, it can be stated that the particle size of the templating agents dramatically influences the pore morphology, hydrophobicity, and acoustics performance of the PVDF foam samples. The PVDF foams possessing medium pore size have exhibited a maximum sound absorption coefficient of 0.89 at a frequency range of 1,000–1,500 Hz, indicating that PVDF foams have great potential for noise-controlling applications.
In this article, the impact of fluid flow and vibration on the acoustics of a subsonic flow is examined. Specifically, it focuses on the noise generated by a convective gust in uniform flow that is scattered by a vibrating plate of limited size. The study analyzes the interaction between acoustics and structures by considering the scattering of sound waves by a soft finite barrier. To achieve this, the Wiener-Hopf technique is utilized for the analytical treatment of the acoustic model. The approach involves performing temporal and spatial Fourier transforms on the governing convective boundary value problem, then formulating the resulting Wiener-Hopf equations. The product decomposition theorem, an extended version of Liouville's theorem, and analytic continuation are employed to solve these equations. Finally, the scattered potential integral equations are computed asymptotically. This study can be significant for understanding the acoustic properties of structures and how they interact with fluid flow in subsonic environments, which could have applications in fields such as aerospace engineering, noise reduction, and structural acoustics.
We numerically and experimentally investigated the behavior of high-frequency underwater ultrasounds reflected by gradient acoustic metasurfaces. Metasurfaces were fabricated with a periodic array of gradient slits along the surface of a steel specimen. The finite element method was adopted for the acoustics–structure interaction problem to design the metasurfaces and simulate the reflected fields of the incident ultrasound. Our metasurfaces yielded anomalous reflection, specular reflection, apparent negative reflection, and radiation of surface-bounded modes for ultrasonic waves impinging on the metasurfaces at different incident angles. The occurrence of these reflection behaviors could be explained by the generalized Snell’s law for a gradient metasurface with periodic supercells. We showed that at some incident angles, strong anomalous reflection could be generated, which could lead to strong retroreflection at specific incident angles. Furthermore, we characterized the time evolution of the reflections using pulsed ultrasound. The simulated transient process revealed the formation of propagating reflected ultrasound fields. The experimentally measured reflected ultrasound signals verified the distinct reflection behaviors of the metasurfaces; strong anomalous reflection steering the ultrasound pulse and causing retroreflection was observed. This study paves the way for designing underwater acoustic metasurfaces for ultrasound imaging and caustic engineering applications using pulsed ultrasound in the high-frequency regime.
Retroreflection is rarely used as a surface treatment in architectural acoustics but is found incidentally with building surfaces that have many simultaneously visible concave right-angle trihedral corners. Such surfaces concentrate reflected sound onto the sound source, mostly at high frequencies. This study investigated the potential for some Indian stepwells (stepped ponds, known as a kund or baori/baoli in Hindi) to provide exceptionally acoustically retroreflective semi-enclosed environments because of the unusually large number of corners formed by the steps. Two cases—Panna Meena ka Kund and Lahan Vav—were investigated using finite-difference time-domain (FDTD) acoustic simulation. The results are consistent with retroreflection, showing reflected energy concentrating on the source position mostly in the high-frequency bands (4 kHz and 2 kHz octave bands). However, the larger stepped pond has substantially less retroreflection, even though it has many more corners, because of the greater diffraction loss over the longer distances. Retroreflection is still evident (but reduced) with non-right-angle trihedral corners (80°–100°). The overall results are sufficiently strong to indicate that acoustic retroreflection should be audible to an attuned visitor in benign environmental conditions, at least at moderately sized stepped ponds that are in good geometric condition.
Erik Nilsson, Sylvain Ménard, Delphine Bard Hagberg
et al.
Sound reduction is complex to estimate for acoustical treatments on ventilation ducts through walls. Various acoustical treatments are available for ventilation ducts, including internal lining (absorption along the inner perimeter), external lagging (external sound insulation), silencer, and suspended ceilings. Previous studies have examined how silencers and the internal lining affect the sound transmission of ventilation ducts. However, there are few theories to predict the effect of external lagging in combination with ventilation ducts and how the total sound reduction is affected. This article aims to investigate different acoustical treatments and develop theoretical models when external lagging with stone wool is used to reduce flanking sound transmission via the surface area of ventilation ducts. Theoretical models are developed for external lagging and compared with measurement data. Measurements and theory are generally in good agreement over the third-octave band range of 100–5000 Hz. The developed models clarify that the distance closest to the wall has the main impact on sound reduction for a combined system with a wall and a ventilation duct. Suspended ceilings and silencers are found to be enough as acoustical treatments for certain combinations of ventilation ducts and walls. However, external lagging seems to be the only effective solution in offices and schools when a large ventilation duct passes through a wall with high sound reduction.
We present a design to achieve antichiral edge states in acoustic systems where edge states on the two parallel edges of a lattice with a strip geometry propagate in the same direction. This peculiar phenomenon is realized by using a honeycomb lattice consisting of acoustic resonators with staggered air flow; i.e., the air flow takes opposite directions in resonators belonging to different sublattices. The existence of antichiral edge states is revealed through full-wave simulations of the band structure and acoustic fields excited by a point source. Furthermore, we compare these antichiral edge states with conventional chiral edge states. It is found that the antichiral edge states are less robust than the chiral ones. Our work offers new possibilities for dispersion engineering and wave manipulations in acoustics.
In this article, a greedy reduced basis algorithm is proposed for the solution of structural acoustic systems with parameter and implicit frequency dependence. The underlying equations of linear time‐harmonic elastodynamics and acoustics are discretized using the finite element and boundary element method, respectively. The solution within the parameter domain is determined by a linear combination of reduced basis vectors. This basis is generated iteratively and given by the responses of the structural acoustic system at certain parameter samples. A greedy approach is followed by evaluating the next basis vector at the parameter sample which is currently approximated worst. The algorithm runs on a small training set which bounds the memory requirements and allows applications to large‐scale problems with high‐dimensional parameter domains. The computational efficiency of the proposed scheme is illustrated based on two numerical examples: a point‐excited spherical shell submerged in water and a satellite structure subject to a diffuse sound pressure field excitation.
Christian Adams, Regine Stutz, Elisabeth Kaiser
et al.
Neonatal incubators provide suitable environmental conditions for premature newborns and allow for medical treatment such as medication and monitoring of vital functions such as blood pressure. The incubator includes several system components such as a control system, an oxygen supply, a scale or flaps and drawers for patient care and storage of medical material, respectively. These system components generate noise such as monitoring alarms, noise of the oxygen supply, or noise due to opening and closing of flaps during medical treatments. The noise leads to a significantly increased sound exposure inside the incubator. Increased sound exposure is known to cause distress and to increase the risk of acute or chronic diseases in the preterm neonate. This paper presents acoustic measurements on an incubator in a neonatal intensive care unit. Several vibration and acoustic measurements are performed inside the incubator as well as in the surrounding environment in order to characterize typical acoustic scenes from everyday life on the neonatal intensive care unit. Based on the measurement results, the scenes are categorized in terms of sound exposure. This forms the basis for a future design for acoustics of the incubator.
In low Mach number aeroacoustics, the known disparity of length scales makes it possible to apply well‐suited simulation models using different meshes for flow and acoustics. The workflow of these hybrid methodologies include performing an unsteady flow simulation, computing the acoustic sources, and simulating the acoustic field. Therefore, hybrid methods seek for robust and flexible procedures, providing a conservative mesh to mesh interpolation of the sources while ensuring high computational efficiency. We propose a highly specialized radial basis function interpolation for the challenges during hybrid simulations. First, the computationally efficient local radial basis function interpolation in conjunction with a connectivity‐based neighbor search technique is presented. Second, we discuss the computation of spatial derivatives based on radial basis functions. These derivatives are computed in a local‐global approach, using a Gaussian kernel on local point stencils. Third, radial basis function interpolation and derivatives are used to compute complex aeroacoustic source terms. These ingredients are necessary to provide flexible source term calculations that robustly connect flow and acoustics. Finally, the capabilities of the presented approach are shown in a numerical experiment with a co‐rotating vortex pair.