Incident Sound Wave Research Articles

Multi-order resonators for acoustic multiband asymmetric absorption and reflection

We propose a multiband asymmetric acoustic absorption and reflection system constructed from a waveguide and multi-order resonators with different radiant modes. Theoretical and experimental results confirm that the coupling of the dark and bright modes at each resonant frequency enables multiband high-efficiency absorption requiring only two functional units. More interestingly, we demonstrate a hybrid multiband asymmetric system based on customized multi-mode pairs. One side of the hybrid multiband system almost completely absorbs the lower frequency incident sound waves while reflecting the higher frequency ones; conversely, the other side effectively reflects the lower frequency ones and absorbs the higher frequency ones. Our design showcases the flexibility of customized multiband asymmetric absorption and also provides an approach for the design of bidirectional wave-manipulation devices.

Simulation-Based Study on Round Window Atresia by Using a Straight Cochlea Model with Compressible Perilymph

The sound stimulus received by the pinna is transmitted to the oval window of the inner ear via the outer ear and middle ear. Assuming that the perilymph in the scala vestibuli and scala tympani is compressible, we report that the sound wave generated in the cochlea due to the vibration of the oval window can be expressed by the combination of even and odd symmetric sound wave modes. Based on this new approach, this paper studies the cause of hearing deterioration in the lower frequency region seen in round window atresia from the viewpoint of cochlear acoustics. Round window atresia is an auditory disease in which the round window is ossified and its movement is restricted. Using the finite element method, a round window atresia model was designed and the acoustic behavior of the round window was discussed corresponding to the level of disease. From this, we report that the healthy round window works as a free-end reflector to the incident sound waves, but it also works as a fixed-end reflector in the case of round window atresia. Next, we incorporated the round window atresia model into a cochlear model and performed a simulation in order to determine the acoustic aspects of the cochlea as a whole. The simulation results indicate that hearing deterioration occurs in a lower frequency range, which is also coincident with the clinical reports (hearing deterioration of approximately 10 to 20 dB below 4000 Hz). Finally, we explain that the cause of hearing deterioration due to round window atresia is considered to be the even sound wave mode enlarging due to the fixed-end reflection at the ossified round window, and, as a result, the odd sound wave mode that generates the Békésy’s traveling wave on a basilar membrane is significantly weakened.

A creeping-wave elliptic-head model that explains non-monotonic envelope interaural time differences

Interaural time and level differences (ITDs and ILDs) created by interactions of impinging sound with the head facilitate horizontal-plane sound localization. ILDs are known to be non-monotonic functions of azimuth, resulting in predictable mislocalization of tonal sources at some frequencies, while ITDs are typically construed as monotonic functions of azimuth. However, recent measurements using a binaural mannequin revealed envelope ITDs (ENV-ITDs) that were non-monotonic functions of azimuth, with the extent of nonmonotonicity dependent on carrier frequency. This study proposes that delayed sound traveling around the back of the head causes this phenomenon. ENV-ITDs created by a time-dependent model of incident sound waves propagating around the front and back of an elliptical head were compared to ENV-ITDs computed using publicly available HRTF libraries for a broadband speech excitation signal. Results from the model align with data from the HRTF libraries, with nonmonotonicities in the ENV-ITD-azimuth functions occurring at nearly identical azimuths and frequencies. These data suggest the veracity of this phenomenon and may have implications for spatial hearing abilities. They also suggest that time-independent models of ITDs that only consider frequency and incident angle, such as spherical head or circular arc length models, could be updated to include such time dependence.

Acoustic Observation, Identification, and Scattering Intensity Measurement of Cold Seep Based on Bubble Resonance

The active “ cold seep “ is indisputable evidence to identify the existence of submarine gas hydrate. Due to the difference in acoustic properties between cold seep bubbles and surrounding seawater, measuring scattering intensity is a new means to detect active cold seep. Cold seep bubbles are the main cause of acoustic scattering, and the scattering ability is closely related to the frequency of the incident sound wave, the radius of the bubble, and the depth of the bubble. The paper introduces a ship-borne cold seep sonar system, which uses the resonance principle of bubbles to measure the scattering intensity at various depths underwater and display it with an intuitive acoustic image. Through the investigation and measurement of the cold seep located in the South China Sea, it is proved that the cold seep sonar system can well identify the submarine cold seep. The measured data are consistent with the theoretical simulation results, confirming that the cold seep bubbles have frequency-selective characteristics for incident sound waves. Compared with other detection methods, it has the characteristics of lossless, fast, and high efficiency.

Sound Wave Insulation Performance of Cylindrical-type Acoustic Metamaterials

In recent years, acoustic metamaterials have been attracting attention as a noise control measure, and various studies have been conducted. They are structures with artificially adjusted characteristic values that contribute to the propagation of the sound. In this study, we devise an acoustic metamaterial with a simple structure that can significantly change the sound propagation characteristics. We fabricated a metamaterial consisting of an array of solid cylinders and investigated the transmission characteristics of incident sound waves propagating through the material both by the finite element analysis. As a result, the insulation effect on sound waves by the material was observed. It was also found that the insulation effect depended on the fill rate and diameter of the cylinder.

Fiber-optic microphone based on bionic silicon micro-electro-mechanical system diaphragm

The bionic sound detector based on the principle of the Ormia ochracea fly’s coupled ears is a miniature directional microphone that is sensitive to the sound pressure gradient. In this work, a silicon micro-electro-mechanical system bionic diaphragm consisting of two interconnected wings is designed and prepared, and a fiber-optic Fabry-Pérot interferometric microphone is constructed using the bionic diaphragm, and the characteristics of this microphone are studied theoretically and experimentally. According to the simulation results, the bionic diaphragm has two vibration modes of rocking and bending, and the rocking-mode displacement amplitude at a given sound pressure depends on the frequency and the propagation direction of the incident sound wave, and the closer to the eigenfrequency of the rocking mode the sound frequency, the greater the amplitude is; the rocking-mode displacement amplitude changes with the propagation direction in the three-dimensional space, resulting in a spindle-shaped distribution, and the long axis of the spindle is parallel to the long axis of the diaphragm, implying that the microphone is most sensitive to the sound wave propagating along the long axis of the diaphragm. The rocking-mode resonance frequency of the fiber-optic bionic microphone is measured to be slightly smaller than the simulated value. The output signal amplitude of the microphone changes with the horizontal azimuth angle of the sound source, producing a figure-8 polar pattern. A linear relationship between the microphone response and the azimuth angle is obtained in a range of 0° – ±60°, and in this angle range the directional sensitivity of the microphone is 39.98 mV/(°).

Mechanism and characteristics of sound scattering modulation by underwater low frequency oscillating vortex flow field

The scattering of sound waves by underwater vortex flow filed is the basic problem of sound waves propagating in complex flow fields, which has important significance in implementing underwater target detection and sound imaging of flow field. The theoretical analysis model and numerical calculation method are established for the problem of sound scattering modulation in underwater low frequency oscillating vortex flow fields, and the generation mechanism and time frequency and space characteristics of the scattering modulation sound field are explored. Firstly, based on the wave equation of the moving medium, in the first-order approximation the wave equation is decomposed into the flow-sound coupling term and the non flow-sound coupling term by introducing a potential function, and the flow-sound coupling term is analyzed in the frequency domain, revealing the underwater oscillating vortex flow field. Secondly, the discontinuous Galerkin numerical calculation method is used to solve the wave equation of the moving medium, and the sound propagation process in the underwater low frequency oscillating vortex flow field is numerically simulated. Under the condition of low Mach number, the effects of incident sound frequency, the oscillation frequency of the vortex flow field, and the scale of the vortex core on the time-frequency and space characteristics of the scattering modulating sound fields of vortex flow field are analyzed, and theoretical analysis model is used to explain the characteristics. The research results show that under the condition of low Mach number, the scattering of sound wave by oscillating vortex flow field can produce a scattering modulated sound field containing the harmonic of oscillating frequency side frequency modulation. The amplitude of the scattered sound pressure changes periodically with time, and the forward scattering is much stronger than the backward scattering. The fundamental frequency scattering modulation is much stronger than the frequency doubling scattering modulation. With the increase of the frequency of the incident sound wave and the scale of the vortex core, the intensity of the scattering modulating sound field increases, and the spatial distribution of the total scattering sound field has symmetry and an obvious main lobe, the main lobe is gradually sharpened, the azimuth angle of the main lobe is close to the propagation direction of the incident wave. When the frequency ratio is much greater than 1, the vortex flow field oscillation frequency has little effect on the spatial distribution of the sound field intensity of scattering modulating sound field.

High sound insulation property of prepared polypropylene/polyolefin elastomer blends by combining pressure-induced-flow processing and supercritical CO2 foaming

Light weight, high sound insulation property polymers have vast potential application in aviation, auto industry and the military application. Pressure induced flow (PIF) process form multi-layer-oriented structure in polymer matrix, and supercritical CO2 foaming (ScCO2) introduce gas into the matrix and increase the impedance mismatch between the phase, both of them enhance the polymer sound insulation property. In this paper, the sound insulation property of polypropylene (PP)/polyolefin elastomer (POE) blended composites prepared by the PIF process and the ScCO2 foaming were measured using the impedance tube. The sound insulation property is up to 103.56 dB in the frequency band of 1000–6000 Hz for 5 mm PIF PP/POE foam material, which it is 67.15 dB higher than PP/POE foam. By employing the multi-scale ffinite element analysis (FEA) modeling method, one two-phase FEA model of the monolith of 0.1 mm size and another 29 mm diameter bulk sample were developed based on SEM image of PIF PP/POE foamed materials at different scales. The monolith FEA model provided the effective material property for bulk sample. The finite element calculation results were consistent with the experimental result, verified the validity of the FEA modelling approach. The FEA simulation results reveal that the multi-layer-oriented structure of PIF material introduce the multi-layer impedance mismatch inside each monolith, and incident sound wave is reflected by those impedance mismatch, and this mechanism produce the high sound insulation property of PIF PP/POE foam materials.

Simulation of detection and scattering of sound waves by the lateral line of a fish

A solvable model of lateral line of a fish based on a wave equation with additional boundary conditions on a set of isolated points is proposed. Within the framework of this model it is shown that the ratio of pressures on lateral lines on different fish flanks, as well as the cross section of sound scattering on both the lines, strongly depends on angles of incidence of incoming sound waves. The strong angular dependence of the pressure ratio seems to be sufficient for the fish to determine the directions from which the sound is coming.

An isogeometric formulation of locally-conformal perfectly matched layer for acoustic scattering problems

This paper presents an isogeometric formulation of the locally-conformal perfectly matched layer (PML) for time-harmonic acoustic scattering problems. The new formulation is a generalization of the conventional locally-conformal PML, in which the NURBS patch supporting the PML domain is transformed from real space to complex space. This is achieved by complex coordinate stretching, based on a stretching vector field indicating the directions in which incident sound waves are absorbed. The performance of the isogeometric PML formulation is discussed through several acoustic scattering problems, spanning from one to three dimensions. It is found that the proposed method presents superior computational accuracy, high geometric adaptivity, and good robustness against challenging geometric features. The geometry-preserving ability inherent in the isogeometric framework could bring extra benefits by eliminating geometric errors that are unavoidable in the conventional PML. Meanwhile, these properties are not sensitive to the location of the sound source or the depth of the PML domain.

Sound reflection of acoustic porous metasurfaces under uniform mean flow

Acoustic metasurfaces are artificial 2D structures with a sub-wavelength thickness that can realize some exotic properties such as non-trivial refraction, broadband and low frequency absorption. However, most relevant studies are still in a static medium, hindering their realistic applications in aviation, where background flow exists. To address it, the effects of mean flow on the acoustic performance of metasurfaces, which is designed based on the generalized Snell's law (GSL) to achieve anomalous reflections, are systemically studied. Firstly, an analytical model of GSL taking the effect of background uniform mean flow into account is built, in which the wavenumbers of both incident and reflected waves are corrected. Then, taking an acoustic porous metasurface for instance, the effectiveness of the derived model is validated by numerical simulations. Results reveal that the reflected waves are deflected in the presence of background flow. The critical incident angle, at which the incident sound wave is converted to surface wave, decreases with the increasing flow velocity. Since the converted surface wave can only propagate along the metasurface, there is little sound energy radiated into far field, which is benefit for the noise attenuation in the presence of flow.

Enhancement of sound absorption in a band frequency using thin porous layer-lined metasurfaces

When a porous layer is installed on a hard wall, sound absorption performance is mainly determined by thickness of the layer. Although material parameters of porous materials are strongly dependent on frequencies, the thickness limitation related to the quarter wavelength of incident sound wave has been a key factor in the treatment of porous layers for noise reduction. This implies that a thicker porous layer is required to absorb lower-frequency sound effectively. To overcome the thickness limitation, metaporous layers, which are named as a compound of sound absorbing porous layers with the concept of metamaterials have received much attentions for alternative implementations of porous layers. Recently, we proposed a new type of metaporous layer for enhancing sound absorption performance in a specified broad frequency band. The proposed metaporous layer is constructed with a thin porous layer backed by a reactive metasurface consisting of an array of bent channels. Formation of sound absorption band is directly determined by the characteristics of scattered sound field from the proposed metaporous layer. Analytical and numerical investigations show that the metasurface is considerably responsible for the enhanced sound absorption in the proposed metaporous layer, while sound dissipation occurs only in the thin porous layer.

Isogeometric locally-conformal perfectly matched layer for time-harmonic acoustics

This paper presents an isogeometric formulation of the perfectly matched layer (PML) for time-harmonic acoustic simulations. The new formulation is a generalization of the conventional locally-conformal PML, in which the NURBS patch supporting the PML domain is transformed from real space to complex space. This is achieved by complex coordinate stretching, based on a stretching vector field indicating the directions in which incident sound waves are absorbed. To solve the difficulty that stretching vectors cannot be defined for control points not lying on the patch, an interpolation method is proposed in which the control-point stretching vectors are calculated by interpolating those at a set of integration points. An automatically matched layer (AML) is also formulated in the isogeometric context. Due to its form-invariant nature, the Helmholtz equation which governs time-harmonic acoustics is kept unchanged in the complex space, except that the Laplacian operator and the sound pressure should be transformed to their complex forms. The performance of the isogeometric PML formulation is discussed through several numerical examples, spanning one to three dimensions and involving both sound radiation and sound scattering problems. It is found that the proposed method presents superior computational accuracy, high geometric adaptivity, and good robustness against challenging geometric features. The geometry-preserving ability inherent in the isogeometric framework brings extra benefits by eliminating geometric errors that are unavoidable in the conventional PML. Meanwhile, these properties are not sensitive to the location of the sound source or the depth of the PML domain. The implementation of the isogeometric PML only requires the replacement of real control-point coordinates by complex ones, making it possible to incorporate the proposed method to various existing numerical routines.

A note on the sound transmission of a spherical sound wave through a double plate structure.

The sound transmission of a spherical wave through a double plate structure is theoretically and numerically analyzed. Unlike the plane wave incidence, it is demonstrated that the sound transmission loss due to the spherical sound wave incidence follows a law of 9 dB/oct for a double-layered infinite plate. For a double-layered finite plate, the sound transmission loss is lying in between 9 dB/oct and 18 dB/oct in most frequency ranges, and depends heavily on the size and position of the plate relative to the source.

Green Sound-Absorbing Composite Materials of Various Structure and Profiling

This article presents thermoplastic sound-absorbing composites manufactured on the basis of renewable raw materials. Both the reinforcing material and the matrix material were biodegradable and used in the form of fibers. In order to mix flax fibers with polylactide fibers, the fleece was fabricated with a mechanical system and then needle-punched. The sound absorption of composites obtained from a multilayer structure of nonwovens pressed at different conditions was investigated. The sound absorption coefficient in the frequency ranging from 500 Hz to 6400 Hz was determined using a Kundt tube. The tests were performed for flat composites with various structures, profiled composites, and composite/pre-pressed nonwoven systems. Profiling the composite plate by convexity/concavity has a positive effect on its sound absorption. It is also important to arrange the plate with the appropriate structure for the incident sound wave. For the composite layer with an added pre-pressed nonwoven layer, a greater increase in sound absorption occurs for the system when a rigid composite layer is located on the side of the incident sound wave. The addition of successive nonwoven layers not only increases the absorption but also extends the maximum absorption range from the highest frequencies towards the lower frequencies.

Sound transmission of acoustic metamaterial beams with periodic inertial amplification mechanisms

This paper presents the improved sound insulation performance obtained from a new type of acoustic metamaterial, built by attaching periodic inertial amplification (IA) mechanisms to a host beam. The IA mechanism is a triangular bar-and-hinge device, which connects a small mass to the host beam at two separate locations. The band structure of a unit-cell and the Sound Transmission Loss (STL) of a finite-length beam are calculated by the spectral element method (SEM), which are validated against finite element references. Numerical simulations show that the STL of the IA acoustic metamaterial beam outperforms that of an ordinary beam across a wide band starting from a very low frequency, which suggests the low-frequency soundproof capacity of the IA acoustic metamaterial beam is greatly enhanced. However, the STL curve also experiences a sudden fall after that band, causing an undesired sound insulation valley. To reveal the underlying physics, the band diagram of the beam is studied together with the trace wavenumber of the incident sound wave. It is found that, due to the collectively amplified inertia of periodic small masses, a wide bandgap is introduced in the band diagram at low frequencies, which effectively suppresses flexural vibration of the beam and eventually leads to the observed STL enhancements. However, as a consequence of this bandgap, band diagram inevitably intersects with acoustic trace wavenumber, creating a coincidence phenomenon. Although happens earlier than classical coincidence, this new coincidence produces a similar STL valley. To mitigate this drawback, a hybrid unit-cell containing two IA mechanisms is designed, which attempts to cover the valley caused by one mechanism with the bandgap induced by the other. The effectiveness of this new design is verified and discussed at last.

Ultraslow medium with an acoustic membrane-like undamped dynamic vibration absorber for low-frequency isolation

An extremely dense and thick sound panel is generally used to block low-frequency sound waves, according to the mass–density law. However, it is possible to block these sound waves with an ultralight thin panel using an ultraslow medium. In this study, theoretical and experimental methods are used to realize an ultrahigh-density acoustic metasurface consisting of thin membranes decorated with a coiled ring. When an incident sound wave induces membrane vibration, the coiled ring also vibrates. The natural frequencies of the two composite vibrating systems provide maximum membrane displacement, thus yielding zero density and extraordinary transmission. Near a frequency ω0=ks∕Ms determined by the elastic modulus ks and mass Ms of the coiled rings, the effective dynamic displacement of the membrane reduces to zero, thus implying maximum density of the membrane. At ω0, the coiled rings act as dynamic vibration absorbers because the induced force exerted by them is opposite to the identical pressure force. Thus, the acoustic metasurface provides a sound transmission loss of 48 dB at 65.7 Hz, a thickness of less than 20 mm, and a surface density of less than 1.27 kg/m2. By arranging eight unit cells in a parallel array, the proposed artificial acoustic waveguide provides a density 2,000 times higher than air density, thus realizing an ultraslow sound medium below 7.5 m/s. This medium can be used for ultralight sound proofing plates operating at the deep subwavelength scale.

Nonlinear vibro-acoustic behavior of cylindrical shell under primary resonances

Nonlinear vibro-acoustic behavior of cylindrical shell excited by an oblique incident plane sound wave under primary resonances is analytically examined in this paper. Donnell’s nonlinear shallow shell theory is used to model the cylindrical shell. Coupled nonlinear differential equations of the system are analytically derived using Galerkin’s approach. The Multiple Scales Method is hence, employed to solve the corresponding nonlinear equations. Then, the effects of crucial design parameters including incident sound wave amplitude, damping ratio and thickness of the shell on the characteristics of the sound transmission loss are studied for different resonance cases. In addition, the effect of detuning parameter on the bifurcation and behavior of the limit cycle under primary resonance is examined. The results show that the detuning parameter is a bifurcation parameter and Neimark–Sacker, flip, and period-3 bifurcations occur when this parameter is varied. Also, according to the results, by getting away from the resonance frequencies, excitation level incorporates no significant effect on the transmission loss of the shell.

Determination of acoustic radiation acting on a spherical rigid particle placed in an elastic tube filled with liquid

Acoustic radiation force effect upon a rigid spherical particle placed in the thin elastic tube is studied. The problem of determination of the acoustic radiation forces acting on an obstacle in an ideal liquid is formulated with respect to the Lagrange coordinate system. Thus, the radiation pressure is defined as time-averaged value of the acoustic pressure over the obstacle surface. This approach is adequate if, at determining of the acoustic pressure in a fluid, the deviation of the pressure from the harmonic law in time domain is taken into account in the obstacle vicinity. An action of the acoustic radiation force on the rigid spherical particle placed in the thin tube with elastic wall is studied here for the case of the incident plane sound wave propagating along the tube axis. Model is developed to describe the response of the system consisting of the compliant infinite thin circular cylindrical tube filled with the ideal compressible liquid and rigid spherical body which is immovable and located on the tube axis under the plane wave propagating along the tube axis. The problem of the hydrodynamic characteristics determination is reduced to the solution of the infinite system of algebraic equations that can be solved by the reduction method. The formula for the acoustic radiation force calculation is derived to characterize the force acting upon rigid spherical particle in the thin compliant elastic cylindrical tube.

DETERMINATION OF THE IMPEDANCE OF A HONEYCOMB RESONATOR BY DEAN’S METHOD AND DIRECT METHOD IN A COMPUTATIONAL EXPERIMENT WITH GRAZING INCIDENCE OF WAVES

The article considers the determination of the impedance of the acoustic liner sample on the basis of numerical simulation of physical processes in a honeycomb resonator with a grazing incidence of sound waves. The computational domain is the test section of a grazing incidence impedance tube with an acoustic liner sample. The liner sample is a single honeycomb resonator with a depth of 14 mm and an open area percent of 4.2%. Numerical simulation is performed based on the direct solution of the non-stationary compressible Navier-Stokes equations in a three-dimensional formulation. The pressure-time and velocity-time signals are recorded in the numerical simulation and processed by Dean's method and the direct method (from the ratio of acoustic pressure to normal acoustic velocity). The comparison of impedances obtained by the two methods demonstrates a good agreement with each other.