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To support purchasing low noise products, sound power and sound energy measurements of sufficient quality need to be routinely made by consumers on a global scale. Sound power measurements using ISO 3744, 3745, and 3746 are conducted in a free field using an acoustic far-field approximation of the intensity integrated over an enveloping measurement surface. Sound power and sound energy measurements generally use a hemispherical, parallelepiped, or cylindrical measurement surface. Those measurement surfaces have limitations and assume that the measurement points lie on the measurement surface often in preferred positions. An alternative approach is to choose microphone positions that optimally satisfy the assumptions of the measurement. The measurement surface should then be fit to the chosen microphone positions. Regression methodologies are available for fitting ellipsoids. The number of microphone positions can be as few as three to fit an ellipsoid. An ellipsoidal measurement surfaces can abut zero, one, two, or three orthogonal reflecting planes. Correction equations for the microphone locations and the angle errors for the microphone orientation and wave propagation direction are shown. This paper will present simulations of sound power, sound energy, and corrections for environmental reflections for ISO 3745 and other measurement surfaces.

On the quest towards efficiently eliminating noises, the development of a subwavelength sound absorber with the capability of free ventilation remains challenging. Here, we theoretically propose and experimentally demonstrate an asymmetric metamaterial absorber constructed by tuned Mie resonators (MRs) with unbalanced intrinsic losses. The lossy MR layer is highly dissipative to consume the sound energy while the lossless one acts as an acoustically soft boundary. Thus, the absorber presents quasi-perfect absorption (95% in experiment) for sound waves incident from the port nearer the dissipative MR and large-amount reflection (71% in experiment) from the opposite port. Moreover, the fluid dynamics investigation confirms the superior character of free air circulation owing to the ultrasparsity (volume filling ratio as low as 5%) of the absorber and its robustness to the velocity of airflows. Due to the multiple-order resonant modes of MR, we further demonstrate the flexibility of a methodology to extend asymmetric absorptions into multibands. Coupled mode analysis is employed to reveal the physical mechanism and further indicates that sparsity can be tuned by attentively controlling the reference leakage factor and intrinsic loss.

Underwater noise produced by rainfall is an important part of underwater ambient noise. The bubbles produced by raindrops are the main noise source of underwater noise. Generally, the sound pressure signal of individual bubbles is easily contaminated by tank reverberation, hydrodynamic flow, and laboratory electrical noise. In order to solve this problem, this study proposes a method for calculating the acoustic energy of the bubble produced by a raindrop when the latter falls onto a plane water surface. For this purpose, a series of experiments was conducted in a 15 m × 9 m × 6 m reverberation tank filled with tap water. The bubble produced by a raindrop behaves as a simple exponentially damped sinusoidal oscillator. Based on the dipole radiation pattern, a formula was derived to predict the sound energy of these bubbles. The damping coefficient of the bubble formed by raindrops is found to differ appreciably from the empirical value of the bubble formed by other mechanisms. The resonance frequency of the bubbles is found to decrease with time. It is due to the rapid increase in the distance between the bubble and the interface. Then, the formula is optimized by using these two improved variables. The experimental results agree well with the theoretical derivation.

Compared with scalar sound field, vector sound field explained the spatial structure of sound field better since it not only presents the sound energy distribution but also describes the sound energy flow characteristics. Particularly, with more complicated interaction among different wavefronts, the vector sound field characteristics of an elastic structure in a shallow water waveguide are worthy of studying. However, there is no reliable prediction method for the vector sound field of an elastic structure with a high efficiency in a shallow water waveguide. To solve the problem, transfer functions in the waveguide have been modified with some approximations to apply for the vector sound field prediction of elastic structures in shallow water waveguides. The method is based on the combined wave superposition method (CWSM), which has been proved to be efficient for predicting scalar sound field. The rationality of the approximations is validated with simulations. Characteristics of the complex acoustic intensity, especially the vertical components are observed. The results show that, with constructive and destructive interferences in the depth direction, there could be quantities of crests and vortices in the spatial structure of time-dependent complex intensity, which manifest a unique dynamic characteristic of sound energy. With more complicated interactions among the wavefronts, a structure source could not be equivalent to a point source in most instances. The vector sound field characteristics of the two sources could be entirely different, even though the scalar sound field characteristics are similar. Meanwhile, source types, source parameters, ocean environment parameters, and geo parameters may have influence on the vector sound field characteristics, which could be explained with the normal mode theory.

In PT-symmetric gratings, asymmetric diffraction can be generated by modulating the ratio between imaginary and real refractive indices. In this paper, a rigorous coupled-wave analysis (RCWA) has been developed to analyze the diffraction properties of acoustic PT-symmetric gratings with two kinds of modulating approaches, including modulating the effective modulus and the effective density. Asymmetric diffraction with both Bragg incident angles and perpendicular incident angles is discussed by using the RCWA method. Results show that the modulation ratio for the diffraction vanishing point changes with the modulation amplitude differently for two kinds of modulating approaches. Moreover, the sound energy will be weaken or be enhanced at Bragg incident angles depending on the sign of incident angles and the modulation ratio.

Both experimental and theoretical studies are conducted in this work to evaluate aeroacoustics damping performances of off-axial double-layer in-duct orifices at low Mach (≤0.0041) and Helmholtz number. The damping performance is characterized by using power absorption coefficient α. It denotes the fraction of incident sound energy being absorbed The effects of 1) mean duct flow (behaving like a bias flow) Mach number M a , 2) the porosities σ 1 and σ 2 of the front- and back-plates (with respect to the flow direction), 3) the off-axial distance dv (normalized with the plate radius) of the orifice relative to the plate’s centre point are experimentally examined first. Varying d v 1 and d v 2 is found to have an impact on the noise damping performance, especially at a higher Ma. Power absorption coefficient α is shown to be increased by 15%, when dv1 = 30% and dv2 = 0 in comparison with that when dv1 = dv2 = 0. In addition, the aeroacoustics damping performances of the double-layer in-duct orifices are compared with that of a single-layer one. The double-layer perforated plates with a smaller diameter orifice (D1,2 ≤ 6 mm) are found to involve with a larger α than that of single-layer orifice plates over a broader Helmholtz number range. Finally, a simplified 1D theoretical model is derived to simulate the experimental measurements. It is based on Rayleigh conductivity. It is found that a qualitatively good agreement is achieved for single-layer in-duct orifice between the theoretical predictions and the experimentally measured ones. As far as double-layer in-duct orifices are concerned, there is some discrepancy between the theoretical and experimental results.

The theoretical basis of noise reduction by noise barrier is Huygens Fresnel's wave theory. When a noise barrier is added in the propagation path between the sound source and the receiving point, the reflection, transmission and diffraction of the noise sound wave will occur when it meets the noise barrier in the propagation process. A part of the sound energy is emitted from the sound source and transmitted to the receiving point through the diffraction of the top structure of the sound barrier; A part of the sound energy can reach the sound receiving point directly through the sound barrier; When a part of the energy reaches the sound barrier, it will be reflected. If the two sound barriers are close to each other, it will be reflected many times. The ability of sound insulation and noise reduction of sound barrier is to reduce the energy of direct and transmission to the sound receiving point by blocking, and reduce the energy of diffraction to the sound receiving point through the upper structure, so as to achieve the effect of sound energy attenuation, The influence of firewall geometry on audible noise distribution of transformer is obtained [1].

Noise annoyance depends not only on sound energy, but also on other features, such as those in its spectrum (e.g., low frequency and/or tonal components), and, over time, amplitude fluctuations, such as those observed in road, rail, or aircraft noise passages. The larger these fluctuations, the more annoying a sound is generally perceived. Many algorithms have been implemented to quantify these fluctuations and identify noise events, either by looking at transients in the sound level time history, such as exceedances above a fixed or time adaptive threshold, or focusing on the hearing perception process of such events. In this paper, four criteria to detect sound were applied to the acoustic monitoring data collected in two urban areas, namely Andorra la Vella, Principality of Andorra, and Milan, Italy. At each site, the 1 s A-weighted short LAeq,1s time history, 10 min long, was available for each hour from 8:00 a.m. to 7:00 p.m. The resulting 92-time histories cover a reasonable range of urban environmental noise time patterns. The considered criteria to detect noise events are based on: (i) noise levels exceeding by +3 dB the continuous equivalent level LAeqT referred to the measurement time (T), criteria used in the definition of the Intermittency Ratio (IR) to detect noise events, (ii) noise levels exceeding by +3 dB the running continuous equivalent noise level, (iii) noise levels exceeding by +10 dB the 50th noise level percentile, (iv) progressive positive increments of noise levels greater than 10 dB from the event start time. Algorithms (iii) and (iv) appear suitable for notice-event detection, that is, those that (for their features) are clearly perceived and potentially annoy exposed people. The noise events detected by the above four algorithms were also evaluated by the available anomalous noise event detection (ANED) procedure to classify them as produced by road traffic noise or something else. Moreover, the assessment of the sonic environment by the Harmonica index was correlated with the single event level (SEL) of each event detected by the four algorithms. The threshold value of 8 for the Harmonica index, separating the “noisy” from the “very noisy” environments, corresponds to lower SEL levels for notice-events as identified by (iii) and (iv) algorithms (about 88–89 dB(A)) against those identified by (i) and (ii) criteria (92 dB(A)).

The ability of sound energy confinement with high-quality factor resonance is of vital importance for acoustic devices requiring high intensity and hypersensitivity in biological ultrasonics, enhanced collimated sound emission (i.e. sound laser) and high-resolution sensing. However, structures reported so far have been experimentally demonstrated with a limited quality factor of acoustic resonances, up to several tens in an open resonator. The emergence of bound states in the continuum makes it possible to realize high quality factor acoustic modes. Here, we report the theoretical design and experimental demonstration of acoustic bound states in the continuum supported by a single open resonator. We predicted that such an open acoustic resonator could simultaneously support three types of bound states in the continuum, including symmetry protected bound states in the continuum, Friedrich-Wintgen bound states in the continuum induced by mode interference, as well as a new type-mirror symmetry induced bound states in the continuum. We also experimentally demonstrated their existence with quality factor up to one order of magnitude greater than the highest quality factor reported in an open resonator., Though bound states in the continuum (BICs) in acoustic systems are attractive for acoustic resonators design, acoustic BICs typically show low Q-factor. Here, the authors report a high performance open acoustic resonator that supports symmetry-protected, Friedrich-Wintgen and mirror symmetry-induced BICs.

The effective elimination of sound energy traveling in open ducts is critical in acoustics, with considerable potential in diverse applications ranging from duct noise control to exhaust system design. However, previous absorber mechanisms are bulky or have limited ventilation or unsatisfactory absorption capability. We propose and experimentally demonstrate the concept of meta-mufflers capable of quasi-perfectly absorbing airborne sound propagating in an open duct while minimizing the influence on airflow because of its vanishing thickness. The mechanism uses coupling among asymmetric metasurfaces to achieve impedance match and subsequent high absorption in a subwavelength scale, which is explained analytically and validated experimentally. The proposed meta-muffler features an uncomplicated design, high attenuation efficiency, and a thickness of nearly two orders of magnitude of wavelength, ensuring the free pass of other entities. Furthermore, our design offers the flexibility of tuning and extending working bandwidth by hybridization of weak resonances. The performance of the proposed muffler is verified via experiments, which correlate excellently with theoretical predictions. The realization of meta-mufflers opens a route to novel acoustic absorbers and could have far-reaching implications in a plethora of important scenarios calling for both perfect sound absorption and open acoustic paths.

[EN] Sound diffusion refers to the ability of a surface to evenly scatter sound energy in both time and space. However, omni-directional radiation of sound, or perfect diffusion, can be impractical or difficult to reach under traditional means. This is due to the considerable size required, and the lack of tunability, of typical quarter-wavelength scattering strategies necessary for producing the required complexity of the surface acoustic impedance. As such, it can be a challenge to design sound diffusing structures that can display near perfect diffusion performance within slim dimensions. In this work, we propose a method for obtaining quasi-perfect and broadband sound diffusion coefficients using deep-subwavelength acoustic diffusers, i.e., metadiffusers. The relation among the geometry of the metasurface, the bandwidth, and the diffusion performance is analytically and numerically studied. For moderate bandwidths, around 1/3 of an octave, the method results in nearly perfect sound diffusion, while for a bandwidth of 2.5 octaves, a normalized diffusion coefficient of 0.8 was obtained using panels 1/30th thinner than traditional phase-grating designs. The ratio between the wavelength and the size of the unit cell was identified as a limitation of the performance. This work demonstrates the versatility and effectiveness of metadiffusers to generate diffuse reflections outperforming those of classical sound diffusers., The collaboration between the authors of this article was made possible thanks to the European Cooperation in Science and Technology (COST) Action CA15125-Design in Noise Reducing Materials (DENORMS). This study was financed by the Royal Opera House, Covent Garden, and the United Kingdom Acoustics Network (UKAN). The authors gratefully acknowledge the ANR-RGC METARoom (No. ANR-18-CE08-0021) project and the project HYPERMETA funded under the program Etoiles Montantes of the Region Pays de la Loire. N.J. acknowledges financial support from Generalitat Valenciana through Grant No. APOSTD/2017/042.

One-way sound-wave propagation has been a research hotspot in acoustics during the past decade, with the production of acoustic diode, acoustic topological insulator, and parity-time-symmetric acoustic metamaterial. Here, we investigate topological pumping for sound waves in a coupled cavity chain along selective paths driven by dynamic modulation. We first showcase the adiabatic passage of sound energy in a heterostructured Su-Schrieffer-Heeger cavity chain. By solving the Dirac-like governing equation, we calculate the eigenspectrum of this model and prove the existence of a topologically protected interface state and end states. From the eigenspectrum, if no level crossing exists, energy transfer of the sound wave (or acoustic topological pumping) is reversed under adiabatic modulation, leading to the release and return of sound energy for a specific topological state. However, when the level crossing occurs, we find that topological pumping between the interface state and end states can be selectively excited, rendering complete transfer of the sound energy between two topological states. Moreover, topological pumping of sound waves can exist along the asymmetric paths under driven modulation. Our work has promising applications for robust, selective, and efficient sound energy transfer in acoustic network systems.

The acoustic black hole (ABH) effect is realized in thin plate structures with a decreasing thickness according to a power-law function, and offers potential applications for structure vibration damping enhancement and free-field noise radiation suppression. In this paper, a wavenumber domain method (WNDM) is proposed for the analysis of vibro-acoustic coupling and internal noise reduction mechanism of a pentahedral cavity enclosed by a flexible plate with a two-dimensional ABH indentation, subject to a point force excitation. The system response of the ABH plate-cavity is computed by a validated finite element model. The relationship between the space-averaged sound energy inside the cavity and the spectra of the structural displacement and the acoustic mode of the cavity is established. This allows revealing a dual physical mechanism behind the observed noise reduction: amplitude reduction and mismatching between the wavenumber spectra of the plate displacement and the acoustic field, which results in a weakened vibro-acoustic coupling. An additional configuration with an ABH embedded in an irregular pentagonal wall of the cavity is examined. Despite the increasing complexity in the geometry of the coupling interface and its coupling with the cavity, numerical analyses confirm the generality of the observed physical phenomena and the applicability of the proposed WNDM to more complex system configurations.

The known methods of acoustical calculation in buildings not fully describe the phenomenon of sound transmission. This paper presents theoretical studies of the sound and vibration in building models, a method of statistical energy analysis (SEА), taking into account nonresonant phenomena and experimental testing of the specified parameters. The SEA-based equations are obtained for the energy balance, the solution of which allows to determine the sound energy in rooms and sound vibration in structures. The energy coupling coefficients, including those allowing for non-resonant sound transmission from adjacent acoustic subsystems, internal losses and the density of mode oscillation are calculated. The results of calculations and measurements of building model fragments, including identical and double partitions, are presented. The proposed method of calculation allows to predict the sound transmission through the partition with regard to adjacent structures.

This work investigates the acoustical intensity generated by the sound radiated from rectangular baffled plates in eight cases with distinct combinations of classical boundary conditions. The primary objective is to identify the regions of these plates that effectively contribute to the radiated sound power into the far-field, especially when the driven frequency occurs below the critical coincidence frequency. This identification is done by filtering the non-propagating waves, both using the (analytical) supersonic intensity method and the (numerical) useful intensity model. The spectral decomposition of the sound energy operator is applied, becoming possible the dimensional reduction of this operator to remove the components that do not propagate. Brief theoretical formulations, both for the plates vibration and the resulting acoustical field, are discussed. The closed form solution of the normal velocity field for the eight cases is given. Then, the supersonic intensity is estimated. On the other hand, a summary of the useful intensity technique is discussed and it is computed for the same cases. In the numerical examples, the comparison of the supersonic intensity, the useful intensity and the classical acoustic intensity is shown. The edge and corner modes are clearly identified and the useful intensity plots showed to fit well with the supersonic intensity ones.

The noise suppression problems within ducts are widespread in aero engines, gas turbines, blowers and various mufflers. It has been well demonstrated that the Cremer impedance can maximize the propagating damping by degenerating the lowest order mode pair into a single mode, and consequently maximize the sound attenuation in both circular and rectangular ducts, however, the Cremer impedance of annular ducts is still unsettled. Therefore, the Cremer concept is further developed to theoretically optimize the acoustic impedance for annular ducts with two degrees of freedom, i.e. inner and outer liner impedances. Through decoupling successfully the radial wavenumber and impedance, the branch point equations are derived in the presence of grazing flow to solve the triple or double eigenvalues, which can be used to design simultaneously the optimal inner and outer impedances. A parametric study is conducted to investigate the effect of the Helmholtz number (kb), the circumferential mode order (m), the radius ratio (RR) and the liner configurations on the optimum radial wavenumber, Cremer impedance and sound transmission loss (TL). Some conclusions are drawn. First, the double eigenvalue solutions can give optimum impedance under more general conditions than triple eigenvalue solution. At small RR, the solutions of outer wall approximate to Cremer impedance of circular ducts, and at large RR and m = 0, the double eigenvalue solutions of both inner and outer walls are identical to Cremer impedance of rectangular ducts with zero transverse mode. Second, with the decrease of kb and the increases of m and RR, the optimum TL exhibits a general tendency of growth. Third, higher TL gain leading by inner liner can be achieved for larger RR and lower m, and whether the inner liner works effectively depends on the sound energy densities near inner and outer walls. Fourth, the RR and kb have larger impacts on the optimum TL than m, thus needing to be given priority to consider in design, the utilization of inner liner needs to comprehensively balance the effectiveness, economical efficiency and the realizability.

The wave equation as derived in Section 7.1 and used so far in this book, is valid for sound propagation in lossless media. The Helmholtz form of this equation is

Stimulated adiabatic passage has been extensively studied to achieve robust and selective population transfer in quantum systems. Recently, the quantum-classic analogy has been rapidly developing and can be considered responsible for the implementation of the adiabatic transfer of sound energy in cavity chain systems. In this article, we investigate the adiabatic transfer of sound energy between two topological end states in the Su-Schrieffer-Heeger (SSH) cavity chain, which can be considered to be the acoustic analog of the quantum chirped-pulse excitation. The topological adiabatic passage in SSH cavity chain has two categories. When the single-cavity resonance frequencies on the sublattices A and B in the SSH cavity chain do not switch their spectrum positions, the topologically protected adiabatic evolution results in the returning passage of the sound excited in one end cavity. When a level crossing with single-cavity resonance frequencies on the sublattices A and B exhibits switch in the frequency spectrum, acoustic energy is observed to be topologically pumped between the two end cavities of the SSH chain.

Target localization using passive azimuth-only measurements obtained by a circular array is difficult. Specifically, when the target and the array travel at constant velocities, there are multiple possible trajectories of the targets. In this condition, the target is unobservable. This paper presents a method to solve this problem in the ocean environment with surface duct. When the sound frequency is below the cut-off frequency, the sound energy in the duct or outside the duct quickly attenuates with the horizontal range at a constant rate. Therefore, the energy difference between two observations of the array implies the horizontal range difference of the target relative to the array. By utilizing this extra measurement, the moving target becomes observable.

The advent of broadband and tunable acoustic metamaterials, amenable to be fabricated into flat panel geometry, offers broad, emerging possibilities for room acoustics. In particular, the fine tuning of surface acoustic impedance becomes possible to optimize specific room acoustic goals. In this initial simulation study we examine the possibility of minimizing the averaged level of energy density (${L}_{e}$), both globally and locally in an enclosed room. The relationship curve between surface impedance and global ${L}_{e}$ shows an optimal value that can reduce ${L}_{e}$ by about $15$--$20\phantom{\rule{0.2em}{0ex}}\mathrm{dB}$ over a wide audible frequency regime. Moreover, partial coverage of the room surface by the optimally tuned acoustic metamaterial already demonstrates considerable effectiveness. We compare and verify our numerical model with the statistical-diffusive acoustics model and find good agreement in the high-frequency regime. In the low-frequency regime, the energy inhomogeneity caused by wave interference can be utilized to create a quiet zone for specific targeted frequency by optimally tuning the surface impedance values at discretized locations on the four walls.

In terms of the overall diffusion sound field, the inherent diffusion state of the space and the scattering coefficient of indoor surfaces work jointly to diffuse sound energy. This study has investigated the impact of surface scattering on reverberation time in differently shaped spaces. First, 10 spaces with the same volume but different shapes were calculated using a computer simulation, Next, two typical spaces were selected to calculate 10 states, in which the volume was multiplied. The calculations results show that the impact of surface scattering on reverberation time in differently shaped spaces follows three laws: in a group of spaces with the same variation pattern, T20 varies with scattering coefficients at a similar rate. In a group of spaces with different variation patterns, there is a difference of more than 5% in the change rate of T20 with scattering coefficients, in imperfect diffusion spaces, decay curves vary in accordance with scattering coefficients. If the scattering coefficients are the same, T20 varies with spatial shapes, when the volume of rectangular spaces and trapezoidal space ranges from 3000 to 30,000 m3, the change rate of T20 is less than 5%. In the present study, spaces were classified by the position combination of shape surfaces. On this basis, we proposed and then graded the concept of &ldquo, morphology diffusivity&rdquo

The acoustical radiosity model (ARM) is a typical algorithm in geometrical acoustics to simulate the sound field in a room of ideally diffusely reflecting boundary. Even in such a room, the sound field decay is a complex process, as the relaxation of the sound field observed in simulations has shown. Based on the Laplace transform of the acoustical radiosity equation, this paper gives a set of properties of the ARM. It shows the system has a series of real or complex conjugate L-eigenvalues and corresponding L-eigenfunctions. Under the relaxation condition, the sound energy decay on the room boundary, generated by finite initial excitation, can be expanded as a summation of decay components, which are composed of real and/or complex conjugate decay modes. Each decay mode is a decaying and oscillating L-eigenfunction corresponding to an L-eigenvalue. The real part of the L-eigenvalue is the exponential decay rate, and the image part is the angular frequency of the oscillation. The reverberant sound field inside the room space has a similar decay structure to the boundary. As an example, the decay structure in a sphere is analyzed. The relaxation of the sound field is explained by the geometrical significance of the sound field decay.

The surface coupling approach is employed to predict the sound field in multiply connected spaces (rooms) with internal noise sources. The coupling is achieved by developing sound pressure and particle velocity at the interfaces between rooms in simple interface functions. Plane surface harmonics can be employed as interface functions where rectangular interfaces are concerned. Using the blocked sound pressure and impedance across the interface surfaces as acoustical descriptors, each room can be identified separately. The acoustical continuity conditions can then be applied to obtain the resulting sound field. Using this approach, the prediction of the sound field created by point sources in three connected parallelepipedic rooms was successfully carried out. The parametric study shows that the normalized harmonics wavenumber equal to 2 times the normalized sound wavenumber enables acceptable sound prediction. Further application was done by predicting sound energy flow of three monopole sources in a rectangular space coupled with a semi-infinite field.

This paper applies ideas originally developed in the field of acoustics of multiscale porous media to the analytical modeling of dissipative silencers. Analytical models for the long-wavelength acoustical properties of different types of dissipative silencers, such as pod silencers, lined ducts, splitters, and bar silencers, are introduced in this work. Some of these models account for the possibility of using different porous materials in a given silencer. For instance, a model for a splitter silencer comprising baffles made of different porous materials is derived. Examples of the sound transmission behavior of the said types of silencers are presented to evidence that the dissipation of sound energy is largely influenced by pressure diffusion occurring in the porous constituents. The predictions of the analytical models are in good agreement with finite element calculations.

This theoretically supplemented experimental research effort is devoted towards demonstrating the feasibility of using heat flux driven transducers, in order to annihilate undesirable acoustic fields directly at the source boundary. Using commonly available materials, a thermophone is produced and deposited on a conventional vibro-acoustic loudspeaker. Unifying the sound energy input from heat and work related processes, a holistic model is derived from first law of thermodynamics - simulating pressure fields at different locations. In the case of thermo-acoustic sound production, the formulation is complemented by an efficiency model based on Fourier heat conduction in a slab with internal unsteady generation. The ensuing sound pressure level emanating from the two sources is predicted for a broad range of measurement locations and relative excitation phase angles. The theoretical estimates are corroborated by experimental results under different conditions. For the prescribed experimental parameters, a heat-flux transducer is shown to successfully diminish the pressure wave generated by a conventional loudspeaker to ambient noise levels.

Helmholtz resonators offer an ideal platform for advanced sound absorbers, but their utility has been impeded by inherent frequency range limitations and the lack of function reconfiguration. Here, we introduce a multi-order Helmholtz resonator (MHR) that allows multiple monopolar resonant modes theoretically and experimentally. The combination of these modularized MHRs further creates reconfigurable multi-band anomalous absorbers in a two-port transparent waveguide while maintaining undisturbed air ventilation. In asymmetric absorption state through coupling of artificial sound soft boundary with preposed MHR, sound energy is almost totally absorbed in multiple frequency ranges when sound waves are incident from one side while it is largely reflected back from the opposite side. Interestingly, the original asymmetric absorber would turn into symmetric bidirectional absorber if one post MHR concatenates after the soft boundary. Using combination of identical MHRs, we demonstrate function selective asymmetric/symmetric absorber in multi-bands, highlighting the potential to use MHRs in the design of diverse devices for more versatile applications.

To measure the air-borne sound and floor impact sound insulation performance, the sound field correction of the receiving room was conducted using reverberation time. Reverberation time is varied by environmental conditions, characteristics of sound source, position of sound source and microphones, etc. The sound field correction method on a rubber ball impact sound was standardized. In this study, the reverberation time was measured and compared using an omni-directional loud-speaker, a half-omni-directional loud-speaker, and a sub-woofer. Reverberation times at below 500 Hz bands became shorter when the height of the Omni-directional loud-speaker was increased. The reverberation times measured with the omni-directional loud-speaker at 1.7 m height were similar with the reverberation times measured with the half-omni-directional loud-speaker. For the measurement of reverberation time in low frequency below 80 Hz band, sub-woofer was installed on the floor. Reverberation times measured with sub-woofer were shorter than reverberation times measured using the half omni-directional loud-speaker, and the deviation was smaller. In order to make a correction for the receiving room sound field condition on light-weight impact sound, the height of the omni-directional loud-speaker should be approximately 1.7 m. In addition, for the sound field correction of the rubber ball impact sound at below 100 Hz bands, the use of sub-woofer for sufficient generation of sound energy in the low-frequency band is needed.

The work of this paper is backgrounded by prediction or evaluation and control of mechanical self-noise in sonar array cavity. The vibratory power flow transmission analysis is applied to reveal the overall vibration level of the fluid-structural coupled system. Through modal coupling analysis on the fluid-structural vibration of the fluid-filled enclosure with elastic boundaries, an efficient computational method is deduced to determine the vibratory power flow generated by exterior excitations on the outside surface of the elastic structure, including the total power flow entering into the fluid-structural coupled system and the net power flow transmitted into the hydroacoustic field. Characteristics of the coupled natural frequencies and modals are investigated by a numerical example of a rectangular water-filled cavity with five acoustic rigid walls and one elastic panel. Influential factors of power flow transmission characteristics are further discussed with the purpose of overall evaluation and reduction of the cavity water sound energy.

Dynamic behavior of a nonlinear composite beam under the action of acoustic incident waves is analyzed in this paper. Frequency responses are obtained in primary resonance condition. The coupled vibroacoustic system is solved by the multiple scale method combined with the Green’s function approach. Two types of geometrical nonlinearities including cubic and quadratic terms are focused. Semi-analytical solutions are obtained in integral forms. Within a parametric study, effects of the ply orientation and material damping on the frequency responses and scattered potential are investigated. It is found that unsymmetric layout induces negative quadric characteristic behavior and applies extra skewness to the frequency responses. It is also proved that the nonlinear composite beam can further dissipate the sound energy with respect to a linear system particularly when it is tuned for an optimal damping.

The noise pollution in pipelines exists for a long time and can’t be ignored in modern industries. It is important to investigate the transfer mechanism of sound energy in pipelines and develop high efficiency mufflers with low penalty of pressure drop. Different with the traditional method, this study is focused on the flow and sound fields synergy principle to investigate the flow noise propagation mechanism in reactive mufflers. In this study, theoretical analysis and numerical simulation methods are coupled to investigate the noise propagation process. In the theoretical analysis aspect, the synergetic relationships between the flow and pressure gradient fields are deduced and the field synergy theory is established. In the numerical simulation aspect, the flow noise propagation process in the simple expansion chamber mufflers is studied. The results show that with the decrease of synergy between flow and sound fields, the work done by the fluid on the wall decreases, which means the exchange of sound energy between the wall and the fluid decreases.

The dissipative splitter silencer is a common means for broadband noise attenuation in flow ducts such as ventilation and gas turbine systems where many higher order modes are cut-on. To predict the effect of a splitter silencer, analytical approaches have obvious advantages over the numerical methods on computation efficiency, but the development of analytical approaches are often based on many simplifications like the infinite length assumption. This paper presents a three-dimensional analytical method to solve the sound propagation in a duct with a finite length splitter silencer in the presence of mean flow. In the developed method, the convergence problem, which is often encountered by using the existing mode-based methods for high-frequency computations of soft-wall eigenmodes, is overcome by applying the equivalent surface source concept. With very low computational cost, the present method is validated to give consistent transmission loss predictions with the numerical results and experimental measurements available in the literature. Further, the performance of a splitter silencer is studied with different higher order incident modes, and reflection versus dissipation analysis is conducted by using the developed method. It is found that the sound attenuation is dependent on the sound energy spatial distributions of the incident sound modes, and the dissipation dominates the transmission loss behavior of the splitter silencer.

Harvesting multi-frequency sound energy from environmental noise is a meaningful topic to supply energy for potential devices. In this work, we constructed an array of Helmholtz resonators (HRs) with cross-linked polypropylene (IXPP) ferroelectret films on the inner walls of HR cavities, whose resonant frequencies range from 300 to 800 Hz and quasi-static piezoelectric coefficient d33 is 230 pC/N. The energy harvesting performance of IXPP films is investigated, both theoretically and experimentally, in a single HR with various sizes, showing the high energy conversion capability close to the resonant frequencies of HRs, e.g., 337, 375, 445, 522, 588, 661, 739, 782, and 795 Hz, in the experiment. By putting one, two, three, and four samples of nine different sized HRs in series connection in order, we measured the average output power of 3.16, 5.31, 7.36, and 8.66 nW at the resonant frequencies. It shows that the output power of IXPP films has been significantly improved at multiple frequencies by series connection of IXPP films. In parallel, the optimal electrical resistance increases in a quasilinear way compared to the number of HRs. These results are helpful for designing efficient sound energy harvesters in the broadband frequency range.

This paper investigates sound propagation in polydisperse heterogeneous porous composites. The two-scale asymptotic method of homogenization is used to obtain a macroscopic description of the propagation of sound in such composites. The upscaled equations demonstrate that the studied composites can be modeled as equivalent fluids with complex-valued frequency-dependent effective parameters (i.e., dynamic viscous permeability and compressibility) as well as unravel the sound energy dissipation mechanisms involved. The upscaled theory is both exemplified by introducing analytical and hybrid models for the acoustical properties of porous composites with different geometries and constituent materials (e.g., a porous matrix with much less permeable and/or impervious inclusions with simple or complex shapes) and validated through computational experiments successfully. It is concluded that the developed theory rigorously captures the physics of acoustic wave propagation in polydisperse heterogeneous porous composites and shows that the mechanisms that contribute to the dissipation of sound energy in the composite are classical visco-thermal dissipation together with multiple pressure diffusion phenomena in the heterogeneous inclusions. The results show that the combination of two or more permeable materials with highly contrasted permeabilities can improve the acoustic absorption and transmission loss of the composite. This paper provides fundamental insights into the propagation of acoustic waves in complex composites that are expected to guide the rational design of novel acoustic materials.

The object of research is the process of elastic interaction of an ultrasonic beam with a cylindrical module enclosing the airplane in the form of two circular shells of the same length coaxially connected by their ends, the hermetical gap between them filled with liquid. One of the problematic areas of the study is that the dislocation of deck aviation in the open waist plane allows the means of detecting the suborbital and atmospheric reconnaissance of the enemy to determine undefined not only the initial coordinates but also its coordinate functions for the entire period of subsequent trajectory travel. Finally, the on-board glider equipment, taking this information as the original one, makes it possible, with anticipation, accurately, to fire at the theoretical trajectory of motion, thereby increasing the probability of hitting the target. Therefore, it is necessary to ensure 100 % masking of deck aviation at open launch positions. This will significantly reduce or completely eliminate and improve the efficiency and life-span of the aircraft as a whole. It is shown that, when the case is subjected to symmetrical oscillations, which greatly exceed the resistance to antisymmetric vibrations, the sound permeability of the case elements will increase solely by bending vibrations. It is revealed that the «acoustic transparency» of the device case serves as an intensive transfer of the sound energy of the bending waves of the case and completely depends on the frequency of the acoustic radiation, as well as the incidence angles. Thus, by creating a circular or ellipsoidal enclosing cylindrical module in the form of two circular shells filled with liquid, it will be possible to quickly disassemble and place it over another object in a matter of hours.

Unsteady heat release coupled with pressure fluctuation triggers the thermoacoustic instability which may damage a combustion chamber severely. This study demonstrates an electroacoustic control approach of suppressing the thermoacoustic instability in a Rijke tube by altering the wall boundary condition. An electrically shunted loudspeaker driver device is connected as a side-branch to the main tube via a small aperture. Tests in an impedance tube show that this device has sound absorption coefficient up to 40% under normal incidence from 100 Hz to 400 Hz, namely over two octaves. Experimental result demonstrates that such a broadband acoustic performance can effectively eliminate the Rijke-tube instability from 94 Hz to 378 Hz (when the tube length varies from 1.8 m to 0.9 m, the first mode frequency for the former is 94 Hz and the second mode frequency for the latter is 378 Hz). Theoretical investigation reveals that the devices act as a damper draining out sound energy through a tiny hole to eliminate the instability. Finally, it is also estimated based on the experimental data that small amount of sound energy is actually absorbed when the system undergoes a transition from the unstable to stable state if the contrpaol is activated. When the system is actually stabilized, no sound is radiated so no sound energy needs to be absorbed by the control device.

Vibrating structures upon a surrounding fluid radiate sound energy. Some of them are desirable like vibrations of musical instruments and many are not like traffic noise. That suggests that the study of its physical mechanism is of importance for the control and reduction of noise. Sound radiation from vibrating structures is also encountered in a number of applications, such as loudspeakers manufacturing, road, rail, marine and airborne vehicles design. The continuous increase of speed in industry and optimization of the weight of pieces go above the domain of validity of the linear theory of structural vibrations, making the classical analytical and numerical tools unable to predict properly the corresponding sound radiation parameters. It is of great interest in such cases to know how the nonlinear vibrations influence the acoustic parameters. In this paper, a new approach is presented for the estimation of the non-linear acoustic radiation of clamped-clamped beams in the neighborhood of the first modes of vibration. Simplified expressions are given for the acoustic indicators using the explicit analytical expressions for the beam non-linear forced response, previously developed. The results allowed the estimation of the effect of non-linearity on the classical acoustic parameters and showed a higher increase in the acoustic indicators, compared with those predicted by the linear theory. This confirms the necessity of taking into account the geometrical non-linearity in order to get accurate estimates of the beams sound radiation at large vibration amplitudes.

It is an important subject in many engineering areas to recognize the vibro-acoustic behaviors of vibrational structures. Many researches have been done to study this characteristics of plane structure with structural intensity (SI) analysis. This paper extends the subject to study this subject by structural intensity approach in cylindrical shell structure and present a method of controlling the sound energy transmission. Structural intensity, a powerful method, can not only quantitatively reflect power flow strength, but also can provide the energy flow directions and distributions. The structural intensity patterns vary under different load and damping conditions, and thus affects the energy level of radiation sound field in cylindrical shell. In this paper, Numerical simulation system was built to represent the simply supported thin cylindrical shell coupled vibration and acoustical radiation structure which excited by harmonic normal point force. We get the corresponding results of structural intensity distribution, sound pressure and acoustic intensity of internal radiation sound field by adjusting damper positions, and calculating the sound field energy. The characteristics of SI of thin cylindrical shell are investigated, and the relationships between SI and sound field are established. We hope that our works could provide researchers new ideas for noise reduction of similar shell structures in potential applications.

This paper presents the first results of the spectral analysis of airborne and underwater sound produced by breaking waves, with the long-term objective to improve breaking wave detectability. The size of an air bubble can be directly linked to the frequency of the sound that is heard using the simple harmonic solution to the Rayleigh–Plesset equation. It indicates the inverse relationship between frequency and bubble size. This relationship has been used successfully to identify wave breaking in general by Manasseh in 2006 [4]. Now this research goes a step farther and examines how the frequency spectrum of the sound changes with time, in an effort to understand the general pattern and from that to deduce an empirical equation that describes the breaking down to turbulence during a wave breaking event. At this point there have been three main indicators identified that are necessary to prove wave breaking when analysing recorded sound: (1) higher frequencies get more pronounced as time passes, (2) amplitude decreases with increasing frequency, and (3) there is a sinusoidal pattern to how the power is distributed throughout the frequencies. This last point is the one that this research focusses on most. It can be concluded from the experimental data that the sinusoidal pattern is most likely due to the probability of how bubbles break down. This probability function depends on the physical properties of the medium the wave is travelling through, or in the case of ocean waves it depends on the properties of the water and air.

Physical mechanisms of active control of sound transmission through orthogonally two ribs stiffened double-panel structure are investigated. This is the continued work of the single rib stiffened case. For the orthogonally two ribs stiffened case, four different cluster mode groups can be coupled with each other, due to the interlaced coupling effects of the horizontal and vertical ribs. One cavity mode can couple with and transmit sound energy to any type of base plate mode of the radiating ribbed plate. Consequently, the main differences of the control mechanism, when compared with the single ribbed case, lie in two aspects. One is that a novel mechanism appears. That is, suppressing and rearranging the cavity modes simultaneously achieves the suppression of the base plate modes. The other is that rearrangement of the cavity modes to rearrange the base plate modes for achieving sound radiation cancellation almost does not appear. The reason is that all types of cavity mode can couple with any one of the base plate modes due to the coupling effects of the two ribs. There is only a need to rearrange several important cavity modes to achieve suppressing the base plate mode of the radiating ribbed plate.

Tactile haptic feedback is an important consideration in the design of advanced human–machine interfaces, particularly in an age of increasing reliance on automation and artificial intelligence. In this work, we show that the typical nanometer-order surface displacement amplitudes of piezoelectric transducers—which are too small to be detectable by the human touch, and constitute a significant constraint in their use for tactile haptic surface actuation—can be circumvented by coupling the vibration into a liquid to drive the deflection of a thermoplastic membrane. In particular, transmission of the sound energy from the standing wave vibration generated along a piezoelectric transducer into a microfluidic chamber atop which the membrane is attached is observed to amplify the mechanical vibration signalling through both the acoustic radiation pressure and the viscous normal stress acting on the membrane—the latter arising due to the acoustic streaming generated as the sound wave propagates through the liquid—to produce 100 μm-order static deflections of the membrane, upon which approximately 0.5 μm dynamic vibrations at frequencies around 1 kHz are superimposed; both these static and dynamic responses are within the perception range for human finger sensation. The large static deformation, the relatively fast response time, and the ability to incorporate a dynamic vibrotactile response together with the small size and potential for integration of the device into large scale arrays make this mechanism well suited for driving actuation in devices which require tactile haptic responses.

Reduction of vibration and sound energy in the form of traveling waves is of vital importance in many applications. Recent development of acoustic metamaterials opens up unusual ways for sound wave manipulation and control. Among acoustic metamaterials, a much newer concept, Acoustic Black Hole (ABH), has been drawing growing attention in recent years, which shows great potential for acoustic energy trapping and dissipation. In a duct ABH with a properly tailored continuous cross-sectional reduction and impedance variation, it is shown that the sound speed can be progressively reduced, which means that sound waves are eventually trapped in the structure. In this paper, such a wave trapping mechanism is further explored in the context of sound transmission problems, in which an exceptional phenomenon—simultaneous reduction of sound reflection and transmission—is realized. The archived trapping mechanism also ensures that little sound waves will be bounced back to the source to jeopardize the overall performance. Transfer matrix method simulations and impedance tube experiments are performed to characterize the behavior of such a structure and to validate the theory. The promising ABH-specific features arising from the proposed design could overcome many existing limitations of traditional noise control devices.

We investigate the harvesting of sound waves by exploiting a 3D-printed gradient-index phononic crystal lens. The concept is demonstrated numerically and experimentally for focusing audio frequency range acoustic waves in air to enhance sound energy harvesting. A finite-element model is developed to design the unit cell dispersion properties and to construct the 3D lens for wave field simulations. Numerical simulations are presented to confirm the focusing of incident plane waves and to study the sensitivity of the refractive index profile to the direction of wave propagation. The theoretical predictions are validated experimentally using a scanning microphone setup under speaker excitation, and a very good agreement is observed between the experimental and numerical wave fields. A circular piezoelectric unimorph harvester is placed at the focal position of the lens, and its performance is characterized with a resistor sweep in the absence and presence of the lens, resulting in more than an order of magnitude enhancement in the harvested power with the lens. The 3D-printed lens presented here substantially enhances the intensity of sound energy via focusing, yielding micro-Watt level power output, which can find applications for wireless sensors and other low-power electronic components.

Bragg scattering phonon crystal and locally resonant acoustic metamaterials were introduced. In order to generate noise reduction, the lattice constant of Bragg scattering phonon crystal should be of the same order of magnitude as the wave length of the sound wave, therefore, its application field is limited. Locally resonant acoustic metamaterials consume sound energy by coupling its own resonant frequencies with those of sound waves at close range. Its size is two orders of magnitude smaller than the wavelength of sound wave; thus, the control of low-frequency noise by small-size acoustic metamaterials is realized. Locally resonant acoustic metamaterials have some extraordinary physical characteristic in the conventional medium for their special acoustic structural units, such as negative refraction and negative mass density. Especially in low frequency band, they have acoustic forbidden band in which the sound wave transmission is prohibited. Acoustic structural unit having resonant characteristics has been developed. Surface-mounted resonant element plate structures and thin film acoustic metamaterials are the normal types of locally resonant acoustic metamaterials. Their research and development provide a new method for low-frequency noise control.

Due to the limited understanding of spatial sound field and contribution of pantograph, a deep analysis is presented to extract understanding the aerodynamic noise characteristics of high-speed trains pantograph system. The near-field and far-field noise are predicted by the acoustic perturbation equations and Ffowcs Williams-Hawkings equation, respectively. The spatial sound propagation is analyzed by proper orthogonal decomposition and cluster-based reduced-order modelling. The flow field results predicted by large eddy simulation show that the flow behind pantograph is governed by hierarchical structures, which occurs to be featured with three layers as well as periodic evolution. The dipole source is dominated in far-field radiated noise while the quadrupole source is negligible, since the intensity of the quadrupole source is less than that of the dipole source. The contribution rates of base-frame, pan-head, groove, upper-arm, horn, lower-arm and rod-insulator are higher than the other components, and the radiated sound energy of them accounts for approximately 90% of the total energy. The noise contribution of pan-head exceeds 10% at the frequency of 1000 Hz. Base-frame is the largest contributor, and more than 6% of noise contribution occurs at the frequency of 630 Hz. The spatial sound propagation includes two major aspects: one is mainly reflected upwind from groove, and the other is propagated with the center of groove. The results highlight the possibility to develop a new design of high-speed trains pantographs, in order to obtain an aerodynamic noise reduction.

It is demonstrated that propagation of the soliton described by the Boussinesq equation along a linear defect of the crystal structure leads to radiation of sound waves (analog of the Vavilov–Cherenkov effect). Radiation that has a continuous spectrum diverges conically from the dislocation line, and the apex angle of the cone is determined by the ratio of the sound speed in the crystal to the soliton speed. With increasing soliton speed, the maximum of the spectral flux density of sound energy is displaced toward higher frequencies. An analytical expression for energy losses is derived.

Although sound radiation from sound-induced vibration and from force-excited vibration of solid structures are similar phenomena in terms of radiating from vibrating structures, the general relationship between them has not been explicitly studied to date. In particular, airborne sound transmission through walls and sound radiation from structurally vibrating surfaces in buildings are treated as different issues in architectural acoustics. In this paper, a fundamental relationship is elucidated through the use of a simple model. The transmission coefficient for random-incidence sound and the radiated sound power under point force excitation of an infinite elastic plate are both analyzed. Exact and approximate solutions are derived for the two problems, and the relationship between them is theoretically discussed. A conversion function that relates the transmission coefficient and radiated sound power is obtained in a simple closed form through the approximate solutions. The exact solutions are also related by the same conversion function. It is composed of the specific impedance and the wavenumber, and is independent of any elastic plate parameters. The sound radiation due to random-incidence sound and point force excitation are similar phenomena, and the only difference is the gradient of those characteristics with respect to the frequency.

The interaction of sound and flow enables an efficient noise damping. Inevitable for understanding of this aeroacoustic damping phenomenon is the simultaneous measurement of flow and sound fields. Optical sensor systems have the advantage of non-contact measurements. The necessary simultaneous determination of sound levels and flow velocities with high dynamic range has major hurdles. We present an approach based on frequency-modulated Doppler global velocimetry, where a high-speed CMOS camera with data rates over 160 MSamples s−1 of velocity samples is employed. Using the proposed system, two-component flow velocity measurements are performed in a three-dimensional region of interest with a spatial resolution of 224 µm, based on single-pixel evaluation, and a measurement rate of 10 kHz. The sensor system can simultaneously capture sound and turbulent flow velocity oscillations down to a minimal power density of 40.5 (mm s−1)2 Hz−1 in a frequency range up to 5 kHz. The presented measurements of the interaction of sound and flow support the hypothesis that the sound energy is transferred into flow energy.

Although the visual and geomagnetic orientation cues used by sea turtle hatchlings during sea-finding have been well studied, the potential for auditory stimuli to act as an orientation cue has not been explored. We investigated the response of sea turtle hatchlings to natural and anthropogenic noises present on their nesting beaches during sea-finding. The responses of hatchling leatherback sea turtles, Dermochelys coriacea, collected from the Sandy Point National Wildlife Refuge, St. Croix, were measured in the presence of aerial acoustic sounds within hatchlings’ hearing range of 50 to 1600 Hz. The highest sound energy produced by beach waves occurs at frequencies 50–1000 Hz, which overlaps with the most sensitive hearing range of hatchling leatherbacks (50–400 Hz). Natural beach wave sounds, which have highest sound energy at frequencies of 50–1000 Hz, may be masked by human conversations (85–650 Hz) and vehicle traffic noise (60–8000 Hz). In the presence of three stimuli, a) beach wave sounds (72.0 dB re: 20 μPa), b) human conversation (72.4 dB re: 20 μPa), and c) vehicle traffic noise (71.1 dB re: 20 μPa), hatchlings exhibited no phonotaxic response (wave sounds: mean angle = 152.1°, p = 0.645; human conversation: mean angle = 67.4°, p = 0.554; traffic noise: mean angle = 125.7°, p = 0.887). These results may be due to the hatchlings being unable to localize sounds in the experimental arena. Visual and auditory cues may also converge to affect sea-finding orientation. Future studies should focus on the localization ability of sea turtles and on the role that sound may play in orientation when combined with other sensory and environmental cues.