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Several studies have been devoted to increasing the attenuation performance of the Helmholtz resonator (HR). One way is by periodic coupling of HRs in a ducting system. In this study, we propose a different approach, where a membrane (or a thin flexible structure in general) is added to the air cavity of a periodic HR array in order to further enhance the attenuation by utilizing the resonance effect of the membrane. It is expected that three attenuation mechanisms will exist in the system that can enhance the overall attenuation, i.e. the resonance mechanism of the HR, the Bragg reflection of the periodic system, and the resonance mechanism of the membrane or thin flexible structure. This study found that the proposed system yields two adjacent attenuation peaks, related to the HR and the membrane respectively. Moreover, extension of the attenuation bandwidth was also observed as a result of the periodic arrangement of HRs. With the same HR parameters, the peak attenuation by the membrane is tunable by changing its material properties. However, such a system does not always produce a wider attenuation bandwidth; the resonance bandwidths of both mechanisms must overlap. [ABSTRACT FROM AUTHOR]

At present, the regulatory technical documents developed in the late 21st century that stipulate design solutions and operating regulations for nuclear power plants do not include provisions for predicting and preventing the occurrence of vibroacoustic resonances of structures involving acoustic standing waves (ASWs) in operational and emergency modes and also under the effect of seismic waves. This results in that the existing feedbacks between neutronic and thermophysical processes under the conditions of coolant density fluctuations caused by ASWs have not been given proper attention. A nuclear reactor digital acoustic model (NRDAC), using which it is possible to identify the sources of self-exciting ASWs and to predict their frequency, has been developed at the NRU MPEI Department of Nuclear Power Plants. The article presents a new method for identifying the ASW frequencies in the acoustic system comprising a pressurized water reactor (in its Russian version known as a VVER, meaning a water-cooled, water moderated power-generating reactor) with the cold and hot leg pipelines connected to it. The models and analysis methods have passed verification in the Novovoronezh NPP unit 3 equipped with a VVER-440 reactor. For measuring the autospectral power densities (ASPD) of pressure pulsations, the main equipment vibration monitoring system SÜS developed by Siemens was used. The predicted values of ASW frequencies are consistent with the signals measured by pressure pulsation sensors. It is shown that the ASW frequencies depend on the operation conditions and may coincide with the pressure fluctuation frequencies caused by the operation of reactor coolant pumps (RCPs) with vibration of fuel assemblies (FAs) and equipment of the NPP reactor coolant system. It is shown that, whatever the number of coolant circulation loops, the nuclear reactor acoustic properties are similar to the properties of a group of simultaneously used Helmholtz resonators. By using the verified procedure, it is possible to optimize the design and layout solutions by constructing equipment able to minimize undesirable cyclic loads. This possibility is the most important one from the viewpoint of ensuring long-term operation of small modular reactors in maneuverable modes. Small NPPs should be highly maneuverable to complement renewable energy sources. The majority of small modular reactors are designed for little-attended operation with long periods of time between refueling outages (2–10 years vs. 12–24 months in large power units) or with the fuel loaded for the entire life cycle. The requirements of ensuring long-term operation in maneuverable modes can be fulfilled subject to minimizing the dynamic loads applied to the reactor plant structures by preventing the occurrence of hydrodynamic instability and resonances of structural vibration with acoustic fluctuations of coolant. [ABSTRACT FROM AUTHOR]

Resonant vessels in ancient and medieval buildings are the subject of some historical and acoustic research today. There have been a number of detailed surveys of European churches, where acoustic pots remain in the buildings. Despite the fact that in medieval Russia the use of built-in vessels was very common in the construction of churches, they have been hardly considered in recent publications. Therefore, the first goal of this paper is to give a brief overview of the Russian experience. Some of the most interesting examples of Russian churches are presented, and among them there may be a world record for the number of the vessels in a single room. The Church of St. Nicholas in Pskov has about 300 pots inserted into the walls, apse and pendentives. The second goal is to study the efficiency of acoustic vessels in an ordinary room. Acoustic measurements were carried out in the Rachmaninov Hall, which is part of the Moscow Conservatory. This chamber concert hall built over 100 years ago has 29 vessels. The first conclusion is that the vessels behave like resonators, their natural frequencies have been identified. The second conclusion is that we found no considerable changes of the acoustics due to the vessels.

Sound energy is all around but not properly utilized despite being a source of electricity. This research was conducted to construct and test an acoustic energy harvester consisting of a Helmholtz resonator and a loudspeaker. The resonator cavity was made of 10 mm-thick cube-shaped acrylic plates with an inner side length of 300 mm while its neck was made of PVC (polyvinyl chloride) pipes with an inner diameter of 55 mm and three length variations at 50 mm, 70 mm, and 90 mm. A 6-inch subwoofer loudspeaker was mounted on the resonator back wall facing the cavity with its terminals connected to a 100-ohm load resistor. The sound waves entering the resonator cavity through the neck were converted into the alternating electric current flowing through the resistor. The test was conducted experimentally by exposing the harvester to sound waves at a maximum sound pressure level (SPL) of 100 dB and frequency variations from 25 Hz to 200 Hz. The root-mean-square (rms) voltages across the resistor were measured to calculate the output rms values for electric power. The results showed seven spectrum peaks which appeared at frequencies of 31 Hz, 37 Hz, 41 Hz, 49 Hz, 58 Hz, 73 Hz, and 82 Hz. Moreover, a shorter neck was also observed to have produced higher output power as indicated by the highest value of 2.75 mW obtained by using a 50 mm-long resonator neck at 37 Hz frequency and 100 dB SPL. These findings showed the acoustic energy harvester used to be effective due to its ability to produce electricity even at low frequencies below 100 Hz.

This paper focuses on the noise reduction of the cylindrical structure at low frequency (130 Hz-180 Hz). The low-frequency noise response spectrum in the cylindrical cavity is obtained by using the Helmholtz resonator (HR) to reduce the noise peak amplitude. The acoustic simulation software Virtual.Lab is used to establish the finite element model of the cylindrical shell with HR, and to obtain the low-frequency acoustic response in the cylindrical cavity. The simulation model is validated by the experimental results. Then, the influence of installation position, the number of installed resonators and the resonators with different resonance frequencies in the cylindrical cavity are discussed. The results indicate that both the noise reduction band and peak amplitude are increased by installing the HR's on the cylinder shell. The noise reduction of the cylinder shell with HR installed on the upper position is larger than other situations. As the number of resonators increased, the frequency range of the noise reduction in the cylindrical cavity gradually increases, and the noise reduction of the cylinder cavity increases first and then decreases.

The generating mechanisms of the mid-high-frequency peaks of a sound pressure level are theoretically investigated in this study with a side-firing front chamber. A mid-frequency peak is caused by the Helmholtz resonance phenomenon of the front chamber, and a high-frequency peak occurs owing to tube resonance. The electromagnetic–mechanical–acoustic coupling method (E–M–A) and resonance equations are used to obtain the sound pressure level and peak frequencies, respectively. Seven side-firing front chambers have been designed with respect to different tube lengths, tube widths and tube heights. Microspeaker samples with different front chamber designs are manufactured to verify the analytical results. The SPL and mid-high peak frequencies comparisons showed good agreement between the analysis and experimental results.

This paper presents a combined experimental and numerical investigation on a novel liner concept for enhanced low-frequency and broadband acoustic attenuation. In particular, two different realizations, derived from conventional Helmholtz resonators (HR) and plate resonators (PR) are investigated, which both deploy flexible materials with material inherent damping. In this context, a comprehensive experimental investigation was carried out focusing the identification and evaluation of various geometric parameters and material properties on the acoustics dissipation and related properties of various materials in a simplified setup of a single Helmholtz resonator with flexible walls (FHR concept). Furthermore, a parameter study based on analytical models was performed for both liner concepts, taking into account material as well as geometric parameters and their effects on transmission loss. In addition, design concepts that enable cylindrical or otherwise curved liner structures and the corresponding manufacturing technologies are presented, while considering essential structural features such as drainage. With respect to the potential application in jet engines, a structural–mechanical analysis considering the relevant load cases to compare and discuss the mechanical performance of a classical HR and the FHR concept liner is presented. Finally, both concepts are evaluated and possible challenges and potentials for further implementation are described.

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Featured Application: A slit-type soundproof panel can be applied to attenuate low-frequency noise and withstand wind load alongside traffic roads. With the rapid increase in automobiles, the importance of reducing low-frequency noise is being emphasized for a comfortable urban environment. Helmholtz resonators are widely used to attenuate low-frequency noise over a narrow range. In this study, a slit-type soundproof panel is designed to achieve low-frequency noise attenuation in the range of 500 Hz to 1000 Hz with the characteristics of a Helmholtz resonator and the ability to pass air through the slits on the panel surface for reducing wind load. The basic dimension of the soundproof panel is determined using the classical formula and numerical analysis using a commercial program, COMSOL Multiphysics, for transmission loss prediction. From the numerical study, it is identified that the transmission loss performance is improved compared to the basic design according to the shape change and configuration method of the Helmholtz resonator. Although the correlation according to the shape change and configuration method cannot be derived, it is confirmed that it can be used as an effective method for deriving a soundproof panel design that satisfies the basic performance. [ABSTRACT FROM AUTHOR]

A model of resonators for which boundary is composed of the Helmholtz resonators is constructed. It is based on the theory of self-adjoint extensions of symmetric operators. We consider a limiting procedure when the number of resonators tends to infinity and look after the spectrum of the Neumann Laplacian. It is shown that there is a sequence of the model systems and, correspondingly, a sequence of eigenfunctions of the model operators which converge to an eigenfunction of the Laplacian with a specific boundary condition. [ABSTRACT FROM AUTHOR]

This paper describes the optimum tuning of damped Helmholtz silencers. In this study, Helmholtz resonators are used as silencers that suppress acoustic resonance in host acoustic fields. Side branch silencers and Helmholtz silencers are commonly known as a type of vibration absorber; however, damped silencers have not been thoroughly studied thus far. Therefore, prior to this paper, we reported the optimum tuning of damped side branch silencers using modal analysis and two fixed point method. In this paper, we additionally describe the optimum tuning of damped Helmholtz silencers in a similar fashion as in our previous paper. The resonance mechanism of Helmholtz silencers is different from that of side branch silencers. The coupled vibration between the host acoustic field and Helmholtz silencer was theoretically analyzed using modal analysis in this study. An equivalent discrete model was obtained using the equations of motion using modal coordinate systems. Using the equivalent discrete model, the open-end correction of the neck of the Helmholtz silencer was considered, and the number of degrees of freedom of the equivalent discrete model was reduced to two to derive optimum tuning conditions using the two fixed point method. The optimum natural frequency ratio and loss factor of the Helmholtz silencer were derived using the vibration model with two degrees of freedom. The theoretical analysis was validated through simulations and experiments.

In this study, a piezoelectric micromachined ultrasonic transducer (PMUT) is integrated with a microliter-sized volume-tunable Helmholtz resonator. The passive Helmholtz resonator is constructed using an SU8 photolithography-defined square opening plate as the neck portion, a 3D-printed hollow structure with a threaded insert nut, and a precision set screw to form the volume-controllable cavity of the Helmholtz resonator. The fabricated piezoelectric films acted as ultrasonic actuators attached to the surface of the neck SU8 plate. Experimental results show that the sound pressure level (SPL) and operation bandwidth could be effectively tuned, and a 200% SPL increase and twofold bandwidth enhancement are achieved when setting the cavity length to 0.75 mm compared with the open-cavity case. A modified Helmholtz resonator model is proposed to explain the experimental results. The adjusting factors of the effective mass and viscous damper are created to modify the existing parameters in the conventional Helmholtz resonator model. The relationship between the adjusting factors and cavity length can be described well using a two-term power series curve. This modified Helmholtz resonator model not only provides insight into this active-type Helmholtz resonator operation but also provides a useful estimation for its optimal design and fabrication.

Helmholtz resonator is considered and widely accepted as a basic acoustic model in engineering applications and research. In this paper, the normal incidence sound absorption characteristics of series and parallel configurations of Helmholtz resonators is studied analytically, numerically and experimentally. The proposed analytical model for series configuration of HRs comprises of Johnson–Champoux–Allard model and transfer matrix method while parallel configuration of HRs is described using parallel transfer matrix method. The results from proposed analytical models fit well with the finite element method (FEM) results obtained from COMSOL multiphysics. Incorporation of parallel configuration and proper tuning of geometric parameters helps to overcome the trade-off between broad band sound absorption and minimum space utilization. Also, the experimental observations of one of the parallel configuration substantiates the FEM results. Moreover, the FEM models are more accountable for the variation in neck position and also provide better visualization of acoustic absorption with frequency. [ABSTRACT FROM AUTHOR]

• Coupling of a Helmholtz resonator with a novel point defect acoustic metamaterial to improve acoustic energy harvesting performance. • The local resonance at the point defect location is further enhanced by introducing the trampoline effect through the improved structure. • The structure achieved a maximum output voltage of 0.64 V and a maximum output power of 1.998 μW under an excitation with an effective sound pressure of 2 Pa and a frequency of 1702 Hz. • The structure also has the advantages of simple processing and small structural dimensions and is expected to be widely used for power supply of micro sensors and other devices. Acoustic energy harvesting technology has attracted much attention for its potential to power wireless sensors. In this paper, we propose a Helmholtz coupled metamaterial and introduce the trampoline effect for efficient acoustic energy harvesting. First, a point defect is introduced on the metamaterial plate and a piezoelectric sheet is mounted to realize the concentration and harvesting of acoustic energy. Next, a Helmholtz resonator is introduced at the defect location to amplify the acoustic pressure and enhance the local resonance effect. Finally, the energy harvesting performance is further improved by introducing a trampoline effect into the structure. The numerical simulation results indicate that the designed structure has a peak output voltage of 0.64 V and a peak output power of 1.99 μW when excited by a plane wave with an effective sound pressure of 2 Pa and a frequency of 1702 Hz. In addition, the physical experimental results agree with the numerical simulation results, which verifies the acoustic energy harvesting performance of the structure. [ABSTRACT FROM AUTHOR]

Samples of sound-absorbing Helmholtz resonator-type liners of circular shape were manufactured from two types of ABS plastic and nylon on the basis of 3D modeling and 3D printing technology. Check samples were made of metal on a numerically controlled machine. Deviations of geometric parameters of the manufactured samples from the design values were determined by visual and dimensional inspection using high-precision equipment. The minimum deviations were obtained for check samples made of metal. The acoustic characteristics of the samples were experimentally determined using an interferometer with normal wave incidence at high sound pressure levels. Numerical simulation of the acoustic processes in the interferometer for the given samples was carried out on the basis of solving full Navier-Stokes equations with account for compressibility. The obtained values of the resonant frequency, impedance and sound absorption coefficient were compared with the experimental ones. It was noted that the impedance values are most sensitive to the deviations of the geometric parameters of the samples from the design values, while the deviations in the sound absorption coefficient and resonance frequency are not so sensitive to them.

To solve the problem of low-frequency noise in the environment, a Helmholtz-type phononic crystal with adjustable cavity structure and labyrinth tubes was designed. The unique design of the labyrinth tube greatly increases the length of the tube, improving low-frequency sound insulation performance, and the design of adjustable cavity structure realizes active regulation of the band structure. The band gap structure and sound insulation characteristics were analyzed by finite element method (FEM) and electro-mechanical-acoustic analogy method. The result shows that, firstly, the structure can generate two complete band gaps in the low-frequency range of 0–500 Hz, and there is a low-frequency band gap with lower limit of 40 Hz. Meanwhile, the structure has excellent sound insulation performance in the range of 0–500 Hz. Secondly, multiple resonant band gaps can be connected by adjusting the structural layout of the cavity through the telescopic screw, so as to achieve the purpose of widening the band gap and active control of environmental noise. Finally, in the periodic arrangement design of the structure, reducing the spacing between cells can effectively increase the bandwidth of band gaps. This design broadens the design idea of phononic crystal and provides a new method to solve the problem of low-frequency noise control.

Abstract 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.

This study proposes to evaluate the acoustical performance of a reactive vehicular silencer made entirely of Helmholtz resonator. Four or seven resonators are idealized to be in a series configuration, all the dimensions except cavity length and resonators are fixed. An algorithm processes inputted characteristic engine noise signal obtained from literature, identify peaks, and calculates ideal cavity length for attenuation. The transmission loss of the system is analytically calculated. Attenuation levels obtained are satisfactory. The required volume of the resonator to achieve the same resonance frequency as low frequency noise peak demands impractical cavity length, proving it to be flawed. It is suggested to use resonators of two or three different diameter for different frequency range in order to overcome this problem.

Noise from jet engines can be reduced by means of a Helmholtz cavity configuration. The resonance that occurs when a flow passes the neck of the Helmholtz resonator will dissipate acoustic energy. The mechanism for such dissipation is mainly due to the vortex shedding that occurs at the neck of the resonator where the vortex structures absorb acoustic energy and subsequently dissipate it through viscous effects. In this work, numerical simulations utilizing the lattice Boltzmann method are used to aid in visualizing the flow behaviour that is associated with Helmholtz cavity-backed acoustic liners. In both experiments and numerical simulations, the 1-neck cavity is found to result in an amplification of an applied acoustic source. For a 4-neck cavity, the configuration is able to achieve acoustic pressure reductions. Differences in the flow behaviour of the 1-neck and 4-neck cavities are detailed in this work. Results show that the stronger vortex shedding that occurs in the 4-neck cavity configuration could explain its increased effectiveness as a Helmholtz cavity-backed acoustic liner. [ABSTRACT FROM AUTHOR]

An accurate triangular spectral element method (TSEM) is developed to simulate acoustic problems in complex computational domains. With Fekete points and Koornwinder-Dubiner polynomials introduced, triangular elements are used in the present method to substitute quadrilateral elements in traditional spectral element method (SEM). The efficiency of discretizing complex geometry is enhanced while high accuracy of SEM is remained. The weak form of the second-order governing equations derived from the linearized Euler equations (LEEs) are solved, and perfectly matched layer (PML) boundary condition is implemented. Three benchmark problems with analytical solutions are employed to testify the exponential convergence rate, convenient implementation of solid wall boundary condition and capable discretization in complex geometries of the present method respectively. An application on Helmholtz resonator (HR) is presented as well to demonstrate the possibility of using the present method in practical engineering. The numerical resonance frequency of HR reaches an excellent agreement with the theoretical result. [ABSTRACT FROM AUTHOR]

Helmholtz resonators (HR) have been proven to have feasibility for sensor applications with good performance. However, for biosensor application, when the amount of test sample is limited, the operating frequency tends to be very high, which may bring challenges of resonance excitation and frequency measurement. In this paper, a modified HR was proposed for biosensor application at a lower frequency, by designing the neck of the HR as a type of an Archimedes spiral, which can increase the neck length as much as possible without occupying much space. The resonant frequency of the proposed HR was derived by treating it as an HR with a straight neck with the equivalent length. An analytical example was calculated and verified by acoustic finite element analysis, and the results clearly showed that the frequency decreases in comparison to that of the conventional HR. Based on the underlying theory, the measurement principle was proposed and the experiment setup was established using a 3D-printed structure. The experiment was done using test solutions with different glucose concentrations, whose results showed consistency with the analytical results and noticeable frequency increases with the glucose concentration. Prospective results of the proposed HR after miniaturization show an operating frequency around 3 kHz when the amount of test sample is 0.1 mL, which will facilitate use of the common acoustic power source and acoustic pressure sensors in the sensing system. [ABSTRACT FROM AUTHOR]

This article presents some experimental results on Helmholtz resonators for large sound amplitudes with two general geometries of their necks: the resonators with classical (cylindrical) and quadratic nonlinear necks. Obtained results for large amplitudes show accelerated amplitude dependency of resonant frequencies of the resonators with modified shape of the neck compared to classical ones. This nonlinear response can be used as a passive controller system with nonlinear restoring forcing function for having broadband frequency absorption. [ABSTRACT FROM AUTHOR]

Acoustic metamaterial attains uncommon material properties over natural material such as negative effective mass density or negative effective bulk modulus or both. The effective bulk modulus of a classical acoustic reactive filter such as Helmholtz resonator has been observed negative, using lumped element model, at resonance point, which desires further investigations. Present research demonstrates and establishes robust method to estimate and measure acoustic metamaterial properties of a Helmholtz resonator analytically, numerically and experimentally in detail. The proposed numerical technique simulates ASTM-2611 standard of measurement to extract corresponding reflection and transmission coefficients to evaluate the effective acoustic metamaterial properties. The subsequent investigations have been carried out by taking eight different configurations of Helmholtz resonators such as parallel and series using similar and dissimilar resonators. Performance of each configuration has been investigated analytically, numerically and validated experimentally. At last, a finite array of Helmholtz resonators has been investigated. From investigations, it has been observed that five configurations of Helmholtz resonators and finite array have negative effective mass density and effective bulk modulus. Summarizing, the present research endeavors analytical, numerical and experimental investigations of different configurations of Helmholtz resonators in detail, which is first in its own kind. [ABSTRACT FROM AUTHOR]

A numerical simulation is carried out to investigate the effect of the Helmholtz resonator capacity on the Hartmann whistle operating at high values of the nozzle pressure ratio using the turbulence model. The results of the present numerical simulations are compared to experimental data. The simulation results show that the frequency and amplitude of the Hartmann whistle with the Helmholtz resonator are obviously lower as compared to the conventional Hartmann whistle. Moreover, the Mach number contours and streamlines indicate that the Helmholtz resonator does not affect the shock-cell structure between the nozzle and the cavity, and the Hartmann whistle with the Helmholtz resonator has a. jet regurgitant mode that is different from the Hartmann whistle with a. straight resonator. The diameter of the Helmholtz resonator is an important factor affecting the fundamental frequency. [ABSTRACT FROM AUTHOR]

• A novel multi-layered corrugated resonator acoustic metamaterial (MLCRAM) is proposed and fabricated. • It is revealed that the MLCRAM exhibits excellent low-frequency broadband sound absorption performance. • The numerical models verified by experiment are developed to reveal the sound absorption mechanism of the MLCRAM. How to design acoustic metamaterials with excellent low frequency broadband sound absorption and mechanical properties is a concern and bottleneck in the field of structural vibration and noise control. Based on the principle of Helmholtz resonator and lightweight multi-functional sandwich structure, we propose a multi-layered corrugated resonator acoustic metamaterial (MLCRAM) and fabricate a sample using 3D printing technology. Through theoretical and simulated studies of the MLCRAM model, it is presumed that the MLCRAM has excellent sound absorption performance in the frequency range of 500 to 1000 Hz, which is mainly attributed to the parallel combination of resonant cavities. Then the experimental test of the sample is also carried out to verify the reliability of both theoretical and simulated methods. By analyzing the complex frequency plane, the influence of coupling effects between resonant cavities on the sound absorption properties of the MLCRAM. Subsequently, the overall sound absorption mechanism of the MLCRAM is further analyzed in terms of sound velocity, sound pressure amplitude, and power dissipation density. It has been demonstrated that MLCRAM exhibits excellent low-frequency broadband sound absorption performance, providing a reference for the design of novel acoustic metamaterials. [ABSTRACT FROM AUTHOR]

With the rapid increase in automobiles, the importance of reducing low-frequency noise is being emphasized for a comfortable urban environment. Helmholtz resonators are widely used to attenuate low-frequency noise over a narrow range. In this study, a slit-type soundproof panel is designed to achieve low-frequency noise attenuation in the range of 500 Hz to 1000 Hz with the characteristics of a Helmholtz resonator and the ability to pass air through the slits on the panel surface for reducing wind load. The basic dimension of the soundproof panel is determined using the classical formula and numerical analysis using a commercial program, COMSOL Multiphysics, for transmission loss prediction. From the numerical study, it is identified that the transmission loss performance is improved compared to the basic design according to the shape change and configuration method of the Helmholtz resonator. Although the correlation according to the shape change and configuration method cannot be derived, it is confirmed that it can be used as an effective method for deriving a soundproof panel design that satisfies the basic performance.

We present a theoretical and experimental study of a resonator of the Helmholtz type for the control of the swell. An experimental demonstration of the shielding effect by a belt made of evenly distributed resonators is given. We then provide in-depth analysis of the Fano resonance resulting from the interference between the dock scattering (the background) and the resonant cavity scattering. This is done thanks to space–time resolved experiments which provides the complex-valued scattering coefficients and amplitude within the resonator. We provide a one-dimensional model derived in the shallow water regime owing to asymptotic analysis. The model contains the two ingredients of the Fano resonance and allows us to exhibit the damping due to leakage. When adding heuristically the damping due to losses, it reproduces the main features of the resonance observed experimentally.

working model of the ocean seismic bottom station. The fiber-optic hydrophone is built on the base of Mach-Zehnder interferometer. Its sensitive arm is wounded on the elastic mandrel. The mandrel material increases acoustic pressure sensitivity of the optical fiber. The developed housing is designed to protect the sensitive optical fiber from mechanical damage. The housing also passes the acoustic signals in water without attenuation in the work frequency range of the fiberoptic hydrophone up to 8 kHz. Method. The theoretical calculations, based on the Helmholtz resonator theory, and mathematical modeling by the finite element method in the ComsolMultiphysics environment were used to develop the protective housing with required parameters. Created models enabled the definition of the protective housing final construction that passes acoustic signals in the required frequency range. Main Results. As a result of mathematical modeling the final construction of the protective housing was chosen. The construction is based on the aluminum cylinder with the external radius equal to 30 mm, the height - 14 cm and the wall thickness - 1 mm and it contains 1900 holes with the radius equal to 1 mm. During the modeling the frequency response of the protective housing was obtained; this response demonstrated its acoustic transparency in water at frequencies up to 8 kHz. The chosen protective housing was fabricated and studied in the working model of the ocean seismic bottom station. Experiment results confirmed the acoustic transparency of the protective housing in the required frequency range. Practical Relevance. The results of this work might be used for the developing and creating of protective housings for fiber-optic hydrophones with the required frequency responses. The developed protective housing is used in the working model of the ocean seismic bottom station and it provides the mechanical protection of the optical fiber in the sensitive element of the fiber optic-hydrophone without distortion of its frequency response.

Acoustic metamaterials have opened fascinating opportunities for manipulation of low-frequency sound and development of compact structures with broadband acoustic performance for noise mitigation applications, room and architectural acoustics. So far, several mechanisms have been studied to achieve perfect sound absorption at subwavelength frequencies, when structural dimensions are much smaller than the wavelength of incident waves. Here, we analyze two alternative approaches based on the use of interacting and coupled resonators in absorbing panels with the aim to reduce the panel thickness. We numerically demonstrate that proper designs of interacting resonators allow extending a single-peak absorption to broadband frequencies, while the use of coupled resonators enables broadband absorption at several frequency ranges. The proposed configurations are inspired by ancient Slavic folk patterns with delicate ligature, multi-layered structure, and concealed meaning. Our results open new possibilities for creating acoustic metamaterials with added art effects, which individualize occupied space and make absorbing panels more appealing for various applications.

In this paper, a mathematical modeling of the physical mechanism underlying the concept of tunable metasurfaces is provided. The scattering properties of two metasurfaces are analyzed. The considered metasurfaces are designed by placing either a Helmholtz resonator or a pair of Helmholtz resonators in a periodic lattice. The subwavelength resonant properties and the concept of hybridization of Helmholtz resonators are exploited in order to shape the scattered waves in unusual way by such metasurfaces. [ABSTRACT FROM AUTHOR]

• Two SDOF (single-degree-of-freedom) models for representing the membrane have been proposed. • The transformation coefficients for describing the acoustic-mechanical coupling have been derived. • A MDOF model for representing the membrane coupled Helmholtz resonator has been developed. • An acoustic metamaterial integrated with membrane coupled Helmholtz resonator has been studied. In recent years, the Helmholtz resonator and membrane are two popular elements that have been vastly employed in the design of energy harvesters and metamaterials. In this paper, a theoretical study of the modelling of the membrane-coupled Helmholtz resonator is presented. The membrane is first represented with lumped parameters as a single-degree-of-freedom piston/centre-mass model. The physical meaning of the effective force-bearing area is explained. The membrane-coupled Helmholtz resonator is then modelled as a multiple-degree-of-freedom system. From the acoustic-mechanical interaction perspective, transformation coefficients are derived to bridge the acoustic and mechanical domains. Inspired by the fact that the membrane-coupled Helmholtz resonator exhibits multiple resonances in the low frequency regime, an acoustic metamaterial system is proposed by integrating the membrane-coupled Helmholtz resonators. A theoretical model of the proposed acoustic metamaterial is developed and multiple band gaps are predicted from the band structure analysis. All the theoretical models presented in this paper have been verified by corresponding finite element models. [ABSTRACT FROM AUTHOR]