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This study addresses the noise challenges associated with roots blowers, pivotal devices in water treatment technology, by developing an innovative duct silencer. To construct an efficient internal structure for noise reduction, a novel rectangular design altering sound propagation direction was implemented. Additionally, an array of Helmholtz resonators giving substantial transmission loss within specific frequency ranges, was used. Experimental validation and acoustic analysis were employed to evaluate the silencer's performance, ultimately confirming its noise reduction capabilities. The optimized silencer through a parametric study of the Helmholtz resonator structure exhibited an impressive transmission loss of 14.75 dB, signifying a remarkable 74 % enhancement over the existing circular silencer design. This remarkable performance substantiated its effectiveness in mitigating noise. To validate the optimized design, a physical prototype was manufactured and subjected to noise measurement experiments, revealing a significant noise reduction of 17.3 dB. [ABSTRACT FROM AUTHOR]

Purpose: Development a new combined material, nanomembrane and melamine foam, for noise attenuation at mid-range frequency to reduce noise inside an airplane cabin. Methods: To be able to investigate how acoustic properties of acoustic foams are affected by nanomaterials, a new type of Helmholtz resonator based on a combination melamine foam and nanomembranes is proposed. Its performance was evaluated experimentally based on ASTM C384 and ASTM C522. Results: The Helmholtz resonator based on an association of nanomembranes doped with carbon nanotubes and melamine foams provided an effective sound absorption coefficient peak (α), ranging from 0.94 to 0.99, in the range of 1200–1600 Hz, a drastic shift from the high frequency range (5500–6000 Hz) commonly detected for melamine foam. The new system sound absorption peak performance at mid-range frequencies (1200–1600 Hz) was obtained with an increase in thickness between 0.05 and 0.09%, a virtual no weight addition to the soundproof material. Moreover, the analytical model based on Langmuir air flow resistivity and the Delany–Bazley equations captured the experimental trend with accuracy. Conclusion: The new material proposed is a valuable option for noise attenuation inside airplane cabin, in special, regional jets. [ABSTRACT FROM AUTHOR]

We present frequency-selective 1-bit bifunctional coding metasurfaces (BCMs) for acoustic-wave field manipulations. Unlike most conventional metasurfaces operating at a single frequency and having only one functionality, the BCM demonstrates the capability to achieve different functionalities at two distinct frequencies. The underlying mechanism is based on the design of the coding units with Helmholtz resonators operating at two frequencies independently. We demonstrate the achievement of different functionalities by the BCMs, involving acoustic focusing, generation of Airy beam, and acoustic splitting. The results are shown through theoretical predictions and finite-element simulations, which show good correspondence. Our work presents a promising metasurface design for pragmatic applications, such as multiple acoustic beam shaping and information preservation. [ABSTRACT FROM AUTHOR]

Based on the turbulence model and the Ffowcs Williams–Hawkings (FW–H) acoustic model, numerical simulations of acoustic characteristics in the Hartmann whistle with the Helmholtz resonator are carried out. The important parameters that control the flow oscillation features of the Hartmann whistle are the stand-off distance, cavity geometry, nozzle pressure ratio, etc. The computational results are compared to experimental data. Under the condition that the jet exit diameter, cavity diameter, nozzle pressure ratio, and stand-off distance remain constant, the mass flow rate and the sound pressure level are calculated as functions of the diameter and length of the Helmholtz resonator. The results show that the sound directivity is similar in the conventional Hartmann whistle and the Hartmann whistle with the Helmholtz resonator, while the sound intensity is higher in the conventional Hartmann whistle. Also, the sound intensity reaches the maximum in the direction perpendicular to the jet. The magnitude of the sound intensity decreases gradually with an increase in the diameter of the Helmholtz resonator, and the decreasing trend in the fundamental resonance frequency is clearly visible. Next, as the length of the Helmholtz resonator increases, the sound intensity first decreases and then increases again. The effect of the resonator length on the fundamental resonance frequency is not large as compared to the resonator diameter. [ABSTRACT FROM AUTHOR]

Introduction: Gradient-based sonic crystals (GBSCs) based on rigid resonators that improve the acoustic attenuation over the traditional sonic crystals (SCs) were recently studied numerically. The present work is a numerical and experimental study of the GBSCs based on rigid and cladded resonators and aims to further improve the attenuation of the GBSCs over the traditional SCs. Methods: Extensive finite element (FE) and experimental studies were conducted on the GBSCs and the traditional uniform SCs using jute, a natural fibrous material as claddings over rigid resonators in the form of felts. The multiple linear regression (MLR) technique was used to find the influence of the airflow resistivity and thickness of the claddings on the attention spectrum of the GBSC. The effect of the angle of incidence of the sound waves on the attenuation by the GBSC and traditional SC was also studied experimentally. Results: Though the claddings were found to improve the overall attenuation by the GBSC, they do not improve the attenuation at all frequencies. Similarly, the claddings were found to be more effective on the GBSC than the traditional Helmholtz resonator-based SC (HRSC). However, the GBSC was found to be less effective than the traditional HRSC if the angle of incidence of the sound waves changes. Conclusions: For a fixed filling ratio of the GBSC resonators, the Bragg scattering and local resonance of a GBSC based on rigid resonators are more effective attenuation mechanisms in the resonance and first Bragg gap regimes than the combined effects of absorption by the fibrous cladding, Bragg scattering and local resonance of a GBSC based on cladded resonators. However, the claddings significantly improve the attenuation post the resonance and first Bragg gaps. Therefore, the claddings should only be used when the targeted frequency regime lies above the resonance and first Bragg gaps. [ABSTRACT FROM AUTHOR]

An array of air chambers embedded on the tunnel track is numerically investigated for different configurations. The air chambers act as an in-tunnel countermeasure to alleviate non-linear steepening of compression waves during propagation process thereby mitigating emission of high amplitude micro-pressure waves (MPWs) from the exit portal. The air chambers are designed to entrap the incoming flow and induce damping behavior so as to reduce wave steepening in tunnels. Initially, qualititative comparisons are made between Helmholtz resonator and damping type air chambers. Thereafter, the quantitative assessment of the compression wave properties are checked in detail. From results, over damped air chamber shows 60 % reduction in peak over pressure for weak compression wave whereas the Helmholtz resonator and critically damped shows 35 % and 22 %, respectively. Similarly, due to the presence of over damped air chamber, 17 % reduction in maximum pressure gradient is noticed while critically damped shows close to 10 % reduction. [ABSTRACT FROM AUTHOR]

The results of theoretical studies of the impedance characteristics of the respiratory system and experimental data on the acoustic characteristics of the lungs of rabbits are presented. The measurement results indicate the possibility of using the Helmholtz resonator as an adequate acoustic model of animal and human lungs in the low-frequency range of sound waves. A physiological interpretation of indicators recorded using impedance measurements is given in relation to the diagnosis of the state of the respiratory system of animals and humans. [ABSTRACT FROM AUTHOR]

In this study, we investigate the acoustic metamaterial (AMM) design for flow noise reduction when using a vacuum cleaner. The AMM was based on the principle of Helmholtz resonators, and metamaterial design variables were defined to reduce noise to approximately 1.5 and 2.5 kHz bands. An acoustic simulation was performed considering the thermoviscous effect using the designed AMM, and a transmission loss in the simulation results was calculated using the four-microphone method. Finally, noise experiments were performed on a vacuum cleaner equipped with metamaterial. Through this, the noise reduction performance of metamaterial predicted through simulations was verified, thereby showing the selective implementation of noise reduction in the desired frequency band. [ABSTRACT FROM AUTHOR]

The question of whether there exists an approximation procedure to compute the resonances of any Helmholtz resonator, regardless of its particular shape, is addressed. A positive answer is given, and it is shown that all that one has to assume is that the resonator chamber is bounded and that its boundary is C 2 . The proof is constructive, providing a universal algorithm which only needs to access the values of the characteristic function of the chamber at any requested point. [ABSTRACT FROM AUTHOR]

Controlling low-frequency noise propagated by power transformers in urban and industrial areas poses a technical challenge in the design of sound absorbers. Although existing acoustic metamaterials are largely successful as single-band absorbers, they are far from optimal approaches to absorb power transformer noise. As a common method of providing multi-band absorbers, the parallel configuration of multiple individual units leads to the coupling effect, resulting in the absorption coefficient drop. In this research, a novel design is proposed to convert a single-band Helmholtz resonator-based sound absorber into a multi-band absorber while maintaining its thickness at 55.1 mm, ~ 1/62 of 100 Hz noise wavelength. The proposed design offers simultaneous perfect absorptions at 100, 200 and 300 Hz frequencies as main components of power transformer noise. Design validation is carried out using finite element modelling in COMSOL Multiphysics and the experimental analyses on the 3D-printed sample. [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]

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]

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]

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]

In horizontal drilling, the cuttings bed is frequently formed to result in severe problems such as backing pressure, binding of bottom hole assembly (BHA) and even sticking of tool. Practice and research show that the smaller particle size is, the more favorable to carry out from bottom of hole in extended reach well or horizontal well. This paper presented a new solution to enhance cuttings transport in mining drilling through a new type of bit called pulsed mill bit (PMB) by using pulse jet mill technique. It provides a promising solution to enhance the efficiencies of rock breaking and cuttings carrying for pulsed jet, sucking cuttings to decrease the chip hold down effect, and reducing cuttings' diameter to eliminate cuttings bed. The design was supported by the calculation models of Helmholtz resonator natural frequency, optimal drilling fluid flow rate of resonator, drilling fluid flow velocity in forward jet channel, critical impact velocity of cuttings and minimum length of the accelerating cavity. Meanwhile, factors affecting optimal drilling fluid flow rate, critical impact velocity of cuttings and minimum length of accelerating cavity were investigated. Case study showed a good consistency between the calculation results and the related theories. It is concluded that optimal drilling fluid flow rate increases with the increase of inlet and outlet cavity's diameter, and decreases with the increase of the diameter of the resonant cavity. Critical impact velocity of cuttings increases with the energy conversion factor (β) and ROP, while decreasing with the final size of the cuttings (dt). This effect is obvious when β<0.2, ROP <3.048 m h−1 and dt<0.3 mm. Minimum length of the accelerating cavity decreases with the increase of the resonator's natural frequency and separation coefficient (α) of drilling fluid flow rate. This study provides a promising solution to remove cuttings bed in horizontal well. [ABSTRACT FROM AUTHOR]

Abstract: The paper considers the problem of sound absorption by a Helmholtz resonator in a room with perfectly hard walls. The resonator parameters are determined that yield the maximum absorption in the vicinity of the lowest eigenfrequency of the room: the coefficient of friction, elasticity, and mass. [ABSTRACT FROM AUTHOR]

Numerical simulation of acoustic processes with an interferometer at high acoustic pressure levels is one of the ways to noise reduction processes using samples of sound-absorbing structures (SAS). In the course of research, an SAS sample consisting of a single Helmholtz resonator of circular shape was used. Studies were conducted at sound pressure levels of 110, 130, 140, and 150 dB. Results obtained with the interferometer with normal wave incidence were taken as a basis. In the calculations, systems of linearized and complete Navier-Stokes equations were used. The results obtained with the linearized Navier-Stokes equations make it possible to determine the acoustic characteristics of the sample for linear modes of operation with sufficient accuracy. The complete system of Navier-Stokes equations, taking into account compressibility, allowed good qualitative and quantitative agreement with the experiment at high acoustic pressure levels. [ABSTRACT FROM AUTHOR]

An experimental study of the generation of pressure fluctuations in a well acoustic emitter with nozzles of various shapes has been carried out. The effect of a smooth nozzle inlet section on the generation amplitude, the optimum jet length, and the outlet diameter has been studied. The formation of a region of reverse currents connecting to the cavity in the nozzle between the channel wall and the jet periphery has been considered. A significant increase in the generation amplitude produced at the smooth nozzle inlet and the formation of a uniform velocity profile in the nozzle channel have been observed. [ABSTRACT FROM AUTHOR]

Absorption characteristics of a Helmholtz resonator positioned at the end wall of a circular duct are considered. The absorption coefficient of the resonator is experimentally investigated as a function of the diameter and length of the resonator neck and the depth of the resonator cavity. Based on experimental data, the linear analytic model of a Helmholtz resonator is verified, and the results of verification are used to determine the dissipative attached length of the resonator neck so as to provide the agreement between experimental and calculated data. Dependences of sound absorption by a Helmholtz resonator on its geometric parameters are obtained. [ABSTRACT FROM AUTHOR]

We studied the inertial attached neck length (end correction) of different Helmholtz resonator configurations based on numerical calculations by the finite element method. First, we determined the eigenfrequency of the resonator, then on its basis, we calculated the attached neck length of the resonator. We analyzed the dependence of the attached neck length on the geometric parameters of the Helmholtz resonator. Using numerical calculations, we approximated the obtained dependences as analytic expressions. [ABSTRACT FROM AUTHOR]

This paper reports a suspended coil, electromagnetic acoustic energy harvester (AEH) for extracting acoustical energy. The developed AEH comprises Helmholtz resonator (HR), a wound coil bonded to a flexible membrane and a permanent magnet placed in a magnet holder. The harvester's performance is analyzed under different sound pressure levels (SPLs) both in laboratory and in real environment. In laboratory, when connected to 50 Ω load resistance and subjected to an SPL of 100 dB, the AEH generated a peak load voltage of 198.7 mV at the resonant frequency of 319 Hz. When working under the optimum load resistance, the AEH generated an optimum load power of 789.65 μW. In real environment, the developed AEH produced a maximum voltage of 25 mV when exposed to the acoustic noise of a motorcycle and generated an optimum voltage of 60 mV when it is placed in the surroundings of a domestic electrical generator. [ABSTRACT FROM AUTHOR]

Helmholtz resonator is generally used to reduce the low frequency noise. However, it has only high transmission loss in a narrow band at the resonance frequency. In order to obtain broad band transmission loss, studies of a silencer using various resonator arrays have been carried out. These previous studies have been limited to low incident sound pressure level conditions with no change of impedance with respect to pressure. In the case of high sound pressure, the resistance increases due to nonlinear behavior at the neck of a resonator. As a result, the desired noise reduction performance cannot be achieved due to the transmission loss decrease. To predict the transmission loss of a silencer using resonator arrays at high sound pressure level, impedance variation depending on the incident sound pressure level should be considered. Furthermore, resonator's numbers and order of arrangement are significant design parameters for design of transmission loss shape. [ABSTRACT FROM AUTHOR]

A method for calculating the number of apertures in an aircraft engine's perforated soundabsorbing structures that present themselves as shells of a rotating body with rectangular and circular guide is presented. Formulas for determining aperture pitch are given. Analytical expressions for calculating the number of apertures in perforated areas with regular triangle and square lattices are presented. [ABSTRACT FROM AUTHOR]

The article presents models of a photoacoustic Helmholtz resonator in which the duct is ended with conical profiles. The analytical descriptions led to poor results, and a partly numerical model was presented, in which cone sections of the duct were stepped-approximated by dividing them into a number of short segments with different diameters. The accuracy of the model was compared to the best models of a photoacoustic Helmholtz cell with a constant diameter duct. The proposed stepped-approximation can be used as well for modeling photoacoustic Helmholtz resonators with other profiles of the duct. [ABSTRACT FROM AUTHOR]

There are very few designs of the open photoacoustic Helmholtz cells, and most of them exhibit very strong penetration of the external acoustic noise inside the cell. So far the best values of external acoustic noise suppression obtained in such cells were reported at the level of about 40 dB to 50 dB. This paper presents an open photoacoustic Helmholtz cell design with a differential signal detection. Both Helmholtz resonator cavities are equipped with microphones and connected with the exterior via duct-buffer-duct structures. The length and diameter of the ducts as well as volume of the acoustic buffers are selected in such a way that the acoustic impedance of the duct-buffer-duct structure at the frequency of light modulation is relatively high. As a result, the resonance of the cell is not damped, while penetration of the external acoustic noise inside the cell is strongly reduced. Preliminary analysis predicts attenuation of the external acoustic noise at the resonance frequency of the cell with a single microphone to be at the level of at least 60 dB. Additional rejection of the external acoustic noise can be obtained with differential detection, which simultaneously doubles the photoacoustic signal component; as in the Helmholtz resonator, pressure changes in the cavities are in counterphase. [ABSTRACT FROM AUTHOR]

The paper describes how mechanical dimensions of a differential open photoacoustic Helmholtz cell affect its operation. The influence of the diameter and length of the internal and external ducts and the volume of the acoustic buffers on the frequency response of the cell, as well as on the attenuation of the external acoustic noise, was studied. The analysis was performed by means of computer simulations based on a model in which loss-corrected transmission line duct definitions were used. The results showed that under proper selection of the cell dimensions, its frequency response at the frequencies around the main resonance is nearly identical as in the case of the conventional, closed Helmholtz resonator. The length of the ducts is only slightly related to the frequency of resonance, and does not noticeably affect the Q-factor of the cell. A decrease of the duct diameters and an increase of the buffer volumes improve attenuation of the external acoustic noise. It should be possible to obtain external acoustic noise rejection as high as 80 dB, which is a substantial improvement in comparison to the previous designs, which reported this value at the level of 40 dB to 50 dB. [ABSTRACT FROM AUTHOR]

Many photoacoustic cells use a Helmholtz structure for amplification of the photoacoustic signal, but very few of them are open cells. Open Helmholtz cells known from the literature exhibit very poor attenuation of the external acoustic noise, which severely limits their applications. Recent research showed that this problem can be overcome in a so-called improved open photoacoustic Helmholtz cell structure. The paper describes how mechanical dimensions of such a cell affect the frequency response of the cell and its penetration by the external acoustic noise. Properties of the cell were investigated by means of computer simulations based on the loss-improved transmission line model. The analysis showed that appropriate selection of the duct dimensions and buffer volumes should result in over 50 dB attenuation of the external acoustic noise, without noticeably affecting the Q-factor and resonance frequency of the cell. [ABSTRACT FROM AUTHOR]

Open photoacoustic cells are often used in continuous-flow photoacoustic measurements. Such cells are sensitive to external noise penetration. The improved open photoacoustic Helmholtz cell has much better external noise attenuation than the previously known designs. This paper describes how mechanical dimensions of such a cell influence its signal-to-noise ratio. The analysis was performed by means of computer simulations based on the loss-improved transmission line model. This research showed that the mechanical parameters affect signal-to-noise noticeably and, if they are properly chosen and applied in the design of the improved open cell, the resulting signal-to-noise ratio may be improved by almost 60 dB in comparison to previous designs of open photoacoustic Helmholtz cells. [ABSTRACT FROM AUTHOR]

When open photoacoustic cells are considered, attenuation of external noise is a serious issue. This paper describes how mechanical dimensions of a differential open photoacoustic Helmholtz cell influence its signal-to-noise (S/N) ratio. The analysis was performed by means of computer simulations based on the loss-improved transmission line model. This research showed that the mechanical parameters noticeably affect the signal-to-noise ratio. According to the presented results, optimal selection of the dimensions is a factor which can substantially improve the signal-to-noise ratio of the examined photoacoustic cell. Due to the very good acoustic properties of the cell, it should be possible to obtain an S/N ratio of over 100 dB. [ABSTRACT FROM AUTHOR]

This paper presents the design of an open photoacoustic Helmholtz cell in which high acoustic volumes with quarter-wave ducts act as high-impedance separators between the main Helmholtz cell structure and the exterior. As a result, penetration of the external acoustic noise into the cell was substantially reduced in comparison to earlier open Helmholtz cell designs. Although the presented cell is not windowless, the photoacoustic background signal resulting from the absorption of the light by the windows is significantly lower in comparison to standard Helmholtz cells. Such an effect was obtained by locating the windows not at the relatively small sample cavity, but at the acoustic buffers, for which the volumes are two orders of magnitude higher. The proposed cell is dedicated for gas or liquid measurements, and its design allows for constant flow of the fluid. Hence, it can be used in continuous, real-time photoacoustic measurements. [ABSTRACT FROM AUTHOR]

High noise levels can be harmful to workers and lead to psychological and physiological ailments. A broadband noise hybridized with multiple pure tones has a high peak value and is detrimental to human hearing in an enclosed machine room. To efficiently reduce the reverberant part of the peak noise level, interest in shape optimization of a Helmholtz resonator attached to the internal wall is coming to the forefront. Research on Helmholtz resonators in dealing with a pure tone has been well-addressed. However, the development of a hybrid sound absorber used in simultaneously reducing both the pure tone and the broadband noise is rare. Therefore, we propose two kinds of hybrid sound absorbers (sound absorber A: a piece of perforate plate + a piece of sound absorbing material + air space + a Helmholtz resonator; sound absorber B: a piece of perforate plate + a piece of sound absorbing material + air space + two Helmholtz resonators) used to deal with a broadband noise hybridized with multiple tones. Here, two case studies for dealing with broadband noise hybridized with one tone and two tones will be introduced. Moreover, a numerical assessment of a sound absorption coefficient and the best optimized design for two kinds of hybrid sound absorbers under a space-constrained situation will be performed. Consequently, a successful approach in eliminating a broadband noise hybridized with multiple tones by using an optimally shaped hybrid sound absorber as well as a particle swarm method within a constrained space has been demonstrated. [ABSTRACT FROM AUTHOR]

The eigenfrequency of a Helmholtz resonator fixed at the wall of a rectangular duct is estimated. Special attention is given to determining the attached length of the resonator neck as viewed from the duct. The dependences of the attached neck length and the eigenfrequency on the duct configuration are analyzed. Theoretical results are compared with the data of numerical calculations performed by the finite-element method. [ABSTRACT FROM AUTHOR]

Some kinds of thermofluid instruments are using Helmholtz resonators for absorbing noises radiated from them. Since the resonance frequency of a Helmholtz resonator depends on the ratio of the resonant volume to the neck size, operating frequency range can be easily tuned and thus the Helmholtz resonator is well-suited for noise control in a wide range of frequency. Besides the noise control application, Helmholtz resonators have been recently applied to acoustic energy harvesting. To date, most of studies on energy harvesting using Helmholtz resonators are employing piezoelectric materials located in the resonant volume and enhancing harvesting efficiency by tuning the resonance frequency and the structure of the added piezoelectric materials. In this study, a piezoelectric (PVDF composite) cantilever was integrated within a Helmholtz resonator. Then, energy harvesting efficiency was maximized by matching and tuning mechanical resonance of the piezoelectric cantilever and acoustic resonance of the Helmholtz resonator. Theoretical analysis including finite element simulation was also performed to confirm our experimental results. To maximize the harvest power, the electrical impedance matching was considered. This study can be applied to acoustic energy harvesting as well as acoustic noise control. [ABSTRACT FROM AUTHOR]

Sound suppression in a wide waveguide with the help of a longitudinal packet resonator is studied. The underlying model takes into account the spatial interaction of fields propagating in the waveguide and the resonator. The suppression process is formalized as the problem of controlling the sound field by varying the medium parameters inside the resonator. [ABSTRACT FROM AUTHOR]

The reason for the decrease in the amplitude of longitudinal vibration combustion self-oscillations in the combustion chamber of a liquid-propellant rocket engine by means of antipulse partitions has been justified. A mathematical model of the development of combustion instability in such a chamber on attachment of a Helmholtz resonator to it has been obtained. The character of the damping of vibration combustion self-oscillations excited by the action of the Crocco mechanisms and negative thermal resistance, when varying the acoustic parameters of the resonator and of the pressure head characteristics of combustion chamber is established. [ABSTRACT FROM AUTHOR]

The attached length of orifices in reactive mufflers has been estimated based on numerical calculations by the finite-element method. The numerical results for a diaphragm in a duct are compared with the theoretical data obtained by Rayleigh, Fock, Karal, and Ingard. The dependence of the attached length on the diaphragm thickness is given. The results obtained are generalized for the case in which the orifice is a Helmholtz resonator neck. The effect of the resonator length on the attached length of the neck is analyzed. [ABSTRACT FROM AUTHOR]

An experimental study is presented of Helmholtz resonators in a dead-end waveguide representing a real hydraulic system with a centrifugal pump. With the example of the discrete spectrum components of the hydrodynamic noise of the waveguide under consideration, it is shown that the application of resonators can both reduce and increase noise levels. [ABSTRACT FROM AUTHOR]

We analyze nonlinear oscillations and waves in a simple model of a granular medium containing inclusions in the form of fluid layers and gas cavities. We show that in such a medium, the velocity of one of the wave modes is low; therefore, the nonlinearity is high and the effects of interaction are more strongly expressed than usual. [ABSTRACT FROM AUTHOR]

This paper is devoted to the study of acoustic flow in the Helmholtz resonator throat. The presented results of a complex investigation on the basis of data of a physical experiment and a mathematical modeling, their comparative analysis, and comparison with theoretical estimations provide a rather complete image of the acoustic flow formation and, in particular, enable one to obtain qualitative and quantitative dependences of the averaged velocity component in the resonator throat at different powers of an exciting sound wave. [ABSTRACT FROM AUTHOR]

The use of a Helmholtz resonator as a photoacoustic cell has many advantages. Its main drawback in high-sensitivity photoacoustic measurements is the relatively low values of Q-factors of such cells. One of the important loss sources which limit the quality factor of photoacoustic Helmholtz resonators is the rapid change of the cross section at the cavity-duct boundaries. This paper presents a design of a cell in which the shape of the duct was modified in such a way that the cross-section change is more gradual. A presented comparison of measurements of the Q-factors shows clearly that the Q-factor of the cell can be improved by almost a factor of two by such a modification of the duct shape. [ABSTRACT FROM AUTHOR]

An adjustable Helmholtz resonator in hydraulics is studied because of obvious lack of studies in the field. First the theory of a Helmholtz resonator is reviewed by examining older studies on the subject. Most of the previous studies have covered Helmholtz resonators in acoustics, but the same basic theory can be applied to hydraulics. After the theory review the test equipment and measurement methods are presented and some results are calculated analytically using the classical model and a modified model that takes the effect of spring mass into consideration. The spring mass is taken into account because the density of hydraulic oil is over 700 times greater than the density of air. In the last section the results of the calculations and measurements are discussed, and it is noted that the results of the experiments agree better with the modified model of classical Helmholtz theory. Some explanations for residual discrepancies are given but also some remarks are presented. [ABSTRACT FROM AUTHOR]

A quasilinear model of the self-oscillations in the Helmholtz resonator is developed. The conditions of existence, uniqueness, and stability of periodic solutions are determined using the Lyapunov-Poincaré, local integral manifold, and averaging methods. In the first approximation, the basic parameters of the self-oscillations are calculated and their qualitative features are studied. A thermodynamic interpretation of the gas self-oscillations in the resonator is given. [ABSTRACT FROM AUTHOR]

We study a two-dimensional analog of the Helmholtz resonator with walls of finite thickness in the critical case, for which there exists a frequency which is simultaneously the limit of poles generated both by the bounded component of the resonator and by a narrow communication channel. Under the assumption that the limit frequency is a simple frequency for the bounded component, by using the method of matched asymptotic expansions, we construct asymptotics for the two sequences of poles converging to this frequency. We obtain explicit formulas for the leading terms of the asymptotics of poles and for the solution of the scattering problem. [ABSTRACT FROM AUTHOR]

Sound absorbing elements commonly used to make quieter spaces are very desirable for high-frequency noise control. It is more attractive if it is possible to produce sound absorbing structures that perform well for a wide frequency range, especially in low-frequencies. In this regard, Helmholtz resonator array is a well-known structure but the maximum performance of each resonator unit in an array panel can only be achieved at resonance frequency that is dependent on its size. Therefore, a resonator array panel with equal size units has insufficient performance in a wide frequency range. Hence, the consideration of suitable sizes for each resonator in an array panel consisting of unequal size units is a key point to widen the frequency range of sound absorption. On the other hand, it seems that engineering science can be developed by nature inspiration. In addition, Fibonacci sequence is a famous mathematical model which describes the shape/ratio of different phenomena which exist in nature. In this paper, Fibonacci sequence is utilized to investigate Helmholtz resonator arrangement to increase both the amount of sound absorption coefficient and frequency bandwidth of absorption without changing the total volume of resonator array with equal size units. Analytical solutions based on the modified equivalent electrical circuit theory is used to calculate the sound absorption coefficient of the proposed panel with numerical experiments in MAPLE and COMSOL Multiphysics softwares to validate the proposed overall approach. Results show that the Fibonacci sequence is an optimum candidate to design a resonator-based sound absorber in low-frequency noise. [ABSTRACT FROM AUTHOR]