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Acoustic silencers are the most effective solution to control noise in ducts. In this paper, we propose a compact acoustic meta-silencer (AMS) based on the synergy between reactive and resistive units that enables the reduction of low-frequency and broadband noise. We first propose a conceptual AMS comprising simple reactive and resistive units to verify its unique sound attenuation performance. To explore its potential for application, we then propose an advanced AMS unit consisting of two independent annular chambers that represent reactive and resistive units, respectively. The synergistic mechanism between reactive and resistive units to achieve superior sound attenuation is revealed. Next, the band structures of the infinite periodic advanced AMS are discussed, and three different types of advanced AMS containing six units are examined. It is demonstrated numerically and experimentally that the optimized AMS with a compact size can achieve a transmission loss higher than 15 dB over a super-wide low-frequency range (290â€"1344 Hz). The work here provides a new avenue for the design of low-frequency and broadband meta-silencers to control the noise in ducts. [ABSTRACT FROM AUTHOR]

We propose a composite acoustic metamaterial consisting of double layer metamaterial plates (DLMPs) lined with porous material (PM). Numerical results show that DLMPs lined with PM can produce a very high diffuse field sound transmission loss (STL) beyond the mass law over an ultra-wide low-frequency range. The unusual behavior is interpreted by explicit formulas. For validation, a large-scale specimen is fabricated using light surface mass density (5.72 kg/m2) and small thickness (5 cm). Measured data show that the specimen possesses an excellent diffuse field STL deeply breaking the mass law at broadband low frequencies (248–1624 Hz), and simultaneously, it has an improved diffuse field STL within a wide low-frequency band (208–850 Hz) compared with the conventional double layer homogeneous plates lined with PM. Owing to the simple construction, light weight, and thinness, the proposed composite metamaterial has promising applications in noise control engineering. [ABSTRACT FROM AUTHOR]

Improvement of sound absorption and insulation using a double-layer metamaterial (DM) in the acoustic free field is proposed in the present paper. The front layer of the DM is composed of a flexible micro-perforated plate with periodic local resonators, the backing layer is a flexible plate attached with periodic local resonators too, and an air gap exists between the double plates. Good agreement is gained between the results of the theoretical prediction and finite element simulation for the DM and the original double-layer plate (DP) while considering the vibroacoustic coupling between the sound excitation and the plates. Both theoretical and simulation results verify that the local resonators can improve the sound absorption and insulation of the DP. The underlying mechanism of the DM is investigated using the acoustic impedance and displacement pattern. Filling the gap with the porous material can further improve the sound absorption and insulation of the DM. Finally, the influences of the number and additional mass ratio of local resonators on the acoustic performance of the DM are investigated, and the practical realization of the DM is verified. The present design shows great potential for practical noise reduction in the free field. [ABSTRACT FROM AUTHOR]

Existing research shows that acoustic BG in a certain frequency range can be realized by installing an expansion chamber on duct system, but the problems of broadband and size limitations at low frequencies remain to be researched. The study of acoustic and elastic wave propagation in artificial periodic structures has received increasing attention for many decades, and the presence of bandgap (BG) in phononic crystals (PCs), which inhibits elastic/acoustic wave propagation within the BG' frequency range, supplies a new way to control noise and vibrations in duct system. Based on PC theory, a duct silencer backed with a gas–liquid expansion chamber is proposed to enhance the acoustic performance of low-frequency noise attenuation. The transfer matrix method (TMM) is used to investigate the acoustic BG properties. The influences on the BG properties of some key parameters are analyzed, and the band formation mechanism is revealed by the law of energy conservation. The results show that silencers with a small size can effectively attenuate ultra-low frequencies and ultra-broad bands. [ABSTRACT FROM AUTHOR]

We propose a hybrid acoustic metamaterial as a super absorber for a relatively broadband low-frequency sound based on a simple construction with deep-subwavelength thickness (5 cm). The hybrid metamaterial absorber is carefully designed and constructed based on a microperforated panel (MPP) and coiled-up Fabry–Pérot channels. It is demonstrated analytically, numerically, and experimentally that over 99% of acoustic absorption could be achieved at a resonance frequency (<500 Hz) with the working wavelength about 30 times larger than its total thickness. It is revealed that the superior absorption is mainly caused by the friction losses of acoustic wave energy in the MPP. The frequency of the absorption peak could be tuned by adjusting the geometry parameters of the MPP and the channel folding numbers. The relative absorption bandwidth could also be tuned flexibly (up to 82%) with a fixed deep-subwavelength thickness (5 cm). The absorber has wide potential applications in noise control engineering due to its deep-subwavelength thickness, relatively broad bandwidth, and easy fabrication. [ABSTRACT FROM AUTHOR]

• Metastructure with low-frequency sound insulation and high-stiffness properties. • Comprehensive analysis of effects of design parameters on STL performance. • Design guideline to achieve broader bandwidth of low-frequency STL. • Experimental verification of sound insulation and high-stiffness properties. According to the mass law, it is impossible to increase sound transmission loss (STL) of conventional structures at low frequencies without increasing their weight. Metastructures are capable to break the limits of the mass law at low frequencies. However, many existing sound insulation metastructures need to be constructed using a host structure with low-stiffness properties, such as a thin plate and a thin membrane. As a result, the metastructures normally have large exposed areas with low-stiffness characteristics and are unable to bear heavy loads, which limits their practical applications. In this work, we propose a high-stiffness plate-type metastructure (HSPM) with both low-frequency sound insulation performance and high stiffness properties. The STL performance of the HSPM is demonstrated analytically, numerically and experimentally, indicating that the HSPM can deeply break the mass law at low frequencies. Owing to the simple construction, high stiffness properties, and high sound insulation performance at low frequencies, the proposed HSPM has promising applications in practical noise control engineering. [ABSTRACT FROM AUTHOR]

This paper describes a duct mounted with multiple hybrid Helmholtz resonators (HHRs) for low-frequency noise control in a piping system. The newly designed HHR is composed of a cavity and a connective neck with a coupling membrane and a perforated plate. Based on the transfer matrix method and equivalent mass-spring model of the membrane, the acoustic impedance of the HHR can be obtained. Sound radiation performance of the unit cell is carefully investigated using a numerical simulation method. Besides, the transmission loss (TL) and bandgap behavior of finite and infinite pipe can be evaluated, respectively. All the theoretical results are verified with corresponding numerical simulation. It is shown that, compared with traditional HR (THR), the HHR exhibit superior TL performance in the low-frequency region, and the energy radiated through the membrane at the first-order resonant frequency is relatively small. Damping caused by the perforated plate has a considerable impact on both the TL and band structure of the periodic pipe. The underlying physical mechanism is then revealed in combination with acoustic modes in the band structure. Additionally, the effect of the perforated plate on the bandgap coupling characteristics is also examined. Eventually, experimental study is also carried out to validate the acoustic performance of the HHRs. Summarizing, this study provides new insights into noise elimination in periodic piping systems. [ABSTRACT FROM AUTHOR]

• Closed-form formulas for direct estimation of the lowest two bandgaps. • Closed-form design formulas for ultra-wide near-coupled pseudo bandgap. • Closed-form design formulas for LR bandgap with maximum attenuation at its center. • Numerical verification of the formulas via design of multi-scale metastructure. Metastructures have received increasing attention because of their unique bandgap behavior that can be utilized to control vibration and noise of mechanical structures. This work is aimed to develop efficient formulas for bandgap estimation and design of flexural beam-type metastructures containing a periodic array of single-frequency force resonators. First, closed-form formulas are developed for a fast yet accurate estimation of the lowest two bandgaps, which may be formed by local resonance (LR) mechanism or Bragg scattering (BS) mechanism. Such formulas are shown to be more general than existing well-known closed-form formulas, which are only applicable to the case of deep-subwavelength LR-type bandgap. Second, closed-form formulas are proposed for a straightforward design of two particular cases of improved bandgaps compared with typical deep-subwavelength LR-type bandgap. One particular case is an ultra-wide near-coupled pseudo bandgap formed by strong interaction of the LR and BS mechanisms. Another particular case is an LR-type bandgap possessing significant attenuation performance at its bandgap center, and also having a broader effective width than the typical deep-subwavelength LR-type bandgap. Moreover, the broad applicability of the developed closed-form formulas is numerically verified by addressing bandgap design problems associated with simple and advanced multi-scale beam-type metastructures. [ABSTRACT FROM AUTHOR]