Propagation Of Low-frequency Acoustic Waves Research Articles

Asymmetric propagation of low-frequency acoustic waves in a granular chain using asymmetric intruders

This paper investigates the asymmetric propagation of acoustic waves in a one-dimensional chain of spherical particles coupled with asymmetric intruders. The chain passes acoustic waves along one direction (forward configuration) whereas partially blocks the acoustic energy propagating along the opposite direction (reverse configuration). A numerical analysis is conducted to investigate this asymmetric propagation by simulating a statically compressed chain of particles interacting via Hertzian contact and subjected to small amplitude periodic displacements at one end. The amount of acoustic energy transmitted through the chain in both configurations is studied and quantified in terms of the acoustic energy transfer ratio, defined as the ratio of the acoustic power at the last particle to the acoustic power at the first particle. The effect of the applied frequency and number of particles in the chain on the transfer ratio is investigated. In addition, a parametric study is performed to evaluate the effects of geometric and material properties on the efficiency of asymmetric acoustic wave propagation in the proposed system. The results show that the proposed design supports asymmetric propagation of low frequency acoustic waves.

Transmission of Low-Frequency Acoustic Waves in Seawater Piping Systems with Periodical and Adjustable Helmholtz Resonator

The characteristics of acoustic wave transmitting in a metamaterial-type seawater piping system are studied. The metamaterial pipe, which consists of a uniform pipe with air-water chamber Helmholtz resonators (HRs) mounted periodically along its axial direction, could generate a wide band gap in the low-frequency range, rendering the propagation of low-frequency acoustic waves in the piping system dampened spatially. Increasing the air volume in the Helmholtz chamber would result in a sharply decrease in the central frequency of the resonant gap and an extension in the bandwidth in the beginning, yet very slowly as the air volume is further augmented. Acoustic waves will experience a small amount of energy loss if the acoustic–structure interaction effect is considered. Also, the structure-borne sound will be induced because of the interaction effects. High pressure loadings on the system may bring in a shrink in the band gap; nevertheless, the features of broad band gaps of the system is still be maintained.

Impedance assessment of a dual-resonance acoustic liner

Acoustic liners are commonly used to reduce noise from commercial aircraft engines. Engine liners are placed in the nacelle inlet and aft bypass duct to attenuate the noise radiated from the engine. Traditional engine liners are constructed of a perforated facesheet over a honeycomb structure to create a quarter-wave absorber. With this design, the low frequency performance of the liner is limited by the depth of the honeycomb. However, with advances in engine design, lower frequency sound absorption is becoming more critical while liner depth must be minimized. Acoustic metamaterials can exhibit unique acoustic behavior using periodically arranged sub-wavelength resonators. Researchers have shown that acoustic metamaterials can effectively block the propagation of low-frequency acoustic waves. Therefore, acoustic metamaterial-inspired concepts are being investigated to improve the low frequency performance of engine liners. A proposed dual-resonance liner is presented here that combines the idea of a Helmholtz resonator metamaterial with a traditional quarter-wave acoustic liner. The low frequency acoustic absorption of a traditional liner can be significantly increased by adding a second, low frequency resonance to the system. The normal incidence absorption coefficient of the proposed liner is more than 10 times larger than a conventional honeycomb liner at the designed Helmholtz resonance frequency while retaining similar performance at higher frequencies.

Atmospheric ducts can transport sound in two directions

When conditions are right, temperature gradients and fast jets of wind can help to establish atmospheric ducts—pathways in the atmosphere that promote the propagation of low‐frequency acoustic waves (infrasound)—across long distances. Atmospheric ducts are part of the basis behind over‐the‐horizon radar, the source of some particularly clear mirages, and a channel through which sounds can travel relatively unperturbed across vast distances.

Impedance assessment of an acoustic metamaterial-inspired acoustic liner

Acoustic liners are commonly used to reduce noise from commercial aircraft engines. Engine liners are placed in the nacelle inlet and aft bypass duct to attenuate the radiated noise from the fan, turbine, combustor, and other sources within the engine. Current engine liners are constructed of a metal perforate facesheet over a honeycomb structure. With this design, the low frequency performance of the liner is limited by the depth of the honeycomb. However, with advances in engine design, lower frequency performance is becoming more critical. Acoustic metamaterials can exhibit unique acoustic behavior using periodically arranged sub-wavelength resonators. Researchers have shown that metamaterials can effectively block the propagation of low-frequency acoustic waves. Therefore, acoustic metamaterial-inspired concepts are being investigated to improve the low frequency performance of engine liners. By combining the idea of a split-ring resonator metamaterial with a traditional quarter-wave acoustic liner, the low frequency acoustic absorption of the device can be significantly increased. The normal incident absorption coefficient of a metamaterial-inspired liner shows an increase from 0.05 to 0.8 at 600 Hz relative to a conventional honeycomb liner with the same depth while retaining the same 0.5 absorption coefficient at 1550 Hz.

Application of the nearly perfectly matched layer to the propagation of low-frequency acoustic waves

Seismic modelling is a very effective method to help us understand the characteristics of the seismic wave propagation in the Earth's interior. However, one tough problem is the presence of spurious reflections from the boundaries of the truncated computational domain, especially the reflections of low-frequency energy. In this paper, we apply the nearly perfectly matched layer (NPML) technique to suppress the artificial reflections from the edges of the 2D model in the case of acoustic waves of a low-frequency source (5 Hz). For the implementation of the finite-difference operator, we employ fourth-order accuracy methods in space and second-order accuracy methods in time. Through the numerical comparison between the NPML and the technique known as the convolutional perfectly matched layer (CPML), which is considered the boundary condition of optimum absorbing, we demonstrate that the NPML has better absorbing performance than the CPML. We suggest that a greater amount of analysis will be needed to study how NPML works facing the wide variety of complex media that are commonly used in seismic modelling.

Low-frequency surface wave propagation and the viscoelastic behavior of porcine skin

A physical model describing the propagation of low-frequency surface waves in relation to the viscoelastic behavior of porcine skin is presented, along with a series of empirical studies testing the performance of the model. The model assumes that the skin behaves as a semi-infinite, locally isotropic, viscoelastic half-space. While the assumption of a semi-infinite body is violated, this violation does not appear to have a significant impact on the performance of the model based on the empirical studies. 1-Hz surface waves in the skin propagate primarily as Rayleigh waves with a wavelength and velocity of approximately 3 m and 3.0 m/s, respectively. The amplitude of the acoustic wave, as measured by tracking the acoustic stress wave-induced shift in a backscattered laser speckle pattern, decreases exponentially with lateral distance from the acoustic source. Using this model of surface wave propagation, the mechanical loss factor or tan delta of the skin is measured to be on the order of 0.14+/-0.07. The results presented are consistent with earlier works on the propagation of low-frequency acoustic waves in biological tissues, and should serve as a theoretical and empirical basis for using the wave characteristics of propagating surface waves in combination with the mechanical behavior of the tissue for biomechanical studies and for potential diagnostic applications.

Local coupled modes and volume scattering in heterogeneous anisotropic shallow water environments

Seafloor bottom/subbottom interactions are significant for low frequency acoustic wave propagation in shallow water environments. We investigate the significance of deterministic and stochastic volume scattering for generally anisotropic seafloor bottoms. The following assumptions are made: (i) the model is laterally heterogeneous with an elastic bottom/subbottom, (ii) anisotropy is hexagonally symmetric, (iii) the bottom/subbottom structure is dominated by deterministic elastic moduli with additional small-scale stochastic variations. The local coupled mode rough interface scattering theory of Park and Odom [Geophys. J. Int. 136, 123 (1999)] is extended by including volume scattering terms. New volume scattering terms are derived by applying perturbation theory to the elastic equations of motion. The acoustic wavefield is expressed as a sum of a mean deterministic wavefield and a small-scale scattered wavefield. Perturbations in the elastic moduli contribute to energy loss and the loss of signal coherence. The local mode formalism brings insight into the efficiency of volume scattering by indicating directly how much energy of coherent mode q is redistributed into scattered mode r. Numerical calculations indicating the effects of volume scattering are presented. [Work supported by ONR.]

Impulse and Low Frequency Acoustic Wave Propagation in Granular Beds

ABSTRACTThe study of sound propagation in granular beds at frequencies exceeding a MHz has been a subject of study for many years. Much remains to be learnt about sound propagation at lower frequencies. We shall present our studies on the problem of impulse propagation and briefly comment on low frequency acoustic propagation in model granular beds using particle dynamical simulations. The following results will be briefly discussed. (i) Impulses propagating as solitary waves in 1-D granular chains with Hertz contacts in the absence of precompression. (ii) The effects of uniform precompression and gravitational loading on wave propagation. (iii) Impulse propagation in 3-D granular beds. The research presented shall highlight the intrinsically nonlinear nature of wave propagation in granular beds.

Linear and Nonlinear Acoustic Wave Propagation in the Atmosphere

This paper describes the investigation of the acoustic wave-propagation theory and numerical implementation for the situation of an isothermal atmosphere. A one-dimensional model to validate an asymptotic theory and an axisymmetric three-dimensional situation to relate to a realistic situation are considered. In addition, nonlinear wave propagation and the numerical treatment are included. It is known that the gravitational effects play a crucial role in the low-frequency acoustic wave propagation. They propagate large distances and, as such, the numerical treatment of those problems becomes difficult in terms of posing boundary conditions that are valid for all frequencies. Our treatment is discussed in detail. Open questions are posed.

Observations of the effects of atmospheric turbulence on low- frequency sound propagation

The propagation of low-frequency acoustic waves in the lower atmosphere is studied using observations of explosive sound sources at ranges of 4.3 and 9.3 km. By observing groups of 30 firings spaced closely in time, the effects of atmospheric turbulence on the propagation are determined; the differences in arrival time between microphone pairs in an approximately 1-km array being the experimental parameter measured. The time difference distributions are found to be Gaussian with standard deviations which show a strong diurnal variation, the increased effect of atmospheric turbulence being evident in mid-day observations. Propagation over nearby paths is shown to be spatially correlated out to separations of several hundred meters and this correlation is shown to be in agreement with the theoretical predictions of Chernov [Wave Propagation in a Random Medium, L. A. Chernov (McGraw–Hill, New York, 1960)]. Using these theoretical relations, atmospheric scale sizes in the range from 200 to 500 m (mean 320 m) are obtained together with acoustic velocity fluctuations with standard deviations in the range from 0.5 to 1.2 m/sec (mean 0.8 m/sec). Subject Classification: [43]28.40, [43]28.60.