Volume 134, Issue 5, November 2013
Index of content:
- STRUCTURAL ACOUSTICS AND VIBRATION 
134(2013); http://dx.doi.org/10.1121/1.4821210View Description Hide Description
An expression for the Green's function at an arbitrary set of passive locations (no applied force) is derived and validated by experiment. Three sets of points are involved, the passive reconstruction points, c, which lie on a virtual boundary and two sets of auxiliary points, denoted a and b, located either side. The reconstruction is achieved using Green's functions forming a “round trip” from and to the reconstruction points via a and b. A two stage measurement procedure is described involving excitation at b and a but with no excitation required at the reconstruction points. A known “round trip” relationship is first introduced which is theoretically exact for points on a multi-point interface between two linear, time invariant subsystems. Experimental results for frequency response functions of a beam-plate structure show that this relationship gives good results in practice. It is then shown that the theory provides an Nth order approximation for the Green's function at arbitrary points, where N is the number of points at b. The expression is validated by reconstructing point and transfer frequency response functions at two passive points on an aluminum plate.
Acoustic waveguiding by pliable conduits with axial cross sections as linear waveguides in two-dimensional sonic crystals134(2013); http://dx.doi.org/10.1121/1.4824122View Description Hide Description
Pliable conduits composed of periodically arranged concentric aluminum tori in air, with their axial cross sections acting as linear waveguides in two-dimensional sonic crystals, are numerically shown to guide acoustic waves in three dimensions in a flexible manner. Waveguide band structures are obtained by exploiting axial symmetry in a super-cell approach through two-dimensional finite-element simulations under the periodic boundary conditions. One isolated band having a bandwidth of 19.66% or 10.10% is observed for each guide, whose cross section is either in square or triangular geometry, respectively. Corresponding mode profiles indicate efficient guiding, as the acoustic energy is mainly concentrated in the hollow-core region of the guides. Transmittance spectra calculated through finite-element simulations are in agreement with the computed guiding bands. Transmittance along the waveguides with square and triangular axial cross sections around mid-band frequencies of their guiding bands varies slightly from −6.05 and −6.65 dB to −5.98 and −8.86 dB, respectively, as the guide length is increased from 10 to 200 periods. Efficient guiding across the smooth bends over circular arcs up to 90 deg is also demonstrated through three-dimensional finite-element method simulations.
Comparisons of two effective medium approaches for predicting sound scattering by periodic arrays of elastic shells134(2013); http://dx.doi.org/10.1121/1.4824340View Description Hide Description
Two effective medium models are presented and used to predict complex reflection and transmission coefficients of finite periodic arrays of resonant elastic shells as well as their effective density and bulk modulus at low frequencies. Comparisons with full multiple scattering theory and measurements show that the self-consistent model fails to correctly predict the shape of the transmission/reflection curves when scatterer resonances are close to the first Bragg bandgap. The low frequency grating model, which neglects the evanescent modes and considers scattered wave propagation only in the same direction as the incident one, gives a much better agreement with both measurements and the full multiple scattering theory. Moreover, because it does not require the wavelength to strongly exceed the size of scatterers, the model gives reliable predictions even at frequencies around the first periodicity related bandgap. In contrast to the self-consistent model, the low frequency grating model is applicable when the resonant scatterers have more than two low frequency resonances.
134(2013); http://dx.doi.org/10.1121/1.4824123View Description Hide Description
This article presents a simulation study on two integrated tuned vibration absorbers (TVAs) designed to control the global flexural vibration of lightly damped thin structures subject to broad frequency band disturbances. The first one consists of a single axial switching TVA composed by a seismic mass mounted on variable axial spring and damper elements so that the characteristic damping and natural frequency of the absorber can be switched iteratively to control the resonant response of three flexural modes of the hosting structure. The second one consists of a single three-axes TVA composed by a seismic mass mounted on axial and rotational springs and dampers, which are arranged in such a way that the suspended mass is characterized by uncoupled heave and pitch-rolling vibrations. In this case the three damping and natural frequency parameters of the absorber are tuned separately to control three flexural modes of the hosting structure. The simulation study shows that the proposed single-unit absorbers produce, respectively, 5.3 and 8.7 dB reductions of the global flexural vibration of a rectangular plate between 20 and 120 Hz.