11,715 research outputs found
A transient boundary element method model of Schroeder diffuser scattering using well mouth impedance
Room acoustic diffusers can be used to treat critical listening environments to improve sound quality. One popular class is Schroeder diffusers, which comprise wells of varying depth separated by thin fins. This paper concerns a new approach to enable the modelling of these complex surfaces in the time domain. Mostly, diffuser scattering is predicted using steady-state, single frequency methods. A popular approach is to use a frequency domain Boundary Element Method (BEM) model of a box containing the diffuser, where the mouth of each well is replaced by a compliant surface with appropriate surface impedance. The best way of representing compliant surfaces in time domain prediction models, such as the transient BEM is, however, currently unresolved. A representation based on surface impedance yields convolution kernels which involve future sound, so is not compatible with the current generation of time-marching transient BEM solvers. Consequently, this paper proposes the use of a surface reflection kernel for modelling well behaviour and this is tested in a time domain BEM implementation. The new algorithm is verified on two surfaces including a Schroeder diffuser model and accurate results are obtained. It is hoped that this representation may be extended to arbitrary compliant locally reacting materials
A hybridizable discontinuous Galerkin method for electromagnetics with a view on subsurface applications
Two Hybridizable Discontinuous Galerkin (HDG) schemes for the solution of
Maxwell's equations in the time domain are presented. The first method is based
on an electromagnetic diffusion equation, while the second is based on
Faraday's and Maxwell--Amp\`ere's laws. Both formulations include the diffusive
term depending on the conductivity of the medium. The three-dimensional
formulation of the electromagnetic diffusion equation in the framework of HDG
methods, the introduction of the conduction current term and the choice of the
electric field as hybrid variable in a mixed formulation are the key points of
the current study. Numerical results are provided for validation purposes and
convergence studies of spatial and temporal discretizations are carried out.
The test cases include both simulation in dielectric and conductive media
Localized Modes in a Finite-Size Open Disordered Microwave Cavity
We present measurements of the spatial intensity distribution of localized
modes in a two-dimensional open microwave cavity randomly filled with
cylindrical dielectric scatterers. We show that each of these modes displays a
range of localization lengths and successfully relate the largest value to the
measured leakage rate at the boundary. These results constitute unambiguous
signatures of the existence of strongly localized electromagnetic modes in
two-dimensionnal open random media
Coherent Virtual Absorption Based on Complex Zero Excitation for Ideal Light Capturing
Absorption of light is directly associated with dissipative processes in a
material. In suitably tailored resonators, a specific level of dissipation can
support coherent perfect absorption, the time-reversed analogue of lasing,
which enables total absorption and zero scattering in open cavities. On the
contrary, the scattering zeros of lossless objects strictly occur at complex
frequencies. While usually considered non-physical due to their divergent
response in time, these zeros play a crucial role in the overall scattering
dispersion. Here, we introduce the concept of coherent virtual absorption,
accessing these modes by temporally shaping the incident waveform. We show that
engaging these complex zeros enables storing and releasing the electromagnetic
energy at will within a lossless structure for arbitrary amounts of time, under
the control of the impinging field. The effect is robust with respect to
inevitable material dissipation and can be realized in systems with any number
of input ports. The observed effect may have important implications for
flexible control of light propagation and storage, low-energy memory, and
optical modulation.Comment: To be published in Optic
Dissipative Chaos in Semiconductor Superlattices
We consider the motion of ballistic electrons in a miniband of a
semiconductor superlattice (SSL) under the influence of an external,
time-periodic electric field. We use the semi-classical balance-equation
approach which incorporates elastic and inelastic scattering (as dissipation)
and the self-consistent field generated by the electron motion. The coupling of
electrons in the miniband to the self-consistent field produces a cooperative
nonlinear oscillatory mode which, when interacting with the oscillatory
external field and the intrinsic Bloch-type oscillatory mode, can lead to
complicated dynamics, including dissipative chaos. For a range of values of the
dissipation parameters we determine the regions in the amplitude-frequency
plane of the external field in which chaos can occur. Our results suggest that
for terahertz external fields of the amplitudes achieved by present-day free
electron lasers, chaos may be observable in SSLs. We clarify the nature of this
novel nonlinear dynamics in the superlattice-external field system by exploring
analogies to the Dicke model of an ensemble of two-level atoms coupled with a
resonant cavity field and to Josephson junctions.Comment: 33 pages, 8 figure
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