70 research outputs found
Non-Hermitian Acoustic Metamaterials: the role of Exceptional Points in sound absorption
Effective non-Hermitian Hamiltonians are obtained to describe coherent
perfect absorbing and lasing boundary conditions. PT -symmetry of the
Hamiltonians enables to design configurations which perfectly absorb at
multiple frequencies. Broadened and flat perfect absorption is predicted at the
exceptional point of PT -symmetry breaking while, for a particular case,
absorption is enhanced with the use of gain. The aforementioned phenomena are
illustrated for acoustic scattering through Helmholtz resonators revealing how
tailoring the non-Hermiticity of acoustic metamaterials leads to novel
mechanisms for enhanced absorption
Acoustic characterization of Hofstadter butterfly with resonant scatterers
We are interested in the experimental characterization of the Hofstadter
butterfly by means of acoustical waves. The transmission of an acoustic pulse
through an array of 60 variable and resonant scatterers periodically distribued
along a waveguide is studied. An arbitrary scattering arrangement is realized
by using the variable length of each resonator cavity. For a periodic
modulation, the structures of forbidden bands of the transmission reproduce the
Hofstadter butterfly. We compare experimental, analytical, and computational
realizations of the Hofstadter butterfly and we show the influence of the
resonances of the scatterers on the structure of the butterfly
Wave propagation in a strongly disordered 1D phononic lattice supporting rotational waves
We investigate the dynamical properties of a strongly disordered micropolar
lattice made up of cubic block units. This phononic lattice model supports both
transverse and rotational degrees of freedom hence its disordered variant
posses an interesting problem as it can be used to model physically important
systems like beam-like microstructures. Different kinds of single site
excitations (momentum or displacement) on the two degrees of freedom are found
to lead to different energy transport both superdiffusive and subdiffusive. We
show that the energy spreading is facilitated both by the low frequency
extended waves and a set of high frequency modes located at the edge of the
upper branch of the periodic case for any initial condition. However, the
second moment of the energy distribution strongly depends on the initial
condition and it is slower than the underlying one dimensional harmonic lattice
(with one degree of freedom). Finally, a limiting case of the micropolar
lattice is studied where Anderson localization is found to persist and no
energy spreading takes place
Invariant currents in lossy acoustic waveguides with complete local symmetry
We implement the concept of complete local symmetry in lossy acoustic
waveguides. Despite the presence of losses, the existence of a spatially
invariant current is shown theoretically and observed experimentally. We
demonstrate how this invariant current leads to the generalization of the Bloch
and parity theorems for lossy systems defining a mapping of the pressure field
between symmetry related spatial domains. Using experimental data we verify
this mapping with remarkable accuracy. For the performed experiment we employ a
construction technique based on local symmetries which allows the design of
setups with prescribed perfect transmission resonances in the lossless case.
Our results reveal the fundamental role of symmetries in restricted spatial
domains and clearly indicate that completely locally symmetric devices
constitute a promising class of setups, regarding the manipulation of wave
propagation.Comment: 11 pages, 5 figure
Duality of bounded and scattering wave systems with local symmetries
We investigate the spectral properties of a class of hard-wall bounded
systems, described by potentials exhibiting domain-wise different local
symmetries. Tuning the distance of the domains with locally symmetric potential
from the hard wall boundaries leads to extrema of the eigenenergies. The
underlying wavefunction becomes then an eigenstate of the local symmetry
transform in each of the domains of local symmetry. These extrema accumulate
towards eigenenergies which do not depend on the position of the potentials
inside the walls. They correspond to perfect transmission resonances of the
associated scattering setup, obtained by removing the hard walls. We argue that
this property characterizes the duality between scattering and bounded systems
in the presence of local symmetries. Our findings are illustrated at hand of a
numerical example with a potential consisting of two domains of local symmetry,
each one comprised of Dirac ? barriers.Comment: 8 pages, 6 figure
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