19,088 research outputs found
Binaural Cues for Distance and Direction of Nearby Sound Sources
To a first-order approximation, binaural localization cues are ambiguous: a number of source locations give rise to nearly the same interaural differences. For sources more than a meter from the listener, binaural localization cues are approximately equal for any source on a cone centered on the interaural axis (i.e., the well-known "cones of confusion"). The current paper analyzes simple geometric approximations of a listener's head to gain insight into localization performance for sources near the listener. In particular, if the head is treated as a rigid, perfect sphere, interaural intensity differences (IIDs) can be broken down into two main components. One component is constant along the cone of confusion (and thus co varies with the interaural time difference, or ITD). The other component is roughly constant for a sphere centered on the interaural axis and depends only on the relative pathlengths from the source to the two ears. This second factor is only large enough to be perceptible when sources are within one or two meters of the listener. These results are not dramatically different if one assumes that the ears are separated by 160 degrees along the surface of the sphere (rather than diametrically opposite one another). Thus, for sources within a meter of the listener, binaural information should allow listeners to locate sources within a volume around a circle centered on the interaural axis, on a "doughnut of confusion." The volume of the doughnut of confusion increases dramatically with angle between source and the interaural axis, degenerating to the entire median plane in the limit.Air Force Office of Scientific Research (F49620-98-1-0108
Radiation recoil from highly distorted black holes
We present results from numerical evolutions of single black holes distorted
by axisymmetric, but equatorially asymmetric, gravitational (Brill) waves. Net
radiated energies, apparent horizon embeddings, and recoil velocities are shown
for a range of Brill wave parameters, including both even and odd parity
distortions of Schwarzschild black holes. We find that a wave packet initially
concentrated on the black hole throat, a likely model also for highly
asymmetric stellar collapse and late stage binary mergers, can generate a
maximum recoil velocity of about 150 (23) km/sec for even (odd) parity
perturbations, significantly less than that required to eject black holes from
galactic cores.Comment: 15 pages, 8 figure
Cosmic spherical void via coarse-graining and averaging non-spherical structures
Inhomogeneous cosmological models are able to fit cosmological observations
without dark energy under the assumption that we live close to the "center" of
a very large-scale under-dense region. Most studies fitting observations by
means of inhomogeneities also assume spherical symmetry, and thus being at (or
very near) the center may imply being located at a very special and unlikely
observation point. We argue that such spherical voids should be treated only as
a gross first approximation to configurations that follow from a suitable
smoothing out of the non-spherical part of the inhomogeneities on angular
scales. In this Letter we present a toy construction that supports the above
statement. The construction uses parts of the Szekeres model, which is
inhomogeneous and anisotropic thus it also addresses the limitations of
spherical inhomogeneities. By using the thin-shell approximation (which means
that the Israel-Darmois continuity conditions are not fulfilled between the
shells) we construct a model of evolving cosmic structures, containing several
elongated supercluster-like structures with underdense regions between them,
which altogether provides a reasonable coarse-grained description of cosmic
structures. While this configuration is not spherically symmetric, its proper
volume average yields a spherical void profile of 250 Mpc that roughly agrees
with observations. Also, by considering a non-spherical inhomogeneity, the
definition of a "center" location becomes more nuanced, and thus the
constraints placed by fitting observations on our position with respect to this
location become less restrictive.Comment: 6 pages, 5 figures, accepted for publication in Phys. Lett.
Inertial waves in a differentially rotating spherical shell
We investigate the properties of small-amplitude inertial waves propagating
in a differentially rotating incompressible fluid contained in a spherical
shell. For cylindrical and shellular rotation profiles and in the inviscid
limit, inertial waves obey a second-order partial differential equation of
mixed type. Two kinds of inertial modes therefore exist, depending on whether
the hyperbolic domain where characteristics propagate covers the whole shell or
not. The occurrence of these two kinds of inertial modes is examined, and we
show that the range of frequencies at which inertial waves may propagate is
broader than with solid-body rotation. Using high-resolution calculations based
on a spectral method, we show that, as with solid-body rotation, singular modes
with thin shear layers following short-period attractors still exist with
differential rotation. They exist even in the case of a full sphere. In the
limit of vanishing viscosities, the width of the shear layers seems to weakly
depend on the global background shear, showing a scaling in E^{1/3} with the
Ekman number E, as in the solid-body rotation case. There also exist modes with
thin detached layers of width scaling with E^{1/2} as Ekman boundary layers.
The behavior of inertial waves with a corotation resonance within the shell is
also considered. For cylindrical rotation, waves get dramatically absorbed at
corotation. In contrast, for shellular rotation, waves may cross a critical
layer without visible absorption, and such modes can be unstable for small
enough Ekman numbers.Comment: 31 pages, 16 figures, accepted for publication in Journal of Fluid
Mechanic
Intersubject Regularity in the Intrinsic Shape of Human V1
Previous studies have reported considerable intersubject variability in the three-dimensional geometry of the human primary visual cortex (V1). Here we demonstrate that much of this variability is due to extrinsic geometric features of the cortical folds, and that the intrinsic shape of V1 is similar across individuals. V1 was imaged in ten ex vivo human hemispheres using high-resolution (200 ÎĽm) structural magnetic resonance imaging at high field strength (7 T). Manual tracings of the stria of Gennari were used to construct a surface representation, which was computationally flattened into the plane with minimal metric distortion. The instrinsic shape of V1 was determined from the boundary of the planar representation of the stria. An ellipse provided a simple parametric shape model that was a good approximation to the boundary of flattened V1. The aspect ration of the best-fitting ellipse was found to be consistent across subject, with a mean of 1.85 and standard deviation of 0.12. Optimal rigid alignment of size-normalized V1 produced greater overlap than that achieved by previous studies using different registration methods. A shape analysis of published macaque data indicated that the intrinsic shape of macaque V1 is also stereotyped, and similar to the human V1 shape. Previoud measurements of the functional boundary of V1 in human and macaque are in close agreement with these results
Shape Coexistence in 78 Ni and the new Island of Inversion
Large Scale Shell Model calculations (SM-CI) predict that the region of
deformation which comprises the heaviest Chromium and Iron isotopes at and
beyond N=40 will merge with a new one at N=50 in an astonishing parallel to the
N=20 and N=28 case in the Neon and Magnesium isotopes. We propose a valence
space including the full pf-shell for the protons and the full sdg shell for
the neutrons; which represents a comeback of the the harmonic oscillator shells
in the very neutron rich regime. The onset of deformation is understood in the
framework of the algebraic SU3-like structures linked to quadrupole dominance.
Our calculations preserve the doubly magic nature of the ground state of 78 Ni,
which, however, exhibits a well deformed prolate band at low excitation energy,
providing a striking example of shape coexistence far from stability
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