9,234 research outputs found
Coping with speaker-related variation via abstract phonemic categories
Listeners can cope with considerable variation in the way that different speakers talk. We argue here that they can do so because of a process of phonological abstraction in the speech-recognition system. We review evidence that listeners adjust the bounds of phonemic categories after only very limited exposure to a deviant realisation of a given phoneme. This learning can be talker-specific and is stable over time; further, the learning generalizes to previously unheard words containing the deviant phoneme. Together these results suggest that the learning involves adjustment of prelexical phonemic representations which mediate between the speech signal and the mental lexicon during word recognition. We argue that such an abstraction process is inconsistent with claims made by some recent models of language processing that the mental lexicon consists solely of multiple detailed traces of acoustic episodes. Simulations with a purely episodic model without functional prelexical abstraction confirm that such a model cannot account for the evidence on lexical generalization of perceptual learning. We conclude that abstract phonemic categories form a necessary part of lexical access, and that the ability to store talker-specific knowledge about those categories provides listeners with the means to deal with cross-talker variation
Evaluations of Space Shielding
A general method of evaluating the efficiency of space vehicle shielding is developed and used to compare various active and passive systems for protection against ionizing radiation. Available permanent magnets are found useless for active shielding, and combined active-passive systems in general are determined to be inefficient. On the other hand, evaluations show that active electrostatic shielding may have possibilities for weight savings if electrical conditions (presently unknown) are favorable therefor in space. Further, a positive potential improvement is calculated for an active shielding system which utilizes superconducting Nb3Sn to provide a confined magnetic flux to deflect incident charged particles; this potential points toward substantial reductions in shield weight for the protection of large vehicles from highly energetic particles. Recommendations are made for further research, particularly for flight experiments to measure directionality of solar flare protons
Realistic Ionizing Fluxes for Young Stellar Populations from 0.05 to twice solar metallicity
We present a new grid of ionizing fluxes for O and Wolf-Rayet stars for use
with evolutionary synthesis codes and single star H II region analyses. A total
of 230 expanding, non-LTE, line-blanketed model atmospheres have been
calculated for five metallicities (0.05, 0.2, 0.4, 1 and 2 solar) using the
WM-basic code of Pauldrach et al. (2001) and the CMFGEN code of Hillier &
Miller (1998). The stellar wind parameters are scaled with metallicity for both
O and W-R stars. We incorporate the new models into Starburst99 (Leitherer et
al. 1999) and compare the ionizing outputs with Schaerer & Vacca (1998) and
Leitherer et al. (1999). The changes in the output ionizing fluxes are
dramatic, particularly below 228 A. We also find lower fluxes in the He I
continuum for Z > 0.4 solar and ages < 7 Myr because of the increased line
blanketing. We test the accuracy of the new models by constructing
photoionization models. We show that for the dwarf O star grid, He I 5876/H
beta decreases between Z = 1 and twice solar in a similar manner to
observations (e.g. Bresolin et al. 1999) due to the increased effect of line
blanketing. We therefore suggest that a lowering of the upper mass limit at
high abundances is not required to explain the observations. For the case of an
instantaneous burst, we plot the softness parameter "eta prime" against the
abundance indicator R_23. The new models are coincident with the data of
Bresolin et al. (1999), particularly during the W-R phase, unlike previous
models which over-predict the hardness of the ionizing radiation.Comment: 21 pages, 15 postscript colour figures, includes mn2e.cls. To be
published in MNRAS. Revised version containing modifications to Tables 1-
Overlay copy technique to provide high-contrast electron micrographs for automatic metallographic analysis
Overlay copy technique to provide high-contrast electron micrographs for automatically determining quantitative microstructure of multiphase specimen
Approximation of the acoustic radiation impedance of a cylindrical pipe
Useful approximation formulae for radiation impedance are given for the
reflection coefficients of both infinitely flanged and unflanged rigid-walled
cylindrical ducts. The expressions guarantee that simple but necessary physical
and mathematical principles are met, like hermitian symmetry for the reflection
coefficient (identical behaviour of positive and negative frequencies) and
causality for the impulse response. A non causal but more accurate expression
is also proposed that is suitable for frequency-domain applications. The
formulae are obtained by analytical and numerical fitting to reference results
from Levine & Schwinger for the unflanged case and extracted from the radiation
impedance matrix given by Zorumski for the infinite flanged case.Comment: Journal of Sound and Vibration (2008) accepte
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A tale of one city: intra-institutional variations in migrating VLE platform
City University London committed in 2009 to make Moodle the Virtual Learning Environment (VLE) at the core of a new Strategic Learning Environment (SLE) comprised of VLE, externally facing website and related systems such as video streaming and virtual classrooms. Previously, the WebCT VLE had been separate from most of the other systems at the institution with very limited connections to other tools. Each of the schools within the institution was able to pursue their own strategy and timeframe for the migration and embedding of Moodle within their subject areas, within an absolute limit of 2 years. This paper outlines the approaches taken by the various schools, highlighting similarities and differences, and draws out common aspects from the project to make recommendations for institutions seeking to undertake similar migrations
Gravitationally Lensed Gamma-Ray Bursts as Probes of Dark Compact Objects
If dark matter in the form of compact objects comprises a large fraction of
the mass of the universe, then gravitational lensing effects on gamma-ray
bursts are expected. We utilize BATSE and Ulysses data to search for lenses of
different mass ranges, which cause lensing in the milli, pico, and femto
regimes. Null results are used to set weak limits on the cosmological abundance
of compact objects in mass ranges from 10 to 10 . A
stronger limit is found for a much discussed universe dominated
by black holes of masses , which is ruled out at the
90% confidence level.Comment: 14 pages, 4 figures, fixed minor corrections. Accepted for
publication in ApJ(L
Elastic moduli approximation of higher symmetry for the acoustical properties of an anisotropic material
The issue of how to define and determine an optimal acoustical fit to a set
of anisotropic elastic constants is addressed. The optimal moduli are defined
as those which minimize the mean squared difference in the acoustical tensors
between the given moduli and all possible moduli of a chosen higher material
symmetry. The solution is shown to be identical to minimizing a Euclidean
distance function, or equivalently, projecting the tensor of elastic stiffness
onto the appropriate symmetry. This has implications for how to best select
anisotropic constants to acoustically model complex materials.Comment: 20 page
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