28,094 research outputs found
Vacuum-isolation vessel and method for measurement of thermal noise in microphones
The vacuum isolation vessel and method in accordance with the present invention are used to accurately measure thermal noise in microphones. The apparatus and method could be used in a microphone calibration facility or any facility used for testing microphones. Thermal noise is measured to determine the minimum detectable sound pressure by the microphone. Conventional isolation apparatus and methods have been unable to provide an acoustically quiet and substantially vibration free environment for accurately measuring thermal noise. In the present invention, an isolation vessel assembly comprises a vacuum sealed outer vessel, a vacuum sealed inner vessel, and an interior suspension assembly coupled between the outer and inner vessels for suspending the inner vessel within the outer vessel. A noise measurement system records thermal noise data from the isolation vessel assembly. A vacuum system creates a vacuum between an internal surface of the outer vessel and an external surface of the inner vessel. The present invention thus provides an acoustically quiet environment due to the vacuum created between the inner and outer vessels and a substantially vibration free environment due to the suspension assembly suspending the inner vessel within the outer vessel. The thermal noise in the microphone, effectively isolated according to the invention, can be accurately measured
Revealing Comparative Advantage: Chaotic or Coherent Patterns Across Time and Sector and U.S. Trading Partner?
We map United States comparative advantage between 1980 and 1995, by trading partner and region, using Balassa's export-based index of Revealed Comparative Advantage (RCA). We find: temporally stable and ubiquitous US comparative advantage in differentiated producer goods (except disadvantage in Japan); somewhat less stable and less sweeping US disadvantage in standardized producer goods; chaotic and diverse patterns of US RCA in consumer goods (especially in the Chinese market). Our most significant findings are surprisingly sharp geographical differences in patterns of US RCA and surprisingly small differences across sub-sectors of 1, 2, and 3-digit SITC classifications - regional, but not sectoral, niche' specialization. The high overall variability across regions in RCA indexes seems unrelated to obvious explanations such as proximity or lingual/historical ties to the US. In producer goods, RCA variability across regions correlates somewhat better with accounts of trade diversion and of regional preferences for and discrimination against US exports. We find only scant evidence of high or increasing variability across disaggregated commodity sub-groups in US RCA indexes. Such variability is often the prediction of theories of comparative advantage that are based on vertical specialization, product differentiation, or scale and agglomeration economies.
Regressed relations for forced convection heat transfer in a direct injection stratified charge rotary engine
Currently, the heat transfer equation used in the rotary combustion engine (RCE) simulation model is taken from piston engine studies. These relations have been empirically developed by the experimental input coming from piston engines whose geometry differs considerably from that of the RCE. The objective of this work was to derive equations to estimate heat transfer coefficients in the combustion chamber of an RCE. This was accomplished by making detailed temperature and pressure measurements in a direct injection stratified charge (DISC) RCE under a range of conditions. For each specific measurement point, the local gas velocity was assumed equal to the local rotor tip speed. Local physical properties of the fluids were then calculated. Two types of correlation equations were derived and are described in this paper. The first correlation expresses the Nusselt number as a function of the Prandtl number, Reynolds number, and characteristic temperature ratio; the second correlation expresses the forced convection heat transfer coefficient as a function of fluid temperature, pressure and velocity
Tidal Excitation of Modes in Binary Systems with Applications to Binary Pulsars
We consider the tidal excitation of modes in a binary system of arbitrary
eccentricity. For a circular orbit, the modes generally undergo forced
oscillation with a period equal to the orbital period (). For an eccentric
orbit, the amplitude of each tidally excited mode can be written approximately
as the sum of an oscillatory term that varies sinusoidally with the mode
frequency and a `static' term that follows the time dependence of the tidal
forcing function. The oscillatory term falls off exponentially with increasing
\b (defined as the ratio of the periastron passage time to the mode period),
whereas the `static' term is independent of \b. For small \b modes (\b
\approx 1), the two terms are comparable, and the magnitude of the mode
amplitude is nearly constant over the orbit. For large \b modes (\b \gta a
few), the oscillatory term is very small compared to the `static' term, in
which case the mode amplitude, like the tidal force, varies as the distance
cubed. For main sequence stars, , , and low order -modes generally
have large \b and hence small amplitudes of oscillation. High overtone
-modes, however, have small overlap with the tidal forcing function. Thus,
we expect an intermediate overtone -mode with \b \sim 1 to have the
largest oscillation amplitude. The dependence on mode damping and the stellar
rotation rate is considered, as well as the effects of orbital evolution. We
apply our work to the two binary pulsar system: PSR J0045-7319 and PSR
B1259-63.Comment: 28 pages of uuencoded compressed postscript. 9 postscript figures
available by anonymous ftp from ftp://brmha.mit.edu/ To be published in ApJ
Floquet engineering of long-range p-wave superconductivity: Beyond the high-frequency limit
It has been shown that long-range {\it p}-wave superconductivity in a Kitaev
chain can be engineered via an ac field with a high frequency [Benito et al.,
Phys. Rev. B 90, 205127 (2014)]. For its experimental realization, however,
theoretical understanding of Floquet engineering with a broader range of
driving frequencies becomes important. In this work, focusing on the ac-driven
tunneling interactions of a Kitaev chain, we investigate effects from the
leading correction to the high-frequency limit on the emergent {\it p}-wave
superconductivity. Importantly, we find new engineered long-range {\it p}-wave
pairing interactions that can significantly alter the ones in the
high-frequency limit at long interaction ranges. We also find that the leading
correction additionally generates nearest-neighbor {\it p}-wave pairing
interactions with a renormalized pairing energy, long-range tunneling
interactions, and in particular multiple pairs of Floquet Majorana edge states
that are destroyed in the high- frequency limit.Comment: 13 pages, 8 figure
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