Multiple Spectral Components Research Articles

Wave Propagation Mechanisms for Intra-Chip Communications

Fundamental wave propagation mechanisms for intra-chip communications in a multilayered IC chip structure are studied theoretically and numerically. The well-known Green's function representation in terms of branch cuts (continuous spectrum), residues (discrete spectrum), and direct source-receiver radiation is used to clarify important wave processes for typical intra-chip antenna structures. It is found that surface waves may or may not provide the dominant propagation channel contribution, depending on frequency and chip structure. We generalize the concept of multipath to include multiple spectral (e.g., discrete and continuous) wave components, appropriate for the near- and intermediate-field problems of interest for chip-scale propagation. The effect of a guiding layer placed either below or above the silicon substrate is considered. It is found that a guiding layer can provide enhanced single-channel signal transmission via the surface wave channel, and thereby reduce dispersion associated with multi wave component effects.

Sympathetic String Modes in the Concert Harp

The concert harp is composed of a soundboard, a cavity with sound holes and 47 strings. When one string is plucked, other strings are excited and induce a characteristic 'halo of sound'. This phenomenon, called sympathetic vibrations is due to a coupling between strings via the instrument's body. These sympathetic modes generate the presence of multiple spectral components in each partial of the tone. Resolution of Fourier analysis does not permit their identification. A high resolution Method, called ESPRIT, is used to separate the spectral components which are very close one to another. Some of the measured spectral components in the analysed partials correspond to the response of sympathetic modes. The eigenfrequencies and mode shapes of these modes are investigated using a suitable model of the instrument : this model is based on a waveguide approach in which bending and longitudinal motions of 35 strings connected to an equivalent beam representing the soundboard are described. Identified experimental sympathetic modes are very well captured by the model.

Encoding of Spectral Correlation over Time in Auditory Cortex

Natural sounds contain multiple spectral components that vary over time. The degree of variation can be characterized in terms of correlation between successive time frames of the spectrum, or as a time window within which any two frames show a minimum degree of correlation: the greater the correlation of the spectrum between successive time frames, the longer the time window. Recent studies suggest differences in the encoding of shorter and longer time windows in left and right auditory cortex, respectively. The present functional magnetic resonance imaging study assessed brain activation in response to the systematic variation of the time window in complex spectra that are more similar to natural sounds than in previous studies. The data show bilateral activity in the planum temporale and anterior superior temporal gyrus as a function of increasing time windows, as well as activity in the superior temporal sulcus that was significantly lateralized to the right. The results suggest a coexistence of hierarchical and lateralization schemes for representing increasing time windows in auditory association cortex.

Multiple intensifications inside the auroral bulge and their association with plasma sheet activities

In this coordinated ground and space study, we report multiprobe measurements from Time History of Events and Macroscale Interactions during Substorms (THEMIS), LANL‐97A, Polar, and ground observatories for a substorm that occurred on 23 March 2007. The THEMIS fleet and LANL‐97A were located in the premidnight, near‐Earth plasma sheet in the radial range from 6.6 to 13 RE, placing the spacecraft into different plasma environments which were subject to different activities. Simultaneous global Polar Ultraviolet Imager images of the aurora revealed a fine structure in the auroral bulge in the form of several time‐delayed regions of brightening. We demonstrate a correspondence between this fine structure and the spatially separated plasma sheet activities (substorm injections with energies >100 keV) by showing that both executed periodic (100–150 s) one‐to‐one correlated modulations. Additionally, the different auroral brightening regions were modulated approximately out of phase to one another, as were the separated plasma sheet activities. The periodic plasma sheet and optical modulations were also one‐to‐one correlated with large‐amplitude (δH ∼ 150 nT) ground Pi2 pulsations. In contrast to the most energetic ions (>100 keV), the lower‐energetic plasma sheet ions executed separate oscillations during the development of the substorm, including the preintensification phase, and showed the following properties. (1) The oscillation periods were different at different spacecraft locations and had a tendency to increase during the evolution of the substorm. During the preintensification phase, multiple (possibly harmonic) spectral components existed. (2) The oscillations were coupled to westward moving perturbations of an energized plasma boundary. The boundary perturbations were likely conjugate to azimuthally spaced auroral forms (“beads”) observed by Polar‐UVI during the preintensification phase and could play a role in the onset of the substorm intensification. (3) The oscillations of the lower‐energetic ions were also one‐to‐one correlated with smaller‐amplitude ground Pi2 pulsations (<15 nT). In conclusion, the combination of these observations allowed us to construct a 3‐D picture of low‐frequency, near‐Earth plasma sheet phenomena associated with a substorm and their connection to aurora and the ground. It appeared that not only one substorm current wedge, but additional current structures existed which started at different times, pulsated out of phase, and mapped from different active regions into the ionosphere. The active space regions appeared to be coupled and transferring energy from one region to the other while pulsating. We propose that the wave‐like structures in the plasma sheet, observed before and during the substorm/intensification phase, and their demonstrated properties support a wave phenomenon (such as a ballooning‐type mode) for the onset and development of the substorm/intensification, rather than directly driven periodic bursty bulk flow activations.

Dynamic coupling of excitatory and inhibitory responses in the medial nucleus of the trapezoid body

The neuronal representation of acoustic amplitude modulations is an important prerequisite for understanding the processing of natural sounds. We investigated this representation in the medial nucleus of the trapezoid body (MNTB) of the Mongolian gerbil using sinusoidal amplitude modulations (SAM). Depending on the SAM's carrier frequency (f(C)) MNTB cells either increase or decrease their discharge rates, indicating underlying excitatory and inhibitory/suppressive mechanisms. As natural sounds typically are composed of multiple spectral components we investigated how stimuli containing two spectral components are represented in the MNTB, especially when they have opposing effects on the discharge rate. Three conditions were compared: SAM stimuli (1) with rate-increasing f(C), (2) with rate-increasing f(C) and an additional unmodulated rate-decreasing pure tone, and (3) with rate-decreasing f(C) and an unmodulated, rate-increasing pure tone. We found that responses under all three conditions showed comparable strength of phase-locking. Adding a rate-decreasing tone to a rate-increasing SAM increased phase-locking for modulation frequencies (f(AM)) of < or = 600 Hz. A comparison of two possible coding strategies--phase-locking vs. envelope reproduction--indicates that both strategies are realized to different degrees depending on the f(AM). We measured latencies for following modulations in rate-increasing and rate-decreasing SAMs using a modified reverse correlation approach. Although latencies varied between 2.5 and 5 ms between cells, a decrease in rate consistently followed an increase in rate with a delay of about 0.2 ms in each cell. These results suggest a temporally precise representation of rate-increasing and rate-decreasing stimuli at the level of the MNTB during dynamic stimulation.

A Virgo high-resolution Hα kinematical survey

New Halpha emission-line observations of the Virgo cluster galaxy NGC 4438 are presented. Fabry-Perot interferometry data at an effective angular resolution of ~2" are used to map the kinematics of the ionized gas in the galaxy. For the first time we obtain a velocity field covering a large area in NGC 4438, much larger than that deduced from previous HI and CO maps. The kinematics of the extended, low surface brightness Halpha filaments to the West of the galactic disk is discussed. We report on the discovery of a northern Halpha structure which is clumpier than the other filaments. Evidence for multiple spectral components through the data-cube are presented in a nuclear shell and in the approaching half of the disk. The role of VCC 1040, a dwarf elliptical galaxy located to the South of NGC 4438, is presented to investigate the origin of a small-scale stellar tail of NGC 4438. It could be due to a minor tidal interaction between the two galaxies.

State-Switched Absorber for Vibration Control of Point-Excited Beams

A system that has the capability to make instantaneous changes in its mass, stiffness, or damping may be termed a state-switchable dynamical system. Such a system will display different dynamical responses dependent upon its current state. State-switchable stiffness may be practically obtained through the control of the termination impedance of piezoelectric stiffness elements. If such a switchable stiffness element is incorporated as part of the spring element of a vibration absorber, the change in stiffness causes a change in the resonance frequencies of the system, thereby instantaneously ‘retuning’ the state-switched absorber to a new frequency. In between state switches, the operation of such a device is passive, being fundamentally a passive vibration absorber. This concept has improved performance over classical passive vibration absorbers or dampers, particularly for disturbances with multiple spectral components. This paper considers the application of such a device for the purpose of vibration control on beams subjected to harmonic point-force excitation.

The X‐Ray Line Emission from the Supernova Remnant W49B

The Galactic supernova remnant W49B has one of the most impressive X-ray emission-line spectra obtained with ASCA. We use both plasma line diagnostics and broadband model fits to show that the Si and S emission lines require multiple spectral components. The spectral data do not necessarily require individual elements to be spatially stratified, as suggested by earlier work, although when ASCA line images are considered, it is possible that Fe is stratified with respect to Si and S. Most of the X-ray-emitting gas is from ejecta, based on the element abundances required, but is surprisingly close to being in collisional ionization equilibrium. A high ionization age implies a high internal density in a young remnant. The fitted emission measure for W49B indicates a minimum density of 2 cm-3, with the true density likely to be significantly higher. W49B probably had a Type Ia progenitor, based on the relative element abundances, although a low-mass Type II progenitor is still possible. We find persuasive evidence for Cr and possibly Mn emission in the ASCA spectrum—the first detection of these elements in X-rays from a cosmic source.

Optimization of chemical shift selective suppression of fat.

Strategies to optimize flip angles for chemical shift selective fat suppression are discussed. Mathematical models for fat suppression in spoiled gradient recalled acquisition, spin echo, and RARE, which incorporate steady state conditions and multiple spectral components of fat, are developed. The optimal suppression flip angle is found to be larger than that determined with a single fat component model by more than 10 degrees due to contributions from unflipped components such as olefinic and glycerol protons that lie outside the suppression band.

Tissue temperature monitoring with multiple gradient-echo imaging sequences.

The inherent sensitivity of multiple gradient-echo sequences to the chemical shift is exploited to rapidly map muscle water frequency shifts caused by ultrasonic heating. The use of multiple echoes is shown to offer several advantages over single gradient-echo approaches previously proposed for temperature measurement. An increase in the effective bandwidth significantly reduces aliasing problems observed with single gradient-echo methods in high temperature applications. Of greater significance is the improved immunity to intrascan motion found for multi-echo versus single echo gradient methods, making the former more attractive for clinical applications. Finally, a sensitivity to the presence of multiple spectral components unavailable with single gradient-echo methods is obtained.

Projection-Reconstruction Spectroscopic Imaging forB0Field Plotting and Shimming without Pulsed Gradients

Shimming is important. Noniterative methods are desirable. Such methods exist for shimming a spectrometer with pulsed field gradients, generally based on field maps made by spin-warp Fourier imaging. For spectrometers with no pulsed gradients (or for cases whereT2is too short to permit echo imaging), an alternative method is presented: projection-reconstruction spectroscopic imaging, which can be accomplished using only the shim coils of a conventional spectrometer. Images so acquired can be used to map the field, even in the presence of multiple spectral components. Noniterative optimization of the axial shims of a GN-300 spectrometer is demonstrated using 1D + 1D spectroscopic images. Prospects for extending the technique to include the radial shims using 3D + 1D spectroscopic images are discussed.

The structural significance of a vocalization that stimulates gaping in black-capped chickadees

In a variety of avian species, vocalizations used to stimulate gaping in nestlings during feeding share similar features such as broad frequency range, multiple spectral components, and sudden onset. It is currently not understood what functional significance, if any, these acoustic features have for close-range communication. One example of such a vocalization is the “squawk” of the black-capped chickadees (Parus atricapillus). The present study analyzes the significant features of the squawk by observing the response of chickadee nestlings to field playbacks of natural and computer-modified squawks. Of the features tested, frequency range appears most critical, but responses to frequency ranges change with age, such that low frequencies are most stimulating around the time of hatching but cease to elicit gaping after 5 days posthatch. Aperiodic stimuli and computer-modified squawks with gradual onsets tend to be less stimulating than natural squawks at some ages, but these differences are not significant at any age. The functional significance of the broad frequency range of the squawk is discussed, including the possible role of accommodating shifting frequency preferences related to ontogenetic changes in auditory sensitivity and elaboration of the vocal repertoire.

Residual stresses of types II and III and their estimation

Residual stresses (RS) are induced in metallic materials by a variety of working and fabrication processes. They are generally classified into three categories (types I, II and III) depending upon their range of influence. Separation of type I and II stresses often requires stress measurements on thin specimens. Type III stresses are related closely to grain fragmentation and micro-strains associated with plastic deformation. Perhaps the oldest and the most rigorous method of estimating these stresses is still the Warren-Averbach analysis developed during the late forties/early fifties. Other techniques involving integral breadths and variance of the profiles have also been developed. In all these methods developed during the early stages, prior to seventies, a major requirement was well-separated non-overlapping profiles. The late sixties and early seventies saw a dramatic increase in computational capabilities with the advent of powerful electronic computers. This led to the introduction of curve-fitting procedures into the field of X-ray diffraction. The most remarkable achievement of this period is the development of the Rietveld Method. Although this method was initially developed to tackle the neutron diffraction profiles, which are as a rule nearly symmetric and Gaussian in nature, the method saw rapid developments during the eighties. At present, techniques based on concepts developed by Rietveld could be applied to essentially asymmetric and non-Gaussian multiple spectral components of X-ray diffraction profiles. Pattern decomposition techniques which separate composite powder diffraction profiles into individual profiles are now available. In combination with single line profile analysis techniques, this provides a powerful tool in the hands of researchers. A typical example of such a single line profile analysis is given.

A novel peridinin—chlorophyll a protein (PCP) from the marine dinoflagellate Alexandrium cohorticula: A high pigment content and plural spectral forms of peridinin and chlorophyll a

A new type of peridinin—chlorophyll a protein (PCP) was isolated from the marine dinoflagellate Alexandrium cohorticula. Unlike previous studies, PCP was obtained as a single component in the presence of a protease inhibitor. The monomer had a molecular mass of 37 kDa with 12 peridinin molecules associated with 2 chl a molecules. This pigment content was much higher than that reported previously. We observed a partial amino acid sequence of the N-terminus that is novel among photosynthetic pigment—protein complexes. Magnetic circular dichroism clearly indicated that chl a in PCP had monomeric features. Multiple spectral components were suggested for both chl a and peridinin. Based on the high pigment content, the optical properties were compared with those for a reported PCP containing 4 chl a and 1 peridinin.

Optical spectroscopic studies of the sorption of UO2+2 species on a reference smectite

The speciation of UO2+2 (uranyl) on a reference smectite (SAz-1 from Cheto, Arizona, USA) has been investigated by electronic emission and Raman vibrational spectroscopies. The spectroscopic studies have been done on uranyl-bearing clays prepared from aqueous solutions of uranyl nitrate in the pH range from ~2.5 to 7 and high initial ionic strength (~0.1–0.3 M). The uranyl loading levels in these samples ranged from ~0.1% to ~53% of the reported cation exchange capacity (~1.2 meq/g). Vibronically resolved emission spectra have been obtained for all samples. These spectra vary significantly in intensity and band-shape as a function of uranyl concentration in the clays and the equilibrium pH of the solutions from which the clays were prepared. For most clay samples the measured emission spectrum is a composite of spectra from multiple uranyl emitters. At the lowest loading levels a uranyl sorption complex with an apparent vibronic spacing of ~750 cm−1 dominates the spectra. At intermediate loading levels an additional uranyl sorption complex also having an apparent vibronic spacing of ~ 750 cm−1 is present at an approximately constant concentration ratio to the species in the most dilute samples. At the highest loading levels, a uranyl sorption complex with a vibronic spacing of ~ 850 cm−1 dominates the spectra. Raman spectra have been obtained for the more concentrated uranyl/clay samples. Two distinct bands (855 cm−1 and 883 cm−1) are seen in the spectral region of the totally symmetric uranyl stretch. The 855 cm−1 band correlates with the dominant high-coverage species, while the 883 cm−1 band arises from an additional sorption complex. Comparison of these results with aqueous solution spectral data suggests that monomeric uranyl moieties are responsible for the observed spectral responses in the clay samples, and the multiple spectral components are a result of occupancy by these moieties in several structurally and/or energetically different sites within the clay. It is proposed that the uranyl species responsible for the dominant components in the emission spectra in the low and intermediate coverage clay sample are sorbed to amphoteric edge site(s). The uranyl species responsible for the dominant component in the emission spectrum and the 855 cm−1 Raman band in the high-coverage clays is proposed to be exchanged into the fixed charge site(s). The additional complex identified by the Raman band at 883 cm−1 is also proposed as a sorption complex at fixed-charge sites.

A feedback model for the source of auroral kilometric radiation

The self‐excited oscillations which result from positive wave feedback are believed to produce the discrete spectrum of auroral kilometric radiation. This would require a closed wave feedback path inside the source, having a length equal to a multiple of the wavelength, and it yields an emission model which is quite analogous to an optical laser oscillator. The end reflections for this feedback path, which would be comparable to those produced by the laser mirrors, are attributed to partial wave reflections at the source boundaries. However, in order to compensate for the wave refraction inside the source, the boundary reflection surfaces must converge with altitude, and this implies that the most likely auroral kilometric radiation source would be a thin, local density enhancement, since the refractive index contours at its boundaries would be expected to slope inward. The ISEE observations of multiple spectral components, which are attributed to separate oscillations at different altitudes in the same enhancement, suggest a source thickness as small as 25 km and an internal wave growth threshold of roughly 40 dB, rather than the 70‐120 dB previously believed necessary to account for auroral kilometric radiation without feedback. But more significantly, the feedback model accounts for numerous aspects of the auroral kilometric radiation behavior, predicts emission at the wave growth saturation level, and leads to the conclusion that auroral kilometric radiation originates at many compact sites, each emitting a nearly monochromatic wave.