scholarly journals Bioinspired Low-Frequency Material Characterisation

2012 ◽  
Vol 2012 ◽  
pp. 1-12 ◽  
Author(s):  
C. Hopper ◽  
S. Assous ◽  
P. B. Wilkinson ◽  
D. A. Gunn ◽  
P. D. Jackson ◽  
...  
Keyword(s):  
Sound Velocity ◽  
Medical Physics ◽  
Low Frequency ◽  
High Sensitivity ◽  
Prediction Errors ◽  
Low Frequencies ◽  
Longitudinal Sound Velocity ◽  
Velocity Prediction ◽  
Thickness Measurements

New-coded signals, transmitted by high-sensitivity broadband transducers in the 40–200 kHz range, allow subwavelength material discrimination and thickness determination of polypropylene, polyvinylchloride, and brass samples. Frequency domain spectra enable simultaneous measurement of material properties including longitudinal sound velocity and the attenuation constant as well as thickness measurements. Laboratory test measurements agree well with model results, with sound velocity prediction errors of less than 1%, and thickness discrimination of at least wavelength/15. The resolution of these measurements has only been matched in the past through methods that utilise higher frequencies. The ability to obtain the same resolution using low frequencies has many advantages, particularly when dealing with highly attenuating materials. This approach differs significantly from past biomimetic approaches where actual or simulated animal signals have been used and consequently has the potential for application in a range of fields where both improved penetration and high resolution are required, such as nondestructive testing and evaluation, geophysics, and medical physics.

scholarly journals Superluminal Motion in CTA 102

1989 ◽  
Vol 134 ◽  
pp. 529-530
Author(s):  
Ann E. Wehrle
Keyword(s):  
Flux Density ◽  
High Speed ◽  
Low Frequency ◽  
Precise Determination ◽  
Flux Variation ◽  
Low Frequencies ◽  
Superluminal Motion ◽  
Low Frequency Variability ◽  
Expansion Speed

Sholomitskii (1965) discovered that the flux density of the quasar CTA 102 varies at low frequencies on a timescale of a few months. Low-frequency variability can be explained by “superluminal flux variation” (Romney et al. 1984): If the intrinsic brightness of a component moving in a relativistically beamed source varies by only a few percent, the observer sees its flux density change by a much larger factor δ3-α when the optically thin blob moves almost directly toward the observer. Such a relativistically beamed source is likely to exhibit superluminal motion if studied with sufficient resolution and sensitivity. Superluminal motion in CTA 102 was discovered by Bååth (1987) who concluded on the basis of maps made at three epochs at a frequency of 932 MHz that two components were separating at a rate of 0.65 milliarcseconds (mas) per year. Using a redshift z = 1.037 and H0 = 100 km s−1 Mpc−1, q0 = 0.5, this expansion speed corresponds to (18 ± 4)h−1c. The extraordinarily high speed led us to make VLBI images of the source at a higher frequency in order to increase the resolution and make a more precise determination of the speed.

Effects of Frequency on the Directional Auditory Sensitivity of Northern Saw-Whet Owls (Aegolius acadicus)

2021 ◽  
pp. 1-11
Author(s):  
Eileanor P. LaRocco ◽  
Glenn A. Proudfoot ◽  
Megan D. Gall
Keyword(s):  
Auditory Evoked Potentials ◽  
Transfer Functions ◽  
Low Frequency ◽  
High Sensitivity ◽  
Directional Hearing ◽  
Directional Sensitivity ◽  
Low Frequencies ◽  
Aegolius Acadicus ◽  
Spatial Locations ◽  
Prey Items

Many animals use sound as a medium for detecting or locating potential prey items or predation threats. Northern saw-whet owls (<i>Aegolius acadicus</i>) are particularly interesting in this regard, as they primarily rely on sound for hunting in darkness, but are also subject to predation pressure from larger raptors. We hypothesized that these opposing tasks should favor sensitivity to low-frequency sounds arriving from many locations (potential predators) and high-frequency sounds below the animal (ground-dwelling prey items). Furthermore, based on the morphology of the saw-whet owl skull and the head-related transfer functions of related species, we expected that the magnitude of changes in sensitivity across spatial locations would be greater for higher frequencies than low frequencies (i.e., more “directional” at high frequencies). We used auditory-evoked potentials to investigate the frequency-specific directional sensitivity of Northern saw-whet owls to acoustic signals. We found some support for our hypothesis, with smaller-magnitude changes in sensitivity across spatial locations at lower frequencies and larger-magnitude changes at higher frequencies. In general, owls were most sensitive to sounds originating in front of and above their heads, but at 8 kHz there was also an area of high sensitivity below the animals. Our results suggest that the directional hearing of saw-whet owls should allow for both predator and prey detection.

scholarly journals Machine-learning based data recovery and its contribution to seismic acquisition: simultaneous application of deblending, trace reconstruction and low-frequency extrapolation

Geophysics ◽  
2020 ◽  
pp. 1-62
Author(s):  
Shotaro Nakayama ◽  
Gerrit Blacquière
Keyword(s):  
Machine Learning ◽  
Incomplete Data ◽  
Field Data ◽  
Low Frequency ◽  
Cost Effective ◽  
Complete Data ◽  
Prediction Errors ◽  
Low Frequencies ◽  
Simultaneous Application ◽  
Frequency Space

Acquisition of incomplete data, i.e., blended, sparsely sampled, and narrowband data, allows for cost-effective and efficient field seismic operations. This strategy becomes technically acceptable, provided that a satisfactory recovery of the complete data, i.e., deblended, well-sampled and broadband data, is attainable. Hence, we explore a machine-learning approach that simultaneously performs suppression of blending noise, reconstruction of missing traces and extrapolation of low frequencies. We apply a deep convolutional neural network in the framework of supervised learning where we train a network using pairs of incomplete-complete datasets. Incomplete data, which are never used for training and employ different subsurface properties and acquisition scenarios, are subsequently fed into the trained network to predict complete data. We describe matrix representations indicating the contributions of different acquisition strategies to reducing the field operational effort. We also illustrate that the simultaneous implementation of source blending, sparse geometry and band limitation leads to a significant data compression where the size of the incomplete data in the frequency-space domain is much smaller than the size of the complete data. This reduction is indicative of survey cost and duration that our acquisition strategy can save. Both synthetic and field data examples demonstrate the applicability of the proposed approach. Despite the reduced amount of information available in the incomplete data, the results obtained from both numerical and field data cases clearly show that the machine-learning scheme effectively performs deblending, trace reconstruction, and low-frequency extrapolation in a simultaneous fashion. It is noteworthy that no discernible difference in prediction errors between extrapolated frequencies and preexisting frequencies is observed. The approach potentially allows seismic data to be acquired in a significantly compressed manner, while subsequently recovering data of satisfactory quality.

scholarly journals A low-frequency ice-penetrating radar system adapted for use from an airplane: test results from Bering and Malaspina Glaciers, Alaska, USA

2009 ◽  
Vol 50 (51) ◽  
pp. 93-97 ◽  
Author(s):  
Howard Conway ◽  
Ben Smith ◽  
Pavan Vaswani ◽  
Kenichi Matsuoka ◽  
Eric Rignot ◽  
...  
Keyword(s):  
Low Frequency ◽  
Radar System ◽  
Ice Thickness ◽  
Radio Waves ◽  
Low Frequencies ◽  
Throat Region ◽  
High Attenuation ◽  
Glacier System ◽  
Dielectric Absorption ◽  
Thickness Measurements

AbstractIce-thickness measurements are needed to calculate fluxes through fast-flowing outlet glaciers in Greenland, Alaska, Patagonia and Antarctica. However, relatively high attenuation of radio waves by dielectric absorption and volume scattering from englacial water hampers detection of the bed through warm deep ice. In the past we have had success measuring ice thickness of temperate glaciers using a ground-based monopulse radar system operating at low frequencies (2 MHz). Here we adapt the same system to operate from an airplane. Test flights over Bering Glacier, Alaska, USA, detected the bed through ice up to 1250m thick. Flights across the Seward–Malaspina Glacier system, Alaska, resolved the ice thickness of Malaspina Glacier, but strong hyperbolic-shaped returns obscured the bed echo through the Seward throat. It is likely that this clutter in the signal was caused by off-nadir returns from chaotic surface crevasses that are ubiquitous in the throat region.

scholarly journals A High-Through Technique to Measure DNA Methylation

2010 ◽  
Vol 3 ◽  
pp. GEG.S5035 ◽  
Author(s):  
Zhiming Zhang ◽  
Jian Gao ◽  
Cheng Qin ◽  
Li Liu ◽  
Haijian Lin ◽  
...  
Keyword(s):  
Dna Methylation ◽  
High Efficiency ◽  
Simple Procedure ◽  
Point Mutations ◽  
Low Frequency ◽  
High Sensitivity ◽  
Snp Analysis ◽  
Allelic Series ◽  
Low Frequencies ◽  
Powerful Approach

MethyLight is a sodium-bisulfite-dependent, quantitative, fluorescence-based, real-time PCR strategy that is used to detect and quantify DNA methylation in genomic DNA. High-throughput MethyLight allows the rapid and sensitive detection of very low frequencies of hypermethylated alleles in populations of alternated individuals. The high sensitivity and specificity of MethyLight can be applied not only to make it uniquely suited disease clinical but also quantitatively assessed of these low-frequency methylation events. Owing to its full of advantages of simple procedure, high efficiency and high sensitivity, MethyLight provides a powerful approach for clinical examination, Gene expression analysis, SNP analysis and allele analysis. Coupled with other techniques, MethyLight can be used immediately in identifying allelic alterations in genes exhibiting expressions correlating with phenotypes, Locating an allelic series of induced point mutations in genes of interest. The development of this technique should considerably enhance our ability to rapidly and accurately generate epigenetic profiles of samples.

On a modified algorithm for the autoregressive recovery of the acoustic impedance

Geophysics ◽  
1984 ◽  
Vol 49 (12) ◽  
pp. 2190-2192 ◽  
Author(s):  
Tad. J. Ulrych ◽  
Colin Walker
Keyword(s):  
Acoustic Impedance ◽  
Low Frequency ◽  
Prediction Errors ◽  
Low Frequencies ◽  
Limited Frequency ◽  
Band Limited ◽  
Ar Modeling ◽  
Ar Process ◽  
Backward Prediction ◽  
Zero Frequency

In a recent paper, Walker and Ulrych (1983) presented an algorithm for the recovery of the acoustic impedance from band‐limited seismic reflection records. The approach used is based on the autoregressive (AR) modeling of the band‐limited frequency transform of the data. This modeling procedure allows prediction of both the high and low missing frequencies. The low frequencies, which are particularly important in the inversion for the acoustic impedance, are determined by considering the low‐frequency band as a gap of missing data which is centered at zero frequency. The gap is filled by minimizing the sum of the squared forward and backward prediction errors which result when the known spectral data are modeled as an AR process.

scholarly journals The Low frequency Space Array (LFSA)

1988 ◽  
Vol 129 ◽  
pp. 459-460
Author(s):  
K. W. Weiler ◽  
B. K. Dennison ◽  
K. J. Johnston ◽  
R. S. Simon ◽  
J. H. Spencer ◽  
...  
Keyword(s):  
High Resolution ◽  
Milky Way ◽  
Low Frequency ◽  
High Sensitivity ◽  
Electromagnetic Spectrum ◽  
Low Frequencies ◽  
Frequency Space ◽  
Physical Limit ◽  
Path Lengths ◽  
Fundamental Physical

At the lowest radio frequencies (≤30 MHz), the Earth's ionosphere transmits poorly or not at all. This relatively unexplored region of the electromagnetic spectrum is thus an area where high resolution, high sensitivity observations can open a new window for astronomical investigations. Also, extending observations down to very low frequencies brings astronomy to a fundamental physical limit where the Milky Way becomes optically thick over relatively short path lengths due to diffuse free-free absorption.

scholarly journals FR II radio galaxies at low frequencies – II. Spectral ageing and source dynamics

2017 ◽  
Vol 469 (1) ◽  
pp. 639-655 ◽  
Author(s):  
Jeremy J. Harwood ◽  
Martin J. Hardcastle ◽  
Raffaella Morganti ◽  
Judith H. Croston ◽  
Marcus Brüggen ◽  
...  
Keyword(s):  
Surface Brightness ◽  
Spatial Scales ◽  
Low Frequency ◽  
Radio Galaxies ◽  
Speed Of Light ◽  
Low Frequencies ◽  
Very Large Array ◽  
Low Surface Brightness ◽  
Initial Electron Energy

AbstractIn this paper, the second in a series investigating Fanaroff–Riley type II (FR II) radio galaxies at low frequencies, we use LOw Frequency ARray (LOFAR) and Very Large Array (VLA) observations between 117 and 456 MHz, in addition to archival data, to determine the dynamics and energetics of two radio galaxies, 3C 452 and 3C 223, by fitting spectral ageing models on small spatial scales. We provide improved measurements for the physical extent of the two sources, including a previously unknown low surface brightness extension to the northern lobe of 3C 223, and revised energetics based on these values. We find spectral ages of $77.05^{+9.22}_{-8.74}$ and $84.96^{+15.02}_{-13.83}$ Myr for 3C 452 and 3C 223, respectively, suggesting a characteristic advance speed for the lobes of around 1 per cent of the speed of light. For 3C 452, we show that, even for a magnetic field strength not assumed to be in equipartition, a disparity of a factor of approximately 2 exists between the spectral age and that determined from a dynamical standpoint. We confirm that the injection index of both sources (as derived from the lobe emission) remains steeper than classically assumed values, even when considered on well-resolved scales at low frequencies. However, we find an unexpected sharp discontinuity between the spectrum of the hotspots and the surrounding lobe emission. We suggest that this discrepancy is a result of the absorption of hotspot emission and/or non-homogeneous and additional acceleration mechanisms; as such, hotspots should not be used in the determination of the underlying initial electron energy distribution.

scholarly journals Properties and magnetic origins of solar S-bursts

2019 ◽  
Vol 622 ◽  
pp. A204 ◽  
Author(s):  
Brendan P. Clarke ◽  
Diana E. Morosan ◽  
Peter T. Gallagher ◽  
Vladimir V. Dorovskyy ◽  
Alexander A. Konovalenko ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Activity ◽  
Solar Radio ◽  
Low Frequency ◽  
High Sensitivity ◽  
Coronal Magnetic Field ◽  
Solar Radio Emission ◽  
Radio Bursts ◽  
Burst Source ◽  
Low Frequencies

Context. Solar activity is often accompanied by solar radio emission, consisting of numerous types of solar radio bursts. At low frequencies (<100 MHz) radio bursts with short durations of milliseconds, such as solar S-bursts, have been identified. To date, their origin and many of their characteristics remain unclear. Aims. We report observations from the Ukrainian T-shaped Radio telescope, (UTR-2), and the LOw Frequency ARray (LOFAR) which give us new insight into their nature. Methods. Over 3000 S-bursts were observed on 9 July 2013 at frequencies of 17.4–83.1 MHz during a period of low solar activity. Leading models of S-burst generation were tested by analysing the spectral properties of S-bursts and estimating coronal magnetic field strengths. Results. S-bursts were found to have short durations of 0.5–0.9 s. Multiple instruments were used to measure the dependence of drift rate on frequency which is represented by a power law with an index of 1.57. For the first time, we show a linear relation between instantaneous bandwidth and frequency over a wide frequency band. The flux calibration and high sensitivity of UTR-2 enabled measurements of their fluxes, which yielded 11 ± 3 solar flux units (1 SFU ≡ 104 Jy). The source particle velocities of S-bursts were found to be ∼0.07 c. S-burst source heights were found to range from 1.3 R⊙ to 2 R⊙. Furthermore, a contemporary theoretical model of S-burst generation was used to conduct remote sensing of the coronal magnetic field at these heights which yielded values of 0.9–5.8 G. Within error, these values are comparable to those predicted by various relations between magnetic field strength and height in the corona.

Sign in / Sign up

Export Citation Format

Share Document