Cut-off Frequency Changes Research Articles

The effect of 3D surface roughness on acoustic wave propagation in a cylindrical waveguide

This paper studies the acoustic wave scattering and attenuation in a cylindrical waveguide (pipe) with wall roughness varying along all three dimensions and roughness height smaller than the acoustic wavelength. Using the decomposition of the acoustic wave field into deterministic and random components, small perturbation method and Fourier transform the analytical solution of a 3-D averaged acoustic wave field is obtained. The correction term describing the mechanism of wave attenuation caused by roughness and determined by the modal cross-talk is also derived. The solution for the plane wave is validated in the frequency range extended well beyond the second cut-off frequency, where the crosstalk between the fundamental and non-axisymmetric modes are observed. The analytical solution is compared with the numerical results obtained with the Monte-Carlo method and Finite Element solver. The numerical study results have demonstrated a close agreement with the analytical solution for the averaged sound field, dispersion curves, and the wave attenuation effect expressed as the wavenumber correction term. A key novelty of this work is a comprehensive analysis of wave dispersion and cut-off frequency changes due to the presence of 3-D wall roughness in a pipe.

Electromagnetic wave propagation in a chiroplasma-filled elliptical waveguide, bounded by a perfect electromagnetic conductor.

This work presents electromagnetic wave propagation in the elliptical waveguides filled with chiroplasma core and an outer perfect electromagnetic conductor boundary. The electromagnetic fields were theoretically examined. The hybrid modes' dispersion relations and power flux density were calculated considering appropriate boundary conditions, and their behaviors were numerically and graphically studied. The effects of the plasma frequency, the cyclotron frequency, the chirality parameter, and the perfect electromagnetic conductor admittance parameter were investigated for the cutoff frequency. Different modes were considered, and the dispersion relation was plotted and studied for each case. It was shown that the cutoff frequency changes with the change of various parameters such as the plasma and cyclotron frequencies, the chirality parameter, and the perfect electromagnetic conductor admittance parameter. In addition, it was shown that for the considered mode, when the plasma or cyclotron frequencies increase, the values of correspond cutoff frequencies have higher or lower values, respectively. Higher values of the perfect electromagnetic conductor admittance parameter or the chirality admittance parameter yield lower or higher cutoff frequencies of the modes, respectively.

Features of using the smoothed instantaneous wave energy history (SIWEH) to analyze the group structure of sea waves

The methodological features of determining the characteristics of wave groups of sea surface waves based on the function smoothed instantaneous waves through energy (SIWEH) are analyzed. The analysis is carried out by numerical simulation based on an analytical model of the group structure of waves. It is shown that when the grouping of sea waves is investigated using the SIWEH function, it is advisable to calculate the Groupiness Factors of Height (GFH) in two stages. At the first stage, transform the SIWEH function into a wave envelope, and at the second stage, calculate the GFH using a known envelope. The previously proposed direct calculation of GFH by the SIWEH function leads to significant errors. The effect of the change in the cutoff frequency of the Bartlett filter at a fixed frequency of dominant waves on the calculated values of GFH is investigated. It is shown that when the cutoff frequency changes ±30%, the GFH changes do not exceed 5%.

Evaluation of butterworth post-filtering effects on contrast and signal noise to ratio values for SPECT images reconstruction

In nuclear medicine, single photon emission computed tomography (SPECT) imaging is very important for the detection or staging of variety of diseases such as myocardial perfusion, bone cancer, thyroid function and many more. To reduce the noise and improved contrast image in SPECT imaging is one of the essential tasks. This study aims to evaluate the optimum value for Butterworth filters to obtain the best physical image with high-quality contrast and reduced noise. In this study, the NEMA 2012/IEC 2008 phantom was imaged using the GE NM/CT Pro Discovery-670 system by implementing routine bone scan acquisition time. The large background compartment of the phantom was filled with 99mTc solution mixed with deionized water using a 5:1 sphere to background activity concentration ratio. The six spheres with different inner diameter and volume were filled with 99mTc solution of 300 kBq/mL with tumor background ratio. This activity concentration corresponds to a total injected activity of 500 MBq of 99mTc. Images in the trans-axial direction were reconstructed by Filter Back Projection technique. The images were examined in terms of hot regions, detectability, and contrast. The contrast and signal to noise ratio (SNR) were calculated for each sphere and COF values. Results show that the optimum parameter for Butterworth filter is at cut-off frequency (COF) of 7 cycle/mm. The hot regions from the body phantom study show that the detectability and contrast vary with the change of COF. The shape of five hot regions shows clarity and easier to draw the region of interest (ROI). Unfortunately, it is difficult to draw on the smallest ROI due to the presence of noise that caused blurring and unclear edges on the images. Statistically, a strong positive correlation was found between the contrast and SNR values for each sphere with coefficient of determination, R2 values above 0.96. The Butterworth filter at COF of 7 cycle/mm increases the contrast and SNR values at 4.4%–7.1%, and 61 to 859 counts, respectively for Sphere 2 to 6, compared to non-filtered images. In conclusion, the optimal parameter of COF for this study can be estimated from the calculation of total counts of contrast and SNR. Besides, findings from this study provides the physicians an important information in delivering the best and optimum diagnosis and treatment to the patients, thus improving the patient's well-being and health.

Comparison of Low-Pass Filters for SPECT Imaging.

In single photon emission computed tomography (SPECT) imaging, the choice of a suitable filter and its parameters for noise reduction purposes is a big challenge. Adverse effects on image quality arise if an improper filter is selected. Filtered back projection (FBP) is the most popular technique for image reconstruction in SPECT. With this technique, different types of reconstruction filters are used, such as the Butterworth and the Hamming. In this study, the effects on the quality of reconstructed images of the Butterworth filter were compared with the ones of the Hamming filter. A Philips ADAC forte gamma camera was used. A low-energy, high-resolution collimator was installed on the gamma camera. SPECT data were acquired by scanning a phantom with an insert composed of hot and cold regions. A Technetium-99m radioactive solution was homogenously mixed into the phantom. Furthermore, a symmetrical energy window (20%) centered at 140 keV was adjusted. Images were reconstructed by the FBP method. Various cutoff frequency values, namely, 0.35, 0.40, 0.45, and 0.50 cycles/cm, were selected for both filters, whereas for the Butterworth filter, the order was set at 7. Images of hot and cold regions were analyzed in terms of detectability, contrast, and signal-to-noise ratio (SNR). The findings of our study indicate that the Butterworth filter was able to expose more hot and cold regions in reconstructed images. In addition, higher contrast values were recorded, as compared to the Hamming filter. However, with the Butterworth filter, the decrease in SNR for both types of regions with the increase in cutoff frequency as compared to the Hamming filter was obtained. Overall, the Butterworth filter under investigation provided superior results than the Hamming filter. Effects of both filters on the quality of hot and cold region images varied with the change in cutoff frequency.

Oklahoma's future wind energy resources and their relationship with the Central Plains low-level jet

Previous work has projected increases in wind speed/wind power density (WPD) over the United States' South-Central Plains, which could be linked to future increases in frequency of the Central Plains Low-Level Jet (LLJ). This work serves to both update projections of this region's wind energy resources, and to assess the LLJ's importance for influencing these resources. Projected changes in several wind energy metrics (including ramp event frequency and cut-off frequency) were assessed specifically over the Oklahoma Panhandle. These projections were computed by using a climatological delta method to statistically downscale changes in near-surface wind speed from multiple climate models onto observations from the Oklahoma Mesonet. The results suggest a pattern of overall reductions in wind speed and WPD, slight reductions in ramp event frequency, and little change in cut-off frequency, with projected changes often being larger and of statistical significance at night and during summer and autumn. Highly distinguishable diurnal and seasonal variability in ramp event and cut-off frequency was found in this work, with these frequencies most notably maximizing during the day. This climatological variability is consistent with that seen in observations of these wind energy metrics. Regression of LLJ characteristics against wind energy metrics identified consistent and conceptually understandable links between increased LLJ frequency and decreased ramp event frequency in future decades. Such a result attests to a more stable and easier to manage future wind energy resource over the Oklahoma Panhandle.

Adaptive measurement of crossover frequencies for intelligibility prediction

In SII theory, frequencies where speech spectra can be divided into two equally-intelligible subbands are called crossover frequencies. These frequencies play a crucial role in SII calculations, and also designate spectral regions that contain important speech recognition cues. Typically, crossover frequencies are found by measuring psychometric curves for speech processed by a series of low-pass and high-pass filters, and then finding the two curves’ intersection: an inefficient, time-consuming process. The present study introduces an up/down quantile estimation algorithm that adaptively steers filter cutoff frequencies toward the crossover frequency. Changes in cutoff frequency are governed by comparisons of block trials for low-pass and high-pass filtered speech that meet theoretical requirements for convergence toward the crossover frequency. Preliminary results for trials with nonsense syllables show that the proposed method’s estimates match those obtained in published trials using the conventional method. Applications in SII measurements and speech recognition cue measurement will be discussed.

A 0.5 μA/Channel front-end for implantable and external ambulatory ECG recorders

There is an increasing demand for low power, low voltage, and small size analog readout bio-potential systems. This paper presents an analog front-end for measuring the intra-cardiac signals in implantable and also external ambulatory ECG cardiac recording applications. The analog front-end benefits from voltage to frequency conversion without conventional analog to digital conversion and thereby achieves a very low power consumption. The frequency averaging digital reconstruction makes an intrinsic low pass filter of which the cut-off frequency changes with the averaging interval and eliminates the need for an analog filter in this system. Furthermore, this feature enables the development of a general purpose analog readout bio-potential measurement system with programmable bandwidth. The proposed IC is fabricated in a 0.18 μm CMOS process and occupies 0.396 mm2. It can measure intra-cardiac bio-potentials with 9.2-bit resolution while consuming just 0.5 μA per channel from a 1 V supply. The input referred noise of the intra-cardiac signal readout channel is 2 μVrms.

Effect of Cut-Off Frequency of Butterworth Filter on Detectability and Contrast of Hot and Cold Regions in Tc-99m SPECT

In SPECT, noise is one of the major limitations that degrade image quality. To suppress the noisy signals in an image, digital filters are most commonly applied. However, in SPECT image reconstruction, selection of an appropriate filter and its functions has always remained a difficult task. In this work an attempt was made to investigate the effects of varying cut-off frequencies and in keeping the order of Butterworth filter constant on detectability and contrast of hot and cold re-gions images. A new insert simulating hot and cold regions which provides similar views in a reconstructed image was placed in the phantom’s cylindrical source tank and imaged. Tc-99m radionuclide was distributed uniformly in the phantom. SPECT data were collected in a 20% energy window centered at 140 keV by a Philips ADAC Forte dual head gamma camera mounted with a LEHR collimator. Images were generated by using the filtered backprojection technique. A Butterworth filter of order 5 with cut-off frequencies 0.35 and 0.45 cycles·cm-1 was applied. Images were examined in terms of hot and cold regions, detectability and contrast. Results show that the hot and cold regions’ detectability and contrast vary with the change of cut-off frequency. With a 0.45 cycles·cm-1 cut-off frequency, a significant enhancement in contrast of cold regions was achieved as compared to a 0.35 cycles·cm-1 cut-off frequency. Furthermore, the detectability of hot and cold regions improved with the use of a 0.45 cycles·cm-1 cut-off frequency. In conclusion, image quality of hot and cold regions affected in a different way with a change of cut-off frequency. Thus, care should be taken in selecting the filter cut-off frequency prior to reconstruction of images; particularly, when both types of regions are expected in the reconstructed image.

Transmission characteristics and potential applications of plasmon-assisted parallel-plated waveguide

Flexible control of terahertz waves is now a research hotspot. Based on the electromagnetic theory the dispersion relation and field distributions in a plasmon-assisted parallel-plated waveguide are deduced. The transmission property of such a waveguide is obtained and confirmed by the full-wave simulation. Results show that the plasmon-assisted parallel-plated waveguide shows a band gap characteristic, and the cutoff frequency of the upper sideband is equal to the plasmon frequency; generally, the thinner the plasmon layer, the higher the cutoff frequency will be, and the narrower the bandwidth will become. Emergence of the band gap is due to the excited surface plasmon polaritons, and the coupling between surface plasmon and the medium in the waveguide. Besides, the influence of plasmon frequency and collision frequency on the transmission properties is investigated, and a method for adjusting the filter characteristic of the waveguide by tuning the plasmon frequency is proposed. Moreover, the plasmon layer is realized by a textured metallic structure, and a sensing model based on the parallel-plated waveguide is designed. Simulation results show that a 0.1 percent change in permittivity of the sample materials filling in the groove will give rise to a significant change of the cutoff frequency, which is 1.8 GHz in average; interestingly, different liquid samples such as nitrogen, gasoline, paraffin, glycerine and water can be identified through detecting the change of cutoff frequency, which further confirms the excellent terahertz sensing characteristic of the proposed sensor. This work may be helpful for the study of terahertz wave transmission, and may have potential applications in the design of terahertz devices.

Tunable low-pass MEMS filter using defected ground structures (DGS)

A tunable bandstop filter applying the micromachined technology is designed, simulated and fabricated. The filter is realized by incorporating defected ground structures structures with the micromachined switches. The surface micromachining fabrication process is employed on the high resistivity silicon substrate to fabricate the filter. The measured 3-dB cut-off frequency changes of the reconfigurable low-pass filter for four different states has been analyzed. The measured insertion loss is very lower in four tuning range of the filter. This work demonstrates the possibility of high-performance compact-size reconfigurable filters at mm-waves using multiple-contact micromachined switches. Simulated and measured results of the proposed filter show good agreements. The entire reconfigurable filter was realized in a small chip size of 1.2mm×1.5mm.

Tunable Low-Pass MEMS Filter Using Defected Ground Structures (DGS)

In this paper, fully monolithic tunable millimeter-wave filters using defected ground structures (DGS) are proposed using the CPW-based periodic structures with novel multiple-contact MEMS switches. Millimeter-wave low-pass filters were designed, fabricated, and tested. The cascaded CPW-based periodic structures, with low-pass intrinsic filtering characteristics, are reconfigured into a self-similar single unit cell by the operation of the novel multiple-contact MEMS switches with single actuation. In the first order tuning, the 3-dB cut-off frequency changes from 8.2GHz to10.5GHz, and the second order tuning is 8.2GHz to 16.8GHz. The tested results show that the pass-band ripple is less than 1.2dB and the maximal out-of-band rejection is better than 27dB. The chip size of the low-pass filter is 2.5mm×1.2mm.

Application of the Polychromatic Defocus Transfer Function to Multifocal Lenses

To model the performance of multifocal lenses in polychromatic lighting. The defocus transfer function (DTF) is a mathematical technique for illustrating the optical transfer function for all levels of defocus at a given wavelength. A polychromatic version of the DTF is developed that accounts for changes in cutoff frequency, reduction in diffraction efficiency, ocular chromatic aberration, and photoreceptor spectral sensitivity. The differences between the monochromatic and polychromatic DTF are illustrated with a diffractive multifocal intraocular lens. Polychromatic analysis shows an increase in depth of field of diffractive lenses relative to assessment at a single wavelength. The polychromatic DTF is a useful tool for analyzing presbyopia treatments under "white-light" viewing conditions and provides feedback to lens designers on anticipated performance.

STUDIES IN NONLINEAR EFFECTS OF SQUEEZE FILM DAMPING IN MEMS STRUCTURES

International Journal of Computational Engineering ScienceVol. 04, No. 03, pp. 477-480 (2003) Testing and CharacterisationNo AccessSTUDIES IN NONLINEAR EFFECTS OF SQUEEZE FILM DAMPING IN MEMS STRUCTURESASHOK K. PANDEY and R. PRATAPASHOK K. PANDEYCranesSci MEMS Lab, Department of Mechanical Engineering, Indian Institute of Science, Bangalore-560012, India Search for more papers by this author and R. PRATAPCranesSci MEMS Lab, Department of Mechanical Engineering, Indian Institute of Science, Bangalore-560012, IndiaCorresponding author. Tel.: +91-80-3942671; +91-80-3608659; fax: +91-80-3600648. Search for more papers by this author https://doi.org/10.1142/S1465876303001551Cited by:8 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail AbstractStructural vibration studies of various MEMS structures have revealed that whenever a suspended flat structure vibrates normal to a fixed substrate with a very small gap between the two surfaces, the squeeze film damping due to fluid present in the gap dominates the dissipation mechanism by orders of magnitude compared with other losses. Since the quality factor is a critical parameter in the design of MEMS devices and it depends on damping, a careful evaluation of the squeeze film damping is necessary. The most often used formulas for evaluating this damping, due to Blech and Griffins, are based on the linearized Reynolds equation. These formulas are applicable for small amplitude oscillations. In this paper, we consider all nonlinear terms in the governing equation of the flow between the plates and study the effects of these terms on the damping characteristics. We show that as the amplitude of vibration increase, the damping force increases faster than the spring force of the fluid and, therefore, the cut-off frequency changes nonlinearly.Keywords:Nonlinear Squeeze film damplingcutoff squeeze numberquality factor References W. E. Langlois, Quart. App. Mathematics 20(2), 131 (1962). Crossref, Google Scholar O. Pinkus and B. Sternlicht , Theory of Hydrodynamic Lubrication ( McGraw-Hill , NY , 1961 ) . Google ScholarJ. J. Blech, J. of Lubrication Technology 105, 615 (1983). Crossref, Google ScholarW. S. Griffin, H. H. Richardson and S. Yamamami, Trans. of ASME 451 (1966). Google ScholarHiroshi Hosaka, Kiyoshi Itao and Susumu Kuroda, Sensors and Actuators A 49, 87 (1995). Crossref, Google ScholarM. Andrews, I. Harris and G. Turner, Sensors and Actuators A 36(2), 79 (1993). Crossref, Google Scholar Y.-J. Yang, M.-A. Gretillat and S. D. Senturia, Effect of Air Damping on the Dynamics of Nonuniform Deformations of Microstructures, Transducers 97, Chicago, 2 (June 1997) 1093-1096 . Google Scholar J. Antunes and P. Piteau, A Nonlinear Model for Squeeze-film Dynamics under Axial Flow, ASME PVP Conference, Symposium on Flow-Induced Vibration 2001, Atlanta, USA, 2001 . Google Scholar J. W. Thomas , Numerical Partial Differential Equations: Finite Difference Methods ( Springer-Verlag , New York , 1995 ) . Crossref, Google Scholar FiguresReferencesRelatedDetailsCited By 8Damping and stiffening forces of a squeeze film between two platesShangyi Chen and Z. C. Feng28 October 2016 | Nonlinear Dynamics, Vol. 87, No. 3A novel approach to the analysis of squeezed-film air damping in microelectromechanical systemsWeilin Yang, Hongxia Li, Aveek N Chatterjee, Ibrahim (Abe) M Elfadel and Ilker Ender Ocak et al.2 November 2016 | Journal of Micromechanics and Microengineering, Vol. 27, No. 1Prediction of pull-in phenomena and structural stability analysis of an electrostatically actuated microswitchC. Sri Harsha, C. S. Prasanth and B. Pratiher17 May 2016 | Acta Mechanica, Vol. 227, No. 9Analysis of squeeze film air damping in MEMS with lattice Boltzmann methodWeilin Yang, Hongxia Li, TieJun Zhang, Aveek N. Chatterjee and Ibrahim M. Elfadel1 May 2016Limits of linearity in squeeze film behavior of a single degree of freedom microsystemShujuan Huang, Diana-Andra Borca-Tasciuc and John Tichy2 November 2013 | Microfluidics and Nanofluidics, Vol. 16, No. 6Interface electronics and controller for an electrostatically actuated MEMS based microgripperMuhammad Bilal Saif, Shafaat A. Bazaz and Nisar Ahmed1 Dec 2009Thermal characterization of microheaters from the dynamic responseBalaji Jayaraman, Navakanta Bhat and Rudra Pratap9 July 2009 | Journal of Micromechanics and Microengineering, Vol. 19, No. 8Coupled nonlinear effects of surface roughness and rarefaction on squeeze film damping in MEMS structuresAshok Kumar Pandey and Rudra Pratap14 July 2004 | Journal of Micromechanics and Microengineering, Vol. 14, No. 10 Recommended Vol. 04, No. 03 Metrics History KeywordsNonlinear Squeeze film damplingcutoff squeeze numberquality factorPDF download

Upper cut-off frequency changes in quasi-periodic VLF emission events

Periodic changes in upper cut-off frequency of quasi-periodic VLF emission events are shown to be consistent with a Coroniti-Kennel type of interaction between geomagnetic micropulsations and the wave-particle interaction process itself. Expected relationships between phase and amplitude of the interrelated phenomena are pointed out and their validity discussed.