Wide Range Of Low Frequencies Research Articles

Analysis of a Geometrical-Stiffening Membrane Acoustic Metamaterial with Individually Tunable Multi-Frequencies

To realize a structure which can be conveniently tuned to multiple and wideband frequency ranges, a geometrical-stiffening membrane acoustic metamaterial (MAM) with individually tunable multiple frequencies is presented. The MAM is realized by a stacked arrangement of two membrane-magnet elements, each of which has a membrane with a small piece of steel attached in the centre. It can be tuned individually by adjusting the position of its compact magnet. The normal incidence sound transmission loss of the MAM is investigated in detail by measurements in an impedance tube. The test sample results demonstrate that this structure can easily achieve a transmission loss with two peaks which can be shifted individually in a wide low-frequency range. A theoretical consideration is analysed, the analysis shows that the magnetic effect related to this distance leads to a nonlinear attractive force and, consequently, nonlinear geometrical stiffening in each membrane-magnet element, which allows the peaks to be shifted. A reasonable design can make the structure have a good application prospect for low-frequency noise insulation where there is a need to adjust the transmission loss according to the spectrum of the noise source.

Active control of low frequency sound absorption of large sized micro-perforated panel absorber by using point source

This paper presents a theoretical investigation on actively controlling the low frequency sound absorption of large-sized micro-perforated panel absorber (MPPA) by using point source placed in the cavity. The large sized active MPPA is an easily implemented and cost-effective scheme for achieving superior low frequency sound absorption in large area. The modal analysis approach is used to establish the theoretical model of such active MPPA. The physical mechanism of active control is analyzed and some physical insights are summarized. Based on these conclusions, the error sensing strategy of the large sized active MPPA is also constructed. Finally, experimental tests are carried out to validate the theoretical modeling and findings. Results show that the sound absorption performance can be significantly improved after control in a wide low frequency range below the cutoff frequency. The optimal position of the point source is at the central of the cavity section, in which case the cutoff frequency is the largest and depends on the length of the short side of the active MPPA. The control mechanism of sound absorption improvement is to enhance the Helmholtz type resonant absorption by suppressing the cavity sound field. The high order cavity mode (except for (0,0,0) mode) has no contribution to sound absorption improvement due to the symmetrical property of their mode shape. The pressure release (PR) and impedance matching (IM) strategies are still applicable for the large sized active MPPA. Relative small sized cavity or low frequency excitation can guarantee the uniformity of the cavity sound field after control, which is conducive to achieve remarkable improvement of sound absorption.

Removal of System and CT Decaying DC Component in Fault Signal Using Selective Interval Integration

The fundamental current component at the secondary side of current transformer (CT) is often accompanied with decaying direct current (DC) component produced by the system and the CT. The discrete Fourier transform (DFT) is known as a powerful tool in digital protection for estimation of the fundamental component in the presence of harmonics. The decaying DC component, unlike harmonics, is not limited to a distinct frequency but include a wide range of low frequencies affecting the DFT output by undesirable oscillations. This paper proposes a method based on two selective interval integration for accurate estimation and elimination of the decaying DC component from the fundamental component of fault current. The two data intervals are separated by half a cycle minus one interval. The proposed algorithm is first tested using data derived by mathematical formulation under different conditions such as sampling frequency, noise, and characteristics of the DC component. The algorithm is also tested by simulation study using PSCAD/EMTDC for different fault conditions in a typical transmission system. In order to test the algorithm performance for practical applications, a Mho distance relay is simulated in the Matlab/Simulink environment. All the test cases verify efficiency, accuracy and speed of the proposed algorithm.

Features of electromagnetic and noise environment in the workplace of the staff of MRI departments: problems of hygienic regulation and control

Introduction. Magnetic resonance imaging (MRI) is one of the most dynamically developing, high-tech diagnostic methods. At the same time, the specific features of the electromagnetic environment and noise exposures at the medical and technical personnel operating and maintaining MRI workplaces are not well understood and are currently not taken into account when conducting a hygienic assessment of working condition in the Russian Federation. The aim of study is scientific substantiation of the need to improve the hygienic assessment of electromagnetic and noise conditions in the workplace of the staff of MRI departments. Materials and methods. An analytical method to evaluate the current domestic and foreign regulatory and methodological documents that determine the hygienic assessment requirements for electromagnetic environment and noise in MRI departments was used. More than 4 thousand measurements of the intensity of electromagnetic fields and noise in 88 rooms in Moscow equipped with different types of MRI (from 0,1 mT to 3 T) and studies of the performing duration various types of work and electromagnetic field and noise exposure on personnel was done. The amplitude-time and frequency parameters of the variables of the electromagnetic field (EMF) and sound pressure detailed studies have been carried out. Results. Significant differences in controlled indicators, acceptable exposure levels, and control methods has revealed by critical analysis of domestic and foreign hygienic regulatory and methodological documents. The studies showed that the medical and technical personnel of MRI departments are exposed with high spatial and temporal gradients static magnetic fields and extreme levels of noise during diagnostic work in the immediate vicinity of the tomograph magnet. The presence of time-varying magnetic fields in a wide range of low frequencies and radio frequencies having a complicated (pulsed) waveform is a feature of the electromagnetic environment in MRI. New standardized control parameters and limit levels for low-frequency ranges magnetic fields for their adequate hygienic assessment are proposed. Conclusion. The studies made it possible to substantiate the proposals the improving of electromagnetic field and noise hygienic assessment at the MRI staff workplaces.

Enhancement of low-frequency magnetic permeability and absorption by texturing flaky carbonyl iron particles

We have fabricated flaky carbonyl iron particles (CIPs) with (200) crystal planes and easy axes preferred to parallel to the flake surfaces by developing a slip-induced grain orientation method. The method involves heat-treatment and subsequent wet milling of spherical CIPs. The heat-treatment in inert atmosphere is used to grow the grain sizes of raw spherical CIPs. During the wet milling process, the large grains in the heat-treated spherical CIPs are preferred to slip along directions {110} and rotate to produce (200) preferred orientation while they are being compressed into flaky shape. The grain orientation factor (a) increases from 0 to 0.42 with grain sizes grown up by controlled heat-treatment temperature. The texture decreases the in-plane magnetic anisotropy field of the flaky CIPs. The aligned flaky textured CIPs brings about an additional magnetic resonance at low frequency with respect to the non-textured counterpart, and exhibit enhanced double magnetic resonances. When a = 0.42, the imaginary permeability peak reaches 3.52 at 0.7 GHz, which is 2 times larger than previously reported values. The reflection loss less than −10 dB for their-based absorbing composite slab at thickness of 1 mm can cover a wide low frequency range between 0.4 and 2 GHz when the volume fraction of aligned textured flaky CIPs changes from 40 to 50%. This work provides an effective route to enhance magnetic permeability and loss at low frequencies, which has been a great challenge in recent years.

Soft metamaterials with dynamic viscoelastic functionality tuned by pre-deformation.

The small amplitude dynamic response of materials can be tuned by employing inhomogeneous materials capable of large deformation. However, soft materials generally exhibit viscoelastic behaviour, i.e. loss and frequency-dependent effective properties. This is the case for inhomogeneous materials even in the homogenization limit when propagating wavelengths are much longer than phase lengthscales, since soft materials can possess long relaxation times. These media, possessing rich frequency-dependent behaviour over a wide range of low frequencies, can be termed metamaterials in modern terminology. The sub-class that are periodic are frequently termed soft phononic crystals although their strong dynamic behaviour usually depends on wavelengths being of the same order as the microstructure. In this paper we describe how the effective loss and storage moduli associated with longitudinal waves in thin inhomogeneous rods are tuned by pre-stress. Phases are assumed to be quasi-linearly viscoelastic, thus exhibiting time-deformation separability in their constitutive response. We illustrate however that the effective incremental response of the inhomogeneous medium does not exhibit time-deformation separability. For a range of nonlinear materials it is shown that there is strong coupling between the frequency of the small amplitude longitudinal wave and initial large deformation.This article is part of the theme issue ‘Rivlin's legacy in continuum mechanics and applied mathematics’.

Deepening subwavelength acoustic resonance via metamaterials with universal broadband elliptical microstructure

Slow sound is a frequently exploited phenomenon that metamaterials can induce in order to permit wave energy compression, redirection, imaging, sound absorption, and other special functionalities. Generally, however, such slow sound structures have a poor impedance match to air, particularly at low frequencies and consequently exhibit strong transmission only in narrow frequency ranges. This therefore strongly restricts their application in wave manipulation devices. In this work, we design a slow sound medium that halves the effective speed of sound in air over a wide range of low frequencies (hence our referral to the microstructure as “broadband”), whilst simultaneously maintaining a near impedance match to air. This is achieved with a rectangular array of acoustically rigid cylinders of elliptical cross section, a microstructure that is motivated by combining transformation acoustics with homogenization. Microstructural parameters are optimized in order to provide the required anisotropic material properties as well as near impedance matching. We then employ this microstructure in order to halve the size of a quarter-wavelength resonator (QWR) or equivalently to halve the resonant frequency of a QWR of a given size. This provides significant space savings in the context of low-frequency tonal noise attenuation in confined environments where the absorbing material is adjacent to the region in which sound propagates, such as in a duct. We employ the term “universal” since we envisage that this microstructure may be employed in a number of diverse applications involving sound manipulation.

The acoustic power of a vibrating clamped circular plate revisited in the wide low frequency range using expansion into the radial polynomials.

This study deals with the classical problem of sound radiation of an excited clamped circular plate embedded into a flat rigid baffle. The system of the two coupled differential equations is solved, one for the excited and damped vibrations of the plate and the other one-the Helmholtz equation. An approach using the expansion into radial polynomials leads to results for the modal impedance coefficients useful for a comprehensive numerical analysis of sound radiation. The results obtained are accurate and efficient in a wide low frequency range and can easily be adopted for a simply supported circular plate. The fluid loading is included providing accurate results in resonance.

Ultra Low-Energy Relaxation Oscillator With 230 fJ/cycle Efficiency

An ultra low-energy oscillator circuit is presented for use in picowatt level systems. The core oscillator uses an 18 transistor 3 stage architecture designed to minimize short circuit current. In addition, a transistor threshold is used to set the trip point as opposed to a voltage reference and comparator scheme, leading to overall energy savings. While operating across a wide range of low frequencies from 18 to 1000 Hz, the oscillator core consumes 110 fJ/cycle at 0.6 V. The circuit is demonstrated alongside an integrated current source to set the reference frequency. The combined system consumes a total power of 4.2 pW at 18 Hz, resulting in 230 fJ/cycle at 0.6 V.

Determining Low-Frequency Earth Return Impedance: A Consistent Electromagnetic Approach

In this paper, we present a new method for determining the earth return impedance. The proposed method, based on electromagnetic field theory, is very accurate and convenient for the analysis of earth behavior over a wide range of low frequencies. It takes into account all crucial physical events, including an exact treatment of the skin and proximity effects within the earth. Combining analytical and numerical procedures for finding the exact current distribution and the earth return impedance per unit length, we have developed an efficient and powerful tool that is described in this paper. In all calculations, real situations were considered, consisting of various soil resistivity values, overhead conductor heights, and combinations of parameters at various frequencies. Although most commonly applied simplified formulas give accurate results at industrial frequencies, significantly worse results are obtained in the presence of higher harmonics. Hence, verification of the developed method was achieved by comparing obtained results with the results of the Carson-Clem simplified formula and the FEM-based calculations. The generality of the developed program indicates that it may also be applied for calculating the earth return impedance in the presence of higher current harmonics.

Broadband attenuation of Lamb waves through a periodic array of thin rectangular junctions

We study theoretically subwavelength physical phenomena, such as resonant transmission and broadband sound shielding for Lamb waves propagating in an acoustic metamaterial made of a thin plate drilled with one or two row(s) of rectangular holes. The resonances and antiresonances of periodically arranged rectangular junctions separated by holes are investigated as a function of the geometrical parameters of the junctions. With one and two row(s) of holes, high frequency specific features in the transmission coefficient are explained in terms of a coupling of incident waves with both Fabry-Perot oscillations inside the junctions and induced surface acoustic waves between the homogeneous part of the plate and the row of holes. With two rows of holes, low frequency peaks and dips appear in the transmission spectrum. The choice of the distance between the two rows of holes allows the realization of a broadband low frequency acoustic shielding with attenuation over 99% for symmetric waves in a wide low frequency range and over 90% for antisymmetric ones. The origin of the transmission gap is discussed in terms of localized modes of the ``H'' element made by the junctions, connecting the two homogeneous parts of the plate.

Transformation Algorithm of Dielectric Response in Time-Frequency Domain

A transformation algorithm of dielectric response from time domain to frequency domain is presented. In order to shorten measuring time of low or ultralow frequency dielectric response characteristics, the transformation algorithm is used in this paper to transform the time domain relaxation current to frequency domain current for calculating the low frequency dielectric dissipation factor. In addition, it is shown from comparing the calculation results with actual test data that there is a coincidence for both results over a wide range of low frequencies. Meanwhile, the time domain test data of depolarization currents in dry and moist pressboards are converted into frequency domain results on the basis of the transformation. The frequency domain curves of complex capacitance and dielectric dissipation factor at the low frequency range are obtained. Test results of polarization and depolarization current (PDC) in pressboards are also given at the different voltage and polarization time. It is demonstrated from the experimental results that polarization and depolarization current are affected significantly by moisture contents of the test pressboards, and the transformation algorithm is effective in ultralow frequency of 10−3 Hz. Data analysis and interpretation of the test results conclude that analysis of time-frequency domain dielectric response can be used for assessing insulation system in power transformer.

Design of a Preamplifier for Capacitive Sensors With Wide Low-Frequency Range and Low Drift Noise

This paper describes a novel DC-coupled single input preamplifier which is robust to drift noise and variation due to the circuit operation conditions. The preamplifier is intended for capacitive sensors such as the capacitive electret film type sensor, which is one of the most sensitive and accurate sensors presently available. There are both DC-coupled and AC-coupled conventional preamplifiers available for such sensors. However, DC-coupled preamplifiers do not always suppress drift noise and variation, while AC-coupled preamplifiers require a large capacitor to expand the frequency range, thus making the large-scale integration (LSI) very large and inducing problems associated with the large capacitor. We describe a DC-coupled preamplifier with wider low-frequency range as well as less drift noise and variation. The design combines a time constant amplifier and rail-to-rail transconductance amplifier, and is implemented in an LSI with 6-GHz fT bipolar 11-V process with 5-V supply voltage. For an LSI with this preamplifier, the output voltage variation due to changes in load current is within ± 35 mV, whereas the variation for a conventional preamplifier is proportional to the load current up to 2.5 V. The output voltage drift due to changes in temperature and load current is restricted to within ± 35 mV for load resistance of 1-5 kΩ , whereas the drift for a conventional preamplifier is 800 mV.

Human sympathetic outflows to skin and muscle target organs fluctuate concordantly over a wide range of time‐varying frequencies

Frequency-domain analyses of simultaneously recorded skin and muscle sympathetic nerve activities may yield unique information on otherwise obscure central processes governing human neural outflows. We used wavelet transform and wavelet phase coherence methods to analyse integrated skin and muscle sympathetic nerve activities and haemodynamic fluctuations, recorded from nine healthy supine young men. We tested two null hypotheses: (1) that human skin and muscle sympathetic nerve activities oscillate congruently; and (2) that whole-body heating affects these neural outflows and their haemodynamic consequences in similar ways. Measurements included peroneal nerve skin and tibial nerve muscle sympathetic activities; the electrocardiogram; finger photoplethysmographic arterial pressure; respiration (controlled at 0.25 Hz, and registered with a nasal thermistor); and skin temperature, sweating, and laser-Doppler skin blood flow. We made recordings at ∼27°C, for ∼20 min, and then during room temperature increases to ∼38°C, over 35 min. We analysed data with a wavelet transform, using the Morlet mother wavelet and wavelet phase coherence, to determine the frequencies and coherences of oscillations over time. At 27°C, skin and muscle nerve activities oscillated coherently, at ever-changing frequencies between 0.01 and the cardiac frequency (∼1 Hz). Heating significantly augmented oscillations of skin sympathetic nerve activity and skin blood flow, arterial pressure, and R-R intervals, over a wide range of low frequencies, and modestly reduced coordination between skin and muscle sympathetic oscillations. These results suggest that human skin and muscle sympathetic motoneurones are similarly entrained by external influences, including those of arterial baroreceptors, respiration, and other less well-defined brainstem oscillators. Our study provides strong support for the existence of multiple, time-varying central sympathetic neural oscillators in human subjects.

The pairwise phase consistency: A bias-free measure of rhythmic neuronal synchronization

Oscillatory activity is a widespread phenomenon in nervous systems and has been implicated in numerous functions. Signals that are generated by two separate neuronal sources often demonstrate a consistent phase-relationship in a particular frequency-band, i.e., they demonstrate rhythmic neuronal synchronization. This consistency is conventionally measured by the PLV (phase-locking value) or the spectral coherence measure. Both statistical measures suffer from significant bias, in that their sample estimates overestimate the population statistics for finite sample sizes. This is a significant problem in the neurosciences where statistical comparisons are often made between conditions with a different number of trials or between neurons with a different number of spikes. We introduce a new circular statistic, the PPC (pairwise phase consistency). We demonstrate that the sample estimate of the PPC is a bias-free and consistent estimator of its corresponding population parameter. We show, both analytically and by means of numerical simulations, that the population statistic of the PPC is equivalent to the population statistic of the squared PLV. The variance and mean squared error of the PPC and PLV are compared. Finally, we demonstrate the practical relevance of the method in actual neuronal data recorded from the orbitofrontal cortex of rats that engage in a two-odour discrimination task. We find a strong increase in rhythmic synchronization of spikes relative to the local field potential (as measured by the PPC) for a wide range of low frequencies (including the theta-band) during the anticipation of sucrose delivery in comparison to the anticipation of quinine delivery.

Temperature dependence of shear wave attenuation in partially molten gabbronorite at seismic frequencies

Torsion oscillatory deformation experiments have been performed at high temperatures (600–170°C) and over a wide range of low frequencies (20–10−2 Hz) on fine-grained gabbronorite samples from the Oman ophiolite in order to determine the shear wave attenuation as a function of temperature and melt fraction. The specimens have a small and uniform grain size (0.25–0.3 mm) and do not contain secondary, hydrated minerals. Measurements of internal friction (Q−1) were performed using a forced oscillatory torsion apparatus at small strains (∽10−7), and with increasing small temperature steps to reduce thermal microcracking. The general dependence of Q−1 to frequency is Q−1∝ω−α, where ω is the angular velocity of forced oscillations and α is an empirical exponent. Below the melting temperature (∽1050°C), α has average values of ∽0.15 at low frequency (≤ 0.5 Hz) and 0.06 at higher frequency. Above the melting temperature, α has average values of ∽0.22 at low frequency and −0.02 at higher frequency. This frequency dependence of Q−1 is attributed to a viscoelastic behaviour due to the diffusion controlled grain boundary sliding, and partially to the squirt flow of the melt-phase wetting grain boundaries. The onset of melting is associated with a markedly higher Q−1 and a stronger dependence of Q−1 on temperature. The melt-related mechanical dissipation process could be a melt squirt flow. The characteristic frequency for the melt squirt flow is ωm ∽ 0.15–300 Hz when the melt pocket aspect ratio is ∽10‒3–10‒2. Around the melting temperature the internal friction can be approximated by an experimental power law Q−1 = A · [ω‒1 · d‒1 · exp(‒Ea/RT)]α with α ∽0.08, A = 34.72 sαμmα and Eα ∽ 873 kJ mol−1.

A Low-Frequency Counter Using Time-Discriminant Connectionist Systems

A new simple, accurate, and instantaneous low-frequency counter based on the time-discriminant connectionist system , is proposed. The new counter could be used as a general-purpose frequency counter to measure a wide range of low frequencies. The accuracy of the proposed counter can be increased by increasing the number and the size of the EPROMS used and by increasing the clock frequency. Measurements from a prototype system agree well with the analysis.

A low-frequency duty-cycle dependent sinusoidal oscillator

An oscillator circuit which incorporates periodically operated switches is presented. The circuit is a modified feedback oscillator consisting of two integrators and an inverter, in which a switch has been put in series with each capacitor. By varying the pulse-switching frequency, the frequency of sinewave oscillations can be proportionally varied over a wide low-frequency range which covers six decades. The technique is general and can be applied to any conventional RC oscillator.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

ZnO transducers on InSb for low-temperature ultrasonics

A reproducible method for the fabrication of ZnO transducers on InSb substrates is described. These transducers are being used for ultrasonic attenuation measurements over a wide low-frequency range (90–600 MHz) at very low temperatures(0.1–4.2 K). They have been characterized by their x-ray diffraction patterns, their insertion loss, and their contribution to the acoustic attenuation in InSb at low temperatures.

Low-frequency conductance of thermally grown SiO2films

A.c. conduction in thermally grown silicon dioxide films is studied in the wide range of low frequencies 10-2 Hz-10 4 Hz. A conductivity-frequency dependence of the form onc.xwn, with values of n smaller than or equal to unity, is observed. A Debye-type loss peak moving to higher frequencies with rise in temperature is also observed in some samples. The activation energy for this process is found to be 1 I eV. The conductivity-frequency dependence may be explained as a hopping process, while the Debyo-type loss peak may be due to the presence of some impurity ion species.