Broadband Frequency Research Articles

Superior broadband sound absorption in hierarchical ultralight graphene oxide aerogels achieved through emulsion freeze-casting

Ultra-lightweight aerogels have profound applications as advanced multi-functional sound-absorbing materials. At present, conventional aerogels are associated with pores with sizes too small for effective sound absorption in the audible range. In this work, we fabricated a hierarchically porous ultralight graphene oxide (GO) aerogel through a novel emulsion freeze-casting process of an air-in-water emulsion. Microstructural analysis of the aerogels reveals a hierarchical pore morphology consisting of highly inter-connected pores produced from the emulsion bubbles and the micro-pores produced by freeze-casting. Sound absorption measurements revealed that an aerogel with a surfactant-to-GO weight ratio of 1:1 displays an average absorption coefficient of up to 0.86 at a broadband frequency range between 250 Hz and 6300 Hz, a vast improvement by up to 58% as compared to conventional GO aerogels. Numerical microstructural models were developed to model the acoustic absorption phenomenon and understand the mechanisms behind enhanced sound absorption. These models identified the enhanced acoustic absorption as attributed to the increased air-wetting area as derived from the high reticulation rate in this hierarchical microstructure. Overall, we demonstrate the potential of leveraging the highly inter-dependent material-process-structure relationships of GO aerogel production to achieve hierarchical GO aerogels as advanced ultralight sound-absorbing materials.

Bi-stable electromagnetic generator with asymmetrical potential wells for low frequency vibration energy harvesting

Efficient energy harvesting from low and broadband frequency has always been a challenging problem. This paper proposes a solution in the form of an electromagnetic bi-stable vibration energy harvester (BVEH) with asymmetric potential barriers, in which the vibrator is attracted to the top magnet and repelled from the bottom magnet. By combining a magnetic spring and a mechanical spring, the BVEH achieves bi-stability with asymmetric potential barriers, enabling large amplitude vibration as the vibrator travels between the two potential wells. The traditional magnetic force model is modified to establish an electromechanical coupling model that accurately predicts the output of the BVEH. Numerical simulation results solved by the Runge-Kutta method show good agreement with experimental results. In addition, a prototype is fabricated and the system parameters are adjusted to achieve high-energy oscillation at the lowest frequency of 4 Hz. Under a sinusoidal excitation with an amplitude of 4 mm, the BVEH's bandwidth is expanded to 6 Hz, and the maximum power can reach 72.76 mW. Finally, the feasibility of the BVEH is demonstrated by powering different electrical appliances. The study provides a new technical approach for energy harvesting under low and broadband frequency vibration excitation.

Ultrawide-Angle and Compact Leaky-Wave Antenna Based on Complex Periodic Structure

A compact antenna with the characteristic of wide-angle frequency beam scanning based on complex periodic structure is proposed. A double gradient spoof surface plasmon polaritons (SSPPs) structure is added to the standard microstrip line for reducing the antenna size. The miniaturization of the proposed antenna is analyzed with the method of dispersion curves and electric field distribution. Periodic changes of the groove depth and trapezoidal gradient structure increase the beam scanning angle from backward to forward and suppress the open stopband effect. The proposed structure is printed on the FR-4 substrate with a compact size of 0.23λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 3.71λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the wavelength of the lowest operating frequency), and has a working frequency band from 12.5 to 42.5 GHz for |S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> |< –10 dB. Meanwhile, a wide beam-scanning angle of 142° (–52° to +90°) and an average gain of 7.9 dBi are obtained by simulation and measurement. The proposed leaky-wave antenna has a potential application in wireless communication system due to its small size, broad frequency band, wide beam scanning angle and low cost.

An Adaptive Broadband Active Impedance Matching Network for Underwater Electroacoustic Transduction System

Since the power supply is limited in ships, the impedance matching network is essential for a high-power underwater electroacoustic transduction system. However, the existing impedance matching networks can barely achieve full impedance matching under a wide frequency change range. This paper proposes a shunt active impedance matching network (SAIMN) to achieve adaptive broadband impedance matching (up to 400Hz) for increasing power utilization from a power amplifier to the transducer. Firstly, the load impedance characteristic and operation principle of SAIMN are introduced mathematically. Secondly, the parameter design method is provided to improve impedance matching performance. After that, a compound control strategy is presented to control the power amplifier and proposed SAIMN. Finally, the simulation and experimental results validate the effectiveness of the proposed SAIMN and its control strategy. The results clearly show that the proposed SAIMN can adaptively match load impedance for the different loads in a broadband frequency range. The power factor of the power amplifier is significantly improved, and the current stress of the switching device is significantly reduced with the proposed SAIMN.

A Mechanism for Large-Amplitude Parallel Electrostatic Waves Observed at the Magnetopause

Large-amplitude electrostatic waves propagating parallel to the background magnetic field have been observed at the Earth’s magnetopause by the Magnetospheric Multiscale (MMS) spacecraft. These waves are observed in the region where there is an intermixing of magnetosheath and magnetospheric plasmas. The plasma in the intermixing region is modeled as a five-component plasma consisting of three types of electrons, namely, two counterstreaming hot electron beams and cold electrons, and two types of ions, namely, cold background protons and a hot proton beam. Sagdeev pseudo-potential technique is used to study the parallel propagating nonlinear electrostatic solitary structures. The model predicts four types of modes, namely, slow ion-acoustic mode, fast ion-acoustic mode, slow electron-acoustic mode and fast electron-acoustic modes. Except the fast ion-acoustic mode, all other modes support solitons. Whereas slow ion-acoustic solitons have positive potentials, both slow and fast electron-acoustic solitons have negative potentials. For the case of 4% cold electron density, the slow ion-acoustic solitons have electric field ∼(40–120) mV m−1. The fast Fourier transforms (FFT) of slow ion-acoustic solitons produce broadband frequency spectra having peaks between ∼100 Hz to 1000 Hz. These theoretical predictions are in good agreement with the observations. The slow and fast electron-acoustic solitons could be relevant in explaining the low-intensity high (&gt;1 kHz) frequency waves which are also observed at the same time.

Broadband cross circularly polarized carpet invisibility cloak based on Pancharatnam–Berry phase reflective metasurface

We present a broadband circularly polarized reflective metasurface carpet cloak. The metasurface unit cell is composed of two crossed H-shaped metal patterns. When the incident wave is left-handed circularly polarization (LCP), the metasurface can achieve high-efficiency cross-polarization conversion in the wide frequency range. The cross-polarization reflection efficiency is higher than 94%. By using the proposed cross-polarized reflection unit cell structure, the phase gradient metasurfaces based on the Pancharatnam–Berry phase principle is proposed, and a broadband reflective metasurface invisibility cloak is constructed. The simulation results show that when the LCP light is incident vertically, the proposed carpet cloak can significantly suppress the scattering and reconstruct the wavefront in the broadband frequency range.

Flexural–torsional vibration reduction of an eccentric phononic crystal pipe conveying fluid

Flexural vibration and torsional vibration pervasively exist in slender structures, which often lead to noise emission and fatigue failure of structures. In order to effectively suppress coupled flexural–torsional vibrations of defective devices transporting fluid, this paper introduces an eccentric fluid-conveying pipe axially made of periodically varying materials, and the flexural–torsional bandgap (BG) characteristics of such phononic crystal pipe system are investigated. With consideration of warping effect, coupled flexural–torsional motion equations are established. The complex band structure of sub-structure and frequency response function and attenuation configuration of topological structure are attained by applying the spectral element method. In comparison with literatures only yielding total merging BG of flexural and torsional vibrations, this paper presents a novel result of completely separate flexural BG and torsional BG, which enables individual manipulation of the two elastic waves. Meanwhile, broadband frequency BGs are harvested in the proposed pipe structure, especially for torsional vibration, demonstrating an excellent vibration isolation property. Complicated regulation mechanism of eccentric, warping effects and some critical parameters is explored. This study is expected to provide a new approach for flexural–torsional vibration reduction of engineering fluid-transporting devices.

Low Frequency Attenuation Characteristics of Two-Dimensional Hollow Scatterer Locally Resonant Phonon Crystals.

The finite element method (FEM) was applied to study the low frequency band gap characteristics of a designed phonon crystal plate formed by embedding a hollow lead cylinder coated with silicone rubber into four epoxy resin short connecting plates. The energy band structure, transmission loss and displacement field were analyzed. Compared to the band gap characteristics of three traditional phonon crystal plates, namely, the square connecting plate adhesive structure, embedded structure and fine short connecting plate adhesive structure, the phonon crystal plate of the short connecting plate structure with a wrapping layer was more likely to generate low frequency broadband. The vibration mode of the displacement vector field was observed, and the mechanism of band gap formation was explained based on the spring mass model. By discussing the effects of the width of the connecting plate, the inner and outer radii and height of the scatterer on the first complete band gap, it indicated that the narrower the width of the connecting plate, the smaller the thickness; the smaller the inner radius of the scatterer, the larger the outer radius; and the higher the height, the more conducive it is to the expansion of the band gap.

Giant panda (Ailuropoda melanoleuca) neonates use broadband calls to communicate with their mothers.

Infant call structure should have evolved to elicit maximum maternal attention and investment. Neonates of giant pandas produce three types of vocalizations reported to be vitally important in the context of mother-infant communications. However, how cubs, 0-15 days old, communicate with their mothers to elicit maternal care remains unknown. We analyzed 12 different call parameters of 3475 squawks, 1355 squalls, and 491 croaks from 11 captive giant panda (Ailuropoda melanoleuca) neonates from age 0 to 15 days. In playback experiments, we also tested whether mothers could detect ultrasound. Our results show that neonates use broadband calls with ultrasonic frequencies up to 65 kHz to convey information about their physiological needs and to attract maternal care. In playback experiments, we tested if mothers reacted differently to broadband calls (BBC) than to artificially altered calls that included only frequencies <20 kHz (AUDC) or calls that included only frequencies >20 kHz (USC). Playback confirmed that, although adult females responded significantly less often to USC, BBC than to or AUDC, they could detect USC, BBC and generally made appropriate behavioral responses, indicating a potential benefit for neonates to utilize ultrasonic and broadband frequencies. Our findings provide a new insight into mother-infant communication in giant pandas and will be helpful for reducing the mortality of cubs, younger than 1 month old, in captivity.

Electromagnetic interference shielding mechanisms of MMG@PVDF composites for a broadband frequency range

Shielding against electromagnetic interference is critical in everyday life, particularly in communication, radar, radio astronomy, and medical equipment. The electromagnetic pollution caused by current electronic devices is increasing rapidly, and it is difficult to achieve a stable and efficient material that can play a significant role at higher frequencies. Due to their lightweight, chemical stability, and outstanding microwave absorbing properties with the inclusion of carbon materials, conductive polymer composites are the most promising EMI shielding materials for overcoming this problem. The PVDF@MMG polymer composite was formed in this research using an ultrasonication process. Fourier transform infrared, thermogravimetric analysis, field emission scanning electron microscopy, vibrating sample magnetometer, and vector network analyzer were used to investigate the physicochemical properties of the composite. At higher frequencies, the obtained polymer nanocomposite demonstrated remarkable shielding effectiveness of 51.74 dB with a low thickness of 3 mmat 12–18 GHz frequency range. The prepared nanocomposite has good EM stability across the frequency range, which is largely due to the filler's good dielectric and magnetic properties in the polymer matrix.

An ultrahigh sensitivity acoustic sensor system for weak signal detection based on an ultrahigh-Q CaF2 resonator

Acoustic sensors with ultrahigh sensitivity, broadband response, and high resolution are essential for high-precision nondestructive weak signal detection technology. In this paper, based on the size effect of an ultrahigh-quality (Q) calcium fluoride (CaF2) resonator, a weak acoustic signal is detected by the dispersive response regime in which an acoustic, elastic wave modulates the geometry and is converted to a resonance frequency shift. Through the structural design of the resonator, the sensitivity reaches 11.54 V/Pa at 10 kHz in the experiment. To our knowledge, the result is higher than that of other optical resonator acoustic sensors. We further detected a weak signal as low as 9.4 µPa/Hz1/2, which greatly improved the detection resolution. With a good directionality of 36.4 dB and a broadband frequency response range of 20 Hz–20 kHz, the CaF2 resonator acoustic sensing system can not only acquire and reconstruct speech signals over a long distance but also accurately identify and separate multiple voices in noisy environments. This system shows high performance in weak sound detection, sound source localization, sleep monitoring, and many other voice interaction applications.

Deep learning-based denoising of acoustic images generated with point contact method

Abstract The versatile nature of ultrasound imaging finds applications in various fields. A point contact excitation and detection method is generally used for visualizing the acoustic waves in Lead Zirconate Titanate (PZT) ceramics. Such an excitation method with a delta pulse generates a broadband frequency spectrum and wide directional wave vector. The presence of noise in the ultrasonic signals severely degrades the resolution and image quality. Deep learning-based signal and image denoising has been demonstrated recently. This paper bench-marked and compared several state-of-the-art deep learning image denoising methods with the classical denoising methods. The best-performing deep learning models are observed to be performing at par or, in some cases, even better than the classical methods on ultrasonic images. We further demonstrate the effectiveness and versatility of the deep learning-based denoising model for the unexplored domain of ultrasound/ultrasonic data. We conclude with a discussion on selecting the best method for denoising ultrasonic images. The impact of this work may help ultrasound-based defects identification equipment manufacturers to adopt a deep learning-based denoising model for more wider and versatile use.

Broadband subwavelength imaging of flexural elastic waves in flat phononic crystal lenses

Subwavelength imaging of elastic/acoustic waves using phononic crystals (PCs) is limited to a narrow frequency range via the two existing mechanisms that utilize either the intense Bragg scattering in the first phonon band or negative effective properties (left-handed material) in the second (or higher) phonon band. In the first phonon band, the imaging phenomenon can only exist at frequencies closer to the first Bragg band gap where the equal frequency contours (EFCs) are convex. Whereas, for the left-handed materials, the subwavelength imaging is restricted to a narrow frequency region where wave vectors in PC and background material are close to each other, which is essential for single-point image formation. In this work, we propose a PC lens for broadband subwavelength imaging of flexural waves in plates exploiting the second phonon band and the anisotropy of a PC lattice for the first time. Using a square lattice design with square-shaped EFCs, we enable the group velocity vector to always be perpendicular to the lens interface irrespective of the frequency and incidence angle; thus, resulting in a broadband imaging capability. We numerically and experimentally demonstrate subwavelength imaging using this concept over a significantly broadband frequency range.

Can Regulating Large Satellite Constellations as Monopolies Improve Sustainability Standards While Providing Effective and Equitable Internet Access?

Large satellite constellations (LSC) in Low Earth Orbit (LEO) provide the means to deliver internet to previously underserved populations. The LSC’s economic potential combined with a growing demand for internet access has led to multiple companies pledging their own internet LSCs in hopes of capitalizing on this need. This, in addition to growing interest in LEO, risks destabilizing the LEO environment if not regulated properly. The current U.S. Orbital Debris Standard Mitigation Practices (USODSMP) are woefully outdated and only sporadically followed by operators. Furthermore, there are no licensing regulations for other key sustainability requirements, such as propulsion and tracking. Accelerated growth of internet LSCs without proper regulation will lead to an unsafe and unsustainable LEO environment. There needs to be a change in how governments regulate private and public LSCs, in particular those providing internet telecommunications. Internet LSCs are the fastest growing population in LEO, often directly competing for orbits and broadband frequencies. In this thesis I investigate whether the U.S. could apply aspects of terrestrial utility infrastructure to better regulate satellite internet. In particular, I question whether the U.S. government could regulate internet LSCs as monopolies. Such regulatory control would allow the U.S. to better enforce and revise sustainability regulations and ensure fair access to internet services. Viewing internet as a utility, as a necessity of modern life to which all citizens deserve access, this thesis proposes the U.S. license a single internet LSC from within its borders. I explored two potential regulatory approaches: one based on the U.S.’s public utility system and one based on European internet infrastructure. I then presented these approaches to five experts in the field of space sustainability and solicited their feedback. From these results, I determined that both of my regulatory approaches are not feasible in their current forms within the U.S.; however, both approaches produce potential partial applications and variations of their initial design that may find traction. This thesis concludes that adopting an interoperability framework in LEO would allow for precise and targeted sustainability, technology, and capacity standards for each orbital corridor, empower regulatory bodies to enforce these standards, and preserve competition among Commerical Satellite Operators (CSOs). Finally, a government internet LSC competing against commercial internet LSCs, akin to terrestrial municipal internet, would demonstrate technological feasibility, satisfy equity requirements, and provide a baseline LSC for CSOs to compete against. To see the complete thesis, please visit https://scholar.colorado.edu/concern/undergraduate_honors_theses/5138jg42t.

Flexible, Stretchable, and Thin Films Based on Functionalized Carbon Nanofiber/Graphene Nanostructures for Electromagnetic Interference Shielding

Flexible electromagnetic shielding materials (ESMs) are becoming progressively important to flexible and conformal electronic devices operating in civil, aerospace, and military industries. There is a legitimate need to fabricate flexible, thin, featherweight, and stretchable materials with higher electromagnetic interference shielding effectiveness (EMI SE) that can handle harsh environments. Graphene nanoplatelets (GNPs) are synthesized by microwave-supported exfoliation of graphite proceeded by water-bath sonication, whereas functionalized carbon nanofibers (FCNFs) are synthesized via acid functionalization using HNO3. The effects of conventional CNF, FCNF, and GNP are observed by embedding in the polyurethane (PU) matrix to obtain flexible, highly stretchable (>400%), lightweight, and thin (300 μm) nanocomposites for enhanced EMI shielding performance. The GFCNF/PU nanocomposite showed uniform distribution and excellent microwave properties at low weight%. An improvement in the total EMI SE (SET) value is noticed by adding GNP (2, 4, and 6 wt %) in FCNF4, and a maximum SET value of 60.55 dB is obtained for FCNF/GNP (4/6 wt %). A negligible change in the value of EMI SE is observed after 500 mechanical bending cycles and bath-sonicating the nanocomposites in distilled water, confirming the efficiency of operating in the complex environmental condition. Further, the value of SET is measured by placing the nanocomposites at different bending radii inside the waveguide to verify the practical applicability of fabricated nanocomposites in conformal devices. EMI shielding is also calculated for nanocomposites through simulation in the CST microwave studio tool (in the X-band), further extending the analysis for broadband frequency (1–18 GHz).

Broadband and high-efficient reflective linear–circular polarization convertor based on three-dimensional all-metal anisotropic metamaterial at terahertz frequencies

In this paper, we proposed a broadband and high-efficient reflective linear–circular​ polarization convertor (LCPC) based on three-dimensional (3D) all-metal anisotropic metamaterial (AMM) in terahertz (THz) region. The LCPC unit-cell is composed of the stand-up inverted U-shaped resonator (USR) deposited on a ground plane. This LCPC design can convert both incident x- and y-polarized (y-pol) waves from linear to circular polarization after reflection in a broadband frequency range of 1.98-4.12THz with an axial ratio (AR) below 3 dB and polarization conversion efficiency (PCE) over 98% on average. The numerical simulation results show that the broadband polarization conversion of the designed LCPC is caused by the overlaps of stronger electric and magnetic dipolar coupling resonances. The polarization conversion properties of the designed LCPC can be adjusted by varying the geometric parameters of the USR. The proposed LCPC design has potential applications in many areas such as spectroscopy, detection, imaging, and communications in the THz region.

Homogeneous Two-Layer Structures for Broadband and Wide-Angle RCS Reduction

A two-layer stacked resonance absorber is proposed to achieve broadband and wide-angle radar cross section reduction. The most prominent structure feature is its simple geometry, which comprises two homogeneous dielectric slabs only. The two slab heights are chosen as one-sixth of a guided wavelength to form a three-way cancellation mode via interface reflections while a high permittivity dielectric is used to achieve low profile. The total thickness is less than 0.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda_{L}$</tex-math></inline-formula> . Because it involves no periodic structure, both monostatic and bistatic scattering levels can be suppressed in a broad frequency band and a wide angular region. A bouncing ray-based high frequency method was developed to perform parametric studies and design optimization in a timely fashion. Measured and simulated results are compared in spectral and angular domains. An 83% 10 dB RCS reduction band is observed. More than 10 dB back scattering suppression is achieved up to ±75° from the surface's normal.

On the Question of Equal Loudness of Sounds of Different Frequencies

Curves of equal loudness for sounds of different frequencies and auditory frequency responses are discussed. It is shown that curves of equal loudness do not describe the auditory frequency response and are applicable only to tonal signals. The auditory frequency response can adapt to the sound spectrum. For music, a broadband frequency response is formed; for speech, a frequency response with low-frequency suppression is used.

The metrological structural resolution for dimensional x-ray CT: analysis of the comparability between the CEB and the PBS method

The metrological structural resolution (MSR) describes the size of the smallest surface feature that can be measured dimensionally with a given accuracy. Several methods to determine the MSR for dimensional x-ray CT (dXCT) have been proposed in the past, two of which are compared and related in this publication, i.e. the curved-edge based (CEB) and the profile-based spectral (PBS) method. Both methods consider the surface structure as being described by a single surface on the relevant local scale and are also suitable for the application to optical or tactile coordinate measurement systems (CMSs). The CEB method evaluates the radii of circular shapes to determine the width of the Gaussian filter that describes the filtering of the surface by the CMS. The PBS method evaluates the instrument transfer function (ITF) determined by means of the measurement of a surface profile with a broad-band spatial frequency spectrum with a finite cut-off frequency. The PBS method yields the threshold wavelength for which the amplitude of the ITF drops below a certain level. While the resulting quantities of the two methods are very different they evaluate the same characteristic of the CMS. In this publication an analytical relation between those results is derived and shown to exist which is used to define the MSR. Simulated CT scans as well as dXCT measurements are performed to verify this relation. The results for the MSR obtained from both methods are consistent and deviations to the expectations based on theory are within a reasonable range.

Active noise control with selective perceptual equalization to shape the residual sound

An active noise control (ANC) system usually contains a control filter to generate an anti-noise sound that destructively interferes with the primary noise. The filtered reference least mean squares (FxLMS) algorithm is the seminal adaptive ANC algorithm that updates the control filter coefficients iteratively. However, the FxLMS algorithm is primarily designed to minimize the power of the residual sound, without consideration to the auditory sensation. Thus, the residual sound can be perceived to be different from the primary noise of a broad frequency band. When an ANC system running the FxLMS algorithm was piloted in a substation’s control room, workers expressed discomfort with the residual sound, even though the noise level was reduced. It was mainly because the workers were annoyed by the residual sound which they identified as an anomaly operating state. In order to resolve this problem, this paper proposes a selective perceptual equalization FxLMS (SPE-FxLMS) algorithm to retain the operating state in the residual sound. The simulation, experiment and subjective test results validate the effectiveness of the SPE-FxLMS algorithm in reducing the noise level and shaping the residual sound to create a sense of auditory familiarity for the workers.