dB Transmission Loss Research Articles

Low terahertz frequency on-chip multi-functional modulator with amplitude and phase modulation

Terahertz amplitude and phase modulation technologies are crucial for terahertz communication, radar, and imaging. However, most current approaches can only achieve either amplitude or phase modulation. In this paper, we present a low terahertz frequency on-chip multi-functional modulator that consists of a hybrid coupler and reflection meta-structure. High-performance amplitude modulation is achieved by combining series resonant absorption of series coupling branch with resonance enhancement of parallel coupling branch in the reflection meta-structure. Meanwhile, the enhanced resonance provides a larger range of phase shifts, enabling effective amplitude and phase modulation in two different frequency regions. Therefore, we realize an amplitude modulation in the range of 115–135 GHz with a minimum transmission loss of 4 dB and a modulation depth of over 10 dB. At the same time, we achieved a continuous phase shift in the 103–113 GHz region, as well as a 180° two-bit phase shift in the 107–109 GHz range with only 5.7 dB transmission loss. Our simple method for terahertz amplitude and phase multi-functional modulation offers the potential to construe terahertz multifunctional integrated systems.

An acoustic design project for final year engineering students using resonant arrays

This paper reviews the creation of a design course for mechanical engineering students at the University of Auckland. The primary objective of the course is to introduce students to the field of acoustics, teaching basic theoretical and experimental acoustics principles, together with product design and fabrication methods, in an engaging and educational manner. During the course, students were assigned the task of developing a noise reduction duct jacket that reduced sound propagation whilst allowing free airflow, all within specified dimensions. Students were required to create designs that attenuated a predefined sound spectrum, with both narrow-band and wide-band components. Over six weeks students developed their designs using mathematical modelling and fabricated their prototypes. The performance of their suppressors was assessed through sound pressure level and impedance tube transmission loss measurements. Students then modified their designs to maximise performance before final submission. Four-person teams produced a diverse range of implementations. The most successful designs achieved impressive performance with peak transmission loss up to 70dB, 2kHz bandwidths of 10 + dB transmission loss and a delta dBA reduction of 21.5 dB. Student feedback indicates a high level of satisfaction with the course, highlighting its effectiveness in imparting key knowledge and skills in acoustics engineering.

Functionalization of glass fiber woven fabrics by indium tin oxide (ITO) coatings for electromagnetic wave absorption

The effects of surface electrical conductivity and cascading order of ITO surface modified glass fiber woven fabrics (GFWF) on the electromagnetic (EM) behavior of hybrid type multilayered absorber structure were investigated. Additional objective was to create a design guideline for a multilayered absorbing structure according to the needs of specific EM wave absorber applications. For this purpose, indium tin oxide (ITO) sols were deposited on GFWFs by spray pyrolysis technique. The electrical conductivity of these conducting layers was controlled to incorporate EM wave absorbing characteristics to glass fabrics. Surface modified fabrics were then used as reinforcement in a hybrid type multilayered EM wave absorbing structure which is a combination of Jaumann and graded type absorbers effective in 8–12 GHz frequency range (X-band). The highest absorption performance design has been formed by cascading six ITO coated fabrics with gradually increasing electrical conductivity gradient from the front to back layer of the structure. The electromagnetic behavior of the multilayered absorber was tested both with and without metallic backing. The absorber design showed ∼−11 dB reflection loss and ∼−9 dB transmission loss corresponding to ∼80 % average absorption without metallic backing, while its reflection loss and average absorption values were ∼−10.5 dB and ∼93 %, respectively, with metallic backing. The front layer of the multilayered structure with the lowest surface electrical conductivity provided impedance matching between air and receiving surface of the structure minimizing reflection, where the gradual increase in surface conductivity of the layers towards the back provided balanced internal reflection and transmission leading to high overall EM wave absorption. Consequently, by controlling the grading of the surface conductivity of the layers through the thickness, responses of the absorbers can be adjusted to provide tailored characteristics for the required EM wave absorption levels.

Broadband muffler by merging negative density and negative compressibility

We present a broadband muffler using a metamaterial consisting of a membrane structure and a Helmholtz resonator. The membrane structure with ventilation exhibited negative effective density in the 290–435 Hz frequency range. The Helmholtz resonator exhibited negative effective compressibility in the 440–600 Hz range. We connected the two frequency ranges to continue the negativity by matching the end frequency of the negative effective density and the start frequency of the negative effective compressibility. The metamaterial muffler showed a wide range of imaginary components of wavenumbers from 290 to 600 Hz. The muffler exhibits 10 dB transmission loss or 10 % power transmission in the frequency range 275–500 Hz. The realization of a metamaterial muffler with a broad and low-frequency performance range is unprecedented when considering the resonant properties of the metamaterial.

A Dual-Layer Full-Phase Frequency-Selective Surface for Continuous 2-D Beam Scanning

Based on the generalized frequency selective surface (FSS) theory, we prove that the dual-layer dual-polarization transmission FSS can achieve 360° phase coverage at a limited loss. As a verification, a dual-layer dual-polarization full-phase unit cell with vias is proposed, which can achieve a phase range of more than 360° under 2 dB transmission loss. In addition, we innovatively apply the proposed dual-layer full-phase FSS to the design of a continuous 2-D beam scanning antenna based on the Risley prism (RP). This scheme effectively reduces the number of dielectric substrate layers required by the phase shifting surface (PSS), reduces the rotating scanning inertia, and effectively improves the aperture efficiency. The measured results show that the maximum scanning range of the 2-D beam scanning antenna can reach ±46° in the elevation plane and 360° in the azimuth plane. When operating at the center operating frequency of 21 GHz, the maximum gain of the antenna is 35.75 dBi with an aperture efficiency of 44.1%.

Wideband Conformal Transmitarrays for E-Band Multi-Beam Applications

Wideband conformal transmitarrays at E-band are developed for multibeam applications in this article. A triple-layer element with double split rings is presented for wideband transmissions, achieving a 360° continuous phase variation range at 74 GHz with less than 2.3 dB transmission loss. A comprehensive design methodology of multibeam conformal transmitarrays is demonstrated for various platforms with different curvatures. To validate the theoretical analysis, conformal transmitarrays with two different curvatures are designed, fabricated, and measured. Multiple radiation beams are realized between ± 30° and ± 45° for the two prototypes, respectively. Good agreement is obtained between simulation and measurement. The 3 dB gain bandwidths are 30% from 66.5 to 90 GHz, and 27.8% from 68 to 90 GHz for the two designs, respectively, covering the entire E-band.

Antiresonant Hollow-Core Inline Fiber Polarizer

The unique characteristics of hollow core fibers lead to low non-linearity and reduced sensitivity to environmental influences, making them attractive platform for designing inline fiber optic devices. Among these, an inline fiber polarizer would be most desired for applications such as gyroscopes, interferometric sensors and telecommunication. Hollow-core fiber polarizers, demonstrated to date, suffer from high insertion loss (>5 dB) for the pass polarization, and typically require tens of meters long nested antiresonant fiber to achieve a meaningful polarization extinction ratio (PER) >20 dB. We report a novel approach that makes use of intricate modal loss spectrum within a resonant band of single-ring antiresonant fiber to realize the first high-performance, discrete hollow-core fiber polarizer. The compact device comprising only 6 cm long fiber offers a high PER of >25 dB with <1 dB transmission loss for the pass polarization. In the present form, the polarizing action takes place over 14 nm and 7 nm bandwidth in the 2 μm and 1 μm wavelength range, but the simplicity of design principles makes it straightforward to shift the operating wavelengths to practically any wavelength band. The device can be fusion spliced with a standard antiresonant hollow-core fiber and a large mode area solid-core fiber with splice loss of 0.14 dB and 0.45 dB respectively. Complete integration of the proposed device into an all hollow-fiber setup would lead to a total insertion loss of 1.3 dB including both input and output splicing, which is the lowest for a discrete hollow-core fiber polarizer reported so far. The results encourage the development of other hollow-core fiber components by exploiting the unique characteristics cultivated in a resonant band of an antiresonant fiber.

A Subwavelength Transmit-Array Lens Element Combining Functions of Phase Modulation and Polarization Conversion

Advanced applications require transmit-array lenses featuring low profiles, covering 360° phase modulation, and, potentially, polarization conversion. Subwavelength metamaterial elements with multilayers manipulate electromagnetic waves with predesigned phase shifts, resulting in the transmit-array lens’s low profile and light weight. Conventional designs separate the transmit-array lens and polarization converter with two functional devices and suffer from high profiles and complicated mechanical assembly. In order to resolve these issues, a subwavelength lens element combining the functions of phase modulation and polarization conversion is proposed. The metamaterial lens element consists of phase modulation, linear-circular polarization conversion modules, and a metallic via structure electrically connecting these two modules. The multifunctional lens element modulates the transmitted phase with a three-bit phase shift and operates in the frequency range from 24.5 to 27 GHz, with less than −10 dB reflection coefficient, 1 dB transmission loss, and a 2.5 dB axial ratio. A transmit-array lens antenna with the proposed element is implemented to validate the abilities of beam focus and polarization conversion. The gain of the lens antenna reaches 26.3 dBi, with a 3 dB beamwidth of 6.7° and −18 dB sidelobe level, and the axial ratio of the converted circular polarization is below 1 dB. The aperture efficiency of the antenna is 45.2%.

Transmission loss of plates with embedded multi-scale and tuned acoustic black holes

The Acoustic Black Hole (ABH) is an emerging method for developing lightweight, high-loss structures and has recently drawn extensive attention. Unlike many conventional methods, ABH implementations are typically able to reduce structure vibration and sound radiation without a net addition of mass. Transmission loss (TL) is a key criterion for panel structure in the noise and vibration industry. Many noise and control applications demand panel structures with high TL and low weight. Plates with embedded ABH cells have the potential to act as good candidates for TL applications. In this work, the transmission loss (TL) of an embedded multi-scale ABH plate was studied. The embedded large and small ABH cells were particularly designed to cut on below and above the critical frequency of the plate, respectively. The results were compared with a uniform plate and an embedded single-scale ABH plate. Discrete tuning masses were attached at the ABH cells' center to manipulate the ABH cut-on modes to increase the TL further. The results show that the damped multi-scale ABH plate achieved a 10 dB TL increase, flattened the TL curve, and nearly eliminated the plate coincidence dip. Manipulating the high loss ABH modes by adding tuning masses (20 g each) demonstrated a 2 dB increase at low frequencies within the mass-law range. Although damping material was applied, adding some mass and an overall weight advantage were still attained compared to the uniform plate. The damped multi-scale ABH plate is 7% lighter than the uniform plate.

Transmission loss of plates with embedded multi-scale and tuned acoustic black holes.

An acoustic black hole (ABH) plate is a lightweight and high loss panel structure for effective reduction of vibration and radiated sound. It is understood that the high loss local ABH modes can be designed at desired frequencies by changing the size of the ABH cell(s). The ABH cell diameter (size) and minimum thickness play dominant roles in the performance of the ABH effect. In addition, attaching tuning masses at the center of the ABH cells has been shown to alter the local ABH modes with the result of improved low-frequency performance. In this work, the transmission loss (TL) of an embedded multi-scale ABH plate was investigated. The embedded large and small ABH cells were particularly designed to cut-on below and above the critical frequency of the plate, respectively. The results were compared with a uniform plate and an embedded single-scale ABH plate. Discrete tuning masses were attached at the ABH cells' center to manipulate the ABH cut-on modes to increase the TL further. The results show that the damped multi-scale ABH plate achieved a 10 dB TL increase, flattened the TL curve, and nearly eliminated the plate coincidence dip. Manipulating the high loss ABH modes by adding tuning masses (20 g each) demonstrated a 2 dB increase at low frequencies within the mass-law range. Although damping material was applied, adding some mass, an overall weight advantage was still attained compared to the uniform plate. The damped multi-scale ABH plate is 7% lighter than the uniform plate.

Acoustic performance and modal analysis for the muffler of a four-stroke three-cylinder inline spark ignition engine

This work presents a concise analytical report on four suggested design modifications (geometrical configuration of the resonating chamber) of a practical muffler model based on acoustic performance and reduction of back pressure. The existing model shows a very low acoustic transmission loss (15.8 dB) with a back pressure of nearly 2.52 × 105 Pa. The second proposed model provided better attenuation of 23.2 dB transmission loss but produced more back pressure (2.70 × 105 Pa). Modification of the second model by tapering of the inlet pipe's outlet accounted for the reduction in back pressure to around 2.30 × 105 Pa and increased the noise attenuation level by 1.2 dB. More significant attenuation of noise (14.8 dB) was observed in the fourth model without any back pressure. The fifth model produced the highest attenuation of 57.1 dB as compared to the other four models with undesirable highest back pressure around 2.83 × 105 Pa.

High-order fiber Bragg grating fabricated by femtosecond laser pulses for high-sensitivity temperature and strain sensing

We have extensively investigated the characteristics of temperature and strain sensing for two high order (the 3rd and 4th order) fiber Bragg Gratings (FBGs). The FBGs were manufactured in a single mode fiber (SMF-28) without hydrogen-loading utilizing femtosecond laser based on a point-by-point direct writing method. The annealing process was studied under the response of temperature up to 900℃. With regarding to the 3rd-order gratings, the experimental sensitivity of temperature (T-sensitivity) was 15 pm/℃ in the range of 25–900N℃ and that of longitudinal strain (S-sensitivity) was 3.7 pm/ ℃ in the range of 0–1824 με. For the 4th-order gratings, T-sensitivity was 13 pm/℃ in the range of 22−900 ℃ and S-sensitivity was 1.1 pm/με in the range of 0–1824 με, the experimental value of which were lower than that of the 3rd-order FBG. During the isothermal annealing process, we achieved that the peak reflectivity of 98 % (-17.3 dB transmission loss) with stability in high-temperature at Bragg wavelength of 1548.5 nm for the 3rd-order gratings and the peak reflectivity of 93 % (-11.6 dB transmission loss) at Bragg wavelength of 1411.3 nm for the 4rd-order gratings. It indicated that the sensitivity of the FBGs depends on the mode order. The proposed FBGs have potential for high-temperature sensing and high-resolution strain sensing. The multi-parametric detection and hazardous environment monitor based on the FBGs can be accomplished in the near future.

Aeroacoustic Attenuation Performance of a Helmholtz Resonator with a Rigid Baffle Implemented in the Presence of a Grazing Flow

To broaden its’ effective frequency range and to improve its transmission loss performance, a modified design of a Helmholtz resonator is proposed and evaluated by implementing a rigid baffle in its cavity. Comparison is then made between the proposed design and the conventional one by considering a rectangular duct with the resonator implemented in the presence of a mean grazing flow. For this, a linearized 2D Navier-Stokes model in frequency domain is developed. After validated by benchmarking with the available experimental data and our experimental measurements, the model is used to evaluate the effects of (1) the width Lp of the rigid baffle, (2) its implementation location/height Hg, (3) its implementation configurations (i.e., attached to the left sidewall or right sidewall), (4) the grazing mean flow Mu (Mach number), and (5) the neck shape on a noise damping effect. It is shown that as the rigid baffle is attached in the 2 different configurations, the resonant frequencies and the maximum transmission losses cannot be predicted by using the classical theoretical formulation ω2=c2S/VLeff, especially as the grazing Mach number Mu is greater than 0.07, i.e., Mu&gt;0.07. In addition, there is an optimum grazing flow Mach number corresponding to the maximum transmission loss peak, as the width Lp is less than half of the cavity width Dr, i.e., Lp/Dr≤0.5. As the rigid plate width is increased to Lp/Dr=0.75, one additional transmission loss peak at approximately 400 Hz is produced. The generation of the 12 dB transmission loss peak at 400 Hz is shown to attribute to the sound and structure interaction. Finally, varying the neck shape from the conventional one to an arc one leads to the dominant resonant frequency being increased by approximately 20% and so the secondary transmission loss peak by 2-5 dB. The present work proposes and systematically studies an improved design of a Helmholtz resonator with an additional transmission loss peak at a high frequency, besides the dominant peak at a low frequency.

Effects of Various Antifouling Coatings and Fouling on Marine Sonar Performance.

There is a rising imperative to increase the operational availability of maritime vessels by extending the time between full docking cycles. To achieve operational efficacy, maritime vessels must remain clear of biological growth. Such growth can cause significant increases in frictional drag, thereby reducing speed, range and fuel efficiency and decreasing the sensitivity of acoustic sensors. The impact that various stages of fouling have on acoustic equipment is unclear. It is also unclear to what extent antifouling techniques interfere with the transmission of acoustic signals. In this study, to examine this effect, neoprene samples were coated with three antifouling coatings, namely, Intersmooth 7460HS, HempaGuard X7 and Hempasil X3. Other neoprene samples were left uncoated but were imbedded with the biocide, 4,5-dichloro-2-octyl-4-isothiazolin-3-one (DCOIT) during the mixing and curing process. Uncoated nitrile samples that had varying levels of fouling from immersion in Port Phillip Bay, Australia, for 92, 156 and 239 days were also extracted. The acoustic properties of these samples were measured using an acoustic insertion loss test and compared to uncoated neoprene or nitrile to ascertain the acoustic effects of the applications of antifouling coatings as well as the fouling growth itself. A T-peel test was performed on all coated samples in an attempt to understand the adhesive properties of the coatings when applied to neoprene. It was found that the application of antifouling coatings had little effect on the transmission characteristics of the neoprene with approximately 1 dB loss. The embedment of DCOIT, however, has a chance of causing aeration in the neoprene, which can heavily hamper transmission. An assessment of the effect of the fouling growth found that light and medium fouling levels produced little transmission loss, whereas more extreme fouling lead to a 9 dB transmission loss. The adhesion properties of the coatings were investigated but not fully ascertained as tensile yielding occurred before peeling. However, various failure modes are presented and discussed in this study.

Numerical investigation of aeroacoustics damping performance of a Helmholtz resonator: Effects of geometry, grazing and bias flow

In this work, the effects of the grazing flow and its geometric dimensions on the aeroacoustics damping performance of a Helmholtz resonator are numerically evaluated. The grazing flow tangentially passing through the neck of the resonator is characterized by Mach number. And it is varied from 0 to 0.1. 2D numerical simulations are conducted by solving linearized Navier–Stokes equations in frequency domain via COMSOL 5.3. The numerical model is first validated by comparing with experimental and theoretical data available in literature, as a low Mach number grazing flow is present only. Good agreement is obtained. The model is then used to examine the effect of the grazing flow and the resonator geometric dimensions on transmission loss (TL) performance. Four key parameters are identified, and they include 1) the neck length, 2) neck diameter, 3) cavity volume and 4) the grazing flow Mach number. It is found that as the grazing flow Mach number is greater than 0.07, increasing the neck length leads to a decreased TLmax. Same observation is found for increased cavity volume Vr. Smaller Helmholtz resonator Vr or shorter neck length Ln is found to be involved with a larger TLmax in the presence of the grazing flow. Further study is then conducted, when there is a joint bias–grazing flow. The bias flow could be injected with respect to the grazing flow in 3 different directions, 1) parallel, 2) perpendicular and 3) counter flow. The bias flow injection direction is found to lead to 5 dB transmission loss difference. Parallel or counter injection of the bias flow is showed to be associated with improved TL performance, since there are counter-rotating vortices produced in the cavity. Finally, increasing the Mach number of the grazing flow leads to the noise damping effectiveness being deteriorated in general, even in the presence of a bias flow. The present work shed lights on the aeroacoustics design of Helmholtz resonators in the presence of a grazing and bias flow.

High Quality Factor, Ultralow Sintering Temperature Li6B4O9 Microwave Dielectric Ceramics with Ultralow Density for Antenna Substrates

Dense Li6B4O9 microwave dielectric ceramics were synthesized at low temperature via solid-state reaction using Li2CO3 and LiBO2. Optimum permittivity ∼ 5.95, quality factor ∼ 41 800 GHz and temperature coefficient of resonant frequency ∼ – 72 ppm/°C were obtained in ceramics sintered at 640 °C with a ultrasmall bulk density ∼2.003 g/cm3 (∼95% relative density, the smallest among all the reported microwave dielectric ceramics). Li6B4O9 ceramics were shown to be chemically compatible with silver electrodes but reacted with aluminum forming Li3AlB2O6 and Li2AlBO4 secondary phases. A prototype patch antenna was fabricated by tape casting and screen printing. The antenna resonated at 4.255 GHz with a bandwidth ∼279 MHz at −10 dB transmission loss (S11) in agreement with simulated results. The Li6B4O9 microwave dielectric ceramic possesses similar microwave dielectric properties to the commercial materials but much lower density and could be a good candidate for both antenna substrate and low-temperature cofire...

Demonstration of acoustic waveguiding and tight bending in phononic crystals

The systematic design, fabrication, and characterization of an isolated, single-mode, 90° bend phononic crystal (PnC) waveguide are presented. A PnC consisting of a 2D square array of circular air holes in an aluminum substrate is used, and waveguides are created by introducing a line defect in the PnC lattice. A high transmission coefficient is observed (−1 dB) for the straight sections of the waveguide, and an overall 2.3 dB transmission loss is observed (a transmission coefficient of 76%) for the 90° bend. Further optimization of the structure may yield higher transmission efficiencies. This manuscript shows the complete design process for an engineered 90° bend PnC waveguide from inception to experimental demonstration.

Femtosecond inscription of phase-shifted gratings by overlaid fiber Bragg gratings.

Two slightly shifted gratings are inscribed, one over the other, in an SMF fiber with a femtosecond laser and a phase mask. The transmission spectrum of the complex structure is similar to that of a phase-shifted grating; yet, the fabrication process is fast and simple compared to standard methods. High-quality semi-phase-shifted gratings with -24 dB transmission loss and <100 pm transmission bandwidth are presented. Their application as highly narrow micro-resonators and notch filters seems feasible.

Investigation of a Switchable Textile Communication System on the Human Body

In this paper, a switchable textile communication system working at 2.45 GHz ISM band is presented and studied for different locations within a realistic on-body environment. A 3D laser scanner is used to generate a numerical phantom of the measured subject to improve the accuracy of the simulations which are carried out for different body postures. For the off-body communications, the system is acting as an aperture coupled microstrip patch antenna with a boresight gain of 1.48 dBi. On-body communication is achieved by using a textile stripline, which gives approximately 5 dB transmission loss over 600 mm distance. The system is switched between on and off-body modes by PIN diodes. Common issues, such as shape distortion and body detuning effects which the textile antenna may experience in realistic use are fully discussed. Robust antenna performance is noted in the on-body tests, and an additional 3 dB transmission coefficient deduction was noticed in the most severe shape distortion case.

Resonant THz sensor for paper quality monitoring using THz fiber Bragg gratings

We report fabrication of THz fiber Bragg gratings (TFBG) using CO(2) laser inscription on subwavelength step-index polymer fibers. A fiber Bragg grating with 48 periods features a ~4 GHz-wide stop band and ~15 dB transmission loss in the middle of a stop band. The potential of such gratings in the design of resonant sensors for the monitoring of paper quality is demonstrated. Experimental spectral sensitivity of the TFBG-based paper thickness sensor was found to be ~-0.67 GHz/10 μm. A 3D electromagnetic model of a Bragg grating was used to explain experimental findings.