Low-frequency Noise Reduction Research Articles

Low-Frequency Noise Reduction in Dual-Fiber Optical Trap Using Normalized Differential Signal of Transmission Lights

The accuracy of long-term position detection in dual-fiber optical trap is limited by low-frequency noise. We propose a technique that can reduce such noise. A position detection system for dual-fiber optical trap using a quadrant photodiode is built. The transmission light from the trapped particle is detected by two photodiodes. The normalized differential signal of transmission lights is demonstrated to carry the noise caused by asymmetric laser power fluctuation irradiating the microsphere, which is proved to dominate low-frequency noise. By subtracting this normalized difference from the position signal, the noise-reduction capability of this technique is remarkably 67% depending on detection bandwidth. It is expected to be applied to integrated and microfluidic fiber-optic trap system.

Research on the technology of noise reduction in hybrid electric vehicle with composite materials

The noise source of hybrid electric vehicle is varied, which makes the frequency band from low-frequency to high-frequency distribution. Using a single material to control the hybrid electric vehicle interior noise to improve the performance of vehicle, noise, vibration, and harshness performance is limited. In this article, the composite noise reduction materials which have the function of damping and acoustic absorption are used to control the vehicle interior noise. First, the noise characteristics of the hybrid electric vehicle are analyzed. The distribution of the hybrid electric vehicle interior noise is obtained. Then, the sound-absorbing properties of polyester-polypropylene bi-component fiber are analyzed. The low-frequency noise reduction principle of butyl rubber damping material is also analyzed. Finally, the noise reduction materials consisting of polyester-polypropylene and butyl rubber are added to the hybrid electric vehicle roof and floor. After improving material, the hybrid electric vehicle interior noise is test. As the result, the car driver’s right ear A-weighting sound pressure levels have been greatly improved. The low-frequency noise below 400 Hz reduced 1.5 dB(A), and the high-frequency noise above 400 Hz reduced 5.2 dB(A). The total noise reduction reached 3.3 dB(A) near the driver’s right ear, which significantly improve the noise, vibration, and harshness performance of hybrid electric vehicle.

The reduction in low-frequency noise of horizontal-axis wind turbines by adjusting blade cone angle

In the present paper, the effect of blade cone angle on low-frequency noise of horizontal-axis wind turbines is studied to investigate the noise reduction, by adjusting the blade cone angle in different wind speeds. In wind turbines, a significant part of the noise is in infrasound range to which exposure could have adverse effects on human health. In this study, a small turbine is selected as a test case, and the sound field is simulated for different blade cone angles from 0° to 10° with a wind speed range between 5 and 25 m/s. The results of flow simulation show that the change in the output power is < 5% in comparison with the planar turbine. In addition, the calculated results for the low-frequency noise demonstrate that the blade cone angle significantly affects the noise value and the directivity pattern. The investigation of the directivity pattern shows that blade cone angle could have opposite effects on different observer positions, and therefore, the noise calculation at only one position is not enough to conclude about the effect of blade cone angle. The obtained directivity also shows that the turbines with blade cone angles of 2.5° and 5° are preferred in order to reduce the maximum noise, in comparison with the planar turbine. Overall, it is concluded that the adjustment of blade cone angle in different wind speeds can be an effective solution for reducing the low-frequency noise.

Low Frequency Noise Reduction with Active Noise Control in Laboratory Settings

Low Frequency Noise Reduction with Active Noise Control in Laboratory Settings

Low-Frequency Noise Reduction by Earmuffs with Flax Fibre-Reinforced Polypropylene Ear Cups

Soldiers and supporting engineers are frequently exposed to high low-frequency (&lt;500 Hz) cabin noise in military vehicles. Despite the use of commercial hearing protection devices, the risk of auditory damage is still imminent because the devices may not be optimally customised for such applications. This study considers flax fibre-reinforced polypropylene (Flax-PP) as an alternative to the material selection for the ear cups of commercial earmuffs, which are typically made of acrylonitrile butadiene styrene (ABS). Different weaving configurations (woven and nonwoven) and various noise environments (pink noise, cabin booming noise, and firing noise) were considered to investigate the feasibility of the proposed composite earmuffs for low-frequency noise reduction. The remaining assembly components of the earmuff were kept consistent with those of a commercial earmuff, which served as a benchmark for results comparison. In contrast to the commercial earmuff, the composite earmuffs were shown to be better in mitigating low-frequency noise by up to 16.6 dB, while compromising midfrequency acoustical performance. Consequently, the proposed composite earmuffs may be an alternative for low-frequency noise reduction in vehicle cabins, at airports, and at construction sites involving heavy machineries.

Analysis and Validation of Low-Frequency Noise Reduction in MOSFET Circuits Using Variable Duty Cycle Switched Biasing

In MOS transistors, low-frequency noise phenomena such as random telegraph signal (RTS), burst, and flicker or $1/f$ noise are usually attributed to the random nature of the trap state of defects present at the gate Si-SiO2 interface. In a previous work, theoretical modeling and analysis of the RTS and 1/ $1/f$ noise in MOS transistor was presented and it was shown that this 1/ $1/f$ noise power can be reduced by decreasing the duty cycle ( $D$ ) of switched biasing signal. In this paper, an extended analysis of this 1/ $1/f$ noise reduction model is presented and it is shown that the RTS noise reduction is accompanied with shift in the corner frequency ( ${f}_{c}$ ) of the 1/ $1/f$ noise and the value of shift is a function of continuous ON time ( $T_{on}$ ) of the device. This 1/ $1/f$ noise reduction is also experimentally demonstrated in this paper using a circuit configuration with multiple identical transistor stages which produce a continuous output instead of a discrete signal. The circuit is implemented in 180 nm standard CMOS technology, from UMC. According to the measurement results, the proposed technique reduces the 1/ $1/f$ noise power by approximately 5.9 dB at switching frequency ( ${f}_{s}$ ) of 1 KHz for 2 stage, which is extended up to 16 dB at ${f}_{s}$ of 5 MHz for six stage configuration.

Analysis of sound absorption performance of an electroacoustic absorber using a vented enclosure

The sound absorption performance of an electroacoustic absorber (EA) is primarily influenced by the dynamic characteristics of the loudspeaker that acts as the actuator of the EA system. Therefore, the sound absorption performance of the EA is maximum at the resonance frequency of the loudspeaker and tends to degrade in the low-frequency and high-frequency bands based on this resonance frequency. In this study, to adjust the sound absorption performance of the EA system in the low-frequency band of approximately 20–80 Hz, an EA system using a vented enclosure that has previously been used to enhance the radiating sound pressure of a loudspeaker in the low-frequency band, is proposed. To verify the usefulness of the proposed system, two acoustic environments are considered. In the first acoustic environment, the vent of the vented enclosure is connected to an external sound field that is distinct from the sound field coupled to the EA. In this case, the acoustic effect of the vented enclosure on the performance of the EA is analyzed through an analytical approach using dynamic equations and an impedance-based equivalent circuit. Then, it is verified through numerical and experimental approaches. Next, in the second acoustic environment, the vent is connected to the same external sound field as the EA. In this case, the effect of the vented enclosure on the EA is investigated through an analytical approach and finally verified through a numerical approach. As a result, it is confirmed that the characteristics of the sound absorption performances of the proposed EA system using the vented enclosure in the two acoustic environments considered in this study are different from each other in the low-frequency band of approximately 20–80 Hz. Furthermore, several case studies on the change tendency of the performance of the EA using the vented enclosure according to the critical design factors or vent number for the vented enclosure are also investigated.In the future, even if the proposed EA system using a vented enclosure is extended to a large number of arrays required for 3D sound field control, it is expected to be an attractive solution that can contribute to an improvement in low-frequency noise reduction without causing economic and system complexity problems.

A hybrid phononic crystal for roof application.

Phononic crystal is a type of acoustic material, and the study of phononic crystals has attracted great attention from national research institutions. Meanwhile, noise reduction in the low-frequency range has always encountered difficulties and troubles in the engineering field. In order to obtain a unique and effective low-frequency noise reduction method, in this paper a low frequency noise attenuation system based on phononic crystal structure is proposed and demonstrated. The finite element simulation of the band gap is consistent with the final test results. The effects of structure parameters on the band gaps were studied by changing the structure parameters and the band gaps can be controlled by suitably tuning structure parameters. The structure and results provide a good support for phononic crystal structures engineering application.

Low-Frequency Noise Reduction by Earmuffs with Coir and Coir/Carbon Fibre-Reinforced Polypropylene Ear Cups

Natural fibres have been extensively studied due to their potential in a wide range of applications. This study aims to demonstrate the viability of composite earmuffs for low-frequency noise reduction in continuous and transient noise environments. Pink noise and aircraft take-off exterior noise were considered for the former and the latter, respectively. The assembly components of the composite earmuffs were kept identical to a commercial earmuff, which served as a reference for results comparison. Based on the profile of the ear cups from the commercial earmuff, composite ear cups were fabricated from coir fibre and coir/carbon fibre fabrics reinforced with polypropylene. In contrast to the commercial earmuff, the composite earmuffs showed improvements in insertion loss at specific frequencies in the respective noise environments. In pink noise, up to 12 dB improvement in insertion loss was achieved. In aircraft take-off exterior noise, up to 8.6 dB improvement in insertion loss was achieved at 160–544 Hz particularly by the coir fibre-reinforced polypropylene earmuff. Consequently, the proposed earmuffs may find applications in areas where noise exposure is predominantly low-frequency—in some vehicle cabins, at airports, and at construction sites, for example.

Very low frequency noise reduction in orthogonal fluxgate

In this paper we present a simple, cheap and effective method to obtain reduction of very low frequency noise in an orthogonal fluxgate in fundamental mode. This method consists in the application of a layer of silicone over the magnetic core of the sensor filling the whole space inside the pick-up coil. In this way we avoid fast variations of temperature to affect the offset of the sensor (which is very sensitive to temperature changes). Eventually the offset will still drift to the same value, but it will do it slower, thus reducing the component of the noise in the sub-Hz region. Using this method we could simply reduce the noise at 100 mHz from 23 to 8 pT/√Hz, with no additional hardware on the excitation or signal conditioning circuits.

Leakage Current and Low-Frequency Noise Analysis and Reduction in a Suspended SOI Lateral p-i-n Diode

In this paper, we present a detailed analysis of leakage current in a silicon-on-insulator (SOI) lateral P+P−N+ (p-i-n) diode suspended on a microheating platform, combining device experimental characterization and numerical simulation. The diode leakage currents have been extensively studied using the back-gate bias as a means to alter the space-charge (SC) condition at the P− region (I-region)/buried oxide interface from accumulation to full depletion, and finally to inversion. Both dark leakage current analysis and low-frequency noise characterization performed on the suspended SOI lateral p-i-n diode indicate device degradation induced by microelectromechanical systems postprocessing (i.e., deep reactive-ion etching or aluminum deposition). A low-temperature (~250 °C) in situ (i.e., using embedded microheater) annealing of SOI lateral p-i-n diode after postprocessing allows reduction of the diode leakage current and optimization of the device performance by neutralizing the interface traps and improving carriers’ lifetime and surface recombination velocity. Numerical simulations have been performed with Atlas/SILVACO for deeper analysis of the leakage current behavior in the lateral p-i-n diode and identification of the generation mechanism dominating the diode leakage behavior. Simulation reveals that the dominant generation rate in the diode depends on the SC conditions, the interface trap density, and the carriers’ lifetime in the I-region. The experimental and simulated behaviors of “as processed” and annealed diode leakage current are shown to be in good qualitative agreement.

Extremely low frequency band gaps of beam-like inertial amplification metamaterials

In this paper, extremely low frequency band gaps of beam-like inertial amplification metamaterials are investigated based on local resonance theory. Inertial amplification mechanism is proposed to obtain extremely low frequency band gaps by altering geometry parameters of the beam-like structures rather than modulating material properties, which allow first lower band gap (BG) to be attained easily compared to traditional local resonance structures. Band structures, frequency response functions (FRFs) plots and vibration modes of the beam-like structures are calculated and analyzed by employing finite element method. Numerical results show that first BG of the structure ranges from 23 Hz to 21 Hz. FRFs are in accordance with the dispersion relationship. It is found that interaction between inertial amplification and traveling wave modes in the proposed structure are responsible for formation of the first BG. This type of beam-like inertial amplification metamaterials has many potential applications in the field of low frequency vibration and noise reduction.

Experimental investigation of the far-field noise due to jet-surface interaction combined with a chevron nozzle

High-speed jets exhausting from engines, especially during takeoff, remain one of the main sources of external aircraft noise. Regulations on aircraft noise levels in regions close to airports are very strict and are forcing the aerospace industry to develop quieter aircrafts. Mixing enhancement devices at the exit of nozzles, such as chevrons, have assisted in the reduction of noise levels for some time. However, installation effects are becoming more intense due to highly integrated aircraft-engine configurations, which are currently employed by the aviation industry as an attempt to increase the aerodynamic efficiency and to reduce noise. Consequently, these installation effects may interfere with the noise reducing performance of such devices. This paper reports experimental investigations involving the combined effect of a flat plate integrated with nozzles, both with and without chevrons, exhausting cold subsonic jets. Assessment of the far-field noise was conducted for different Mach numbers and several configurations formed by relative positions between the nozzle and the plate. In general, it was observed that the acoustic benefit of chevrons, as found in isolated jets, remains effective in most of installed configurations. Nevertheless, for highly integrated configurations the low-frequency noise reduction provided by the chevron nozzle becomes almost negligible, whereas the increase of high-frequency noise becomes substantial and the shielding effect is observed to decrease.

Low-frequency active noise control of an underwater large-scale structure with distributed giant magnetostrictive actuators

A light and thin underwater large-plate active acoustic structure is developed that satisfies the particular requirements of high pressure resilience, low frequency and high efficiency encountered in underwater work environments. A low-frequency miniaturized active control unit, with a thickness of less than 50mm, is designed using giant magnetostrictive material (GMM). The noise reduction performance is measured with an active control system based on a multi-channel adaptive filter. The active control system is developed within a LabVIEW environment and can achieve significant levels of noise reduction within time intervals of less than one second achieving absorption coefficients far exceeding 0.8 even under high pressures. The new active-control system incorporates hardware and software components and represents a novel technology for low-frequency underwater noise reduction.

Low-noise tunneling-magnetoresistance vector magnetometers with flux chopping technique

A concept for a low-noise three-axis magnetometer consisting of tunneling magnetoresistance (TMR) sensors and a flux chopper was designed, implemented, and characterized in this work. The TMR sensors used in this study were the model of TMR2102D from Multidimension Technology Inc. Three TMR sensors were aligned orthogonally on a printed circuit board (PCB) and mounted inside a cylindrical flux chopper. The cylindrical flux chopper including a soft magnetic shielding tube and enameled copper wires was 16mm in length and 8mm in diameter. The shielding tube was made of a cobalt-based soft magnetic ribbon, Metglas-2714A, from Metglas Inc. The flux chopper modulated the external magnetic flux density using the fluxgate effect, which made the sensors respond to the quasi-static field at the chopping frequency. The demodulated output showed a reduction in low-frequency noise to the level of 0.17nT/√Hz@1Hz. To demonstrate the technical feasibility for electronic compass application, the demodulated output of the vector magnetometer prototype was recorded by rotating the device in Earth’s magnetic fields about a fixed axis. The Cartesian components Bx, By, and Bz of the Earth’s fields at various azimuth angles were retrieved by performing a calibration algorithm to correct the non-orthogonality caused by the misalignment of TMR sensors and the chopper. The calibrated outputs were linear and orthogonal to each other with an angle error less than 1° and nonlinearity of 0.7%, indicating that the chopping technique is useful to realize an extremely low-noise three-dimensional magnetometer for the geomagnetic application.

Low frequency band gap characteristics of double-split Helmholtz locally resonant periodic structures

A double-split Helmholtz periodic structure with the characteristic of local resonance is designed and constructed in this paper. The double-split periodic structural cell which can be divided into internal and external cavities is adopted in structure. In such a kind of structure, the resonating area is remarkably expanded while the inner cavity is continuously enlarged. Thus, a satisfactory feature of low frequency resonance can be obtained. At the same time, the adjustability of band gap is achieved by the designed adjustment of the arc length of the inner cavity, therefore, the effect of sound insulation in a specific low frequency band can be achieved. In the analyses of the mechanism and factors of the generation of low frequency band gap, the mathematical model of the upper and lower limits of the band gap is established by using the electric circuit analogy. And some comparative analyses between the methods of electric circuit analogy and finite element method are carried out. The result suggests that a satisfactory feature of low frequency band gap is presented, and the first band-gap ranges from 86.9 Hz to 138.2 Hz. The low frequency band gap can be influenced by the arc length of inner cavity, the space between inner and outer cavities, and the interaction of the structural cells in the periodic arrangement. The longer the arc length of the inner cavity, the lower the low frequency band gap will be; the longer the distance between inner and outer cavities, and the higher the frequency of band gap, the worse the low frequency effect will be; the lower limit of low frequency band gap cannot be influenced by reducing the space between individual structures, on the contrary, the width of low frequency band gap can be sharply increased. Plenty of practical and theoretical support in the field of low frequency noise reduction is offered in the research.

Multi-cavity locally resonant structure with the low frequency and broad band-gaps

A multi-cavity periodic structure with the characteristic of local resonance was proposed in the paper. The low frequency band-gap structure was comparatively analyzed by the finite element method (FEM) and electric circuit analogy (ECA). Low frequency band-gap can be opened through the dual influence of the coupling’s resonance in the cavity and the interaction among the couplings between structures. Finally, the influence of the structural factors on the band-gap was analyzed. The results show that the structure, which is divided into three parts equally, has a broader effective band-gap below the frequency of 200 Hz. It is also proved that reducing the interval between unit structures can increase the intensity of the couplings among the structures. And in this way, the width of band-gap would be expanded significantly. Through the parameters adjustment, the structure enjoys a satisfied sound insulation effect below the frequency of 500Hz. In the area of low frequency noise reduction, the structure has a lot of potential applications.

Turbine Aerodynamic Low-Frequency Oscillation and Noise Reduction Using Partial Shrouds

The design of highly efficient low-pressure gas turbines in compliance with strict emission regulations for pollutants and noise, as well as long component lifetime, requires the detailed knowledge of potential excitations in the flow. The analysis of fast response measurement data from a 1.5-stage low-pressure axial turbine in the frequency domain shows multiple relevant excitation and noise sources in the flowfield, in addition to the primary rotor–stator interaction. Especially nonsynchronous low-frequency modes at 10% of the blade-passing frequency, occurring in the tip shroud exit cavity, are found to communicate with the main flow and are amplified through the downstream stator row by a flow instability in the interaction area of the passage vortex and boundary layer. Full-annular unsteady computational fluid dynamics simulations are carried out in addition to the experiments to probe the origin of the cavity modes. They predict the rotor–stator interaction well, but they struggle to resolve low-frequency oscillations in the cavities. Out of the four tested configurations, a shroud trailing-edge cutback is the most efficient option to prevent the formation of the cavity modes. The tradeoff for a low-frequency fluctuation reduction of 12 dB using a trailing-edge partial shroud is a total–total stage efficiency drop of 0.7%.

Design and experimental tests of active control barriers for low-frequency stationary noise reduction in urban outdoor environment

Active noise control (ANC) techniques are based on the emission of an antiphase signal in order to cancel the noise produced by a primary source. ANC has been successfully applied especially for reducing noise in confined environments, such as headphones and ducts. In this study, we present an application of ANC concepts to the design of an anti-noise barrier for an outdoor environment and its experimental testing. Even though passive techniques are effective in noise reduction at middle-high frequencies, they become less efficient at low frequencies (below 300Hz) due to the limited dimensions of commonly deployable barriers. In this paper, we analyze the properties of a low-cost active noise system able to efficiently operate on stationary, almost pure-tone, low-frequency noise, such as that produced by electrical transformers and reactors in power and transformation plants. A prototype has been implemented and on-the-field experimental tests have been carried out. The results (confirmed also by numerical simulations) demonstrate a remarkable efficiency in the far field, with a reduction up to 15dB with respect to the absence of the ANC system.

Membrane- and plate-type acoustic metamaterials.

Over the past decade there has been a great amount of research effort devoted to the topic of acoustic metamaterials (AMMs). The recent development of AMMs has enlightened the way of manipulating sound waves. Several potential applications such as low-frequency noise reduction, cloaking, angular filtering, subwavelength imaging, and energy tunneling have been proposed and implemented by the so-called membrane- or plate-type AMMs. This paper aims to offer a thorough overview on the recent development of membrane- or plate-type AMMs. The underlying mechanism of these types of AMMs for tuning the effective density will be examined first. Four different groups of membrane- or plate-type AMMs (membranes with masses attached, plates with masses attached, membranes or plates without masses attached, and active AMMs) will be reviewed. The opportunities, limitations, and challenges of membrane- or plate-type AMMs will be also discussed.