Low-frequency Noise Attenuation Research Articles

Surface EMG signal normalisation and filtering improves sensitivity of equine gait analysis

Low-frequency noise attenuation and normalisation are fundamental signal processing (SP) methods for surface electromyography (sEMG), but are absent, or not consistently applied, in equine biomechanics. The purpose of this study was to examine the effect of different band-pass filtering and normalisation conventions on sensitivity for identifying differences in sEMG amplitude-related measures, calculated from leading (LdH) and trailing hindlimb (TrH) during canter, where between-limb differences in vertical loading are known. sEMG and 3D-kinematic data were collected from the rightBiceps Femorisin 10 horses during both canter leads. Peak hip and stifle joint angle and angular velocity were calculated during stance to verify between-limb biomechanical differences. Four SP methods, with and without normalisation and high-pass filtering, were applied to raw sEMG data. Methods 1 (M1) to 4 (M4) included DC-offset removal and full-wave rectification. Method 2 (M2) included additional normalisation relative to maximum sEMG across all strides. Method 3 (M3) included additional high-pass filtering (Butterworth 4thorder, 40 Hz cut-off), for artefact attenuation. M4 included the addition of high-pass filtering and normalisation. Integrated EMG (iEMG) and average rectified value (ARV) were calculated using processed sEMG data from M1 – M4, with stride duration as the temporal domain. sEMG parameters, within M1 – M4, and kinematic parameters were grouped by LdH and TrH and compared using repeated measures ANOVA. Significant between-limb differences for hip and stifle joint kinematics were found, indicating functional differences in hindlimb movement. M2 and M4, revealed significantly greater iEMG and ARV for LdH than TrH (P<0.01), with M4 producing the lowestP-values and largest effect sizes. Significant between-limb differences in sEMG parameters were not observed with M1 and M3. The results indicate that equine sEMG SP should include normalisation and high-pass filtering to improve sensitivity for identifying differences in muscle function associated with biomechanical changes during equine gait.

Analysis of an Ultra-Low Frequency and Ultra-Broadband Phononic Crystals Silencer with Small Size

Existing research shows that acoustic BG in a certain frequency range can be realized by installing an expansion chamber on duct system, but the problems of broadband and size limitations at low frequencies remain to be researched. The study of acoustic and elastic wave propagation in artificial periodic structures has received increasing attention for many decades, and the presence of bandgap (BG) in phononic crystals (PCs), which inhibits elastic/acoustic wave propagation within the BG’ frequency range, supplies a new way to control noise and vibrations in duct system. Based on PC theory, a duct silencer backed with a gas–liquid expansion chamber is proposed to enhance the acoustic performance of low-frequency noise attenuation. The transfer matrix method (TMM) is used to investigate the acoustic BG properties. The influences on the BG properties of some key parameters are analyzed, and the band formation mechanism is revealed by the law of energy conservation. The results show that silencers with a small size can effectively attenuate ultra-low frequencies and ultra-broad bands.

A case study of acoustic efficiency of existing noise barrier in reducing road traffic noise in school area

The study investigates the effectiveness of existing noise wall barrier installed in a school for shielding noise from heavy traffic. The objectives of this study are to investigate the effect of this barrier on the characteristic of traffic noise spectra, and evaluate its efficiency on broadband noise insertion loss and low frequency noise attenuation. The barrier is of absorptive type with slightly aging condition where small holes and cracks in some places are spotted in both surfaces of wall. The barrier is made of panels of fibre cement mortar infill with absorbent materials with the height is 4m x 0.25m thickness x 132 m length with the distance of the barrier to the highway as main noise source is 17 m. Investigation of the efficiency of barrier started with the determining of noise frequency spectra near the barrier at 0.5m in front of barrier, and 0.5 m and 6m behind the barrier, all at five points along the barrier length. The efficiency of barrier was determined by its insertion loss of broadband noise and attenuation of low frequency in the range 20 to 200 Hz. The results showed that barrier changed the characteristic of traffic noise spectrum. It was found that barrier efficiently achieved insertion loss of 5 dBA or above if the receiver was at distance more than 3.5m behind the wall. At 6m meter behind wall, although the barrier was considered effective but it was failed in reducing the sound pressure level below the World Health Organisation (WHO) permissible limit for school area and attenuating the low frequency noise sufficiently. These results highlighted that the barrier unable to combat the noise disturbance in school playing field area.

Low-Frequency Noise Reduction of Rocket Fairings Using Horn-Shaped-Neck Helmholtz Resonators

The low-frequency noise attenuation technology inside rocket fairings using Helmholtz resonators with horn-shaped neck is constructed. The acoustic performance of a Helmholtz resonator with horn-shaped neck is investigated by methods combining theory, simulation, and experiment. A one-dimensional analytical method is developed to determine the resonance frequency under rigid-wall assumption, whereas an electroacoustic analogy approach is applied to consider the effect of compliant wall, and the theoretical analysis is verified by three-dimensional finite element method. Moreover, the influence of different neck types and wall thickness on the resonance frequency is compared. The main mechanism of sound absorption of a Helmholtz resonator is also studied, and experimental verification on this is carried out in an impedance tube. Results indicate that the resonant frequency can be controlled by neck shape and wall thickness, and the viscous damping at the neck is the main noise-reduction mechanism of this kind of Helmholtz resonator. The finite element model of a rocket fairing is established, and the response characteristic of a fairing cavity is obtained, based on which the certain size horn-shaped-neck Helmholtz resonators are designed and made out. The availability of Horn-shaped-neck Helmholtz resonators on low-frequency noise reduction is demonstrated by test and simulation.

Improving Adaptive Filters for Active Noise Control Using Particle Swarm Optimization

The excessive exposure to certain kinds of acoustic noise can lead to health problems. To avoid this situation, the use of noise attenuation devices is a standard solution. Among those devices, the active noise control (ANC) systems have gained prominence over the years, mainly due to the technological development and costs reduction of electronic components. Despite good performance of ANC concerning low-frequency noise attenuation, the convergence speed for this kind of system is still an important issue when it deals with real-time applications in dynamic environments. This article presents an alternative solution to accelerate the active attenuation system response. This solution is based on the use of sets of coefficients, which are employed during the adaptive filter initialization and are obtained via a training process with particle swarm optimization (PSO). Two objective functions were tested: one based on the response time itself and the other one based on the magnitude reduction of the residual noise. The coefficients obtained through this process provided response time reductions up to 98.3% concerning adaptive filters initialized with null coefficients. The article is an extended version of the conference paper Accelerating the Convergence of Adaptive Filters for Active Noise Control Using Particle Swarm Optimization, published in LA-CCI 2017.

The effect of cut-off frequency when high-pass filtering equine sEMG signals during locomotion

High-pass filtering (HPF) is a fundamental signal processing method for the attenuation of low-frequency noise contamination, namely baseline noise and movement artefact noise, in human surface electromyography (sEMG) research. Despite this, HPF is largely overlooked in equine sEMG research, with many studies not applying, or failing to describe, the application of HPF. An optimal HPF cut-off frequency maximally attenuates noise while minimally affecting sEMG signal power, but this has not been investigated for equine sEMG signals. The aim of this study was to determine the optimal cut-off frequency for attenuation of low-frequency noise in sEMG signals from the Triceps Brachii and Biceps Femoris of 20 horses during trot and canter. sEMG signals were HPF with cut-off frequencies ranging from 0 to 80 Hz and were subjected to power spectral analysis and enveloped using RMS to calculate spectral peaks, indicative of motion artefact, and signal loss, respectively. Processed signals consistently revealed a low-frequency peak between 0 and 20 Hz, which was associated with motion artefact. Across all muscles and gaits, a 30–40 Hz cut-off fully attenuated the low-frequency peak with the least amount of signal loss and was therefore considered optimal for attenuating low-frequency noise from the sEMG signals explored in this study.

Structural-acoustic interaction of a three-dimensional panel–cavity–duct system with non-uniform boundary restraints

Plate drum-silencer has received a lot of research attention due to its excellent low frequency noise attenuation and flow-through characteristics. The majority of existing studies are limited to the classical or uniform elastic boundary conditions, while there is little effort devoted to the study of boundary restraint non-uniformity effect on the vibro-acoustic behavior of such silencing system. Motivated by the current limitation, the structural-acoustic coupling model of a three-dimensional panel–cavity–duct silencer is established, in which the elastic boundary restraint can be set as arbitrary distribution function. Energy principle is formulated to describe the vibro-acoustic dynamics of such 3-D panel–cavity–duct silencing system, with the admissible field functions constructed as the superposition of standard Fourier series and the auxiliary edge/interface smoothed terms. All the coupled system response information can be derived in conjunction with Rayleigh–Ritz procedure. Numerical examples are presented to validate the proposed model through the comparison with those from other approaches. The coupling effects of boundary restraining coefficient and plate bending stiffness on sound attenuation performance of such cavity–backed plate–duct silencer are then discussed and analyzed. The results show that the relationship of translational restraints at the duct entrance and exit, kx0 and kxLx, can be obtained through property inverse proportional functions to achieve the optimal sound attenuation. There will be significant improvement in some special frequency band compared with the uniform restraints distribution, especially for the plate with lower bending stiffness The experimental study is also performed to verify the theoretical prediction from current model. In reality, the arbitrary non-uniform elastic edge restraints represent the most general class of boundary conditions, and can provide more optimal space for such drum silencer design.

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 attenuation in seismic and microseismic data using mathematical morphological filtering

Low-frequency noise is one of the most common types of noise in seismic and microseismic data. Conventional approaches, such as the high-pass filtering method, utilize the low-frequency nature and distinguish between signal and noise based on their different frequency contents. However, conventional approaches are limited or even invalid when the signal and noise shares the same frequency band. Moreover, high-pass filtering method will suppress not only low-frequency noise but also low-frequency signal when they overlap in a same frequency band, which is extremely important in the inversion process for building the subsurface velocity model. To overcome the drawbacks of conventional high-pass filtering approach, we developed a novel method based on the mathematical morphology theorem to separate signal and noise using their differences in morphological scale. We extracted empirical relation between morphological scale and frequency band so that the mathematical morphology based approach can be conveniently used in low-frequency noise attenuation. The proposed method is termed as the mathematical morphological filtering (MMF) method. We compare the MMF approach with high-pass filtering and empirical mode decomposition (EMD) approaches using synthetic, reflection seismic and microseismic examples. The various examples demonstrate that the proposed MMF method can preserve more low-frequency signal than the high-pass filtering approach, and is more efficient and causes fewer artefacts than the EMD approach.

Design and optimization of membrane-type acoustic metamaterials using genetic algorithms

One of the most difficult problems in noise control is the attenuation of low frequency noise. Typical solutions require barriers with high density and/or thickness. Membrane-type acoustic metamaterials are a novel type of engineered material capable of high low-frequency transmission loss despite their small thickness and light weight. These materials are ideally suited to applications with strict size and weight limitations such as aircraft, automobiles, and buildings. This paper reviews computationally efficient dynamic models for designing membrane-type acoustic metamaterials that have been proposed by the authors, based on the impedance-mobility approach. The computational efficiency of the impedance-mobility models compared to finite element methods enables implementation in design tools based on a graphical user interface and in optimization schemes. Genetic algorithms are used to optimize the unit cell design for a variety of noise reduction goals, including maximizing transmission loss for broadband, narrow-band, and tonal noise sources. The degree to which the optimal configurations extend to systems of multiple unit cells and unit cells of different shapes is explored.

Investigation of aircrew noise exposure levels and hearing protection solutions in helicopter cabin

High noise levels in the helicopter cabin adversely affect aircrew communication and reduce comfort in the short-term and may lead to hearing loss in the long-term if flight helmets cannot provide sufficient protection to the aircrew. A cabin noise exposure survey has been performed on a Royal Canadian Air Force CH-147F Chinook heavy lift helicopter to evaluate the noise environment and noise protection performance of the flight helmet. Investigation results showed that the low-frequency noise attenuation provided by the Royal Canadian Air Force flight helmet was marginal in high-speed flight conditions that generate loud cabin noise. Therefore, in-canal earphone integrated with active noise cancellation capability was investigated to provide enhanced noise protection and improve clarity in voice communication. Simulation and proof-of-concept test results verified that active noise cancellation in-canal earphones can serve as a feasible technical solution to provide enhanced noise attenuation to mitigate the low-frequency N/rev tonal noise generated by the aerodynamic pressure from the helicopter rotor blades.

Validation of low frequency noise attenuation using locally resonant patches.

Since conventional silencers in acoustic ducts have problems of size limitations at low frequencies and being prone to high backpressure, locally resonant aluminum patches are introduced in acoustic duct walls aiming at creating frequency stop bands in the low frequency region (below 1 KHz). With these flush mounted patches, promising noise reductions, with no such drawbacks, can be obtained, building on local resonance phenomenon implemented in acoustic metamaterials techniques. The objective of the current paper is to experimentally validate the performance of an array of flexible side-wall-mounted patches inside ducts. The experimental results are compared with Analytical Green's function method as well as Numerical Finite Element Method and a close agreement was found. The results show that the presence of the patches singly or periodically can play a prominent role in designing any acoustic bandgap materials. The effect of the arrays of patches on the effective dynamic density and bulk modulus has also been investigated.

ハイブリッドANCシステムを用いた能動的遮音壁(ウォーターベッド効果による増音の抑制及び各種音場における減音効果の考察)

In this paper, the hybrid active noise control (HB-ANC) system applied in the active noise barrier (ANB) is improved, and the noise attenuation performace of the ANB is invetigated under four different sound field configurations. This hybird ANB reduces the diffraction noise from top of the ANB and the noise propagated to the head position of the target person behind the ANB simultaneously to achieve higher noise attenuation. However, the waterbed effect of the feedback control part of the HB-ANC system will cause noise enhancement at some frequencies in the control area. In order to solve this problem, the HB-ANC system is improved by adding a filter to cancel the noise enhancement caused by the waterbed effect. Then, the noise attenuation performance of the hybrid ANB under four sound field configurations is evaluated by simulations. The sound field configurations are set up to investigate how the diffraction from side of the ANB and the reflection influence the performance of the ANB. An experiment in the anechoic chamber is also conducted. The results indicate that the low frequency noise attenuation can be obtained in a wide area behind the ANB, and at the head position of the person, the noise attenuation can be ensured within a wider frequency range, even for the sound fields where the uncontrolled diffraction from side and the reflection exist.

Passive control of resonances by nonlinear absorbers

A new passive technique for reducing vibration and noise using purely nonlinear absorbers is stud- ied. Compared to classical passive systems (Frahm absorbers, Helmholtz resonator), a single nonlinear absorber works in a limited range of forcing but in a wide band of frequency. In this paper, a simple system with two degrees of freedom is used to analyze beginning of 'Targeted Energy Transfer TET phenomenon and to define desired working for Nonlinear Energy Sink (NES). An analytical formula of level of forcing for beginning of TET is established by a simplified approach. retical analysis with multiple time scale and complexifica- tion method. The results are however hardly transposable for specific grounded system used in (5). We will here follow a simplified approach by determining nonlinear modes and periodic responses of system and by us- ing a harmonic-based numerical method to analyze their stability. Looking for first destabilization of reso- nance peak of periodic response and appearance of additional branches of periodic regimes at a higher amplitude of forcing, we analyze beginning of TET phenomenon and finally define the desired working zone for NES. An analytical formula of level of forcing for beginning of TET (the threshold for be- ginning of TET) is established by a simplified approach. The intended application of this study is to treat low frequency noise attenuation, where no effective disspative mechanism exists.

Design and Acoustic Performance of a Drum Practice Music Studio

Minimising the impact of music practice and “Garage Band” performances upon neighbours in a residential area is challenging in terms of noise emissions from musical instruments, but particularly so when drums and percussion instruments are involved. Normal residential building façades and roofing designs offer limited low frequency noise attenuation and domestic building construction methods can severely compromise the performance of seemingly adequate partition construction details. This paper presents the results of design, construction and testing activities for a private drum studio that was required to meet stringent boundary noise emission targets in order to comply with local council Development Application requirements. High transmission loss lightweight partition test data is provided for the as-built final installation, along with details of cavity absorption, panel damping and vibration isolation treatments that contributed to maximising façade sound reduction performance. A range of room internal absorption treatments, including low frequency “tube traps”, corner traps and diffusers were successfully employed to achieve compliance with BBC recommended reverberation times for small recording studios

Analysis and design of composite/folded variable area perforated tube resonators for low frequency attenuation

Variable area concentric-tube-resonator mufflers may be used effectively for attenuating the odd harmonics. This ensures a broad-band transmission loss (TL) spectrum. In this paper, configurations that consist of variable area resonators within a uniform area muffler shell for enhancing the low-frequency noise attenuation are investigated. The variable area resonator elements within a muffler allow multiple elements, where the sum of their acoustic lengths is more than the geometric length of the muffler. This shifts the overall TL spectrum toward lower frequency. The underlying idea of incorporating the extended inlets and outlets in these resonators, is to reproduce the broadband attenuation. It has the effectiveness of dissipative ducts without the use of any absorptive materials. The results have been validated by means of three-dimensional finite element analysis.

Combined Feedback Design of Active Noise Control and Face Velocity Control Based on a Novel Secondary-Path Model of Speaker-Duct Systems

Active noise control (ANC)-based systems provide good low-frequency noise attenuation by destructive interference between the secondary signal from an acoustic actuator (speaker) and the primary noise of the acoustic field (duct). A novel secondary-path model that described by two cascaded transfer functions and two disturbances for speaker-duct systems is first developed in this study. One disturbance is imposed on face velocity of actuator speaker that is usually ignored in ANC systems while the other is on sound pressure at a given location in the duct. The transfer functions are identified with measurements from a developed velocity sensor and a microphone, respectively. A combined design of ANC and face velocity control (FVC) based on the identified secondary-path model is further proposed to set up an adaptive feedback ANC/FVC controller with a modified filtered-X recursive least square (FXRLS) algorithm to update controller coefficients to reduce noise levels near microphone location. Results of experiments show that the adaptive feedback ANC/FVC controller has a noise reduction performance of ∼27-46dB for noise at a frequency between 100-200Hz, as compared with ∼10-35dB for that of the conventional feedback ANC design. Furthermore, the adaptive feedback controller reveals a good capability to reduce the noise level for time-varying noise of a frequency sweeping from 100 to 200Hz. Our data demonstrate a substantial improvement in the low-frequency noise attenuation using the developed controller that includes commonly overlooked factors and support the feasibility of our proposed approach in practices.

Active Attenuation of Low Frequency Noise Radiated from Tunnel Exit of High Speed Train

A pressure wave occurs at the instant when a high speed train enters into a long tunnel. The wave propagates downstream to the tunnel exit and low frequency noise is radiated from the exit to outer space. The low frequency noise causes a lot of problems1 to the residents living near the exit and has a close relation with the pressure gradient of the pressure wave. To attenuate the low frequency noise, an active cancellation system rather than a passive one is developed. This research uses a model tunnel to examine the characteristic of the pressure wave and investigates the possibility to reduce the low frequency noise by reducing the pressure wave gradient with active cancellation.

Optimum attenuation of low frequency noise by proper tuning of a prismatic array of waveguides

An open, prism-shaped network of rigid strips can constitute an effective sound barrier for long wavelength sounds [1]. Its physical parameters may be chosen so that the delaying strip medium is perfectly matched to the particular frequency of the noisiest component of the incident sound. The phase lag introduced by the delay medium over free space propagation may then, with a few strips, reach a 180° value creating dipole line-sources below the prism vertex line-source, which cancel the low-frequency diffraction above the obstacle, thus protecting areas behing this barrier. This study deals with the evaluation of three theories which are likely to predict, for various 1 10 scale models the first stopband frequency where 180° phase lag is most easily obtained.