Solution For Noise Reduction Research Articles

Predicting the impact of wind turbine noise: general overview and results of the PIBE project

The PIBE project aims to improve wind turbine noise prediction methods and to explore new solutions for noise reduction. This five years project brought together French experts in aeroacoustics, sound propagation, experimental noise characterization and wind engineering, and was structured around three work packages. The first one aimed to study amplitude modulation phenomena and focused particularly the characterization of dynamic stall noise. Specific aerodynamic and acoustic measurements were carried out in a wind tunnel, showing the influence of several stall regimes on noise production. The second work package focused on quantifying the uncertainties of noise prediction methods. It developed an open-access online application (WindTUNE) that quantifies uncertainties on noise prediction of a wind farm, and a parametric and uncertainties calculation tool for the engineering application Code-TYMPAN. This axe also produced an open-access database of a 400 days campaign of meteorological and acoustical measurements around a wind farm. The last work package investigated new noise reduction devices using blades with modified leading and/or trailing edges. The efficiency of the solutions were characterized in a wind tunnel, both acoustically and aerodynamically. The paper presents the main results obtained at the end of the project.

A new method for the characterization of perforated facings using a wide frequency range Impedance Sensor

Perforated facings (like fabrics, micro-perforated screens, ...) are widely employed in noise reduction solutions. Following previous works, it has been shown that they can be described as a porous media with identical cylindrical perforations. Then, a classical five parameters JCA model can be employed to predict the acoustical behaviour of perforated facings and, due to their small thicknesses, these five parameters can be obtained from only two independent quantities: the perforation average radius and the perforation rate. The CTTM and LAUM have recently developed a new approach for the measurement of JCA porous model parameters. This approach relies on the properties of two innovative apparatuses: an Impedance Sensor which allows for accurate impedance measurements over a wide frequency range combined with a patented compact anechoic termination. These devices are used to propose an improvement of an existing method for perforated facings characterisation. By using a non-resonant acoustic load and the Impedance Sensor, an accurate estimation of facing bulk magnitude can be done over a wide frequency range. Then the possible impact of the facing vibrational mode and of the acoustic load resonances is reduced, allowing for a more accurate estimation of facing parameter. Results are presented and discussed for different samples.

Anthropogenic noise and impact on marine species : Acoustic metamaterials as a reduction solution

Anthropogenic underwater noise generated by offshore maritime activities poses a significant threat to nearby ecosystems due to elevated sound levels. Efforts have to be undertaken on noise reduction solutions, depending both on the offshore activities and on the marine species. In this context, leveraging the recent advances in underwater acoustic metamaterials seems crucial, due to their special properties in terms of scattering and absorption. This endeavor aligns with advancing Marine Renewable Energies while minimizing environmental impact. In this communication, a synthetic review is first proposed to define the various maritime works targeted, as well as their associated sound levels and frequency ranges of emission. In a second part, a cartography is outlined by classifying different marine species according to their hearing type, depth of immersion and auditory threshold. Then, in relation with the two previous analyses, solutions based on metamaterials for deep sea immersions are proposed. In particular, a multi-periodic arrangement of bars is proposed that exhibits several band gaps (i.e. frequency ranges in which propagation is forbidden), in order to limit the transmission of sound waves in a large frequency range. The concept can be easily extended depending on the targeted offshore activities and marine species.

Optimization of a metal additively manufactured hydraulic silencer with array resonators

As mobile machines shift to electrification, hydraulic circuit noise, previously masked by combustion engine noise, has emerged as a significant concern. To address acoustic comfort issues, this paper investigates an innovative solution for high-performance noise reduction in hydraulic circuits. The proposed silencer design incorporates an optimized array of Helmholtz resonators, strategically arranged to broaden the narrow band characteristics of single resonators. The chosen configuration effectively reduces pressure pulsations across a broad mid-frequency spectrum. While this design is well-established in air medium, its application in a hydraulic oil medium is pioneering. Fabricated through metal additive manufacturing, the silencer undergoes testing on a dedicated bench. Transmission Loss measurements validate both the concept and the optimization methodology.

Annoyance assessment of the combined effect of railway noise and vibration including physiological measurements

The prediction of annoyance due to railway transportation in residential areas is crucial to correctly dimension noise and vibration reduction solutions. In situ annoyance assessment for multiple exposition scenarios requires to convince many residents to take annoyance tests combined with noise and vibration measurements inside their homes. Such approach is not adapted to large-scale experiments. Therefore, the VibAcouPhysio project aims at studying annoyance due to railway noise and vibration in laboratory conditions. A vibrating platform combined with a high-quality sound reproduction system has been designed for this project to reproduce the noise and vibrations of a train pass-by. Annoyance is estimated by multiple means including questionnaires, cognitive tasks performance assessment and various physiological measurements. Twenty participants were exposed to various train pass-by signals as if they were seating at a table inside their housing. The results show a significant correlation between physiological metrics, the annoyance evaluated in the questionnaires and the physical indicators derived from the noise and vibration signals. The main purpose of the VibAcouPhysio project is to rank various physical indicators describing noise and vibration signals according to their correlation to the perceived or measured annoyance.

Multifunctional metamaterials for simultaneous sound absorption and flexural vibration isolation

Controlling the noise and vibration simultaneously in some engineering scenes is extremely essential to eliminate harmful effects on the humans and equipments. To overcome this challenge, a multifunctional metamaterial plate with both airborne sound absorbtion and flexural vibration isolation simultaneously is investigated theoretically, numerically, and experimentally. The plate consists of a host plate and a Helmholtz resonator, where Helmholtz resonance is used to absorb sound waves and its external solid domain plays as role of a local resonator to isolate the flexural vibrations. The theoretical models were established to characterize the mechanism of the sound absorption and vibration isolation. The experimental results demonstrated that the proposed metamaterial plate can achieve perfect sound absorption at 649 Hz and a flexural wave band gap at 289–470 Hz. Besides, although the sound absorption and vibration isolation functions are integrated in the same component, our theory identifies several critical geometrical parameters which can manipulate the absorption peak and the flexural wave bandgap, allowing us to design its frequency of multiple functions as desired. This work provides a promising solution for noise and vibration reduction in engineering.

Sound Characteristics and Mechanical Properties of Metal Powder Chamber Structures Composite Materials Formed by Laser Powder Bed Fusion

Granular damping technology is a widely used method in aerospace and other industries to reduce vibration and noise. However, the technique still faces challenges such as secondary processing, additional structure, single structure, and residual stress. This article introduces a new composite material, the metal powder chamber structure (MPCS), formed through the laser powder bed fusion technology. The article explores the effects of factors like arrangement mode, volume fraction, and powder chamber diameter on the sound and mechanical properties of MPCS. The study shows that MPCS improves the sound insulation effect of the material and defies the mass law of sound insulation. The sample 4B40 using a body‐centered cubic arrangement has a sound insulation coefficient of 52 dB. The sound absorption efficiency curve presents a double peak, and the sample 6F60 exhibits the highest sound absorption efficiency of 0.76. MPCS changes the deformation stage of the material stress–strain curve, and different MPCS also alters the compression deformation mode of the composite material. By studying the effects of MPCS on sound and mechanical properties, it is hoped to provide a better and more sustainable solution for vibration and noise reduction.

Serrations as a Passive Solution for Turbomachinery Noise Reduction

Aircraft engine noise has become a significant concern for air operators to address. Engineering strategies have resulted in the development of easily applicable solutions, known as “passive solutions”, that do not necessitate real-time control. These solutions include the incorporation of corrugations or cutouts at critical locations on the engine’s aerodynamic surfaces. Realistic solutions, whether approached numerically or tested at small scales, as well as computational models, have been found to closely match experimentally observed behaviors, both in 2D and 3D scenarios. The identified geometries serve as promising starting points for devising combined concepts that may offer even better performance under specific flow conditions.

Numerical solution and experimental investigation for hydro-acoustic analysis and noise reduction assessment of ship ducted propeller

Ship noise emission has many environmental impacts. Recognizing noise-generating source and investigating noise reduction techniques is the first step in elimination or reduction of these harmful effects. Studies show that ship propeller is one of the main sources of ship noise emission. Ducted propeller is one of the well-known solutions for noise reduction. In this paper, results of numerical solution for governing mathematical equations of noise emission in far field is presented and sound pressure level is calculated using Ffowcs Williams and Hawkings (FWH) equations. Numerical solution is validated by valid research resources. Propeller noise emission is also calculated by experimental measurements and filtering ambient noise frequencies. Finally, a comparison has been made between numerical solution and experimental results and the effect of duct on noise reduction is calculated. The measured SPL emitted from the propeller in CFD approach (solving the FWH) shows 28% reduction for ducted propeller in comparison with the propeller without the duct. In experimental approach (after removing the ambient noise) shows 37% reduction for ducted propeller in comparison with the propeller without the duct.

Performance Evaluation of Balcony Designs for Mitigating Ground Level Noise

This study aims to tackle the challenge of high noise levels on balconies while preserving natural ventilation. Eight innovative balcony designs, incorporating elements like diffuser edges, undulating ceilings, Helmholtz resonators, grooves, or sound traps, were evaluated via finite element (FE) modeling. The insertion loss results showed that for many balcony designs, noise reduction in the balcony could deteriorate beyond an elevation of 8 m. However, the front jagged and full wavy ceiling designs were shown to be more robust in noise attenuation across balconies on different floors. The jagged ledge and grooved parapet designs yielded an overall 1.5 dBA lower SPL at the exterior regions, compared to other designs, which implies that the designs are less acoustically detrimental to nearby residential blocks as they tend to diffract and absorb incident noise. The jagged ledge design is more effective for lower floors while the jagged ceiling design is more effective for higher floors. A combination of the protruded jagged ledge for the lower floor and jagged balcony ceiling for the higher floor would result in the lowest noise ingress over three stories of residential units: this would be capable of achieving more than 3 dB noise reduction and would offer viable options for improving balcony noise mitigation, by providing valuable insights to architects and designers seeking practical solutions for outdoor noise reduction. Our study highlights that whereas the spectrum characteristics of acoustic absorption materials may be less tunable, and where reduced head space is traded for thicker material for greater ab-sorption and added affixation and maintenance cost, the jagged ledge and ceiling curvatures can actually be shape-tuned, say for every 3 to 4 floors up the high-rise to more effective reduce noise ingress and possibly improve the architecture façade outlook.

A Resonator Noise Reduction Solution for a Centrifugal Gas Compressor

The purpose of this work is to reduce the noise generated by a compressor that conveys methane gas. After certain measurements were conducted a high level of noise was observed in the 2000-3000 Hz range, therefore a solution for noise reduction at the source is addressed and presented in this paper. The research method is based on designing resonators to be applied on the stator of a centrifugal compressor used in a natural gas distribution station. First, the calculations are made on resonators with air as the working fluid and then are validated through real measurements in a Kundt tube. After validation, the working fluid is changed with gas, calculations are made once again, and acoustic simulations are performed. To facilitate acoustic simulations and reduce computational time, a simplified stator geometry was employed. This simplified model encompassed the region starting from the rotor's gas exit, where the resonators were deployed. The purpose of the acoustic simulation was to validate the frequency range influenced by the resonators and to estimate the overall noise reduction. Depending on the operating regime of the compressor, the rotor fundamental can vary within the frequency domain of 2000 – 3000 Hz. This broadband domain requires the usage of several resonators with different resonant frequencies. The proposed solution obtained an average value of attenuation, excluding the peaks of the attenuation, in the frequency domain of 2000 - 3000 Hz, of 9 dBA. If the fundamental frequency coincides with a resonance of the resonator, higher attenuation can occur. Also, fundamental attenuation can lead to attenuation of the harmonics.

Complex-valued impedance tiles to reduce noise emanating through openings in mechanical systems

We present acoustic metasurfaces to achieve high noise reduction for an open structure, which refers to a structure with openings, by manipulating complex impedances along unit cells called impedance tiles. These acoustic metasurfaces are applied to inner walls of the open structure to suppress noise emanating through openings. Each impedance tile is made up of subwavelength Helmholtz resonators (HRs), and its effective impedance is tuned to minimize the noise level outside the open structure at an arbitrary target frequency by adjusting the geometrical parameters of those HRs. Rather than absorbing incident waves, the proposed metasurfaces significantly affect the magnitude and phase of scattered waves, causing the sound energy to be trapped near their surfaces and dissipated by visco-thermal losses in narrow orifices of the impedance tiles. As a result, the metasurfaces enable more effective noise reduction for the open structure compared to impedance-matching conditions, which absorb sound waves impinging on the surface. Experimental results show that the sound pressure level reduction exceeding 12.6 dB is verified for a single source at a target frequency of 360 Hz with λ/31-thick metasurfaces fabricated via three-dimensional printing. Furthermore, we demonstrate that the proposed metasurfaces are also useful in reducing noise when the number of target frequencies is more than one or when multiple sources are distributed in different locations, thereby offering a potential solution for noise reduction in a wide range of mechanical systems with open structures.

Ultra-broadband low-frequency high-efficiency acoustic energy harvesting with metamaterial-enhanced loudspeakers

Acoustic energy harvesters (AEHs) open up opportunities to recycle noise waste and generate electricity. They provide potential power solutions to a wide range of sensors. However, the practicality of AEHs has long been limited by their narrow bandwidths and low efficiencies. In this study, we present an ultra-broadband AEH and a highly efficient AEH that transforms sound energy into usable electrical power. Our broadband device comprises an electrodynamic loudspeaker driver and an optimized acoustic metamaterial matching layer and is capable of converting 7.6% to 15.1% of total incident sound energy from 50 to 228 Hz. Moreover, we demonstrate that by replacing the loudspeaker surround with a lower-loss material such as PDMS, the energy conversion rate can be significantly increased to 67%. The proposed broadband AEH has a fractional bandwidth eight times the state-of-the-art, while the proposed highly efficient AEH has a peak efficiency three times the state-of-the-art. The outstanding performance makes our designs cost-effective and scalable solutions for noise reduction and power generation.

State of the art about solutions for tram noise reduction in the framework of the Life SNEAK project

The LIFE SNEAK project, started in September 2021, aims at the reduction of noise from road traffic that mainly affects densely populated urban areas where the noise and vibrations produced by the tram overlap with noise produced by road traffic. Applicative measures will be designed and tested in a pilot case of the city of Florence, such as low-noise and vibration surfaces, with life cycle costs comparable to those of traditional surfaces, and measures to reduce tram noise aiming to obtain substantial reductions in noise and annoyance. Referring to tram noise, in the first phase of the project, specific attention has been dedicated to the state-of-the-art analysis concerning possible measures to perform noise reduction with specific attention to noise due to wheel-rail contact and "squeal noise" phenomena that mainly occur in urban environments close to curves with small radius. In this paper, the results of the state-of-the-art analysis are presented with particular attention to the use of sound-absorbing panels to be applied on the tram (bogie skirts).

Vibroacoustic metamaterials as add-on solution for noise reduction in existing housing structures

Vibroacoustic metamaterials (VAMM) have the potential to reduce unwanted noise components in a very targeted and narrow-band frequency range. Many VAMM concepts are based on mechanical resonators that act as vibration absorbers in their natural frequency and thus absorb energy that would otherwise be radiated in the form of airborne sound and perceived as noise. Often, during the design phase, it is not yet possible to adequately assess which surfaces will be acoustically problematic and in which frequency ranges disturbing noise components are going to be. In addition, many VAMM concepts can only be manufactured using additive manufacturing (AM) processes, due to their complex geometry. However, AM parts often have strongly anisotropic material behavior, depending on the manufacturing process, which makes a prediction of the vibroacoustic behavior even more difficult. Direct integration into casing structures during the design phase is therefore not practical and economically feasible in most cases. This paper therefore investigates the extent to which retrofitted resonators can be effectively used in existing casing structures. For this purpose, FDM-printed bending resonators made of ABS and PLA are used, which have already been measured with regard to their natural frequencies in a previous publication. Different variants are attached to a demonstrator housing and the surface vibration velocity is measured using a 3D laser scanning vibrometer, and compared with the basic variant without resonators. The radiated airborne sound is also measured. Furthermore, a comparison is made with a dynamic FEM simulation in order to be able to evaluate its prediction quality, in particular with regard to the additively manufactured resonators.

A new method to reduce motion artifact in electrocardiogram based on an innovative skin-electrode impedance model

The electrocardiogram (ECG) is sensitive to human body motions when it is measured with dry metal electrodes. A common hypothesis in previous literature dealing with these subjects postulate that this is due to the skin-electrode impedance variability. This paper aims to verify this hypothesis by investigating the origin of the noise in the ECG signal and finding suitable solutions to reduce this disturbance. For this reason, experiments are proposed here to explore the relationship between the skin-electrode impedance and the ECG for several motions. Results demonstrate that the noise due to human body motions in ECG equally comes from the electrochemical equilibrium break of the reduction–oxidation reaction at the skin-electrode interface. Moreover, it has been shown that motions lead to sweat thickness variations and equally to skin-electrodes capacitance variations. To model this phenomenon, an electrical model based on a variable capacitor is introduced. Finally, a significant noise reduction solution based on the monitoring of this capacitor is proposed.

Frequency stop-band optimization in micro-slit resonant metamaterials

An optimized unit cell design of a micro-slit resonant metamaterial is proposed to increase the size of the frequency stop-bands and to enhance the sound absorption at normal incidence. Micro-slit resonant metamaterials offer a compact and lightweight solution for low-frequency noise reduction, in contrast to traditional methods such as absorptive foams. A combination of numerical and semi-analytical solutions based on dispersion and absorption curves is presented. A novel algorithm allows for the decoupling of wave types from raw numerical data in the dispersion curves, without using the stiffness and mass matrices. A thorough optimization process of unit cell designs with genetic algorithms is performed. Focus is given to the first frequency stop-band, located in the frequency range of application of micro-slit resonant metamaterials. The process shows that relatively large resonators (with respect to the unit cell total area) produce a larger first frequency stop-band, whereas slit size, and resonator stiffness have a negligible effect. The optimized design increases the first frequency stop-band by 20% and the second stop-band by 25% compared to the literature standard. The absorption curves at normal incidence of acoustic waves are derived numerically for a rigid and elastic frame of the metamaterial backed by a cavity. These curves are validated by the JCAPL semi-analytical model. The optimized unit cell design shows a 9% increase in the first peak of absorption coefficient compared to the literature standard at a cavity depth of 30 mm, and an increase of 10% at a cavity of size 53 mm. Stop-band behavior does not influence sound absorption at normal incidence of acoustic waves in the frequency range of interest.

A light-weight periodic plate with embedded acoustic black holes and bandgaps for broadband sound radiation reduction

Conceiving lightweight structures with low vibration and sound radiation properties is an important topic. The concept of Acoustic Black Hole (ABH) offers new impetus to tackle this problem. Most existing ABH structures are based on simple ABH cells. Apart from the reduced structural strength, systematic ABH effects occur typically above the cut-on frequency of the ABH element, which is perceived as a bottlenecking problem. To tackle the problem, this paper examines the sound radiation properties of a plate comprising periodically tangled ABH cells. Through combining ABH effects with sub-wavelength bandgaps (BGs), numerical and experimental studies show that the plate exhibits reduced sound radiation properties in an ultra-broad frequency range, far below the cut-on frequency of an ABH element. This is owing to the tangled nature of the ABH elements, which extends the actual dimension of the ABH, lowers its onset frequency and reduces the sound radiation efficiency through creating slow waves. Inside the BGs, the reduced sound radiation is mainly due to the redistribution of the vibration energy, basically confined to the excitation area. Capitalizing on the combined ABH and BG features alongside improved mechanical properties, the proposed structure shows promise as a light-weight solution for broadband noise reduction.

Refined clock-jitter reduction in the Sagnac-type time-delay interferometry combinations

The ongoing development of the space-based laser interferometer missions is aiming at unprecedented gravitational wave detections in the millihertz frequency band. The spaceborne nature of the experimental setups leads to a degree of subtlety regarding the otherwise overwhelming laser frequency noise. The cancellation of the latter is accomplished through the time-delay interferometry technique. Moreover, to eventually achieve the desired noise level, the phase fluctuations of the onboard ultra-stable oscillator must also be suppressed. This can be fulfilled by introducing sideband signals which, in turn, give rise to an improved cancellation scheme accounting for the clock-jitter noise. Nonetheless, for certain Sagnac-type interferometry layouts, it can be shown that resultant residual clock noise found in the literature can be further improved. In this regard, we propose refined cancellation combinations for two specific clock noise patterns. This is achieved by employing the so-called geometric time-delay interferometry interpretation. It is shown that for specific Sagnac combinations, the residual noise diminishes significantly to attain the experimentally acceptable sensitivity level. Moreover, we argue that the derived combination, in addition to the existing ones in the literature, furnishes a general-purpose cancellation scheme that serves for arbitrary time-delay interferometry combinations. The subsequential residual noise will only involve factors proportional to the commutators between the delay operators. Our arguments reside in the form of the clock noise expressed in terms of the coefficients of the generating set of the first module of syzygies, the linear combination of which originally constitutes the very solution for laser noise reduction.

Improved spatialization performance for joint speech dereverberation and noise reduction in binaural hearing aids

ObjectiveIn this work we propose a unified solution for joint speech dereverberation, noise reduction and spatial preservation of either diffuse noise fields or single-point noise sources in binaural hearing aid applications, which is based on the multichannel Wiener filter (MWF) with preservation of the interaural coherence (MWF-IC) method. MethodsThe original interaural coherence (IC) auxiliary penalty cost function in the MWF-IC, which comprises information about both interfering noise and late reverberation component, is modified to consider the interference noise information only. It provides a new regularization form for preserving both interaural time difference (ITD) and IC binaural cues of the residual noise. This approach is theoretically supported by recent findings about the relationship between IC and ITD binaural cues for point acoustic sources. ResultsExtensive simulation results with objective measures, as well as psychoacoustic experiments with normal hearing volunteers indicate that the proposed method outperforms the preservation of spatial localization cues of its original version, keeping approximately the same performance in terms of noise-reduction and speech-dereverberation. ConclusionThe proposed scheme expands the MWF-IC original spatial cue preservation capability from only diffuse noise fields to either diffuse noise fields or a single-point noise source. SignificanceThis is the first MWF-based speech dereverberation and noise reduction method for binaural hearing aid that provides perceptually relevant preservation of acoustic scenarios comprised of one speech source and either a single-point noise source or a diffuse noise field.