Environmental Noise Research Articles

Enhancing railway noise reduction through advanced multilayer acoustic absorbent systems with tunable resonators

Railway noise poses a significant environmental challenge, prompting the need for effective mitigation strategies. In response, noise barriers are often employed to reduce noise near railways. This research focuses on the use of porous materials, more specifically porous concrete, for acoustic absorption, particularly in the context of railway noise reduction. A novel approach is proposed, wherein porous multilayer metamaterial systems are developed, with each layer's macroscopic parameters strategically optimized to enhance the overall sound absorption curve. Additionally, to broaden the frequency range of noise attenuation, tunable resonators are seamlessly integrated into the multilayer system. The analytical framework for analyzing both the multilayer system and resonators involves employing equivalent fluid models and the analytical transfer matrix method. Linear regressions were employed to establish relationships between porosity, density, and other macroscopic parameters, enabling the derivation of the sound absorption curve solely from these parameters. Through systematic optimization and integration of resonators, it is expected that, the proposed approach demonstrates significant advancements in railway noise reduction efficacy across a wide frequency range. This research aims to contribute to the development of more efficient and versatile acoustic absorbent systems for mitigating railway noise pollution.

Identification of radiated modes by a small-scale ducted fan

Various industrial applications involve fluid flow in ducted fans. Turbojet engines or air condition systems are just a few examples. The long-term objective is to reduce the associated noise pollution generated by rotor-stator interaction within a small-scale duct. Modal detection strategies help to understand the characteristics of sound radiation of such systems involving fluid flow. In this work, the authors analyze the modal amplitudes generated by a rotor-stator interaction in the presence of a background mean flow. The inverse approach detecting the modal amplitudes is applied using experimentally as well as numerically determined sound pressure values outside the circular duct. The results are discussed and compared against mode amplitudes estimated by an in-duct microphone array whose data is measured simultaneously with that of the far-field one. Some physical interpretations regarding varying amplitudes are discussed. The approach is finally used to assess the impact of homogeneous and heterogeneous stator vanes on the rotor-stator generated noise.

A numerical model calibrated by in-situ measurement for predicting the underwater radiated noise of an azimuth thruster structure of a passenger ship

In a global context of limiting the impact of noise pollution on the behavior of marine fauna, the understanding of acoustic radiation phenomena from underwater sound sources of a passenger ship is of primary importance. Although diesel engines and propellers are considered to be ones of the main sources of ship noise, our observations have shown that immersed motors contribute significantly to the noise radiated into the water by passenger ships at low speeds. This contribution summarizes a semi-empirical method for estimating the noise radiated into the water from an azimuth thruster-type structure. An analysis of the vibroacoustic behavior in water of the structure excited by an internal acoustic source is detailed. Beyond the complexity of performing full-scale measurements in a noisy environment, the analysis of the measurements allows to estimate the vibration modes of the immersed structure. A numerical model of the dedicated structure, calibrated on the basis of an experimental modal analysis, is used to estimate the pressure field radiated by the structure into the water. As a result of these analyses, a simplified model of radiated noise in the water is developed for use in nominal thruster operation.

Application of a novel visual representation method for urban noise source monitoring

Increasing urban mobility has led to a rise in the number of vehicles, resulting in urban noise pollution, which can cause health problems. Conventional methods for noise control can be adopted to reduce noise near the sources. However, this is not cost-effective to reduce overall noise level in the urban area. Therefore, it is preferable to encourage drivers to voluntarily reduce noise by applying methods such as cracking down on road noise sources. To this end, a real-time urban noise source monitoring system is needed, but processing large amounts of data can be difficult. In this study, a novel visual representation method, DoAgram, is applied for urban noise source monitoring. DoAgram can represent the time, frequency, and spatial information of the sound source within a 2D color graph. Therefore, the long-term spatial information of the sound source acquired from the sensor array node can be transmitted as a single image. Also, one can restore spatial information of the source from an image through a decoding process corresponding to the encoding method. One can find that the proposed method can be utilized as a database that can efficiently conduct urban noise source monitoring.

Relation between measured noise levels and traffic flow

Road-traffic flow parameters are a critical input to urban noise pollution evaluation frameworks such as CNOSSOS-EU. The accuracy and resolution of the traffic flow improves the quality of the noise exposure assessment. Highly resolved traffic flow within an urban network is difficult to reliably measure or predict. The quality and availability of such traffic information may be improved using data from low-cost noise sensors that are mounted beside representative segments of the road network. A model is presented, which relates the noise levels measured at a single noise sensor with the local traffic flow conditions. The noise data is obtained from a noise sensor mounted alongside a road, which is part of a test-bed in Stockholm, Sweden. The local traffic flow is obtained from commercial vehicle-counting sensors mounted alongside the noise sensor. Effect of seasonality is observed when comparing models trained on different subsets of the training data. The effect of seasonality on the noise levels is investigated.

Modelling of passengers behaviour under the influence of noise in public transport buses: a structural equation approach

This study aims to investigate the impact of noise pollution levels on passengers during travel. Specifically, the aim was to explore the interconnections between noise-induced annoyance, sensitivity to noise, sleep disturbances, and physical health problems using a structural equation modelling (SEM) approach. 14 routes were selected for detailed analysis across different time periods. The noise levels were observed, and the impact of these levels on passengers was assessed through a questionnaire survey to know their perception towards noise and its impacts. A total of 252 responses from passengers were collected through the personal interview. Findings reveal that noise pollution levels exceeded 70 dBA on most of the routes. A significance test (t-test) conducted on 15-minute frequency intervals showed no significant difference in noise pollution levels during morning, evening, and afternoon periods. Subjective analysis indicates 31% of passenger were found to be highly annoyed due to noise within public buses. Additionally, 32% and 9% of passengers reported moderate and minimal annoyance, respectively, attributed to traffic and engine noise inside buses. Structural equation approach showed that sensitivity to noise affects the annoyance level most with coefficient 0.76, followed by physical health factors and sleep disturbance with coefficient 0.48 and 0.44 respectively.

Investigation of ultrasonic propagation in high attenuation porous materials

This research examines ultrasonic wave propagation in air-saturated plastic foams used for noise pollution reduction, questioning the effectiveness of current models such as the well known Johnson-Champoux-Allard model at high frequencies. These foams present a challenge for existing models to accurately depict visco-inertial and thermal interactions within their pores. The study highlights the model's failure to align experimental results with theoretical predictions. It introduces novel parameters denoted as Σ and V for viscous effects, and Σ′ and V ′ for thermal effects, to improve the representation of fluid-sturcture interactions. These parameters suggest a more significant boundary layer effect within the pores than previously considered. This approach aims to provide additional physical context for the principles governing the behavior of highly attenuating porous media and to explore new avenues for material characterization that surpass the limitations of existing models.

Generalization of noise insulation afforded by common window designs with active noise control exposed to moving noise source

A window device that can provide noise insulation while maintaining natural ventilation is highly desirable and is especially sought after in heavily noise-polluted and hot-humid climates cities, such as Southeast Asian cities. However, a solution to provide such a window is non-trivial because the design principles guiding acoustics and ventilation are diametrically opposed. While louvre windows have traditionally been used in communal, healthcare, and some educational setting to provide a balance between acoustics and ventilation benefits, it is still sorely lacking to provide sufficient insertion loss without sacrificing ventilation entirely. A new plenum window design has recently garnered research and market interest to tackle the dichotomous problem. Additionally, the application of active noise control (ANC) in windows is gaining popularity and offers potential solutions to improve noise control without compromising ventilation. However, it remains unclear how different traditional and modern window designs compare in terms of their acoustic performance, especially when exposed to non-stationary noise sources such as urban traffic, a major noise pollutant. Thus, the present study seeks to provide a generalization of insertion loss provided by different window designs exposed to moving sources including a tangent study on ANC implementation via a time-domain approach.

An action-oriented participatory research methodology involving residents, elected representatives and researchers: a case study in Rezé, France

The management of sound environments is a major challenge for the quality of life of residents (a definition here including city users). Local authorities have regulatory tools at their disposal to manage urban noise pollution, but these are sometimes far removed from the residents' perceptions. The aim of the SonoRezé project is to build on a citizen science approach, based on a triptych of "Local authorities - Residents - Researchers", working together on the diagnosis and management of urban sound environments in the city of Rezé, in Nantes metropolitan area. The approach consists in participatory noise measurements using the NoiseCapture mobile application, and the definition and implementation of actions jointly developed by residents, elected representatives and researchers, with the aim of improving the city's sound environments. The article will present the interdisciplinary and multi-stakeholder methodology involved. Then, two initiated actions will be outlined and discussed: (i) a scientific mediation initiative in schools focusing on the development of perceptual sound maps, (ii) an action on aircraft noise, precising the methodology involving the three parties used to construct the action, as well as some of the results produced, namely a sensitive map, analyses of measurement data and the production of alternative indicators.

A fundamental investigation on the sound absorption characteristics of acoustic meta-surface type baseboards

The sound of footsteps generated in corridors propagates into surrounding spaces, sometimes becoming a problem of noise pollution. Therefore, the aim of this study is to reduce footstep noise by incorporating sound absorption mechanisms into the baseboard of corridors. We focus on the acoustic metasurface that has gained significant attention in recent years in order to absorb a relatively wide range of frequencies. In this study, we proposed acoustic metasurface type baseboards by utilizing multiple resonators with different resonance frequencies. The proposed models feature parallel arrangement of multiple resonators with straight or inclined slits. The absorption performance of these models is evaluated through theoretical analysis, numerical simulations, and experiments. Results suggest that acoustic metasurface type baseboards is found to be able to absorb the dominant frequencies in the sound of footsteps. In addition, increasing the number of resonators and incorporating slanted perforations in the neck contribute to improved absorption performance. Furthermore, the possibility of thinning the models through slanted perforations is also suggested.

Modelling of courtyard acoustics using the nodal discontinuous Galerkin finite element method to validate a newly developed PML

Evaluating the acoustics of courtyards has gained increasing attention from a sustainable urban design perspective, which requires numerical predictions tools for estimation of noise pollution and assessment of the influence of acoustic conditions. Accurate modeling of the acoustics of a courtyard is scarce because of the variety of factors to include: meteorological effects, building material properties, and geometry of the inner façades. We consider the nodal Discontinuous Galerkin finite element method framework in a 3D static and uniform medium and use it to model the sound propagation in a courtyard. A new and efficient Perfectly Matched Layer formulation is implemented at the open boundary of the courtyard model to absorb outgoing acoustic waves and this way limiting the size of the computational domain. The numerical results are validated against experimental measurements carried out in a scaled model of a courtyard with an elementary design to implement an accurate corresponding numerical model. The experiments were conducted in an anechoic chamber where the meteorological effects are controlled, and the courtyard facades are modelled as flat surfaces with known material properties.

Geometric optimization for localized attenuation of subsonic slit-based open acoustic barriers

In recent years, many solutions have been developed to reduce noise from different sources. One of these solutions is acoustic barriers. An acoustic barrier is a structure designed to reduce noise transmission between areas by blocking the direct path of sound. Open acoustic barriers are a valuable solution for technical and industrial plants. They allow airflow and visibility. Given the need for localized attenuation in these environments, it is possible to use numerical models to improve effectiveness and thus increase insertion loss in a region of interest. This study has used numerical simulation tools to provide degrees of freedom in placing the different elements of a subsonic slit-based open barrier. It has been shown that by rotating some of the elements and determining an area of interest, it is possible to improve the effectiveness of the barrier. These results allow using all the elements with the same topology to individually attack noise pollution problems in their location and frequency range.

Development of low-cost MEMS microphone array for sound localization in urban environments

Understanding noise pollution and mitigating its impact on the quality of life in urban environment is a major challenge from a societal and health point of view. In order to monitor and ultimately understand these noisy environments a process of long term acoustic measurements and analysis is required. These urban environments are noisy and obtaining a clear recording of a desired acoustic signal is difficult hence an effective approach is the use of beamforming theory coupled with a microphone array. In this paper a novel approach utilizing compact low-cost microphone arrays equipped with MEMS sensors for continuous sound localization in urban areas is proposed. This approach focuses on leveraging the affordability and scalability of MEMS sensors to deploy distributed acoustic sensor networks across diverse urban locations. The aim is to capture real-time acoustic data, enabling comprehensive monitoring of sound sources and their spatial distribution within the urban environment. This sensor unit can be upgraded with a camera, i.e. acoustic camera, which will provide information on the conditions of the environment where the measurements are performed. This low-cost solution facilitates long-term monitoring, providing valuable insights into the dynamic nature of urban soundscapes.

Human voice absorbing system by the use of aeroacoustics metamaterials

The increase of calls meetings has intensified vocal noise pollution, particularly in open spaces and during transportation. This poses challenges not only for speakers seeking confidentiality but also for disturbed neighbours. Both workplace and transportation environments suffer from the disruptive nature of vocal noise, with direct implications for cognitive performance in office settings. To answer to that problem, Skyted propose an innovative acoustic mask designed to reduce voice noise by up to 35 dB at a distance of 1 meter. This technology comes from meta-materials initially developed at ONERA for aircraft engine applications. Adapted and modeled for human vocal use, Skyted's mask ensures optimal comfort, breathability, and bidirectional attenuation. The numerical models based on FEM, and analytical (Matrix Transfert based on) are compared to measurements made for those specific application.

Analysis of representation and generalization capabilities of pre-trained audio models in urban environments

In the last decade, urban noise pollution has become a significant environmental concern that can be mitigated with the help of audio detection algorithms for classifying different sources of noise and creating more informative noise maps. In this context, machine learning, particularly transfer learning, is an essential technology that enables accurate analysis of urban noise sources. However, the choice of the pre-trained model used to compute audio embeddings can significantly influence the performance of downstream classification tasks. This paper aims to compare the embeddings of various pre-trained models on different data collection campaigns in the context of the Sons al balcó project and quantify the robustness of audio representations. To achieve this, we develop metrics and statistically test the presence of distribution shifts in learned latent features. To evaluate the quality of the embeddings, we perform both qualitative and quantitative analysis using dimensionality reduction methods and assess the performance on downstream tasks using data from different collection campaigns. Results highlight major differences between general purpose and specific models. Our findings suggest the need for careful consideration during the choice of the pre-trained model to use in audio event detection applications.

Assessing and mitigating environmental noise pollution: a case study of a metallurgy company

This study aims to analyze the environmental acoustic and vibrational scenario surrounding a metallurgy company, focusing on an industrial unit. Initially, assessments were conducted, including sound pressure measurements according to the ABNT NBR 10.151:2019 standard and internal procedures, comparing the results with regional legal limits. The second analysis focused on particle vibration velocity peaks collected in June 2022, comparing them with the standards established by DD No. 215/2007/E of CETESB. Upon identifying nocturnal noise levels above the limit at two monitored points, an additional step was taken. The internal noise of the main plant sources was characterized, creating noise and conflict maps with legislation. These maps assess noise in the inhabited areas surrounding the industry, identifying critical sources for exceeding limits. Conceptual acoustic solutions were proposed, including the necessary attenuation to ensure that external noise from the factory complies with normative limits in the inhabited areas nearby.

Experimental analysis of eigenmode frequency of an hydrofoil on trailing edge hydroelastic coupling generating tonal noise

The majority of anthropogenic noise in the oceans is due to commercial, military or tourist motorized navigation. Lifting surfaces such as hydrofoils, propeller blades, rudders and stabilizers can be prone to flow induceed vibrations and, under certain conditions, to produce tonal noise. Reducing vibration and noise generated by this type of structures, is therefore an important issue to limit noise pollution and to improve the acoustic discretion of ships. This study, carried out at the French Naval Academy Research Institute, highlights the hydroelastic coupling mechanism responsible of trailing edge vibrations and of lifting surface tonal noise evolving in laminar to turbulent transition regime. This experimental research was carried out in a hydrodynamic tunnel with a NACA0015 hydrofoil, for angles of incidence ranging from 0° to 12° and Reynolds numbers varying from 200,000 to 1,200,000. Laser vibrometry was used to specify the vibrational behavior of two identical hydrofoil made of aluminum and brass in order to estimate the influence of material and eigenfrequency on tonal noise generation. The results show that, contrary to the same mechanism in air, tonal noise of hydrofoil was strongly dependent of the natural frequency of the profile.

Noise in a UK intensive care unit: a retrospective cohort study

Excessive noise in hospital Intensive care units (ICU) has been associated with adverse health effects and human factor impacts acknowledged to negatively affect both patients and healthcare providers. Aims: Evaluation of the acoustic environment of the Royal Liverpool University Hospital (RLUH) ICU versus WHO and UK guidelines. Methods: Noise measurements were taken from a six-bedded bay within the RLUH ICU between 15th June and 1st July 2022. This audit focussed on noise data > 87dBA, in accordance with the national exposure limit established by UK Noise Regulations. A retrospective review of relevant patients' records was conducted to identify potential noisy events during the audit period. Results: The highest and lowest LAeq value was 77.9 and 63.5dBA, respectively. All LAeq and LAmax noise measurements recorded exceeded recommended guidelines. Noise fell below recommended daytime guidelines less than 1% of the time. The top potential causes of noise were patient repositioning/personal care, medication administration and suctioning. Conclusion: Noise levels in the RLUH ICU considerably exceed national and international guidelines. These findings highlight the need to address noise pollution in the ICU setting. Hospital staff and policy makers should consider implementing strategies and interventions for noise reduction in ICUs.

Assessment of annoyance from traffic noise in a school and a hospital

Traffic noise can cause significant annoyance. We administered noise annoyance surveys in two Florida facilities located near a highway and a busy road while traffic noise levels were being measured. In Trilogy school, thirty middle & high school students rated the annoyance caused by traffic noise inside the school to be an average of 1.9 (SD 2.0) on a scale from 0 to 10. Traffic noise was just as disturbing as other sounds inside the school. However, the fraction of students reporting any level of traffic noise disturbance was significantly higher outdoors (80%) than indoors (48%). Additionally, traffic noise caused some difficulty hearing the teacher for 9.7% of students. In Shands Hospital, the average ratings of annoyance from traffic noise given by 107 staff (M=1.1, SD=2.0) and 104 patients (M=1.2, SD=1.4) were not higher in areas of the hospital that were closer to the road or the helicopter landing pad compared to the other areas of the hospital. Traffic noise was substantially less disturbing than other noise inside the hospital, e.g., tube station. However, traffic noise caused moderate to extreme annoyance for 12.4% of patients and some degree of sleep disruption for 27.9% of patients.

Coupling model between building post structure and floors with the spectral element method

With the densification of urban area and public transport, the structure-borne noise disturbance due to railway traffic is a major issue. In order to mitigate it, ground-borne vibration propagation in building has to be investigated. The case of post-beam constructions necessitates 3D modeling with choices between complex, precise and computationally time-consuming approaches (3D finite element method for example), and fast but less accurate approaches like the statistical energy analysis (SEA) which need a strong modal density. For such post-beam constructions, the spectral element method (SEM) appears to be a good compromise. This method allows reducing the number of elements to represent the truss structure compared to the finite element method. However, the building walls and floor are difficult to consider with the SEM. In this paper, the challenge of coupling post-beam structures and floors is examined, a method is proposed and results are discussed. The aim is to develop a method applicable to different buildings while keeping advantage of SEM low discretization.