On the difference-to-sum power ratio of speech and wind noise based on the Corcos model

The difference-to-sum power ratio was proposed and used to suppress wind noise under specific acoustic conditions. In this contribution, a general formulation of the difference-to-sum power ratio associated with a mixture of speech and wind noise is proposed and analyzed. In particular, it is assumed that the complex coherence of convective turbulence can be modelled by the Corcos model. In contrast to the work in which the power ratio was first presented, the employed Corcos model holds for every possible air stream direction and takes into account the lateral coherence decay rate. The obtained expression is subsequently validated with real data for a dual microphone set-up. Finally, the difference-to- sum power ratio is exploited as a spatial feature to indicate the frame-wise presence of wind noise, obtaining improved detection performance when compared to an existing multi-channel wind noise detection approach.Comment: 5 pages, 3 figures, IEEE-ICSEE Eilat-Israel conference (special session

Computational Acoustic Beamforming for Noise Source Identification for Small Horizontal Axis Wind Turbines

This thesis develops a computational acoustic beamforming (CAB) method for identification of sources of small wind turbine noise. The methodology consists of three components: computational fluid dynamic (CFD), acoustic propagation and acoustic beamforming components. Each component of the CAB method is validated on the component level. The numerical results agree well with the experimental data for the validation of each component. The CAB method is then validated on the whole system level using the NACA 0012 airfoil trailing edge noise case. The predicted acoustic maps are in excellent agreement with the corresponding observed acoustic maps obtained from wind-tunnel experiments. It is found that the spatial resolution of the acoustic maps increases with increasing frequency. It is also found that the Archimedean spiral array has a better spatial resolution than the star array at all frequencies of interest. Furthermore, an Archimedean spiral array exhibits better signal to noise ratio (SNR) at frequencies below 1000 Hz, but poorer SNR at frequencies above 1000 Hz when compared to the performance of a star microphone array. Following these validation studies, the CAB methodology was applied to the identification of noise sources generated by a commercial small wind turbine (WINPhase 10 wind turbine). Despite the coarse grid and large time step used in the CFD simulations, the simulated aerodynamic results (wind turbine power output) and the aeroacoustic results (A-weighted SPL spectra) are in good agreement with some field measurements for this wind turbine. The simulated acoustic maps reveal that the blade tower interaction and the wind turbine nacelle are two possible noise generation mechanisms in the range of frequencies between 200 and 630 Hz for this small wind turbine

A design of experiment approach to 3D-printed mouthpieces sound analysis

Nowadays additive manufacturing is affected by a rapid expansion of possible applications. It is defined as a set of technologies that allow the production of components from 3D digital models in a short time by adding material layer by layer. It shows enormous potential to support wind musical instruments manufacturing because the design of complex shapes could produce unexplored and unconventional sounds, together with external customization capabilities. The change in the production process, material and shape could affect the resulting sound. This work aims to compare the music performances of 3D-printed trombone mouthpieces using both Fused Deposition Modelling and Stereolithography techniques, compared to the commercial brass one. The quantitative comparison is made applying a Design of Experiment methodology, to detect the main additive manufacturing parameters that affect the sound quality. Digital audio processing techniques, such as spectral analysis, cross-correlation and psychoacoustic analysis in terms of loudness, roughness and fluctuation strength have been applied to evaluate sounds. The methodology herein applied could be used as a standard for future studies on additively manufactured musical instruments

Measurement and magnitude-based system identification of tornado-associated infrasound

Recent evidence indicates that acoustic waves at frequencies below human hearing (infrasound) are produced during tornadogenesis and continue through the life of a tornado. The currently available tornadic infrasound data remains sparse, which has prevented the identification of the fluid mechanism responsible for its production. To increase the probability of detection, this thesis presents an adaptation of fixed infrasound sensing technology to equip storm chasers that are in regular proximity to tornadoes with mobile infrasound measurement capabilities. This approach has and continues to increase the quantity of samples while also reducing the long-range propagation-related uncertainties in the measurement analysis. This thesis describes the design and deployment of the Ground-based Local INfrasound Data Acquisition (GLINDA) system - a system which includes a specialized infrasound microphone, GPS receiver, and an IMU package for data remote collection. This thesis additionally presents analyses of measurements during an EFU tornado and a significant hail event as collected by the GLINDA system. The measured signal from the EFU tornado event notably produced an elevated broadband signal between 10 and 15 Hz consistent with past observations from small tornadoes. Frequency peak identification approaches are discussed in context of the EFU tornado. Utilizing the tornado event's frequency response magnitude as the output and a selection of wind noise pressure and energy models considered for acoustic response forcing inputs, frequency response system identification approaches are utilized to develop a set of transfer function models for the tornado acoustic production mechanism

Amélioration de loi de paroi de Simulation aux Grands Échelles pour des applications aéroacoustiques

Le bruit de train d’atterrissage, généré par l’interaction de l’écoulement turbulent avec des corps solides et le décollement de la couche limite, sont les sources principales de bruit d’un avion en phase d’atterrissage. Les données expérimentales existantes ne sont pas suffisantes pour fournir les informations détaillées sur ces mécanismes de génération du bruit, et, depuis des années, les simulations numériques ont prouvé être un moyen efficace pour la prévision du bruit de ce type. Comparée à la Simulation Numérique Directe et aux modèles à moyenne de Reynolds, la Simulation aux Grandes Échelles (SGE), est un compromis efficace entre la précision des résultats et le coût de calcul. Cependant, la prévision de l’écoulement dans la couche limite turbulente reste un défi en SGE. En effet, les simulations existantes résolvent souvent les plus petites échelles aux parois, nécessitant alors un maillage très raffiné proche des surfaces, augmentant considérablement le coût de calcul. Par conséquent, un modèle de paroi qui est capable de reconstituer la contrainte de ci- saillement à la paroi sur la base de données extraites à une certaine distance au-dessus du paroi est nécessaire pour réduire les coûts. La revue de litérature met en évidence le modèle analytique proposé par Afzal [6] qui considère les effets de gradient de pression défavorables avec un surcoût négligeable. Outre les effets de gradient de pression, la couche limite lami- naire dans la partie amont du cylindre avant la transition vers la turbulence pose un autre problème. L’utilisation des lois de paroi pour la couche limite turbulente peut être imprécise et même changer complètement le régime d’écoulement. Pour surmonter cet obstacle, un modèle a été proposé dans ce travail pour estimer la contrainte de cisaillement de la paroi dans la couche limite laminaire lorsque le gradient de pression est important. Un capteur de transition basé sur le modèle de sous-maille a été utilisé pour déclencher l’utilisation de la loi de paroi turbulente. L’écoulement d’un cylindre circulaire dans le régime critique a été considérée comme une première validation de la loi d’Afzal et son extension. La valeur du nombre de Reynolds choisi correspond à la configuration de l’écoulement qui se trouve sur la jambe principale du train d’atterrissage LAGOON. L’écoulement complexe du cylindre est examiné par une SGE résolue, qui a ensuite été utilisée extensivement comme base de données de validation intense pour la loi d’Afzal et son extension. Tous les modèles de paroi sont capables de prédire correctement la moyenne et le RMS de la pression pariétale de la simulation de référence. L’utilisation des lois turbulentes sur toute la surface du cylindre entraîne une contrainte de cisaillement de la paroi inférieure dans la région laminaire et supérieure dans la région turbulente par rapport à la simulation résolue. L’extension de la loi d’Afzal fournit une prédiction améliorée dans les régions laminaires et turbulentes. Comme dans les systèmes du train d’atterrissage réels, il existe des interactions entre ses composants cylindriques, tels que la barre de traction avec la jambe principale. L’expérience canonique de barreau-profil pour une telle interaction, est donc sélectionnée comme deuxième cas de validation. Les simulations avec loi de paroi montrent des résultats acoustiques en champ lointain en bon accord avec les messures. Enfin, des SGE avec ces modèles de paroi ont été effectuées sur la configuration LA- GOON#1. En général, toutes les simulations prédisent précisément la pression moyenne pariétale. Cependant, l’application d’un modèle pour la couche limit turbulente partout prévoient des valeurs RMS et des spectres de pression plus élevés sur le périmètre de la roue depuis la première position de mesure expérimentale. Une transition plus précoce se produit systématiquement. L’extension de la loi d’Afzal retarde la transition et permet de mieux prédire le spectre de pression des parois, à la fois sur la surface de la roue et sur la jambe principale. Toutes les simulations sont capables de récupérer les spectres de pres- sion des parois dans la région séparée. Malgré ces divergences sur le développement de la couche limite, toutes les simulations prédisent une valeur OASPL acceptable dans le champ lointain, avec une amélioration notable de l’extension de la loi d’Afzal.Abstract: Airframe noise, generated through the interaction of turbulent flow with solid bodies such as landing gears becomes the main contributor to the airplane noise during approach and landing phases, since significant progress has been made on the noise reduction of turbo-jet engines. The existing experimental data haven’t been able to provide sufficiently detailed information on airframe noise mechanism and numerical simulations have been considered as an effective method in understanding both aerodynamic and noise generation mechanisms. Among different numerical methods, Large Eddy Simulation (LES) is considered as the best trade-off between predictive accuracy and computational cost. However, wall-bounded flows at high Reynolds number remain the most crucial challenge for LES since the resolution of the boundary layer dominates the computational cost which is close to Direct Numerical Simulations. One solution to overcome this difficulty is the use of wall models to provide boundary conditions for the LES simulation. The classical logarithmic-law is not suitable in simulations of landing gear flows in which the longitudinal adverse pressure gradient have significant effects. A new analytical wall model (proposed by Afzal [6]) which accounts for the adverse pressure gradient effect has been considered to tackle the noise prediction of a realistic landing gear. Another challenge of such flows is the presence of the laminar state boundary layer. The use of wall models for the turbulent boundary layer can be inaccurate and even change completely the flow regime. To overcome this obstacle, a model has been proposed in this work to approximate the wall-shear stress in the laminar boundary layer when important pressure gradient effects are present. A transition sensor based on the subgrid-scale model has been used to trigger the use of wall law for the turbulent boundary layer. The benchmark of the circular cylinder flow in the critical regime has been considered as a first validation for the above wall models. The flow at such a critical Reynolds number combines complex features: large favorable and adverse pressure gradient, separation and turbulence transition and flow reattachment. This flow regime is also the most relevant for landing gear flow applications because of the Reynolds number range involved on its components. The complex cylinder flow has been investigated by a wall-resolved LES which has then been used extensively as validation database for Afzal’s law and its extension. All the wall-models are able to predict the mean and the RMS wall pressure distributions of the reference simulation. The use of a turbulent wall model on the entire surface results in lower wall-shear stress in the laminar region and higher in the turbulent region compared with the resolved simulation. The extended model shows improved prediction of the shear stress in both laminar and turbulent regions. All of the models recover the dipole pattern with similar OASPL levels as in the wall-resolved simulation. Since in actual landing gear systems, there are actually interaction between various cylindrical components such as the tow bar with the main strut for instance. The canonical experiment for such an interaction, the rod-airfoil interaction is therefore selected as a second validation case. These models show reasonable aerodynamic and acoustic results compared with the experimental references. Finally, wall-modeled LES has been performed on a modeled landing gear configuration. In general, the mean wall pressure profiles are accurately predicted by all the simulations. However, turbulent wall models predict higher rms and spectra of pressure on the wheel perimeter since the first experimental measurement position. Earlier transition systematically occurs. The extended Afzal’s law delays the transition and shows improved prediction of the wall pressure spectra both on the wheel surface and on the main leg. All the models are able to recover the wall-pressure spectra in the separated region. Despite these discrepancies on the boundary layer development, all the simulations predict satisfactory OASPL in the far-field with a significant improvement from the extended Afzal’s law

An investigation of the wind noise reduction mechanism of porous microphone windscreens

Wind energy is a green way to produce electricity without carbon emissions. However, the infrasound and low frequency audible sound radiated by wind turbines may adversely affect the nearby communities. To investigate the impact of wind farm noise and to understand its noise generation mechanism and propagation, the sound level of wind farm noise must be measured under windy conditions. However, it is often a challenge to measure wind turbine noise under windy conditions in quiet rural residential areas due to wind noise, especially for infrasound and low frequency audible sound. Wind noise is the pseudo sound pressure generated on microphones due to turbulent pressure fluctuations and is indistinguishable from the acoustic signals to be measured. Various microphone windscreens have been utilized to reduce wind noise. However, the physical mechanism of wind noise reduction by windscreens has been unclear to date. The aim of this PhD research is to investigate the mechanisms of wind noise generation and the wind noise reduction mechanism of porous microphone windscreens, and then develop a new compact acoustic measurement system that is insensitive to wind noise. To achieve this objective, a critical literature review is first presented to summarise the state-of-the-art research results in the field of wind noise and its reduction. Then, the research is focused on three aspects: the mechanisms of wind noise generation, the wind noise reduction mechanism of porous microphone windscreens, and wind noise reduction with a compact spherical microphone array. In the first aspect of this thesis, the generation mechanism of wind noise is explored and two theoretical models are proposed to predict wind noise spectra. One model is for outdoor atmospheric turbulence where the Reynolds number based on the Taylor microscale varies from 4250 to 19500, and the other is for indoor fan generated turbulent flows where the Reynolds number based on the Taylor microscale is estimated to be around 432. The proposed theoretical models are validated with existing simulations and experimental results from the literature, as well as measurement results conducted as part of this thesis in a car park for outdoor wind noise and in a laboratory for wind noise from an axial fan. In the second aspect of this thesis, the mechanism of wind noise reduction by porous microphone windscreens is investigated. It is shown that the wind noise reduction of porous microphone windscreens is caused by viscous and inertial forces introduced by the porous structure. Simulation results indicate that the design of porous microphone windscreens should take into account both turbulence suppression inside and wake generation behind the windscreens to achieve optimal performance. Besides, porous windscreens are found to be the most effective in attenuating wind noise in a certain frequency range, where the windscreen diameter is approximately 2 to 4 times the turbulence wavelengths. It is also found that the wind noise reduction is related to the spatial decorrelation of the wind noise signals provided by porous microphone windscreens. The simulation findings are validated with measurement results from an axial fan in a laboratory. In the last aspect of this thesis, a method for wind noise reduction with the spherical microphone array is proposed, and the effect of wind noise on the beamforming performance of a spherical microphone array is investigated. The characteristics of the wind noise is explored and compared with the sound signals in the spherical harmonics domain, based on which a spherical harmonics domain low pass filter method is proposed to reduce wind noise without degrading the desired sound signal. Experimental results demonstrate the feasibility of the proposed method. On the other hand, the effects of wind noise on the beamforming performance of the spherical Plane Wave Decomposition (PWD), Delay and Sum (DAS) and Maximum Variance Distortionless Response (MVDR) beamformers are studied. The experimental results demonstrate that the MVDR beamformer is insensitive to wind noise and able to localise the sound source direction under windy conditions. In summary, two theoretical models are proposed in this PhD research to predict the wind noise spectra in outdoor, large Reynolds number, atmospheric turbulence and indoor, small Reynolds number, turbulent flows, respectively; the physical mechanism of wind noise reduction by porous microphone windscreens is found to be related to the spatial decorrelation effect on the wind noise signal due to the porous structure, and it is demonstrated that the design of porous windscreens should take into account both turbulence suppression inside and wake generation behind the windscreen to achieve optimal performance; the effect of wind noise on the beamforming performance of a spherical microphone array is investigated and a spherical harmonic domain low pass filtering method is proposed to attenuate wind noise without degrading the desired sound signal

Recent Advances in Signal Processing

The signal processing task is a very critical issue in the majority of new technological inventions and challenges in a variety of applications in both science and engineering fields. Classical signal processing techniques have largely worked with mathematical models that are linear, local, stationary, and Gaussian. They have always favored closed-form tractability over real-world accuracy. These constraints were imposed by the lack of powerful computing tools. During the last few decades, signal processing theories, developments, and applications have matured rapidly and now include tools from many areas of mathematics, computer science, physics, and engineering. This book is targeted primarily toward both students and researchers who want to be exposed to a wide variety of signal processing techniques and algorithms. It includes 27 chapters that can be categorized into five different areas depending on the application at hand. These five categories are ordered to address image processing, speech processing, communication systems, time-series analysis, and educational packages respectively. The book has the advantage of providing a collection of applications that are completely independent and self-contained; thus, the interested reader can choose any chapter and skip to another without losing continuity

Reduction of wind induced microphone noise using singular spectrum analysis technique

Wind induced noise in microphone signals is one of the major concerns of outdoor acoustic signal acquisition. It affects many field measurement and audio recording scenarios. Filtering such noise is known to be difficult due to its broadband and time varying nature. This thesis is presented in the context of handling microphone signals acquired outdoor for acoustic sensing and environmental noise monitoring or soundscapes sampling.Thethesis presents a new approach to wind noise problem. Instead of filtering, a separation technique is developed. Signals are separated into wanted sounds of specific interest and wind noise based on the statistical feature of wind noise. The new technique is based on the Singular Spectrum Analysis methodwhich has recently seen many successful paradigms in the separation of biomedical signals, e.g., separating heart soundfrom lung noise. It has also been successfully implemented to de-noise signals in various applications.The thesis set out with particular emphasison investigating the factor that determines and improves the separability towards obtaining satisfactory results in terms of separating wind noise components out from noisy acoustic signals. A systematicapproach has been established and developed within the framework of singular spectral separation of acoustic signals contaminated by wind noise. This approach, which utilisesa conceptual framework, has, in its final form, three key objectives; grouping, reconstruction and separability. This approach is offered through introducing new mathematical models particularly for window length optimisation along with new descriptive figures.The research question has therefore been addressed considering developing algorithms according to updated requirements from method justification to verification and validation of the developed system. This thesis follows suitable testing criteria by conducting several experiments and a case-study design, with in-depth analysis of the results using visual tools of the method and related techniques.For system verification, an empirical study using testing signals thatintroduces a large number of experiments has been conducted. Empirical study with real-world sounds has been introduced next in system validation phase after rigorously selecting and preparing the dataset whichis drawn from two main sources: freefield1010 dataset, internet-based Freesound recordings. Results show that microphone wind noise is separable in the singular spectrum domain after validating and critically evaluating the developed system objectively. The findings indicate the effectiveness of the developed grouping and reconstruction techniques with significant improvement in the separability evidenced by w-correlation matrix.The developed method might be generalised to other outdoor sound acquisition applications