Abstract
Comparison of mechanical vibration and acoustic noise in the open-air MRI
This article analyzes and compares spectral properties of an acoustic noise produced by mechanical vibration of the gradient coils during scanning in the open-air magnetic resonance imager (MRI) working in a weak magnetic field. Selection of a usable type of the vibration sensor and the noise pickup microphone for measurement in weak magnetic field conditions is also discussed. The changes in acoustic noise spectral properties caused by loading of the holder of the lower gradient coils by the weight of the examined person lying in the scanning area of the MRI device are analyzed, too. The achieved results will be first of all used to design a correction filter for noise suppression in the speech signal recorded simultaneously with three-dimensional (3D) human vocal tract scanning. Finally, the article describes measurement and determination of the time delay between the resulting noise signal and the vibration impulses originated in the gradient coils. The obtained results will serve for description of the influence of the vibration on the acoustic noise and the way it travels through the plastic holder in the MRI device scanning area.
A comparative study of the effectiveness of vibration and acoustic emission in diagnosing a defective bearing in a planetary gearbox
While vibration analysis of planetary gearbox faults is relatively well established, the application of acoustic emission (AE) to this field is still in its infancy. For planetary-type gearboxes, it is more challenging to diagnose bearing faults due to the dynamically changing transmission paths which contribute to masking the vibration signature of interest. This study is aimed to reduce the effect of background noise while extracting the fault feature from AE and vibration signatures. This has been achieved through developing of internal AE sensor for helicopter transmission system. In addition, series of signal processing procedure has been developed to improved detection of incipient damage. Three signal processing techniques including an adaptive filter, spectral kurtosis, and envelope analysis were applied to AE and vibration data acquired from a simplified planetary gearbox test rig with a seeded bearing defect. The results show that AE identified the defect earlier than vibration analysis irrespective of the tortuous transmission path.
Nonlinear secondary noise sources for passive defect detection using ultrasound sensors
This article introduces the concept of secondary noise sources for passive defect detection and localization in structures. The proposed solution allows for the exploitation of the principle of Green’s function reconstruction from noise correlation, even in the absence of an adequate ambient noise. The main principle is to convert a part of low-frequency modal vibrations into high-frequency noise by exploiting the frictional contact nonlinearities. The device consists of a mass-spring resonator coupled to a flexible beam by a rough frictional interface. The extremity of the beam, attached to the surface of a plate, excites efficiently flexural waves in the plate up to 30 kHz when the primary resonator vibrates around its natural frequency, that is, a few dozens of hertz. A set of such devices is placed at random positions on the plate surface, and low-frequency excitation is provided by a shaker. The generated high-frequency noise is recorded by an array of eight piezoelectric transducers attached to the plate. A differential correlation matrix is constructed by subtracting correlation functions computed from noise signals at each sensor pairs, before and after the introduction of a local heterogeneity mimicking a defect. A simple array processing then allows for the detection and estimation of the defect location from this differential correlation matrix. Beyond the successful proof of concept, influence of experimental parameters, such as the number of secondary sources or the variability of the position of the shaker application point, is also investigated.
Wavenumber–frequency deconvolution of aeroacoustic microphone phased array data of arbitrary coherence
Deconvolution of aeroacoustic data acquired with microphone phased arrays is a computationally challenging task for distributed sources with arbitrary coherence. A new technique for performing such deconvolution is proposed. This technique relies on analysis of the array data in the wavenumber–frequency domain, allowing for fast convolution and reduced storage requirements when compared with traditional coherent deconvolution. A positive semidefinite constraint for the iterative deconvolution procedure is implemented and shows improved behavior in terms of quantifiable convergence metrics when compared with a standalone covariance inequality constraint. A series of simulations validates the method’s ability to resolve coherence and phase angle relationships between partially coherent sources, as well as determines convergence criteria for deconvolution analysis. Simulations for point sources near the microphone phased array show potential for handling such data in the wavenumber–frequency domain. In particular, a physics-based integration boundary calculation is described and can successfully isolate sources and track the appropriate integration bounds with and without the presence of flow. Magnitude and phase relationships between multiple sources are successfully extracted. Limitations of the deconvolution technique are determined from the simulations, particularly in the context of a simulated acoustic field in a closed-test-section wind tunnel with strong boundary layer contamination. A final application to a trailing edge noise experiment conducted in an open-jet wind tunnel matches best estimates of acoustic levels from traditional calculation methods and qualitatively assesses the coherence characteristics of the trailing edge noise source.
A nonlinear circular ring model with rotating effects for tire vibrations
Rolling noise contributes significantly to the noise inside cars. This noise comes from the tire/road contact and for low frequencies (0–400 Hz), and it is mainly transmitted into the cabin through structural vibrations. Thus, estimating this noise requires modeling the tire vibrations by taking into account the rotating effects and the contact with rough surfaces. Concerning the model of rolling tire, a formulation of a deformable solid is constructed using an arbitrary Lagrangian Eulerian approach. This formulation is applied on a new simplified tire model which is a circular ring including shear stresses and nonlinear effects due to the vehicle load. This model is successfully validated by comparison with finite element method (FEM) results.
Dominant modal decomposition method
This article deals with the automatic decomposition of experimental frequency response functions (FRFs) of mechanical structures. The decomposition of FRFs is based on the Green function representation of free vibratory systems. After the determination of the impulse dynamic subspace, the system matrix is formulated and the poles are calculated directly. By means of the corresponding eigenvectors, the contribution of each element of the impulse dynamic subspace is determined and the sufficient decomposition of the corresponding FRF is carried out. With the presented dominant modal decomposition (DMD) method, the mode shapes, the modal participation vectors, and the modal scaling factors are identified using the decomposed FRFs. Analytical example is presented along with experimental case studies taken from machine tool industry.
Vibration and acoustic characteristics of a city-car engine fueled with biodiesel blends
A number of studies have demonstrated that biodiesel is a more environmentally sustainable fuel than petroleum-derived fuels since it is a renewable source of energy and it allows to reduce undesired exhaust emissions (e.g. unburned HC, CO, and particulate matter). However, specialized literature highlights there is still the need to further investigate performance, emissions, and noise, vibration, and harshness (NVH) characteristics of engines equipped with up-to-date technologies fueled with biodiesel blend. The aim of this article is to investigate the vibro-acoustic behavior of a small displacement engine, mainly employed in micro-cars, fueled with blends of distilled biodiesel (obtained from used cooking oil) and ultra low sulfur diesel fuel up to 40% by volume. Demands for reducing chemical and noise pollutions, traffic congestion, and parking difficulties in urban areas make the micro-cars one of the possible solutions for the future urban environment, especially if the engine is fueled with biodiesel blends for their potential of reducing the pollutant emissions. An original methodology developed by the authors for in-cylinder pressure characterization via nonintrusive measurements is here applied to evaluate the impact of biodiesel content on the combustion process and therefore on engine vibration and noise emissions. The data processing in frequency domain allowed to extract the components mainly related to the combustion events. Concerning vibration signals: for all blends, the vibration amplitudes increase with the increase in engine speed values; B40 is characterized by highest values of root mean square (RMS) of accelerometer signal almost in the complete engine operative field. RMS values obtained for B10 are the lowest ones in most of the investigated engine operative conditions. Concerning noise radiation: the Noise Index was used to evaluate the components of the emission where the combustion energy demonstrated to be concentrated. The results show an increase in Noise Index for all blends with the increase in engine speed. B10 is characterized by the highest values in most of the test conditions. B40 values demonstrated the opposite behavior.
Resolution and quantification accuracy enhancement of functional delay and sum beamforming for three-dimensional acoustic source identification with solid spherical arrays
Functional delay and sum (FDAS) is a novel beamforming algorithm introduced for the three-dimensional (3D) acoustic source identification with solid spherical microphone arrays. Being capable of offering significantly attenuated sidelobes with a fast speed, the algorithm promises to play an important role in interior acoustic source identification. However, it presents some intrinsic imperfections, specifically poor spatial resolution and low quantification accuracy. This article focuses on conquering these imperfections by ridge detection (RD) and deconvolution approach for the mapping of acoustic sources (DAMAS). The suggested methods are referred to as FDAS + RD and FDAS + RD + DAMAS. Both computer simulations and experiments are utilized to validate their effects. Several interesting conclusions have emerged: (1) FDAS + RD and FDAS + RD + DAMAS both can dramatically ameliorate FDAS’s spatial resolution and at the same time inherit its advantages. (2) Compared with the conventional DAMAS, FDAS + RD + DAMAS enjoys the same super spatial resolution, stronger sidelobe attenuation capability and more than 200 times faster speed. (3) FDAS + RD + DAMAS can effectively conquer FDAS’ low quantification accuracy. Whether the focus distance is equal to the distance from the source to the array center or not, it can quantify the source average pressure contribution accurately. This study will be of great significance to the accurate and quick localization and quantification of acoustic sources in cabin environments.
Beamforming in a reverberant environment using numerical and experimental steering vector formulations
The effect of acoustic reflections on beamforming maps and their correction is investigated in this article. By replacing the usual steering vector expression in the beamforming algorithm with an adapted one, the effect of reflections can be reduced. Two formulations of the steering vectors are considered. The first makes use of an experimental Green’s function, which is obtained by measuring simultaneously the signal of a speaker and of a 31-channel acoustic array in a hard-walled test-section. The second formulation is based on the assumption that the reflections can be modeled by a set of monopoles located at the image source positions. This numerical model is first validated by comparing the obtained Green’s function with the experimental one. Then, the beamforming algorithm is modified using the different steering vector formulations. In addition, the deconvolution algorithm Clean-SC has been used and implemented with the different formulations. The best results in terms of location and resolution accuracy were obtained when using the experimental Green’s function formulation.
Bridge the semantic gap between pop music acoustic feature and emotion: Build an interpretable model
Music emotion recognition (MER) is an important topic in music understanding, recommendation, retrieval, and human computer interaction. Great success has been achieved by machine learning methods in estimating human emotional response to music. However, few of them pay much attention in semantic interpretation for emotion response. In our work, we first train an interpretable model between acoustic audio and emotion. Filter, wrapper, and shrinkage methods are applied to select important features. We then apply statistical models to build and explain the emotion model. Extensive experimental results reveal that the shrinkage methods outperform the wrapper methods and the filter methods in arousal emotion. In addition, we observed that only a small set of the extracted features have the key effects to arousal. However, most of our extracted features have small contribution to valence music perception. Ultimately, we obtain a higher average accuracy rate in arousal compared with that in valence.
Short-term effects of simultaneous cardiovascular workout and personal music device use on the outer hair cell function of young adults
Numerical modeling of acoustic stimulation induced mechanical vibration enhancing coal permeability
Mechanical vibration is a major effect generated by acoustic stimulation inside a coal sample for enhancing its permeability. The numerical simulation based on the staggered-grid finite differential method (FDM) is applied to simulate three-dimensional (3D) wave propagation in a fractured coal. Two parameters, shear wave energy (SE) and variable width of cleat (DW), are introduced and implemented in the numerical model to explicitly visualize and evaluate the acoustic stimulation effects. The polarized wave induced wave dynamics in a coal sample with a cleat/fracture is numerically simulated and analyzed. Especially, the energy trapping and dynamic variation of fracture width in coal for the cleat/fracture with three different filled media (i.e. air, water, and weak mineral) are numerically analyzed and compared with each other subjected to different incident wave angles, and the optimal stimulation parameters are obtained through such sensitivity analysis. Simulation results show that the coal property (i.e. cleat and its filled media) and incident wave angles are crucial for acoustic stimulation to produce physical damages around coal cleats and enhance the permeability of fractured coal samples.
Loss-induced enhanced transmission in anisotropic density-near-zero acoustic metamaterials
Anisotropic density-near-zero (ADNZ) acoustic metamaterials are investigated theoretically and numerically in this article and are shown to exhibit extraordinary transmission enhancement when material loss is induced. The enhanced transmission is due to the enhanced propagating and evanescent wave modes inside the ADNZ medium thanks to the interplay of near-zero density, material loss, and high wave impedance matching in the propagation direction. The equi-frequency contour (EFC) is used to reveal whether the propagating wave mode is allowed in ADNZ metamaterials. Numerical simulations based on plate-type acoustic metamaterials with different material losses were performed to demonstrate collimation and subwavelength imaging enabled by the induced loss in ADNZ media. This work provides a different way for manipulating acoustic waves.
A window into the brain mechanisms associated with noise sensitivity
Noise-sensitive individuals are more likely to experience negative emotions from unwanted sounds and they show greater susceptibility to adverse effects of noise on health. Noise sensitivity does not originate from dysfunctions of the peripheral auditory system, and it is thus far unknown whether and how it relates to abnormalities of auditory processing in the central nervous system. We conducted a combined electroencephalography and magnetoencephalography (M/EEG) study to measure neural sound feature processing in the central auditory system in relation to the individual noise sensitivity. Our results show that high noise sensitivity is associated with altered sound feature encoding and attenuated discrimination of sound noisiness in the auditory cortex. This finding makes a step toward objective measures of noise sensitivity instead of self-evaluation questionnaires and the development of strategies to prevent negative effects of noise on the susceptible population.
On the generalization to swirling flows of Lighthill’s eighth-power law. Part II: Laws of intensity of radiation for acoustic-vortical waves
The Lighthill–Proudman theory of sound generation by turbulence, inhomogeneities, and dissipation in a medium at rest is extended to an axisymmetric mean flow with (1) arbitrary unidirectional shear velocity profile and (2) arbitrary angular velocity of rotation, both depending only on the radius and also (3) isentropic conditions. The forced acoustic-vortical wave equation through (1) the wave operator describes the propagation of coupled acoustic-vortical waves (Part I) and (2) the forcing terms specify the compressive, shear, and swirl wave sources that model the generation of acoustic-vortical waves by turbulence, inhomogeneities, and entropy production (Part II). The energy balance is considered for acoustic-vortical waves including the energy density, flux, and production. The dimensional scaling for the wave sources and the energy flux leads to a law of intensity of radiation that generalizes the Lighthill eighth-power law of aeroacoustics by allowing for: (1) compressive, shear, and swirl wave sources; (2) acoustic and vortical waves and their couplings; and (3) monopoles, dipoles, quadrupoles, and multipoles of any order.
Vortex shedding noise from a beveled trailing edge
Coherent vortex shedding from blunt and beveled trailing edges generates tonal noise, which is usually undesired. To obtain a better understanding of the noise generation under such conditions, the flow field around a beveled trailing edge was characterized for Reynolds numbers based on the bluntness ranging from 2.5 × 104 to 5.1 × 104. Flow field statistics were obtained by means of planar high-speed two-component and stereoscopic particle image velocimetry measurements. The development of the shear layers and vortex roll-up is described in this study. Related length scales, the vortex formation length, and wake thickness parameter were derived from the measurements. Noise emission due to vortex shedding was predicted from an analytic solution, derived from diffraction theory and the reversed Sears’ problem, and compared with acoustic phased array measurements. This approach has previously been shown to provide accurate results for sharply truncated edges, but questions with regard to the applicability with different trailing edge geometries remained open. The prediction required the auto-spectral density, correlation length, and convective velocity of the upwash velocity component in the vortex formation region. Direct application with data obtained from particle image velocimetry measurements showed an overestimation of about 20 dB when compared with the acoustic measurements. The results thus showed that the prediction of vortex shedding noise based on the simplified wake model and diffraction theory is not generally applicable.
Numerical analysis of aerodynamic noise mitigation via leading edge serrations for a rod–airfoil configuration
Noise produced by aerodynamic interaction between a circular cylinder (rod) and an airfoil in a tandem arrangement is investigated numerically using incompressible large eddy simulations. Quasi-periodic shedding from the rod and the resulting wake impinges on the airfoil to produce unsteady loads on the two geometries. These unsteady loads act as sources of aerodynamic sound and the sound radiates to the far field with a dipole directivity. The airfoil is set at zero angle of attack for the simulations and the Reynolds number based on the rod diameter is Red = 48 K. Comparisons with experimental measurements are made for (1) mean and root mean square surface pressure on the rod, (2) profiles of mean and root mean square streamwise velocity in the rod wake, (3) velocity spectra in the near field, and (4) far-field pressure spectra. Curle’s acoustic analogy is used with the airfoil surface pressure data from the simulations to predict the far-field sound. An improved correction based on observed spanwise coherence is used to account for the difference in span lengths between the experiments and the simulations. Good agreement with data is observed for the near-field aerodynamics and the far-field sound predictions. The straight leading edge airfoil is then replaced with a test airfoil with a serrated leading edge geometry while maintaining the mean chord. This new configuration is also analyzed numerically and found to give a substantial reduction in the far-field noise spectra in the mid- to high-frequency range. Source diagnostics show that the serrations reduce unsteady loading on the airfoil, reduce coherence along the span, and increase spanwise phase variation, all of which contribute to noise reduction.