Abstract
On the completeness and the linear dependence of the Cartesian multipole series in representing the solution to the Helmholtz equation
The (Cartesian) multipole series, that is, the series comprising monopole, dipoles, quadrupoles, and so on, can be used, as an alternative to the spherical or cylindrical wave series, in representing sound fields in a wide range of problems, such as source radiation and sound scattering. The proofs of the completeness of the spherical and cylindrical wave series in these problems are classical results, and it is also generally agreed that the Cartesian multipole series spans the same space as the spherical waves: a rigorous mathematical proof of that statement has, however, not been presented. In this work, such a proof of the completeness of the Cartesian multipole series, both in two and three dimensions, is given, and the linear dependence relations among different orders of multipoles are discussed, which then allows one to easily extract a basis from the multipole series. In particular, it is concluded that the multipoles comprising the two highest orders in the series form a basis of the whole series since the multipoles of all the lower source orders can be expressed as a linear combination of that basis.
Acoustic properties of multiple cavity resonance liner for absorbing higher order duct modes
This article describes analytical and experimental studies conducted to investigate the acoustic properties of axially nonuniform multiple cavity resonance liner for absorbing higher order duct modes. A three-dimensional analytical model is proposed based on transfer element method. The model is assessed by making a comparison with results of a liner performance experiment concerning higher order modes propagation, and the agreement is good. According to the present results, it is found that the performance of multiple cavity resonance liner is related to the incident sound waves. Moreover, an analysis of the corresponding response of liner perforated panel-cavity system is performed, in which the features of resonance frequency and dissipation of the system under grazing or oblique incidence condition are revealed. The conclusions can be extended to typical non-locally reacting liners with single large back-cavity, and it would be beneficial for future non-locally reacting liner design to some extent.
Prediction of far-field wind turbine noise propagation with parabolic equation
Sound propagation of wind farms is typically simulated by the use of engineering tools that are neglecting some atmospheric conditions and terrain effects. Wind and temperature profiles, however, can affect the propagation of sound and thus the perceived sound in the far field. A better understanding and application of those effects would allow a more optimized farm operation toward meeting noise regulations and optimizing energy yield. This article presents the parabolic equation (PE) model development for accurate wind turbine noise propagation. The model is validated against analytic solutions for a uniform sound speed profile, benchmark problems for nonuniform sound speed profiles, and field sound test data for real environmental acoustics. It is shown that PE provides good agreement with the measured data, except upwind propagation cases in which turbulence scattering is important. Finally, the PE model uses computational fluid dynamics results as input to accurately predict sound propagation for complex flows such as wake flows. It is demonstrated that wake flows significantly modify the sound propagation characteristics.
Fast and persistent adaptation to new spectral cues for sound localization suggests a many-to-one mapping mechanism
The adult human auditory system can adapt to changes in spectral cues for sound localization. This plasticity was demonstrated by changing the shape of the pinna with earmolds. Previous results indicate that participants regain localization accuracy after several weeks of adaptation and that the adapted state is retained for at least 1 week without earmolds. No aftereffect was observed after mold removal, but any aftereffect may be too short to be observed when responses are averaged over many trials. This work investigated the lack of aftereffect by analyzing single-trial responses and modifying visual, auditory, and tactile information during the localization task. Results showed that participants localized accurately immediately after mold removal, even at the first stimulus presentation. Knowledge of the stimulus spectrum, tactile information about the absence of the earmolds, and visual feedback were not necessary to localize accurately after adaptation. Part of the adaptation persisted for 1 month without molds. The results are consistent with the hypothesis of a many-to-one mapping of the spectral cues, in which several spectral profiles are simultaneously associated with one sound location. Additionally, participants with acoustically more informative spectral cues localized sounds more accurately, and larger acoustical disturbances by the molds reduced adaptation success.
Human recognition of familiar voices
Recognizing familiar voices is something we do every day. In quiet environments, it is usually easy to recognize a familiar voice. In noisier environments, this can become a difficult task. This article examines how robust listeners are at identifying familiar voices in noisy, changing environments and what factors may affect their recognition rates. While there is previous research addressing familiar speaker recognition, the research is limited due to the difficulty in obtaining appropriate data that eliminates speaker-dependent traits, such as word choice, along with having corresponding listeners who are familiar with the speakers. The data used in this study were collected in such a fashion to mimic conversational, free-flow dialogue, but in a way to eliminate many variables such as word choice, intonation, or non-verbal cues. These data provide some of the most realistic test scenarios to-date for familiar speaker identification. A pure-tone hearing test was used to separate listeners into normal hearing and hearing-impaired groups. It is hypothesized that the results of the normal hearing group will be statistically better. Additionally, the aspect of familiar speaker recognition is addressed by having each listener rate his or her familiarity with each speaker. Two statistical approaches showed that the more familiar a listener is with a speaker, the more likely the listener will recognize the speaker.
Linear transformation method to control flexural waves in thin plates
In this article, the linear transformation method (LTM) to control flexural waves propagating in thin plates is presented. Unlike earlier studies, only a small number of homogeneous materials with no requirement of in-plane forces or pre-stress are needed, which tremendously simplifies the implementation of devices for flexural waves. An invisibility cloak with homogeneous materials is studied to confirm the validity of the present approach and to show its imperfection due to impedance mismatch at interfaces. Required materials can be further simplified as layered isotropic materials using the effective medium theory. Finally, the LTM can be extended to the case of flexural waves propagating in anisotropic thin plates. This method opens a promising avenue toward the realization of advanced structured shields and other devices.
Head wave correlations in ambient noise
Ambient ocean noise is processed with a vertical line array to reveal coherent time-separated arrivals suggesting the presence of head wave multipath propagation. Head waves, which are critically propagating water waves created by seabed waves traveling parallel to the water-sediment interface, can propagate faster than water-only waves. Such eigenrays are much weaker than water-only eigenrays and are often completely overshadowed by them. Surface-generated noise is different, whereby it amplifies the coherence between head waves and critically propagating water-only waves, which is measured by cross-correlating critically steered beams. This phenomenon is demonstrated both experimentally and with a full wave simulation.
How to measure community tolerance levels for noise
Relationships between noise exposure and transportation noise induced annoyance have been studied extensively for several decades. The annoyance due to aircraft noise exposure is in this article assumed to be influenced by the day–night yearly average sound level (DNL). It has long been recognized that the annoyance also depends on non-DNL factors, but this is complicated—resulting in a variety of different modeling strategies. Motivated by this, the community tolerance level (CTL) was introduced in 2011 for a loudness-based psychometric function. It is a single parameter that accounts for the aggregate influence of other factors. This article suggests and investigates different methods for the measurement of the CTL. The methods are illustrated on data found in the literature, on recent surveys around two Norwegian airports, and on simulated data. The results from the presented methods differ significantly. An elementary method is shown to give a measurement of the CTL with smaller uncertainty and is recommended as a replacement for the originally suggested least-squares method. Methods for evaluating the measurement uncertainty are also presented.
Enhanced near-field acoustic holography for larger distances of reconstructions using fixed parameter Tikhonov regularization
This article evaluates the performance of various regularization parameter choice methods applied to different approaches of near-field acoustic holography when a very near-field measurement is not possible. For a fixed grid resolution, the larger the hologram distance, the larger the error in the naive near-field acoustic holography reconstructions. These errors can be smoothed out using an appropriate order of regularization. This study shows that using a fixed/manual choice of regularization parameter, instead of automated parameter choice methods, reasonably accurate reconstructions can be obtained even when the hologram distance is 16 times larger than the grid resolution.
Wavenumber transform analysis for acoustic black hole design
Acoustic black holes (ABHs) are effective, passive, and lightweight vibration absorbers that have been developed and shown to effectively reduce the structural vibration and radiated sound of beam and plate structures. ABHs employ a local thickness change that reduces the speed of bending waves and increases the transverse vibration amplitude. The vibrational energy can then be effectively focused and dissipated by material losses or through conventional viscoelastic damping treatments. In this work, the measured vibratory response of embedded ABH plates was transformed into the wavenumber domain in order to investigate the use of wavenumber analysis for characterizing, designing, and optimizing practical ABH systems. The results showed that wavenumber transform analysis can be used to simultaneously visualize multiple aspects of ABH performance including changes in bending wave speed, transverse vibration amplitude, and energy dissipation. The analysis was also used to investigate the structural acoustic coupling of the ABH system and determine the radiation efficiency of the embedded ABH plates compared with a uniform plate. The results demonstrated that the ABH effect results in acoustic decoupling as well as vibration reduction. The wavenumber transform-based methods and results will be useful for implementing ABHs into real-world structures.
Effects of noise and acoustics in schools on vocal health in teachers
Previous studies on the influence of noise and acoustics in the classroom on voice symptoms among teachers have exclusively relied on self-reports. Since self-reported physical conditions may be biased, it is important to determine the role of objective measurements of noise and acoustics in the presence of voice symptoms and to assess the association between objectively measured and self-reported physical conditions at school with the presence of voice symptoms among teachers. In 12 public schools in Bogotá, we conducted a cross-sectional study among 682 Colombian school workers at 377 workplaces. After signing the informed consent, participants filled out a questionnaire on individual and work-related conditions and the nature and severity of voice symptoms in the past month. Short-term environmental measurements of sound levels, temperature, humidity, and reverberation time were conducted during visits at the workplaces, such as classrooms and offices. Logistic regression analysis was used to determine associations between work-related factors and voice symptoms. High noise levels outside schools (odds ratio (OR) = 1.83; 95% confidence interval (CI): 1.12–2.99) and self-reported poor acoustics at the workplace (OR = 2.44; 95% CI: 1.88–3.53) were associated with voice symptoms. We found poor agreement between the objective measurements and self-reports of physical conditions at the workplace. This study indicates that noise and acoustics may play a role in the occurrence of voice symptoms among teachers. The poor agreement between objective measurements and self-reports of physical conditions indicate that these are different entities, which argue for inclusion of physical measurements of the working environment in studies on the influence of noise and acoustics on vocal health.
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.
An explicit time-domain approach for sensitivity analysis of non-stationary random vibration problems
This article presents an explicit time-domain method for sensitivity analysis of structural responses under non-stationary random excitations. Based on time-domain explicit expressions of dynamic responses, a new and more concise time-domain explicit expression of response sensitivity is derived using the direct differentiation method (DDM). Then, a more efficient algorithm for direct construction of the explicit expression of response sensitivity is developed based on the physical meanings of the coefficient matrices in the formulation. The adjoint variable method (AVM) is further used to establish the explicit expression of the sensitivity of an arbitrary response. Finally, based on the time-domain explicit expressions for both dynamic response and its sensitivity, an efficient time-domain approach is proposed to calculate the sensitivity of variance responses of a structure subjected to non-stationary random excitations. Numerical examples of different structural systems under non-stationary random excitations are presented to demonstrate the accuracy and efficiency of the proposed method.
Time-varying singular value decomposition for periodic transient identification in bearing fault diagnosis
For rotating machines, the defective faults of bearings are generally represented as periodic transient impulses in acquired signals. The extraction of transient features from signals has been a key issue for fault diagnosis. However, the background noise reduces identification performance of periodic faults in practice. This article proposes a time-varying singular value decomposition (TSVD) method to enhance the identification of periodic faults. The proposed method is inspired by the sliding window method. By applying singular value decomposition (SVD) to the signal under a sliding window, we can obtain a time-varying singular value matrix (TSVM). Each column in the TSVM is occupied by the singular values of the corresponding sliding window, and each row represents the intrinsic structure of the raw signal, namely time-singular-value-sequence (TSVS). Theoretical and experimental analyses show that the frequency of TSVS is exactly twice that of the corresponding intrinsic structure. Moreover, the signal-to-noise ratio (SNR) of TSVS is improved significantly in comparison with the raw signal. The proposed method takes advantages of the TSVS in noise suppression and feature extraction to enhance fault frequency for diagnosis. The effectiveness of the TSVD is verified by means of simulation studies and applications to diagnosis of bearing faults. Results indicate that the proposed method is superior to traditional methods for bearing fault diagnosis.
On the dimension of complex responses in nonlinear structural vibrations
The ability to accurately model engineering systems under extreme dynamic loads would prove a major breakthrough in many aspects of aerospace, mechanical, and civil engineering. Extreme loads frequently induce both nonlinearities and coupling which increase the complexity of the response and the computational cost of finite element models. Dimension reduction has recently gained traction and promises the ability to distill dynamic responses down to a minimal dimension without sacrificing accuracy. In this context, the dimensionality of a response is related to the number of modes needed in a reduced order model to accurately simulate the response. Thus, an important step is characterizing the dimensionality of complex nonlinear responses of structures. In this work, the dimensionality of the nonlinear response of a post-buckled beam is investigated. Significant detail is dedicated to carefully introducing the experiment, the verification of a finite element model, and the dimensionality estimation algorithm as it is hoped that this system may help serve as a benchmark test case. It is shown that with minor modifications, the method of false nearest neighbors can quantitatively distinguish between the response dimension of various snap-through, non-snap-through, random, and deterministic loads. The state-space dimension of the nonlinear system in question increased from 2 to 10 as the system response moved from simple, low-level harmonic to chaotic snap-through. Beyond the problem studied herein, the techniques developed will serve as a prescriptive guide in developing fast and accurate dimensionally reduced models of nonlinear systems, and eventually as a tool for adaptive dimension-reduction in numerical modeling. The results are especially relevant in the aerospace industry for the design of thin structures such as beams, panels, and shells, which are all capable of spatio-temporally complex dynamic responses that are difficult and computationally expensive to model.
Genetic optimization of a plane array geometry for beamforming. Application to source localization in a high-speed train
Thanks to its easy implementation and robust performance, beamforming is applied for source localization in several fields. Its effectiveness depends greatly on the array sensor configuration. This article introduces a criterion to improve the array beampattern and increase the accuracy of sound source localization. The beamwidth and the maximum sidelobe level are used to quantify the spatial variation of the beampattern through a new criterion. This criterion is shown to be useful, especially for the localization of moving sources. A genetic algorithm is proposed for the optimization of microphone placement. Statistical analysis of the optimized arrays provides original results on the algorithm performance and on the optimal microphone placement. An optimized array is tested to localize the sound sources of a high-speed train. The results show an accurate separation.
Noise reduction by the application of an air-bubble curtain in offshore pile driving
Underwater noise pollution is a by-product of marine industrial operations. In particular, the noise generated when a foundation pile is driven into the soil with an impact hammer is considered to be harmful for the aquatic species. In an attempt to reduce the ecological footprint, several noise mitigation techniques have been investigated. Among the various solutions proposed, the air-bubble curtain is often applied due to its efficacy in noise reduction. In this article, a model is proposed for the investigation of the sound reduction during marine piling when an air-bubble curtain is placed around the pile. The model consists of the pile, the surrounding water and soil media, and the air-bubble curtain which is positioned at a certain distance from the pile surface. The solution approach is semi-analytical and is based on the dynamic sub-structuring technique and the modal decomposition method. Two main results of the article can be distinguished. First, a new model is proposed that can be used for predictions of the noise levels in a computationally efficient manner. Second, an analysis of the principal mechanisms that are responsible for the noise reduction due to the application of the air-bubble curtain in marine piling is presented. The understanding of these mechanisms turns to be crucial for the exploitation of the maximum efficiency of the system. It is shown that the principal mechanism of noise reduction depends strongly on the frequency content of the radiated sound and the characteristics of the bubbly medium. For piles of large diameter which radiate most of the acoustic energy at relatively low frequencies, the noise reduction is mainly attributed to the mismatch of the acoustic impedances between the seawater and the bubbly layer. On the contrary, for smaller piles and when the radiated acoustic energy is concentrated at frequencies close to, or higher than, the resonance frequency of the air bubbles, the sound absorption within the bubbly layer becomes critical.
Effect of noise-reducing components on nose landing gear stability for a mid-size aircraft coupled with vortex shedding and freeplay
In the pursuit of quieter aircraft, significant effort has been dedicated to airframe noise identification and reduction. The landing gear is one of the main sources of airframe noise on approach. The addition of noise abatement technologies such as fairings or wheel hub caps is usually considered to be the simplest solution to reduce this noise. After touchdown, noise abatement components can potentially affect the inherently nonlinear and dynamically complex behavior (shimmy) of landing gear. Moreover, fairings can influence the aerodynamic load on the system and interact with the mechanical freeplay in the torque link. This article presents a numerical study of nose landing gear stability for a mid-size aircraft with low noise solutions, which are modeled by an increase in the relevant model structural parameters to address a hypothetical effect of additional fairings and wheel hub caps. This study shows that the wheel hub caps are not a threat to stability. A fairing has a destabilizing effect due to the increased moment of inertia of the strut and a stabilizing effect due to the increased torsional stiffness of the strut. As the torsional stiffness is dependent on the method of attachment, in situations where the fairing increases the torsional inertia with little increase to the torsional stiffness, a net destabilizing effect can result. Alternatively, it is possible for the case that if the fairing were to increase equally both the torsional stiffness and the moment of inertia of the strut, then their effects could be mutually negated. However, it has been found here that for small and simple fairings, typical of current landing gear noise abatement design, their implementation will not affect the dynamics and stability of the system in an operational range (Fz ⩽ 50,000 N and V ⩽ 100 m/s). This generalization is strictly dependent on size and installation methods. The aerodynamic load, which would be influenced by the presence of fairings, was modeled using a simple vortex-shedding oscillator acting on the strut. The stability boundary was found to remain unaltered by vortex shedding. Significantly, however, the addition of freeplay in the torque link was found to cause shimmy over the more typical operating conditions studied here. Unlike the no-freeplay case, there was a suppressed stabilizing effect of increased torsional stiffness of the strut caused by the presence of fairing. No interaction between the vortex shedding and the freeplay on the stability threshold was observed.
On the efficiency and robustness of damping by branching
This article investigates the mechanism referred to as damping by branching (DBB), and its ability to attenuate the response of an oscillating structure, in the range of large amplitudes. More specifically, we give an experimental proof of this mechanism and we show that it is very robust against variations of the physical parameters and constituents: nature of the damping mechanism and geometrical structure. These results motivated the design of the tuned mass branched damper (TMBD) which is shown to be a little more efficient than the classical tuned mass damper (TMD) in some domains of the parameter space.
Noise of high-performance aircraft at afterburner
The noise from a high-performance aircraft at afterburner is investigated. The main objective is to determine whether the dominant noise components are the same or similar to those of a hot supersonic laboratory jet. For this purpose, measured noise data from F-22A Raptors are analyzed. It is found, based on both spectral and directivity data, that there is a new dominant noise component in addition to the usual turbulent mixing noise. The characteristic features of the new noise component are identified. Measured data indicate that the new noise component is observed only when the rate of fuel burn of the engine is increased significantly above that of the intermediate power setting. This suggests that the new noise component is combustion related. The possibility that it is indirect combustion noise generated by the passage of hot spots from the afterburner through the nozzle of the jet is investigated. Because flow and temperature data were not measured in the F-22A engine tests, to provide support to the proposition, numerical simulations of indirect combustion noise generation due to the passing of an entropy wave pulse (a hot spot) through a military-style nozzle are carried out. Sound generation is observed at the front and at the back of the pulse. This creates a fast and a slow acoustic wave as the sound radiates out from the nozzle exit. Quantitative estimates of the principal directions of acoustic radiation due to the emitted fast and slow acoustic waves are made. It is found that there are reasonably good agreements with measured data. To estimate the intensity level (IL) of the radiated indirect combustion noise, a time-periodic entropy wave train of 15% temperature fluctuation is used as a model of the hot spots coming out of the afterburner. This yields an IL of 175.5 dB. This is a fairly intense noise source, well capable of causing the radiation of the new jet noise component.
An experimental investigation of acoustic black hole dynamics at low, mid, and high frequencies
The acoustic black hole (ABH) has been developed in recent years as an effective, passive, and lightweight method for attenuating bending wave vibrations in beams and plates and reducing the sound radiation and structural-acoustic response of structures. The ABH effect utilizes a local change in the plate or beam thickness to reduce the bending wave speed and increase the transverse vibration amplitude. Attaching a viscoelastic damping layer to the ABH results in effective energy dissipation and vibration reduction. Surface-averaged mobility and radiated sound power measurements were performed on an aluminum plate containing an array of 20 two-dimensional ABHs with damping layers and compared with a similar uniform plate. Detailed laser vibrometer scans of an ABH cell (including the ABH and surrounding homogeneous plate) were also performed to analyze the vibratory characteristics of individual ABH cells and compared with mode shapes calculated using finite elements. The results showed that the surface-averaged mobility was reduced by up to 14 dB for the fully damped ABH plate compared with a uniform reference plate while also reducing the mass of the plate. The results demonstrated that the dynamics of plates with embedded ABHs can be characterized by low, mid, and high frequency ranges, with low-order local ABH modes contributing significantly to low-frequency ABH performance. The effects of damping layer thickness and diameter were also investigated to assess ABH performance optimization. It was shown that the damping layer can have the added benefit of mass loading the ABH and enhancing low-frequency performance. The results will be useful for designing the low-frequency performance of future ABH systems and describing ABH performance in terms of design parameters.
On the use of ultrasound-based technology for cargo inspection
A new guided wave imaging application for fast, low-cost ultrasound-based cargo scanning system is proposed. The ultimate goal is the detection of high atomic number, shielding containers used to diminish the radiological signature of nuclear threats. This ultrasonic technology has the potential to complement currently deployed X-ray-based radiographic systems, thus enhancing the probability of detecting nuclear threats. An array of ultrasonic transceivers can be attached to the metallic structure of the cargo to create a guided Lamb wave. Guided medium thickness and composition variation creates reflections whose placement can be revealed by means of an imaging algorithm. The knowledge of the reflection position provides information about the shielding metallic container location inside the cargo. Moreover, due to the low coupling between metallic and nonmetallic surfaces, only the footprint of metallic containers shows up in the imaging results, thus avoiding false positives from plastic or wooden assets. As imaging capabilities are degraded if working with dispersive Lamb wave modes, the operating frequency is tuned to provide a trade-off between low dispersion and real-time image resolution. Reflected waves in the guided domain bounds may limit the performance of imaging methods for guided media. This contribution proposes a solution based on real-time Fourier domain analysis, where plane wave components can be filtered out, thus removing nondesired contributions from bounds. Several realistic examples, scaled due to limited calculation capabilities of the available computational resources, are presented in this work, showing the feasibility of the proposed method.
Nonlinear effects on tonal sound in lined ducts
Porous sound absorbers are a familiar and supposedly well-understood feature of the noise control palette. As in many other aspects of engineering, however, aerospace applications require a degree of mastery well beyond the common place: sound absorbers must be designed to be effective at high noise levels within strict mass and volume constraints. In the process of fine-tuning sound absorbers for jet aircraft, it was discovered that high incident sound pressure levels cause the performance of lined ducts to diverge from small-signal predictions. The underlying physics was not fully understood prior to the early 1980s, at which point National Aeronautics and Space Administration (NASA)-sponsored research disclosed and quantified the primary mechanism. Further research by others continues building on that foundation down to the present. This article gives a brief overview of the topic, summarizes one of the research efforts, and adapts its findings for lined duct applications.