Abstract
An investigation of multi-rate sound decay under strongly non-diffuse conditions: the crypt of the Cathedral of Cadiz
Francesco Martellotta, Lidia Álvarez-Morales, Sara Girón and Teófilo Zamarreño
Multi-rate sound decays are often found and studied in complex systems of coupled volumes where diffuse field conditions generally apply, although the openings connecting different sub-spaces are by themselves potential causes of non-diffuse behavior. However, in presence of spaces in which curved surfaces clearly prevent diffuse field behavior from being established, things become more complex and require more sophisticated tools (or better combinations of them) to be fully understood. As an example of such complexity, the crypt of the Cathedral of Cadiz is a relatively small space characterized by a central vaulted rotunda, with five radial galleries with flat and low ceiling. In addition, the crypt is connected to the main cathedral volume by means of several small openings. Acoustic measurements carried out in the crypt pointed out the existence of at least two decay processes combined, in some points, with flutter echoes. Application of conventional methods of analysis pointed out the existence of significant differences between early decay time and reverberation time but was inconclusive in explaining the origin of the observed phenomena. The use of more robust Bayesian analysis permitted the conclusion that the late decay appearing in the crypt had a different rate than that observed in the cathedral, thus excluding the explanation based on acoustic coupling of different volumes. Finally, processing impulse responses collected by means of a B-format microphone to obtain directional intensity maps demonstrated that the late decay was originated from the rotunda where a repetitive reflection pattern appeared between the floor and the dome causing both flutter echoes and a longer reverberation time.
Vibration-based damage detection in wind turbine blades using phase-based motion estimation and motion magnification
Aral Sarrafi, Zhu Mao, Christopher Niezrecki and Peyman Poozesh
Vibration-based structural health monitoring (SHM) techniques are among the most common approaches for structural damage identification. The presence of damage in structures may be identified by monitoring the changes in dynamic behavior subject to external loading and is typically performed by using experimental modal analysis (EMA) or operational modal analysis (OMA). These tools for SHM normally require a limited number of physically attached transducers (e.g. accelerometers) in order to record the response of the structure for further analysis. Signal conditioners, wires, wireless receivers and a data acquisition system (DAQ) are also typical components of traditional sensing systems used in vibration-based SHM. However, instrumentation of lightweight structures with contact sensors such as accelerometers may induce mass-loading effects and for large-scale structures, the instrumentation is labor intensive and time-consuming. Achieving high spatial measurement resolution for a large-scale structure is not always feasible while working with traditional contact sensors, and there is also the potential for a lack of reliability associated with fixed contact sensors in outliving the life-span of the host structure. Among the state-of-the-art non-contact measurements, digital video cameras are able to rapidly collect high-density spatial information from structures remotely. In this article, the subtle motions from recorded video (i.e. a sequence of images) are extracted by means of phase-based motion estimation (PME), and the extracted information is used to conduct damage identification on a 2.3-m long Skystream® wind turbine blade (WTB). The PME and phase-based motion magnification approach estimates the structural motion from the captured sequence of images for both a baseline and damaged test cases on a WTB. Operational deflection shapes of the test articles are also quantified and compared for the baseline and damaged states. In addition, having proper lighting while working with high-speed cameras can be an issue; therefore, image enhancement and contrast manipulation has also been performed to enhance the raw images. Ultimately, the extracted resonant frequencies and operational deflection shapes are used to detect the presence of damage, demonstrating the feasibility of implementing non-contact video measurements to perform realistic structural damage detection.
Development of a morphological convolution operator for bearing fault detection
Yifan Li, Xihui Liang, Weiwei Liu and Yan Wang
This article presents a novel signal processing scheme, namely morphological convolution operator (MCO)-lifted morphological undecimated wavelet (MUDW), for rolling element bearing fault detection. In this scheme, an MCO is first designed to fully utilize the advantage of the closing and opening gradient operator and the closing–opening and opening–closing gradient operator for feature extraction as well as the merit of excellent denoising characteristics of the convolution operator. The MCO is then introduced into MUDW for the purpose of improving the fault detection ability of the reported MUDWs. Experimental vibration signals collected from a train wheelset test rig, and the bearing data center of Case Western Reserve University are employed to evaluate the effectiveness of the proposed MCO-lifted MUDW on fault detection of rolling element bearings. The results show that the proposed approach has a superior performance in extracting fault features of defective rolling element bearings. In addition, comparisons are performed between two reported MUDWs and the proposed MCO-lifted MUDW. The MCO-lifted MUDW outperforms both of them in detection of outer race faults and inner race faults of rolling element bearings.
Sliding window denoising K-singular value decomposition and its application on rolling bearing impact fault diagnosis
Honggang Yang, Huibin Lin and Kang Ding
The performance of sparse features extraction by commonly used K-singular value decomposition (K-SVD) method depends largely on the signal segment selected in rolling bearing diagnosis, furthermore, the calculating speed is relatively slow, and the dictionary becomes so redundant when the fault signal is relatively long. A new sliding window denoising K-SVD (SWD-KSVD) method is proposed, which uses only one small segment of time-domain signal containing impacts to perform sliding window dictionary learning and select an optimal pattern with oscillating information of the rolling bearing fault according to a maximum variance principle. An inner product operation between the optimal pattern and the whole fault signal is performed to enhance the characteristic of the impacts’ occurrence moments. Finally, the signal is reconstructed at peak points of the inner product to realize the extraction of the rolling bearing fault features. Both simulation and experiments verify that the method could extract the fault features effectively.
Transient response of the human ear to impulsive stimuli: a finite element analysis
Jing Zhang, Jiabin Tian, Na Ta and Zhushi Rao
Nowadays, the steady-state responses of human ear to pure tone stimuli have been widely studied. However, the temporal responses to transient stimuli have not been investigated systematically to date. In this study, a comprehensive finite element (FE) model of the human ear is used to investigate the transient characteristics of the human ear in response to impulsive stimuli. There are two types of idealized impulses applied in the FE analysis: the square wave impulse (a single positive pressure waveform) and the A-duration wave impulse (both of positive and negative pressure waveforms). The time-domain responses such as the displacements of the tympanic membrane (TM), the stapes footplate (SF), the basilar membrane (BM), the TM stress distribution, and the cochlea input pressure are derived. The results demonstrate that the TM motion has the characteristic of spatial differences, and the umbo displacement is smaller than other locations. The cochlea input pressure response is synchronized with the SF acceleration response, while the SF displacement response appears with some time delay. The BM displacement envelope is relatively higher in the middle cochlea, and every portion of BM vibrates at its best frequency approximately. The present results provide a good understanding of the transient response of the human ear.
Fin whale density and distribution estimation using acoustic bearings derived from sparse arrays
Danielle V Harris, Jennifer L Miksis-Olds, Julia A Vernon and Len Thomas
Passive acoustic monitoring of marine mammals is common, and it is now possible to estimate absolute animal density from acoustic recordings. The most appropriate density estimation method depends on how much detail about animals’ locations can be derived from the recordings. Here, a method for estimating cetacean density using acoustic data is presented, where only horizontal bearings to calling animals are estimable. This method also requires knowledge of call signal-to-noise ratios, as well as auxiliary information about call source levels, sound propagation and call production rates. Results are presented from simulations and from a pilot study using recordings of fin whale (Balaenoptera physalus) calls from Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) hydrophones at Wake Island in the Pacific Ocean. Simulations replicating different animal distributions showed median biases in estimated call density of less than 2%. The estimated average call density during the pilot study period (December 2007–February 2008) was 0.02 calls hr−1 km2 (coefficient of variation, CV: 15%). Using a tentative call production rate, estimated average animal density was 0.54 animals/1000 km2 (CV: 52%). Calling animals showed a varied spatial distribution around the northern hydrophone array, with most detections occurring at bearings between 90° and 180°.
Externalization of remote microphone signals using a structural binaural model of the head and pinna
James M Kates, Kathryn H Arehart, Ramesh Kumar Muralimanohar and Kristin Sommerfeldt
In a remote microphone (RM) system, a talker speaks into a microphone and the signal is transmitted to the hearing aids worn by the hearing-impaired listener. A difficulty with RMs, however, is that the signal received at the hearing aid bypasses the head and pinna, so the acoustic cues needed to externalize the sound source are missing. The objective of this article is to process the RM signal to improve externalization when listening through earphones. The processing is based on a structural binaural model, which uses a cascade of processing modules to simulate the interaural level difference, interaural time difference, pinna reflections, ear-canal resonance and early room reflections. The externalization results for the structural binaural model are compared to a left–right signal blend, the listener’s own anechoic head-related impulse response (HRIR) and the listener’s own HRIR with room reverberation. The azimuth is varied from straight ahead to 90° to one side. The results show that the structural binaural model is as effective as the listener’s own HRIR plus reverberation in producing an externalized acoustic image and that there is no significant difference in externalization between hearing-impaired and normal-hearing listeners.
How the acoustic resonances of the subglottal tract affect the impedance spectrum measured through the lips
Noel Hanna, John Smith and Joe Wolfe
Experimental determinations of the acoustic properties of the subglottal airway, from the trachea below the larynx to the lungs, may provide useful information for detecting airway pathologies and aid in the understanding of vocal fold auto-oscillation. Here, minimally invasive, high-precision impedance measurements are made through the lips (7 men and 3 women) over the range 14–4200 Hz during inspiration, expiration and with a closed glottis. Closed glottis measurements show the expected resonances and anti-resonances of the supraglottal vocal tract. As the glottis is gradually opened and the glottal inertance decreases, maxima in the subglottal impedance increasingly affect the measured impedance spectrum, producing additional pairs of maxima and minima. The pairs with the lowest frequency appear first. Measurements during a cycle of respiration show the disappearance and reappearance of these extrema. For a wide glottal opening during inspiration, and for the frequency range 14–4200 Hz, the impedance spectrum semi-quantitatively resembles that of a single, longer duct, open at the remote end, and whose total effective length is 37 ± 4 cm for men and 34 ± 3 cm for women. Fitting to simple models of the subglottal tract yields mean effective acoustic lengths of 19.5 cm for the men and 16.0 cm for the women in this study.
Nonlinear finite element analysis of transient behavior of delaminated composite plate
Chetan Kumar Hirwani, Subrata Kumar Panda and Trupti Ranjan Mahapatra
Experimental determinations of the acoustic properties of the subglottal airway, from the trachea below the larynx to the lungs, may provide useful information for detecting airway pathologies and aid in the understanding of vocal fold auto-oscillation. Here, minimally invasive, high-precision impedance measurements are made through the lips (7 men and 3 women) over the range 14–4200 Hz during inspiration, expiration and with a closed glottis. Closed glottis measurements show the expected resonances and anti-resonances of the supraglottal vocal tract. As the glottis is gradually opened and the glottal inertance decreases, maxima in the subglottal impedance increasingly affect the measured impedance spectrum, producing additional pairs of maxima and minima. The pairs with the lowest frequency appear first. Measurements during a cycle of respiration show the disappearance and reappearance of these extrema. For a wide glottal opening during inspiration, and for the frequency range 14–4200 Hz, the impedance spectrum semi-quantitatively resembles that of a single, longer duct, open at the remote end, and whose total effective length is 37 ± 4 cm for men and 34 ± 3 cm for women. Fitting to simple models of the subglottal tract yields mean effective acoustic lengths of 19.5 cm for the men and 16.0 cm for the women in this study.
Orbit response recognition during touchdowns by instantaneous frequency in active magnetic bearings
Mindong Lyu, Tao Liu, Zixi Wang, Shaoze Yan, Xiaohong Jia and Yuming Wang
During touchdowns of active magnetic bearings (AMBs), the violent collision between rotors and touchdown bearings (TDB) can cause damages to both parts. Orbit response recognition provides a way for the AMB controller to automatically switch the control algorithm to actively suppress the rotor–TDB vibration and promptly relevitate the rotor during touchdowns. A novel method based on Hilbert transform (HT) is proposed to recognize the orbit responses (pendulum vibration, combined rub and bouncing and full rub) in touchdowns. In this method, the rotor suspension status is monitored by the AMB controller in real-time. When touchdown is detected, the rotor displacement signal during the sampling period is intercepted, and the instantaneous frequency (IF) is calculated by HT. Then, the local variance of IF during the sampling period is calculated, and it is compared with the threshold value. Combined rub and bouncing can be identified for it has the largest local variance. Finally, the mean value of IF during the sampling period is calculated and is compared with the other threshold value. Pendulum vibration can be identified, for it has a lower and fixed mean value, while full rub has a larger value. The principle of the recognition method is demonstrated by the simulated results of a thermo-dynamic model. The results reveal that the method is feasible in recognizing the orbit responses and can be implemented in the AMB controller to help switch the control algorithms automatically in case of touchdowns.
Surge vibration-induced nonlinear behavior regulation of power amplifier for magnetic bearing in a 315 kW centrifugal compressor
Yin Zhang, Shiqiang Zheng, Chen Ma, Cheng Chen and Can Wang
The severe vibration induced by surge and rotating stall is an obstacle to the stability of a magnetically suspended centrifugal compressor (MSCC). In order to suppress the severe vibration caused by surge instability, this article focuses on compressor surge performance improvements enabled by power amplifier control improvements which result in increased dynamic load capacity (DLC) of the systems axial thrust magnetic bearing. A complete discrete-time model of the active magnetic bearing (AMB) power amplifier, composed of three piecewise linear intervals is developed. A comprehensive view of the dynamic evolution process from stable state to bifurcation for the power amplifier is also analyzed. In order to stabilize the unstable periodic orbits in the power amplifier, a time-delay feedback control (TDFC) method is introduced to enhance the stability of the power amplifier, while the MSCC is subjected to the surge instability. Simulation results show that the stable region of the power amplifier is extended significantly using the TDFC method. Finally, the experimental investigations performed by an MSCC test rig demonstrate the effectiveness of the proposed solution under the conditions of modified surge and mild surge.
Band gap characteristics of nonrotating passive periodic drill string
Yaser Alsaffar, Sadok Sassi and Amr Baz
A new class of drill strings is investigated whereby strategically designed and placed periodic inserts are utilized to filter out the vibration transmission along the drill strings. Such mechanical filtering capabilities allow the vibrations to propagate along the periodic drill string only within specific frequency bands called the ‘pass bands’ and completely block it within other frequency bands called the ‘stop bands’. The design and the location of the inserts are selected to confine the dominant modes of vibration of the drill string within the stop bands generated by the periodic arrangement of the inserts in order to completely block the propagation of the vibrations. A finite element model (FEM) that simulates the operation of this new class of drill strings is developed to describe the complex nature of the vibration encountered during drilling operations. Experimental prototype of the passive periodic drill string was built and tested to demonstrate the feasibility and effectiveness of the concept of periodic drill string in mitigating undesirable vibrations. The experimental results are used to validate the developed theoretical model and to develop a scalable design tool that can be used to predict the dynamical behavior of this new class of drill strings.
Evaluation of psychoacoustic annoyance and perception of noise annoyance inside university facilities
Edgar Tristán Hernández, Ignacio Pavón García, Juan Manuel López Navarro, Isaac Campos-Cantón and Eleazar Samuel Kolosovas-Machuca
The levels of noise produced in university facilities by students, increases the noise annoyance. The quality of life and the academic performance of university students could be influenced by this factor. Unfortunately, as far as methodology is concerned, there are no regulations or standards that allow for the correct evaluation of noise annoyance at educative facilities. In this work, a method for the evaluation of noise annoyance and an indicator of noise annoyance are presented. In order to obtain a numerical value, a percentage index and a verbal index that represents the noise annoyance in a specific area at university facilities, psychoacoustic annoyance (PA) and evaluation of perception of noise annoyance has been related. Resulting from this correlation, an indicator of noise annoyance was proposed. The results were associated with this indicator. The method and the proposed indicator allow for deeper evaluation of noise annoyance and facilitate the development of appropriate actions against noise.
Active-suspension design for a special road-rail vehicle based on vehicle-track coupled model using genetic algorithm
Roohollah Talebitooti and Reza Bayat
In this article, a proportional–integral–derivative (PID) controller for a special road-rail vehicle is developed. The dynamic model is derived to properly consider the vehicle and track vibrations. This model contains the effects of track elasticity which makes it more reliable and precious compared to traditional models. The vehicle model contains a two-dimensional 10 degree of freedom (DOF) vehicle model, and the track model consists of 40 sleepers. In addition, the effect of the sleeper number on car-body vibration is investigated. The results show that the present vehicle-track-coupled model is more efficient in controller design than the traditional ones. Finally, tuning of controller gains is performed with the aid of a genetic algorithm, in order to achieve a well-organized active suspension.
Estimation of viscous damping parameters of fiber-reinforced plastic plates using finite element model updating
Subhajit Mondal and Sushanta Chakraborty
A combined numerical-experimental technique has been implemented using the finite element model updating technique to estimate a set of proportional viscous damping parameters for determining the global responses of fiber-reinforced plastic (FRP) plates over a chosen frequency range of interest. The experimentally determined frequencies and mode shapes are used to update the homogenized anisotropic in-plane material constants, before estimating the damping parameters from the directly observed frequency response functions (FRFs). Gradient-based inverse sensitivity method has been implemented for the parameter estimation. Existing FRP structures may degrade considerably due to environmental effects over the long period of existence—changing the material and damping properties significantly compared to their initial values after fabrication—thus requiring updating. The estimated viscous damping parameters using the current technique reproduces higher values of modal damping factors for FRP plates. For nonviscous damping, estimation of a set of relaxation factors produces a more realistic estimate of modal damping factors. Increased value of the relaxation factors make the model coincide with the viscous one. A numerically simulated plate problem has been presented along with the experimental validation.
Improved numerical modeling of fiber-reinforced plastics I-beam from experimental modal testing and finite element model updating
Asim Kumar Mishra, Althaf Mohammed and Sushanta Chakraborty
Fiber-reinforced plastics (FRPs) is increasingly being used in infrastructural applications like bridges, chemical plants and so on, where the environment can limit the expected service life of structures made of conventional materials such as reinforced concrete, steel or timber. Advantages of FRP over conventional constructional materials are its high specific strength and specific stiffness, ease with which it can be molded to various shapes, corrosion resistance, lower lifecycle cost, durability and so on. Estimation of accurate dynamic responses of FRP structures is very important from their operation point of view. Such dynamic responses are functions of material properties, boundary conditions, geometry and applied loading. FRP being an anisotropic and layered composite material, a large number of elastic material property parameters are to be determined. Moreover, its structural fabrication and material fabrication at constituent level being one unified process, the actual existing material property parameters may vary considerably from those specified in established standards or determined from characterisation tests. This approach attempts at establishing a non-destructive technique based on experimental modal testing and finite element model updating to estimate the elastic material parameters of an ‘I’ beam made of FRP, thereby making the prediction of dynamic responses more accurate. Static load test on the beam and characterisation tests on samples cut from actual structure are conducted to assess the performance of this updating exercise. This approach can also be used to non-destructively monitor degradations of elastic material properties over time and thus can be used for health monitoring of existing FRP structures.
Radiation characteristics of two semi-infinite overlapped cylindrical waveguides with impedance loading
Ahmet Demir and Alinur Büyükaksoy
The radiation of plane waves by two coaxial overlapping pipes is investigated in the case where the walls of the pipes lying in the overlap region are impedance boundaries. By using the eigenfunction expansion in the overlap region and using the Fourier transform technique elsewhere, we obtain directly two uncoupled modified Wiener–Hopf equations of the second kind, whose solutions involve two set of infinitely many unknown expansion coefficients satisfying two systems of linear algebraic equations. Numerical solution of these systems are obtained for various values of the parameters of the problem, whereby are studied the effects of these parameters on the radiation phenomenon.
Perfect absorption of sound by rigidly backed high-porous materials
Noé Jiménez, Vicent Romero-García and Jean-Philippe Groby
We present the conditions to observe perfect sound absorption by rigidly backed layers of rigid-frame high-porous materials. We theoretically analyze different configurations of increasing complexity: a single layer of high-porous material, a layer of high-porous material with an air gap (air plenum) and an optimized multilayer structure. First, we show that to obtain normal incidence perfect sound absorption at the first so-called quarter-wavelength resonance of a single rigidly backed high-porous layer, the thickness of the material is strongly related to its flow resistivity. In particular, perfect sound absorption is observed when the quarter-wavelength resonance is around the characteristic Biot frequency of the high-porous media. We found that the optimal thickness of the layer is 4.64 smaller than the perfectly absorbed wavelength when using one-parameter empirical models for the effective parameters of the high-porous material. Then, we analyze the behavior of the structure for oblique angles of incidence, showing that perfect sound absorption is also produced for other incident angles and frequencies. Second, an air gap is introduced between the high-porous layer and the rigid backing. The gap allows to produce perfect sound absorption for structures thicker than the optimal one and for media with large intrinsic losses, that is, for materials with high flow resistivity. Finally, optimized multilayer structures are proposed, which present broadband and perfect sound absorption. The existence of perfect absorption is related to the impedance matching, which is produced when the intrinsic losses of the system exactly compensate the leakage of the structure due to its resonance. Thus, to observe perfect sound absorption, in addition to and material properties, the total thickness of any multilayered porous structure is constrained to its quarter-wavelength resonance because of the lack of deep-subwavelength resonance in the system.
Acoustic pulses in media with power-law hysteretic nonlinearity
Andrey Radostin
This article presents analytical results on propagation of acoustic unipolar pulses in media with hysteretic nonlinearity characterized by a power law with an arbitrary exponent. Exact solutions for pulse profiles are derived for one-dimensional (1D) propagation in the case of integer power-law exponents. For fractional ones, numeric and approximate analytical solutions are obtained. These results are used for interpretation of experimental data from the study by Y. Yasumoto, et al. (Acta Acustica united with Acustica 30(5) (1974) 260–267), where propagation of acoustic pulses in aluminum samples with different degree of annealing was observed. It is found that the dependence of the received pulse amplitude on the excitation amplitude corresponds to exponent n = 4 in the hysteretic equation of state, the nonlinear parameters of which increase with increasing temperature annealing.
Semi-empirical prediction of noise from non-zero pressure gradient turbulent boundary layers
Steven AE Miller
Turbulent boundary layers with non-zero pressure gradients are present on almost all aerospace flight vehicles and radiate acoustic waves. A semi-empirical mathematical model is developed to predict acoustic radiation from non-zero pressure gradient turbulent boundary layers. The arguments of the mathematical model are the turbulent statistics and mean flow, which are numerically derived from steady Reynolds-averaged Navier–Stokes equations closed by an algebraic Reynolds stress model. Predictions are conducted for four subsonic Mach numbers and five non-dimensional pressure gradients. The turbulent statistics and mean flow relative to the zero pressure gradient boundary layer are quantified. Predictions of acoustic radiation are compared with a previously developed analytical model and a well-validated large eddy simulation. Finally, relative changes of the power spectra for four Mach numbers and four non-dimensional pressure gradients are compared with corresponding zero pressure gradient flows.