[Show abstract][Hide abstract] ABSTRACT: We show that sound waves can resonantly transmit through Bragg bandgaps in an acoustical duct periodically attached with an array of Helmholtz resonators, forming within the normally forbidden band a transparency window with group velocity smaller than the normal speed of sound. The transparency occurs for the locally resonant frequency so much close to the Bragg one that both the local-resonance-induced bandgap and the Bragg one heavily overlap with each other. The phenomenon seems an acoustical analog of the well-known electromagnetically induced transparency by quantum interference. Different from the Fano-like interference explanation, we also provide a mechanism for the transparency window phenomenon which makes it possible to extend the phenomenon in general.
[Show abstract][Hide abstract] ABSTRACT: Tuning the extraordinary acoustical transmission is shown to be practically feasible simply by controlling acoustical impedances induced by surface evanescent waves. We demonstrate this idea with an example of making a sound tunnel in an acoustical waveguide with a subwavelength short throat and a catenoid horn working below its cutoff frequency. The throat acting as a resonant aperture assists sound waves effectively tunneling through the normally barred horn, leading to resonant transmission of sound waves within an adjustable narrow band. The example may find its applications for highly efficient acoustical filters and transmitters.
[Show abstract][Hide abstract] ABSTRACT: A windowed average technique is designed as an efficient assistance of empirical mode decomposition, aimed especially at extracting components with temporally variant frequencies from heavily noisy signals. Unlike those relying on detection of such points as local extrema that are highly sensitive to noise interference, the present method evaluates a local mean curve that reflects the slow variation of a signal in longer time scales by locally integral average over a sliding window. It adapts to variation of signal component in a broad frequency range by making the window width variable in response to the variation. The enhanced performance and robustness of the new algorithm with respect to noise resistance are demonstrated in comparison with other EMD-based methods, and examples of processing both speech and underwater acoustic signals are given to show the success of extracting time varying information.
Mechanical Systems and Signal Processing 04/2011; 25(3-25):812-820. DOI:10.1016/j.ymssp.2010.10.007 · 2.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We propose a mechanism for soliton creation from resonantly excited localized waves via supratransmission in band gaps of nonlinear lattices. A nonlinear localized wave, which is formed by and vibrates around an impurity with an intrinsic frequency, is found to undergo a local resonance when subject to an external forcing. Under the resonance, an instability develops that leads to the efficient emission of solitons at a much lower rate than that in uniform lattices with no impurity.
[Show abstract][Hide abstract] ABSTRACT: An approach is proposed specially for capturing fine dynamic structures of speech fundamental frequency F <sub>0</sub> that may vary in such a nonmonotonic way as those of the third tones in Chinese speech. It first estimates the rough trend of variation of a F <sub>0</sub> contour by means of the cepstrum technique, and then, utilizes the trend as a reference to track the variation and calculates the detailed contour from a few of intrinsic mode functions that are decomposed by the ensemble empirical mode decomposition. Intensive evaluation and direct comparisons with existing methods are conducted with the standard Chinese Mandarin database, showing the effectiveness of the proposed method in acquiring accurate and reliable F <sub>0</sub> contours from speech signals even heavily contaminated with noise.
[Show abstract][Hide abstract] ABSTRACT: The empirical mode decomposition (EMD) is applied to extract information of modulation from signals contaminated by noise. The EMD method is capable of recovering the amplitude-modulated components from strong background noise in an adaptive way, and achieves better performance than traditional methods. We further propose a modified EMD technique by estimating the local mean of a signal via windowed average. This method alleviates the unfavorable influence of noise disturbance effectively in the process of sifting and yields a remarkable improvement for the modulation extraction. Finally, by utilizing this novel technique, we achieve the adaptive and effective extraction of the modulated cavitation noise from ship-radiated noise.
Mechanical Systems and Signal Processing 10/2010; 24(7-24):2124-2136. DOI:10.1016/j.ymssp.2010.03.013 · 2.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An analytical theory for sound transmitting through apertures that are slits or holes periodically pored on one- or two-dimensional rigid panels is developed in small-aperture approximation, with all coefficients of reflection and transmission given explicitly in concise and easily calculable forms. We utilize acoustical impedance to quantitatively describe the effect of sound diffraction by both surfaces of a perforated slab on the aperture resonance. We show that diffraction induced reactance X<sub> a </sub> , which is acoustically inertant (X<sub> a </sub>>0) for incident wavelength λ longer than the period Λ of the perforated slab, can become infinitely large as λ approaches to Λ . We further show that the singularity of X<sub> a </sub> not only causes the already known full reflection of acoustic waves at λ=Λ , but also drastically changes the aperture resonance leading to the extraordinary acoustical transmission that was observed in recent experiments. With this understanding, tuning the resonant transmission becomes practically feasible in applications of the resonant transmission phenomenon.
[Show abstract][Hide abstract] ABSTRACT: Classification for ship-radiated underwater sound is one of the most important and challenging subjects in underwater acoustical signal processing. An approach to ship classification is proposed in this work based on analysis of ship-radiated acoustical noise in subspaces of intrinsic mode functions attained via the ensemble empirical mode decomposition. It is shown that detection and acquisition of stable and reliable nonlinear features become practically feasible by nonlinear analysis of the time series of individual decomposed components, each of which is simple enough and well represents an oscillatory mode of ship dynamics. Surrogate and nonlinear predictability analysis are conducted to probe and measure the nonlinearity and regularity. The results of both methods, which verify each other, substantiate that ship-radiated noises contain components with deterministic nonlinear features well serving for efficient classification of ships. The approach perhaps opens an alternative avenue in the direction toward object classification and identification. It may also import a new view of signals as complex as ship-radiated sound.
The Journal of the Acoustical Society of America 07/2010; 128(1):206-14. DOI:10.1121/1.3436543 · 1.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An acoustical theory is developed for sound transmitting through subwavelength apertures. We show that the excitation of evanescent high-order modes induces, on each end of an aperture (either a slit or a hole), an additional acoustical reactance, which is singular as incident wavelength approaches one of cutoff wavelengths of high modes. The anomaly of the induced reactance greatly changes the resonant behaviors of the aperture, and makes the conjugate impedance matching possible at a wavelength slightly longer than the cutoff, thus leading to the extraordinary full transmission.
[Show abstract][Hide abstract] ABSTRACT: Modulation analysis is an important issue in target classification and identification for ship-radiated noise. However, the modulated cavitation noise sought for analyzing is always submerged under strong ambient noise and difficult to be separated out. In this paper, an approach is proposed to extract the modulated cavitation noise adaptively by combining empirical mode decomposition and singular value decomposition. The results for both synthetical and practical signals demonstrate the practicability and effectivity of the approach.
The Journal of the Acoustical Society of America 12/2009; 126(6):3106-13. DOI:10.1121/1.3244987 · 1.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A novel sifting method based on the concept of local integral mean of a signal is developed for empirical mode decomposition (EMD), aiming at decomposing those modes whose frequencies are within an octave. Instead of averaging the upper and lower envelopes, the proposed technique computes the local mean curve of a signal by interpolating data points that are local integral averages over segments between successive extrema of the signal. With the sifting method, EMD can separate intrinsic modes of oscillations with frequency ratios up to 0.8, thus considerably improving the frequency resolving power. Also, it is shown that the integral property of the sifting considerably accelerates the convergence of the sifting iteration and remarkably enhances the robustness of EMD against noise disturbance.
[Show abstract][Hide abstract] ABSTRACT: The well-known Bragg resonance in periodic waveguides always leads to the creation of the so-called Bragg gap, within which sound propagations are effectively forbidden. Here we report the possibility of sound energy transmission in the Bragg gap via the high-order transverse modes, which penetrate through the forbidden band due to the interactions between different sound modes in an acoustic duct with periodically corrugated walls. The theoretical analysis indicates that in the waveguides with transverse scales comparable to its period, the guided wave modes can interact with the Bragg gap so that the forbidden band undergoes an abnormal change, giving rise to both a considerable compression in the band width and a sharp descent of the transmission loss on the upper edge of the stopband. The experiment confirms the existence and the significance of the interacting effect, and the measurements of the transmission loss and the radial distribution of sound fields agree quite well with the theoretical predictions.
[Show abstract][Hide abstract] ABSTRACT: The non-Bragg resonance of surface water waves is investigated, both theoretically and experimentally, in a trough with square-wave corrugated sidewalls. Unlike the familiar Bragg resonances, the non-Bragg resonances occur far from the edges of the Brillouin zone and open additional forbidden bands. The experimental observations confirm the existence of these resonances, and the measurements for the transmission properties showing both Bragg and non-Bragg band gaps agree fairly well with the theoretical predictions obtained by the plane-wave expansion method. It is shown that both Bragg and non-Bragg resonances highly depend on the symmetry of the corrugations on the opposite sidewall. As the relative shift between the two corrugations increases from zero to the half period of the corrugations, the Bragg gap shrinks and vanishes, but the non-Bragg gap varies in the opposite way, reaching its maximum, which is impressively wide and much more efficient in reflecting water waves.
[Show abstract][Hide abstract] ABSTRACT: The band gaps in periodic structures are usually regarded as being induced by the Bragg resonances. Until the recent years, the non-Bragg nature resonances were not taken into account in analysing and computing the band gaps, though it can exist in all kinds of waveguides with periodic structures. Here, the resonance-induced band gaps in a periodic acoustic duct are investigated extensively and a graphical method is introduced to analyse the dependence of these resonances on the duct geometry. With this method, it becomes quite easy to estimate the frequency band gaps of the waveguide and shape the band structures by choosing the proper geometric parameters. Our analysis show that the location and the width of the band gap are closely related to the wavenumber and the amplitude of the wall corrugations, and the non-Bragg resonance can result in the obvious band gap when the wall wavenumber is close to the cut-off frequency of the first mode.
Journal of Sound and Vibration 06/2008; 313(3-5-313):830-840. DOI:10.1016/j.jsv.2007.11.055 · 1.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A non-Bragg nature forbidden band is experimentally observed in an axially symmetric hard-walled duct with a periodically varying cross section. Unlike the familiar Bragg ones, the observed bandgap is found to result from the interference of sound wave modes having different transverse standing-wave profiles, the so-called non-Bragg resonance. The experiments also show that the non-Bragg band can be comparably wider than the Bragg one; furthermore, the sound transmission loss within the band can be much more effective, exhibiting the great significance of the non-Bragg resonance in wave propagation in periodic waveguides.
[Show abstract][Hide abstract] ABSTRACT: Based on recently observed nonlinear dynamic features of human speech, the local projection (LP) method, originally developed for noisy chaotic time series, is generalized and adapted to the enhancement of Chinese speech. The analysis of minimum embedding dimensions estimated by the false nearest neighbor algorithm shows that all the basic phonemes and syllables in Chinese can be faithfully embedded in some low-dimensional phase space. Over-embedding is applied to reconstruct the dynamics of continuous speech in some extended phase space of higher dimension, thus solving the problem of nonstationarity in continuous speech. A generalization of the LP method, named the local subspace method, is presented for speech enhancement in the phase space. It is demonstrated that, the local subspace method is essentially an extension of the well-known linear subspace technique in the local phase space, and the LP method is the least square case of this generalization. Noise reduction is then carried out in the local phase space. Results show that the LP method, with 2 or 3 iterations, achieves better performances than the local subspace method. For both isolated and continuous speech with additive white noise, experiments show the superiority of the LP method over two other popular algorithms.
Signal Processing 10/2007; 87(10-87):2431-2445. DOI:10.1016/j.sigpro.2007.03.020 · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The bifurcation behaviors of a parametrically excited solitary wave are investigated via Faraday's water tank experiment. It is observed that, as the driving frequency fd is decreased or/and the driving amplitude Ad is increased, the standing (but vertically oscillatory) solitary wave becomes modulationally unstable, leading to the temporal modulation of the vertical oscillation and the emergence of very low subharomic components on the frequency spectrum. Further lowering fd or/and increasing Ad will cause the modulational oscillation unstable and then, the peak of the solitary wave becomes rocking along the trough in the longitudinal direction. These bifurcations also give rise to the emission of continuous waves resulting in complex wave patterns and complicated fluctuations, especially for the quite low fd and large Ad. A possible route from solitary waves to chaos via bifurcations and mode competitions is therefore suggested on the basis of these observations.
[Show abstract][Hide abstract] ABSTRACT: A new reconstruction algorithm in a finite form based on the Rytov transform is presented for acoustical diffraction tomography. Applying the Rytov transform to the governing differential wave equation necessarily introduces the so-called generalized scattering. Our analysis shows that the generalized scattered wave is asymptotically equivalent to the physically scattered wave, and also satisfies the Sommerfeld radiation condition in the far field. Using the method of formal parameter expansion, we further find that all other terms in the expansion of the object function vanish except the first- and second-order ones, and thus reach a finite form solution to the diffraction tomography. Our computer simulation confirms the effectiveness of the algorithm in the case of the scattering objects with cylindrical symmetry, also shows its limitations when it applies to the strong scattering.
[Show abstract][Hide abstract] ABSTRACT: An exact analytic solution for a solitary wave of arbitrary height is attained by series expansions of flow variables based on parameter ε=k2h2, (k being the wave number of the solitary wave on water of uniform depth h) by orders in O(εn) up to n=25. Its convergence behavior is found first to yield a set of asymptotic representations for all the flow variables, each and every becoming highest in accuracy at O(ε17). For n>17, the field variables and wave parameters, e.g., wave amplitude, have their errors continue increasing with n, but, in sharp contrast, all the wave integral properties including the excess mass first undergo finite fluctuations from O(ε17) to O(ε20), then all converge uniformly beyond O(ε20) in a group of tight bundle within the range 0ε0.283, with ε=0.283 corresponding to the highest solitary wave with a 120° vertex angle. This remarkable behavior of series convergence seems to have no precedent, and furthermore, is unique in ε, not shared by the exact solutions based on all other parameters examined here.
Physics Letters A 01/2006; 350(1):44-50. DOI:10.1016/j.physleta.2005.10.006 · 1.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A unified perturbation theory is developed here for calculating solitary waves of all heights by series expansion of base
flow variables in powers of a small base parameter to eighteenth order for the one-parameter family of solutions in exact
form, with all the coefficients determined in rational numbers. Comparative studies are pursued to investigate the effects
due to changes of base parameters on (i) the accuracy of the theoretically predicted wave properties and (ii) the rate of
convergence of perturbation expansion. Two important results are found by comparisons between the theoretical predictions
based on a set of parameters separately adopted for expansion in turn. First, the accuracy and the convergence of the perturbation
expansions, appraised versus the exact solution provided by an earlier paper  as the standard reference, are found to depend,
quite sensitively, on changes in base parameter. The resulting variations in the solution are physically displayed in various
wave properties with differences found dependent on which property (e.g. the wave amplitude, speed, its profile, excess mass,
momentum, and energy), on what range in value of the base, and on the rank of the order n in the expansion being addressed. Secondly, regarding convergence, the present perturbation series is found definitely asymptotic
in nature, with the relative error δ(n) (the relative mean-square difference between successive orders n of wave elevations) reaching a minimum, δ
, at a specific order, n=n
, both depending on the base adopted, e.g. n
=11-12 based on parameter α (wave amplitude), n
=15 on β (amplitude-speed square ratio), and n
=17 on ∈ ( wave number squared). The asymptotic range is brought to completion by the highest order of n=18 reached in this work.