Acoustic Source Characterization Research Articles

Analysis of the load selection on the error of source characteristics identification for an engine exhaust system

Linear time-invariant assumption for the determination of acoustic source characteristics, the source strength and the source impedance in the frequency domain has been proved reasonable in the design of an exhaust system. Different methods have been proposed to its identification and the multi-load method is widely used for its convenience by varying the load number and impedance. Theoretical error analysis has rarely been referred to and previous results have shown an overdetermined set of open pipes can reduce the identification error. This paper contributes a theoretical error analysis for the load selection. The relationships between the error in the identification of source characteristics and the load selection were analysed. A general linear time-invariant model was built based on the four-load method. To analyse the error of the source impedance, an error estimation function was proposed. The dispersion of the source pressure was obtained by an inverse calculation as an indicator to detect the accuracy of the results. It was found that for a certain load length, the load resistance at the frequency points of one-quarter wavelength of odd multiples results in peaks and in the maximum error for source impedance identification. Therefore, the load impedance of frequency range within the one-quarter wavelength of odd multiples should not be used for source impedance identification. If the selected loads have more similar resistance values (i.e., the same order of magnitude), the identification error of the source impedance could be effectively reduced.

Supplemental Material for Acoustic Source Characteristics, Across-Formant Integration, and Speech Intelligibility Under Competitive Conditions

Supplemental Material for Acoustic Source Characteristics, Across-Formant Integration, and Speech Intelligibility Under Competitive Conditions

IC-Engine Exhaust and Intake System Acoustic Source Characterization

The paper gives an overview of techniques used for characterization of IC-engines as acoustic sources of exhaust and intake system noise. Some recent advances regarding nonlinear source models are introduced and discussed. To calculate insertion loss of mufflers or the level of radiated sound information about the engine as an acoustic source is needed. The source model used in the low frequency plane wave range is often the linear time invariant one-port model. The acoustic source data is obtained from experimental tests or from 1-D CFD codes describing the engine gas exchange process. The IC-engine is a high level acoustic source and in most cases not completely linear. It is therefore of interest to have models taking weak non-linearity into account while still maintaining a simple method for interfacing the source model with a linear frequency domain model for the attached exhaust or intake system. The use of source characterization in acoustic design of mufflers is also briefly discussed.

Acoustic Source Characterization for Prediction of Medium Speed Diesel Engine Exhaust Noise

To achieve reliable results when simulating the acoustics of the internal combustion engine (IC-engine) exhaust system and its components, the source characteristics of the engine must be known. In the low frequency range only plane waves propagate and then one-port source data can be determined using, for example, the acoustic multiload method. For the medium speed IC-engines used in power plants and ships, the exhaust duct noise often needs to be analyzed up to 10 kHz, i.e., far beyond the plane wave range, and it is then more appropriate to use acoustic power to characterize the source. This power should ideally be measured under reflection-free conditions in the exhaust duct. The results from an earlier study showed that a suitable way to characterize the source for any frequency is to determine the in-duct sound power by extending the plane wave formulation with frequency band power weighting factors. The aim of this study is to apply this high frequency range method in situ to a real test engine. Another aim is to define, theoretically, how to combine the source data in the low frequency plane wave range with those in the high frequency nonplane wave range using a source sound power formulation.

Effect of standoff distance on the reconstruction of in-duct velocity field and regeneration of pressure field

Identification of in-duct acoustic source characteristics is essential in the design of fluid machinery system for reducing and predicting the flow-generated noise. To this end, the inverse estimation method can be employed by using the measured sound field and matrix formulation for wave propagation within a duct. In this paper, the effect of the distance between source and measurement plane is investigated. At each standoff distance, pressures are measured at three planes with two different spacings to widen the estimation frequency range, and measurements are conducted with three different standoff distances. Modal decomposition is applied to estimate modal amplitudes, and the result is used to reconstruct the velocity field at the source plane and to obtain the regenerated pressure field at the measurement planes. It is shown that the modal amplitude identified by measured pressure field at the short standoff distance, i.e., at nearfield, can yield an accurate reconstructed velocity field of the source and regenerate the pressure field with smaller error, which is similar to the other inverse techniques such as equivalent source method and nearfield acoustical holography. A field reduction example by suppressing some parts of source velocity field is shown for demonstrating the effectiveness of the method.

Acoustic Source Data for Medium Speed IC-Engines

Knowledge of the acoustic source characteristics of internal combustion engines (IC-engines) is of great importance when designing the exhaust duct system and its components to withstand the resulting dynamic loads and to reduce the exhaust noise emission. The goal of the present study is to numerically and experimentally investigate the medium speed IC-engine acoustic source characteristics, not only in the plane wave range but also in the high frequency range. The low frequency acoustic source characteristics were predicted by simulating the acoustic multiload measurements by using a one-dimensional process simulation code. The low frequency in-duct exhaust noise of a medium speed IC-engine can be quite accurately predicted. The high frequency source data is estimated by averaging the measured acoustic pressures with different methods; using the simple cross-spectra averaging method seems promising in this instance.

On the characterization of acoustic two-port sources using multi-load method

A multi-load method for determining the source data of acoustic two-port sources is presented. By eliminating the term of source strength, the scattering matrix can then be obtained by solving a set of nonlinear algebraic equations, and the source strength is determined by the scattering matrix and the directly measured spectrum matrix corresponding to one of the acoustic loads. Numerical simulations indicate that the proposed method is effective. The method has been tested on a loudspeaker and an axial flow fan in a duct. The source data obtained by this method show reasonable agreement with that measured by the direct method. The multi-load method avoids using external sources, and it can be used as an alternative method when the external source is not easy to realize in practice.

Research on Acoustic Emission Signal of Three Dimensional Braided Composite Material

This paper discusses the application and experimental method of acoustic emission for the three-dimensional (3D) braided composite material under flexural testing. It describes the feature of acoustic emission when applied to three-dimensional braided composite material under flexural testing. In order to know the materials damage position precisely, it is very important to collect the signal of acoustic emission source. The results of flexural experiment show that it is precise on the research of acoustic emission source location by the method of wave form analysis .During the collecting signal of acoustic emission , it is compared between parameter analysis and wave form analysis. According to the attenution degree of signal, it is selected signal of acoustic emission source exactly. It is the method to extract the precise acoustic emission signal by the different algorithm of wavelet analysis. We know that humans have been using materials by testing activities support, these tests have developed several centuries. From the traditional rough test to support materials and materials used in all aspects of science and technology, modern, scientific program, at present, there are mutual dependencies between the progress of scientific knowledge and the development of test methods. The 3D braided composite material is a kind of complex structure. Because of the many components and weaving material preparation process characteristic, material mechanics behavior is very complicated. It is characteristics of the 3D braided composite material, that the application on composites is widely more and more. The 3d braided composites have been used widely in aerospace, aviation, transportation, chemical, sports, medical care and other fields, so it is extremely vital significance for research on mechanics performance analysis of 3D braided composite material. Current studies of damage fracture behavior of composite materials are used in homogeneous materials research methods, namely the mechanical test and microscopic observation, but the research cannot distinguish and identify different damage fracture source, due to the combination of mechanical test parameters of the complex fracture micro-mechanism not sensitive. Microscopic observation is in fact observation later, local fracture surface morphology research, so can not observe the interaction of numerous fracture source and micro behavior, and can not study in the character of fracture source of composite materials, therefore the source faults already can not adapt to the traditional methods of composite material damage fracture process. From external or internal force effect material and structure produced deformation or fracture with elastic wave form, the strain energy release phenomenon called acoustic emission or stress wave [1]. Acoustic emission testing method is a kind of material internal defects or potential defects in the dynamic changes in movement, the damage detection method is real-time monitoring and can reflect the characteristics of acoustic emission sources in the load of the dynamic response. The AE information can directly reflect defects and changes [2, 3].

Directivity and frequency control of an intense underwater laser acoustic source for navy applications.

We develop an intense laser acoustic source, in which a tailored laser pulse can compress underwater at a predetermined remote location. Optical compression results in laser-induced breakdown (LIB), localized heating, and acoustic shock generation. Recent experiments include near-field acoustic source characterization using lens-focused 400-, 800-, 532-, and 1064-nm pulses of Ti:sapphire and Nd:YAG lasers. Sound pressure levels over 215-dB were achieved using a compact laser. We have demonstrated control of the shape of the LIB plasma volume, and thereby control of the acoustic frequency spectrum and acoustic source directivity. By superposition of volume elements within the LIB, the acoustic pulse duration in a given propagation direction is determined by the parallel dimension of the LIB, divided by the parallel acoustic transit time across the LIB. Thus, the shape of the LIB strongly affects the acoustic pulse duration and directivity, and aspherical LIB volumes result in strongly anisotropic acoustic sources. In our experiments, the LIB volume shape was varied by laser pulse length, energy, optical bandwidth, and focusing angle. We also studied acoustic propagation in a 30 000-gal bubbly salt water tank. [This work is supported by the U.S. Office of Naval Research.]

Using speech models for separation in monaural and binaural contexts.

When the number of sources exceeds the number of microphones, acoustic source separation is an underconstrained problem that must rely on additional constraints for solution. In a single‐channel environment the expected behavior of the source—i.e., an acoustic model—is the only feasible basis for separation. We have developed an approach to monaural speech separation based on fitting parametric “eigenvoice” speaker‐adapted models to both voices in a mixture. In a binaural, reverberant environment, the interaural characteristics of an acoustic source exhibit structure that can be used to separate even without prior knowledge of location or room characteristics. For this scenario, we have developed MESSL, an EM‐based system for source separation and localization. MESSL’s probabilistic foundation facilitates the incorporation of more specific source models; MESSL‐EV incorporates the eigenvoice speech models for improved binaural separation in reverberant environments.

Investigation on acoustic emission source of bridge crane

In order to draft an acoustic emission testing standard for bridge cranes, the characteristics of acoustic emission sources were studied by performing AE tests for more than 20 bridge cranes. The attenuation curve and linear locations of the AE signals on the box beam were investigated. The results indicate that the linear location method can be used effectively in AE source location in the box beam structure. AE sources of bridge cranes arise mainly from cracking, plastic deformation, structural friction and the noise of vehicle and trolley. The characteristics of each acoustic emission source are shown respectively in this paper.

The reflection of geodynamic processes in local geoacoustic emissions

This paper is concerned with the correlation of fast local processes that generate geoacoustic emissions and the regional character of excitation in the geologic medium prior to large earthquakes. It is shown that this comparison is possible based on the unifying process of vertical energy transport due to the earth’s degassing. For the first time here, we examine local geoacoustic characteristics by analyzing Poincare diagrams and developing difference equations that describe dynamic changes in the characteristics of acoustic emission sources. Based on this concept, and incorporating the continuity of geoacoustic emissions, we suggest a decompressional model of seismoacoustic noise and geoacoustic emissions as a component of the noise.

Applicability of acoustic noise correlation for structural health monitoring in nondiffuse field conditions

Ambient noise correlation offers an elegant way of passively caracterize a medium. The principle is that in diffuse field condition, Green’s function between two sensors can be retrieved from the cross-correlated received signals. Here, the technique is applied in conditions where convergence toward Green’s function is not ensured. Since the obtained information is sensible to the medium state, it is still exploitable for damage detection provided a baseline version is available for comparison. This requires the identification of the acoustic source characteristics at the instant of measurement, which could be achieved using the signal measured at a “reference” sensor.

Research for Characteristics of Rotating Dipole Acoustic Source in Spatial Acoustic Field

Formula for calculating the acoustic pressure of spin dipole acoustic source at any point in space was deduced on the base of frequency-domain solution of turning point acoustic source and acousticfield in free space .Which discussed the acoustic field characteristics during the harmonic dipole source rotating and studied the impact on the acoustic field and acoustic pressure at different source frequency, rotating frequency. Study shows that: dipole acoustic field is of an intense space directivity, the characteristics of acoustic field and acoustic source are closely related.

Intense underwater laser acoustic source for Navy applications.

An intense remote underwater laser acoustic source is under development at the Naval Research Laboratory. In a novel configuration, a tailored intense laser pulse can be designed to propagate many meters underwater and compress at a predetermined remote location. Controlled compression of these optical pulses is governed by a combination of optical group velocity dispersion and nonlinear Kerr self-focusing. Optical compression can result in laser-induced breakdown, localized heating, and acoustic shock generation. Recent experiments include near-field acoustic source characterization using lens-focused 400 and 800 nm pulses of a Ti:sapphire laser, as well as 532 and 1064 nm pulses of a YAG laser. Sound pressure levels over 210 dB were achieved using a compact laser. Acoustic source characterization includes measurements of photoacoustic energy conversion efficiency, acoustic power spectrum, and directivity. Nonlinear optical studies included the precise measurement of the Kerr index of water at 400 and 800 nm, as well as conditions for optical filament generation, and their effects on acoustic signals. Planned experiments include tests in a bubbly salt water tank. Experimental results will be presented, and laser sources and techniques for underwater acoustic generation will be compared. [This work is supported by the U.S. Office of Naval Research.]

Supportless unidirectional acoustic sources

The results of studying the physical characteristics of unidirectional acoustic sources used in active sound control systems are presented. A discrete unidirectional source in the form of two phased monopoles and a planar array of such unidirectional sources are considered. One-dimensional boundary-value problems with two (the two-point problem) and three (the three-point problem) controlled parallel planar boundaries between homogeneous media with arbitrary impedances are studied. The boundaries (two or three) are subjected to the action of external forces. The case of the zero sum of external forces applied to the controlled boundaries corresponds to a supportless unidirectional source. It is shown that a unidirectional source can be created within the two-point boundary-value problem, whereas a supportless unidirectional source can be created within the three-point problem. Such parameters as transparency, small size, absence of support, and broad frequency band can be achieved for a unidirectional source in the form of two piezoelectric layers with the same impedance and velocity of sound as those of the surrounding medium.

Acoustic source characterization of impulsive Strombolian eruptions from the Mount Erebus lava lake

We invert for acoustic source volume outflux and momentum imparted to the atmosphere using an infrasonic network distributed about the erupting lava lake at Mount Erebus, Ross Island, Antarctica. By modeling these relatively simple eruptions as monopole point sources we estimate explosively ejected gas volumes that range from 1,000 m 3 to 24,000 m 3 for 312 lava lake eruptions recorded between January 6 and April 13, 2006. Though these volumes are compatible with bubble volumes at rupture (as estimated from explosion video records), departures from isotropic radiation are evident in the recorded acoustic wavefield for many eruptions. A point-source acoustic dipole component with arbitrary axis orientation and strength provides precise fit to the recorded infrasound. This dipole source axis, corresponding to the axis of inferred short-duration material jetting, varies significantly between events. Physical interpretation of dipole orientation as being indicative of eruptive directivity is corroborated by directional emissions of ejecta observed in Erebus eruption video footage. Although three azimuthally distributed stations are insufficient to fully characterize the eruptive acoustic source we speculate that a monopole with a minor amount of oriented dipole radiation may reasonably model the primary features of the recorded infrasound for these eruptions.

Underwater acoustic generation with narrow and broadband lasers

Underwater laser acoustic sources, generated by intense pulsed lasers on above-water and underwater platforms, are under investigation. In a novel configuration, a tailored intense broadband laser pulse can be designed to propagate many meters underwater and compress at a predetermined remote location. Controlled compression of these optical pulses is governed by a combination of optical group velocity dispersion (GVD) and nonlinear Kerr self-focusing, resulting in photoionization, localized heating, and shock generation. Recent and ongoing experiments include near-field acoustic source characterization using lens-focused pulses of a broadband 400 nm Ti:sapphire laser, as well as 1064 nm and 532 nm narrowband YAG laser pulses. Also, the nonlinear optical Kerr index of water at 400 nm was precisely measured. Acoustic source characterization includes measurements of photoacoustic energy conversion efficiency, acoustic power spectrum, and directivity. Experimental results will be presented, and laser sources and techniques for underwater acoustic generation will be compared. [This work is supported by the U.S. Office of Naval Research.]

Acoustic near-field conditions in an ESEM/AFM hybrid system

The local distribution of acoustic waves generated by a locally modulated electron beam is investigated in a hybrid system consisting of an Environmental Scanning Electron Microscope (ESEM) and an Atomic Force Microscope (AFM). Depending on the modulation, different acoustic source characteristics with different near-field conditions are generated which can be used in acoustic near-field techniques, for example to detect ferroelectric domain structures.

Transverse Cracking in a Cross-ply Composite Laminate - Detection in Acoustic Emission and Source Characterization

Transverse cracking detection in a uniaxially loaded symmetric cross-ply hybrid composite laminate is studied using modal-based acoustic emission (MAE) method. Transverse cracking in the hybrid laminate is identified in conjunction with the finite element analysis and acoustic emission (AE) testing on unidirectional carbon/epoxy specimens. Higher wave modes are involved in various damage events in both types of specimens. Wave attenuation in the hybrid laminate is observed to be much stronger than in the unidirectional carbon/epoxy specimen, resulting in fewer AE events detected. A material failure kinetics-based characterization of the transverse cracking process is proposed. Results of dynamic response analysis of the hybrid laminate show good agreement with the AE experiment data.