摘要:

Matched-field processing is a method for inversion of the acoustic field utilizing its spatial coherence. In this work, a matched-field processor is introduced that incorporates the spatial coherence of the acoustic field not only at a single frequency but across frequencies as well. The new processor is suitable for multitonal sources and does not require knowledge of the source spectrum which is typically unavailable in passive estimation problems. A performance evaluation on source localization under low signal-to-noise ratios shows that the new processor is significantly superior, under certain circumstances, to the conventional incoherent Bartlett processor especially in cases involving receiver arrays with a small number of phones.

ISSN:0001-4966    

DOI:10.1121/1.423954

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A novel approximation for the delays used in focused beamforming is presented that is based on weighting the terms of the conventional Fresnel approximation. The objective is to enlarge the validity region of the Fresnel approximation in order to investigate wider scanning regions. A least-squares solution provides the exact weight values by the minimization of the mean-square error computed over the desired scanning region and on the basis of the array geometry. The attractive results obtained by applying the proposed approximation to a linear array are presented.

ISSN:0001-4966    

DOI:10.1121/1.423286

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A simple and noncomputational intensive method is proposed for elastically characterizing an isotropic material. The Poisson’s ratio and the shear modulus are determined from the axisymmetric vibrations of a cylinder with a length equal to its diameter. These vibrations are excited by means of a wide spectrum impact. The optical system used allows the simultaneous detection of several vibration modes. The out-of-plane displacement is detected by speckle interferometry. From the resulting vibration displacement spectrum, the two lowest frequencies are obtained, both corresponding to axisymmetric modes—specifically, the first symmetric mode and the first antisymmetric mode. The values of both dynamic elastic constants are obtained by comparing previously computed nondimensional natural frequencies and the measured frequencies. The method requires only one experiment and needs no electronic computation. The results obtained for an aluminum cylinder are coherent and confirm the appropriateness of the method. The relative difference between the Young’s modulus values calculated by the proposed method and applying the elementary theory to a long rod of the same material is 0.27%. Comparing the shear modulus value obtained by this method and the calculated one from the lower frequency in torsion, a relative difference of 4.6% is found.

ISSN:0001-4966    

DOI:10.1121/1.423266

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The determination of the modulus of the pupil function in reflection acoustic and optical microscopy with the help of spherical particles is demonstrated. The theoretical examination shows that this method can give good results if the pupil function drops smoothly towards the edge. For a pupil with a sharp edge this technique can give the accurate pupil function only by imaging large spheres. The dependence of the accuracy of the method on the size of the spherical particle is analyzed. Numerical and experimental results obtained for the confocal scanning optical microscope and the scanning acoustic microscope are examined.

ISSN:0001-4966    

DOI:10.1121/1.423268

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Frequency-dependent acoustic center correction values are required to obtain accurate microphone calibrations in the free-field by the reciprocity technique. These values were determined for IEC type LS2aP microphones at normal incidence by utilizing the theoretical inverse relationship between the sound pressure amplitude at the acoustic center of a receiver and the distance between acoustic centers of source and receiver. A dynamic signal analyzer was used to measure the gain factor between the amplified output voltage of the receiver and the source input voltage at 500-Hz intervals in the extended frequency range 2–50 kHz. This procedure allowed all the data for a microphone pair to be gathered within several hours for microphone diaphragm separations from 101–311 mm at 10-mm intervals. At each frequency, the reciprocal of this gain factor as a function of microphone diaphragm separation was fit to a straight line after correction for atmospheric effects, including attenuation of sound caused by atmospheric absorption. Mean acoustic center correction values (from four microphones combined in six pairs) were calculated using the parameters obtained from the fit and found to be in good agreement with published values calculated by the boundary element method, as well as with values predicted by scaling published values of the acoustic center correction for microphones with a geometrical configuration similar to that of IEC type LS1P microphones.

ISSN:0001-4966    

DOI:10.1121/1.423288

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A mathematical model is developed for the coupling of two finite plates at an arbitrary angle for the prediction of the dynamic response and power flow at the coupling edge and at any cross section. The coupling at the joint edge considers bending, out-of-plane shear, and in-plane longitudinal (perpendicular to the joint edge) vibration. No constraint is imposed on the in-plane displacement perpendicular to the coupling edge. The exact solution for flexural mode shapes and resonance frequencies of rectangular plates with one edge free and the other edges simply supported is considered. This exact solution satisfies both the displacement and force boundary conditions, consequently it is used in the coupling of plate panels. An approximate relation is derived for prediction of the flexural resonance frequencies of higher-order modes with a reasonable accuracy. An approximate solution is presented for the in-plane response of the same plate panels when excited by in-plane forces perpendicular to the free edge. A simple expression is given for an approximate estimation of the cut-off frequencies for the different in-plane modes. The frequency response of the individual plate panels is presented as receptance functions for both flexural and in-plane vibration which are used in the plate coupling. The vibrational power injected into a plate by out-of-plane concentrated force and moment is computationally investigated. The importance of power input by a small moment excitation in some resonant bands even at low frequencies is highlighted. The spatial distribution of the components of flexural power transmitted by shear and moment across a section of the plate are investigated and the reason for the circulatory patterns of active power flow in plates is illustrated. The power flow characteristics in two plates coupled at an arbitrary angle is examined by a computational example. The effect of the coupling angle on the components of input power and power flow across the coupling edge is investigated. It is shown that the coupling of the two plates is mainly due to moment at frequencies up to the cut-off frequency of the first in-plane mode. Above this frequency, the coupling is due to out-of-plane shear and in-plane vibration with a diminishing participation of the moment in transmitting vibrational power through the coupling edge.

ISSN:0001-4966    

DOI:10.1121/1.423270

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

This paper discusses distributed parameter sensors designed to extract vibration modes of a structure, as well as its active modal control. Compared to conventional point sensors such as accelerometers or displacement sensors, the distributed parameter sensors are superior to point sensors provided that they are properly designed. First, this paper overviews a conventional modal filter method using point sensors, and enumerates the problems they possess. To overcome the drawbacks of the point sensor-based modal filtering, a novel modal filtering technique based upon distributed parameter PVDF film sensors is proposed. This paper begins by discussing a design methodology for modal filtering using two-dimensional distributed sensors. Then, taking into consideration the applicability of the sensors, a design procedure for modal filtering using one-dimensional sensors is presented, the number, location, and shaping functions for the one-dimensional modal sensors being clarified. Furthermore, the modal filtering using the one-dimensional sensors is found to be applicable for two-dimensional structures subject to any “classical” boundary condition. Experimental results verify the capability of the desired modal filtering for both isolated and degenerate structural modes. Finally, using the modal sensors, experiments on active modal control are conducted, showing a significant suppression of the targeted mode without causing instability of the control system.

ISSN:0001-4966    

DOI:10.1121/1.423271

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

This work investigates and compares the potential of adaptive feedforward and feedback methods for a hybrid active/passive radiation control using smart foam. The radiating structure is a vibrating plate mounted in a rigid baffle in an anechoic chamber. The smart foam, designed to reduce sound by the action of the passive absorption of the foam (which is effective at higher frequencies) and the active input of an embedded PVDF element driven by an oscillating electrical input (which is effective at lower frequencies), is positioned on the plate. The first test consists of using a single-input single-output (SISO) adaptive feedforward LMS controller to minimize the error sensor signal provided by a microphone in the close proximity of the active element under narrow-band excitation and broadband random excitation. For feedforward control, two different reference signals are considered: the voltage sent to the piezoceramic actuator driving the plate (disturbance) and the signal from an accelerometer directly mounted on the plate (more realistic in practice). In the latter case, the effect of the smart foam on the reference signal (or acceleration level) can be taken into account (feedback removal). An adaptive feedback controller is also implemented to avoid the use of a reference signal. In this case, a reference signal is obtained from the error signal using the internal model approach. The results from these three different control methods are compared in terms of the sound attenuation achieved. For broadband excitation, a feedforward adaptive control with an external reference is shown to be more efficient for this arrangement than a feedback adaptive control.

ISSN:0001-4966    

DOI:10.1121/1.423290

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A single number rating index to qualify the acoustical protection of a noise barrier that accounts for the influence of the ground is developed. This is an extension of a previous index for semi-infinite barrier developed by Pfretzschner [Pfretzschner et al., Acust. Acta Acust.82, 504–508 (1996)]. This index is based on the asymptotic value found for the insertion loss or the acoustic attenuation as physical magnitude, and it is uniquely dependent on the barrier, noise source, and acoustic characteristics of the ground in the source semi-space. The new index agrees well with measured data on a scale model under laboratory conditions, and a direct experimental measuring procedure could be envisaged. In this way the proposed index, which accounts for extrinsic properties (acoustical protection) of barriers, would complete the two previous indices dealing with intrinsic properties (transmission loss and acoustical absorption).

ISSN:0001-4966    

DOI:10.1121/1.423273

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Plant uncertainty is one of the major contributing factors that could affect the performance as well as stability of active noise control (ANC) systems. Plant uncertainty may be caused by either the errors in modeling, computation, and measurement, or the perturbations in physical conditions. These factors lead to deviations of the plant from the nominal model, which will in turn affect the robustness of the control system. In this paper, the effects due to changes in physical conditions on the ANC system are investigated. The analysis is carried out in terms of performance and robustness by using a general framework of theH∞robust control theory. Thesizeof plant uncertainty is estimated according to the infinity norm of the perturbations to physical conditions, which provides useful information for subsequent controller design that accommodates both performance and stability in an optimal and robust manner. The guidelines for designing the ANC systems with reference to plant uncertainties are also addressed.

ISSN:0001-4966    

DOI:10.1121/1.423274

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP