摘要:

At frequencies where the thickness of an elastic plate is more than a wavelength, the propagation of the two lowest Rayleigh–Lamb modes in an elastic plate can be viewed as the propagation of a Rayleigh surface wave over two weakly coupled, surface-wave waveguides. That is, a Rayleigh wave launched on one surface gradually transfers to the other and then back. It does so in a length we call the beatlength. Measurements of the beatlength for brass plates are reported as a function of frequency and thickness. This phenomenon is readily excited and persists over a wide range of thicknesses and frequencies

ISSN:0001-4966    

DOI:10.1121/1.419554

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This paper presents direct field measurements of acoustic modes guided by the interface between two transparent solids. The measurement technique is based on the acousto-optical interaction inside the solid between the acoustic field and the probe laser beam of an interferometer. The main advantage of the method is its ability to measure acoustic strain fields in areas of difficult access with the classic detection methods. Moreover, it gives complete information about the dilatation strain field inside the solid, e.g., amplitude and phase. The propagation of a real velocity mode (Stoneley wave) is first illustrated. Then the situation of complex velocity modes is investigated for a Plexiglas–fused quartz slip interface. This material combination supports two possible interface modes theoretically. These modes are simultaneously observed and the differences between their behavior are measured.

ISSN:0001-4966    

DOI:10.1121/1.420066

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

New methods to experimentally determine the electroelastic coefficients of radially polarized piezoelectric ceramic cylindrical shells (RPPCCSs) of arbitrary length to radius ratio are presented. The real parts ofε33T,s11E,s12E,andg31are explicitly expressed in terms of quantities which can be easily measured. The quantities can be obtained from the measured frequency-dependent input electrical admittances (IEAs) of RPPCCSs with free ends. The theoretical IEA based on the two-dimensional equations of state and the membrane approximation is used to derive the explicit expressions. Some methods cannot be used when the resonance frequencies, which are required as inputs, cannot be identified. This occurs when there are several resonances in a narrow frequency band or losses are high. However, the other methods can be used. The methods are validated by using simulated data. The required input data are computed using the theoretical expression for the IEA and an assumed set of complex electroelastic coefficients. It is then shown that the real parts of the coefficients of RPPCCSs with a small or large length to radius ratio and low or high mechanical losses can be accurately determined by using the input data. The effect of measurement errors on the overall accuracy of the experimentally determined values is also discussed. The methods are further validated by the good agreement over a wide frequency range between measured IEAs and the IEAs computed using the experimentally determined values of coefficients.

ISSN:0001-4966    

DOI:10.1121/1.420067

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

An eigenfunction approach is presented to determine the response of radially polarized piezoelectric cylindrical shells of finite length to electrical excitation. The equations of motion of the piezoelectric cylinder are first derived by using the membrane approximation. Then they are solved by expressing the displacement distribution as the sum of the static solution and a weighted sum of a complete set of functions—the eigenfunctions of the short-circuited cylinder. The static solution is necessary to exactly satisfy the boundary conditions of the electrically excited cylinder. Moreover, in some cases, the series solution does not converge to the correct displacement distribution when some other terms, instead of the static solution, are used to satisfy the boundary conditions. The weights or displacement coefficients are determined by using the orthogonal property of the eigenfunctions. Finally, the input electrical admittance is determined by using the equation of state and the displacement distribution. Analyses of cylinders with various boundary conditions are presented to illustrate the approach. Numerical results are used to show that the series solutions converge very rapidly to the closed form solutions.

ISSN:0001-4966    

DOI:10.1121/1.420068

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The use of such materials as PVDF and piezorubber in making smart acoustically active surfaces is discussed and demonstrated. By using one layer as a sensor and applying the signal detected in this layer through an amplifier and filter to a second driving layer it is possible to control either the reflection or transmission of plane waves striking the surface normally. By using two driving layers, both transmission and reflection can be simultaneously controlled. When transmissions are eliminated the control of reflection is independent of changes in backing impedance. By adding a second sensing layer along with two driving layers, reflections and radiation from a vibrating surface can be eliminated simultaneously. Stability problems involved in using pressure and/or velocity sensors in various control algorithms are discussed.

ISSN:0001-4966    

DOI:10.1121/1.420102

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

As airborne, correlation-based, ultrasonic sensors are becoming more widely used for robotic applications, it becomes increasingly important to have a good understanding of how the transducer’s filtering properties affect the performance of the sensor system. Using well-known results from acoustics, a simple, yet accurate, filtermodel for the Polaroid transducer is described. In the course of this derivation the reason for the accuracy of the often used “moving piston” approximation is also clarified. This filtermodel is then used to analyze the characteristic “peak-doubling” from which the correlation based range sensor used in the tri-aural sensor suffers. The predicted errors are compared with actual measurements by the real sensor system and are found to be in good agreement. Finally, the use of broad beamwidth transducers to avoid these errors is proposed, and it is and argued that this approach has some distinct advantages compared to alternative solutions.

ISSN:0001-4966    

DOI:10.1121/1.420069

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

An actuator—sensor tile has been developed for use in studies examining the issues involved in actively controlling the reflection and transmission characteristics of generic underwater structures. The tile is 25 cm square and contains a full area actuator, acoustic pressure sensor, and (acoustic particle) velocity sensor. This paper presents the predictive models used in the tile design. Issues addressed include transducer performance, spatial sampling, near-field sensing, internal resonances, and both direct and extraneous coupling mechanisms, all of which can contribute to complicate the system transfer functions. The acoustic characteristics of the final tile were evaluated at the NRL Laboratory for Structural Acoustics (LSA) and these results are compared with predictions.

ISSN:0001-4966    

DOI:10.1121/1.420103

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The nonlinear effects causing distortion in condenser microphones are investigated and modeled with special attention paid to microphones made with silicon micromachining technology. A model of the distortion, based on the quasistatic properties, is described and solved numerically using finite differences. The results of the model show that the distortion is mainly caused by both the inverse relation between the microphone capacitance and the deflection of the diaphragm, as well as the nonlinear electrostatic forces generated by the applied dc bias voltage. The results of the model are compared with measurement results on a silicon/polyimide microphone, and good agreement is found.

ISSN:0001-4966    

DOI:10.1121/1.420070

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A new strategy based on the direct use of strain sensing is proposed for the minimization of the acoustic radiation from a vibrating plate. This strategy uses active structural acoustic control because it eliminates the need for far-field acoustic sensors. The cost function is defined as the radiated acoustic power and the discrete strain information is given directly, for example, by fiber optic sensors embedded in the structure. Two approaches are presented to express the cost function in terms of strain information. The first one is based on plate’s displacement reconstruction from the strain field while the second is based on integration by parts of the wave-number transform of the displacement. Proper concentration of the control effort is obtained with the cost function defined in the wave-number domain. Both approaches are first validated with respect to an analytical solution for the calculation of the radiated acoustic power from structural strain information. It is shown that good agreement is obtained with at least two strain sensors per structural wavelength. Optimal control of acoustic radiation from a simply supported plate is then performed using different configurations of sensors and actuators. The control is performed using simulated piezoelectric actuators and results show that both approaches are well suited for active control applications as they offer good performance over a wide frequency range while requiring only few sensors.

ISSN:0001-4966    

DOI:10.1121/1.420071

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The free vibration analysis of symmetrically laminated thick rectangular plates is examined. Thep-Ritz method is employed in which sets of uniquely defined polynomials are used as the admissible trial displacement and rotation functions. The energy integral expressions of the laminates are derived by incorporating the shear deformation using Reddy’s higher-order plate theory [J. Appl. Mech. ASME51, 745–752 (1984)]. The formulation is basically applicable to rectangular laminates with any combination of free, simply supported, and clamped boundary conditions. To evaluate the validity and to demonstrate the applicability of the proposed method, a series of free vibration analyses of laminated composite plates is reported. Wherever possible, the accuracy of this analysis is validated through comparison with available results. Efforts are made to interpret the results to provide physical insight to the problem.

ISSN:0001-4966    

DOI:10.1121/1.420072

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP