摘要:

Double-exposure holographic interferometry is applied to measure the surface displacements of ultrasonic transducers. Interferograms revealing detail surface displacements are produced using iron-doped lithium niobate crystals as the recording medium. Lead zirconate titanate disks poled in the longitudinal thickness mode possessing deposited, shaped electrodes are examined. Transducers with circular, strip, rectangular, and rosette electrode shapes are investigated. The displacement measurements reveal that the electrode “drags” the rest of the transducer surface. Thus the unmetallized portion of the transducer also contributes to the acoustic field. Pressure field calculations using the experimentally measured displacements are performed. It is shown that proper geometrical design of the electrode yields reduction of the sidelobes in the far field and smoothing of the maxima and minima in the near field, and improved focusing via reduced acoustic beam divergence. Most importantly, this work reveals that the unmetallized portion of the transducer contributes significantly to the displacement and to the resulting pressure fields.

ISSN:0001-4966    

DOI:10.1121/1.418008

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A one-dimensional material model of lead magnesium niobate (PMN) is presented. The model includes saturation phenomenology, but excludes hysteresis and dispersion. (Constant temperature is assumed.) It is shown that the strain can be taken to respond as an exactly quadratic function in the electric displacementDthroughout the saturation regionand yet still deduce the full observed response of the material, including flattening of the curve, when the strain is expressed as a function of electric fieldE. The model developed here is shown to be compatible with experimental measurements previously acquired by other researchers.

ISSN:0001-4966    

DOI:10.1121/1.418009

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Nonlinear transducer equations for the six major types of electroacoustic transducers (electrostatic, variable reluctance, moving coil, piezoelectric, electrostrictive, and magnetostrictive) are formulated in a common way and solved approximately by perturbation analysis for indirect drive, i.e., current drive of electric field transducers and voltage drive of magnetic field transducers. The direct drive analysis, i.e., voltage drive of electric field transducers and current drive of magnetic field transducers, was given in J. Acoust. Soc. Am.98, 1596–1611 (1995). Expressions and graphs are given for some of the indirect drive harmonics which apply to all six transducer types. The solutions for direct and indirect drive are compared, and some of the differences are discussed with emphasis on the question of which drive gives lower harmonic distortion. No answer to this question has been found that is applicable to all nonlinear mechanisms in all transducers at all frequencies. However, it is shown that for transducers of all types with low electrical loss operated near resonance direct drive usually gives lower second harmonics. The exceptions occur when mechanical springs are the dominant nonlinearity and the electromechanical coupling is very high; then indirect drive gives lower second harmonics. It is shown how the results, although based on simple lumped parameter models, can be applied in some cases to more complex transducer structures.

ISSN:0001-4966    

DOI:10.1121/1.418010

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Investigations into the field characteristics of an air-coupled ultrasonic capacitance transducer have been performed for a range of transducer configurations. The field of a 2-MHz bandwidth silicon backplate capacitance transducer has been scanned in air using a 1-mm-diam miniature detector at frequencies of up to 1.5 MHz. The radiated peak sound-pressure field is compared to theory based on a plane piston approach for various driving signals, namely pulsed and tone burst excitation. Aperture modifications, such as an annulus and a zone plate, have also been investigated and the devices have been shown to behave as predicted by theory.

ISSN:0001-4966    

DOI:10.1121/1.418011

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Any nonuniformity in the flatness of a crystal element used in an ultrasonic transducer may lead to significant variations in transducer response. An accurate estimation of such variations could obviously be a measure of the nonuniformity involved, whatever its extent may be. Studies are reported on the effect of nonparallelism between two major plane surfaces of the crystal element on transducer response by studying the normal displacement amplitude pattern and its distribution over one of the vibrating (plane) surfaces at resonance. Measurements are carried out in air using a phase-locked laser interferometry technique where the effect of tapering in disk thickness on the transducer performance is manifested through the variation in vibrational patterns. Results of these measurements are compared with those obtained from electrical (conductance) response to highlight the importance of the former. Other comparison measurements, such as the conventional method of determination of acoustic response and pulse waveforms of the radiated signals in water, are also reported as a function of tapering in disk thickness. A correlation is discussed between results obtained from different measurements.

ISSN:0001-4966    

DOI:10.1121/1.418012

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This paper presents a result on the measurements of longitudinal and Rayleigh wave velocities in a concrete plate using horizontally polarized conical transducers. The characteristics of horizontally polarized surface responses due to a point source applied at a half-space and a plate are analyzed and discussed. The analyses show that the horizontal component of a skimming longitudinal wavefront due to a point source is much larger than its vertical component. This suggests the utilization of a horizontal component of a surface response to detect the skimming longitudinal wavefront. In addition, theoretical analysis shows that by using the horizontal component to determine the Rayleigh wave velocity, the source-to-receiver distance can be reduced significantly. Homemade horizontally polarized conical transducers were fabricated to demonstrate the feasibility of the present method on measuring the dynamic concrete elastic constants. Experimental results on the measurements of the horizontal component of a surface response due to a point impact were in good agreement with the theoretical prediction. In conclusion, it is noted that, based on this proposed method, concrete dynamic elastic constants can be determined through a single transient elastic wave experiment.

ISSN:0001-4966    

DOI:10.1121/1.418423

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

An analysis is made of the problem of vibrations of an infinite beam on elastic foundation excited by a moving and vibrating mass. The solution is presented within the framework of a beam theory which includes the effects of shear deformation and rotatory inertia. An example is provided where the displacement is calculated.

ISSN:0001-4966    

DOI:10.1121/1.417978

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Discrete-frequency axial-flow fan noise reduction using active noise control is described. The unique aspect of the current research is the use of the fan itself as the antinoise source in the active noise control scheme. This is achieved by driving the entire fan unit axially with an electrodynamic shaker which mechanically couples the solid surfaces of the fan to the acoustic medium. The fan unit is thus transformed into a crude loudspeaker. A near-field microphone serves as an error sensor, where transfer function measurements between the electrical input to the shaker and the electrical output of the microphone are found to be reasonably free of phase distortions and linear. A feedforward algorithm utilizing the output of a tachometer as a reference signal is used. The experimental apparatus is composed of a baffled fan unit in a free field. A small cylindrical flow obstruction is placed on the inlet side of the fan to enhance noise emissions at the blade-pass frequency and harmonics. The experiment successfully demonstrates the concept of active control of tonal fan noise using a shaken fan as the cancellation source. For the fan operating in a planar baffle, the fundamental blade-passage frequency sound-pressure level at the location of the error sensor is reduced by 20 dB, while the second and third harmonic levels are reduced by 15 and 8 dB, respectively. Placing a cabinet enclosure over the baffled fan did not affect these results significantly, and free-field sound power measurements indicate similar level reductions with the active control in operation.

ISSN:0001-4966    

DOI:10.1121/1.417979

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The performance of the energy density control algorithm for controlling a broadband noise is evaluated in a one-dimensional enclosure. To avoid the noncausality problem of the control filter, which often happens in a frequency domain optimization, analyses presented in this paper are undertaken in the time domain. This approach provides the form of the causally constrained optimal controller. Numerical results are presented to predict the performance of the active noise control system, and indicate that improved global attenuation of the broadband noise can be achieved by minimizing the energy density, rather than the squared pressure. It is shown that minimizing the energy density at a single location yields global attenuation results that are comparable to minimizing the potential energy. Furthermore, unlike controlling the squared pressure, the energy density control does not demonstrate any dependence on the error sensor location for this one-dimensional field. A practical implementation of the energy-based control algorithm is presented. Results show that the energy density control algorithm can be implemented using the two sensor technique with a tolerable margin of performance degradation.

ISSN:0001-4966    

DOI:10.1121/1.417980

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A one-parameter model(ξ)for stop-and-go noise is developed. The physics of the phenomenon is based on the idea of thelinear density of radiated energy. The methodology ofξdetermination has been worked out and illustrated. The noise interaction with the surface of the ground is considered to be the only wave phenomenon that disturbs geometrical spreading. The model makes possible the calculation of the correction term in the time-average sound level,LAT,as an adjustment to the constant speed model of traffic noise.

ISSN:0001-4966    

DOI:10.1121/1.417981

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP