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The generation of sound by absorption of laser light in water is analyzed for the case of lasers pulsating at high repetition rates. It is shown that pulsating the laser at any arbitrary repetition rate is, in general, not a very effective way to produce a narrow-band signal. In order to produce a signal with a narrow frequency band, one should instead pulsate the laser at an optimum repetition rate that is determined by the optical frequency of the laser and, to a lesser extent, by the laser beam diameter. Expressions for the optimum repetition rate are derived from both a frequency domain analysis and a time domain analysis. It is found that, with the present laser technology, significant gain in signal-to-noise ratio can be achieved by pulsating the laser at its optimum repetition rate, as compared with the more conventional methods for generation of narrow-band signals, such as the method of laser intensity modulation. As a result, it now seems possible to generate continuous thermoacoustic highly collimated sound beams with high repetition pulsed lasers in such a way that these signals are easily detectable several kilometers away from the source. Spatially periodic laser deposition configurations on the water surface are also discussed, and it is shown that further improvement in signal-to-noise ratio is achievable, in principle, for a spatial periodicity tuned to the optimum temporal periodicity of the repetition rate of the pulsed laser. [ABSTRACT FROM AUTHOR]

It is shown that optical fibers offer a convenient way to control, both spatially and temporally, the laser light used to generate ultrasound in a solid. Proper phasing between the acoustic signals received at the transducer can enhance the ultrasonic signal in a particular direction. The principle is validated experimentally, with two optical fibers guiding the light emitted by a cw argon-ion laser. [ABSTRACT FROM AUTHOR]

A high-power laser is used to produce via thermal expansion a long thermoacoustic broadside array in water by deflecting a ruby-laser beam (optical wavelength 0.6943 μm) off a rotating mirror to a body of water. The rotation of the mirror results in a rectilinear motion of the source over the water surface. Such a source is commonly referred to as a moving thermoacoustic array (MTA) and its penetration depth into the water is inversely proportional to the coefficient of absorption of the laser intensity in water. The sound field radiated by the ruby MTA (long array) is analyzed experimentally for subsonic, sonic, and supersonic source velocities. The results obtained with the long ruby MTA are compared with those obtained with a much shorter glass-neodymium MTA (optical wavelength 1.06 μm). Results reported here include directivity patterns, both in a plane perpendicular to the water surface and in a plane parallel to the water surface, and sound-pressure level (SPL) dependence on distance between the source and the receiver. [ABSTRACT FROM AUTHOR]

The question is raised as to whether the analysis of the generation of sound by a laser beam moving over a water surface at the sound speed c for an interminable time period requires consideration of nonlinear effects. A principal consideration in this regard is whether the linear acoustics theory predicts a pressure waveform that is bounded in the asymptotic limit when the laser irradiation time is arbitrarily large. It is shown that a bounded asymptotic limit exists when the upper boundary condition corresponds (as is more nearly appropriate) to that of a pressure release surface, but not when it corresponds to that of a rigid surface. The asymptotic solution to the appropriate inhomogeneous wave equation is given exactly for the former case, and it is shown that the highest asymptotic amplitudes, given specified laser power and beam radius a, occur in the limit of a very small light absorption coefficient μ. In this limit, the peak amplitude is independent of μ and occurs at a depth of 0.88/μ. An approximate solution for the pressure waveform at intermediate times establishes that the characteristic time for buildup to the asymptotic limit is of the order of 2.5/(cμ2a). If this time is substantially shorter than the time that a plane-wave pulse with the asymptotic waveform would take to develop a shock wave, then accumulative nonlinear effects are of minor importance. [ABSTRACT FROM AUTHOR]

The generation of sound by a moving laser source is investigated both theoretically and experimentally. The analysis is restricted to sound waves generated exclusively through the thermal mechanism. The model is based on the impulse response of a thermoacoustic source as described in a previous paper [J. Acoust. Soc. Am. 78, 2074-2082 (1985)]. The results presented here include pressure waveforms, directivity patterns, sound level dependence on source velocity, and spreading curves for subsonic, transonic, and supersonic source velocities. In general, experimental results are in good agreement with theoretical predictions. [ABSTRACT FROM AUTHOR]

When an intensity-modulated laser beam illuminates a column of water, it produces, via thermal expansion, a highly directive sound beam. The pressure field radiated by such a thermoacoustic source is analyzed, first by determining the impulse response of the system. The thermoacoustic pressure is then expressed as a convolution between the impulse response and the time derivative of the laser intensity. The analysis is valid in both the nearfield and the farfield of the source and, in principle, for any spatial or temporal distribution of the laser intensity. This approach provides valuable insight into the nearfield of this type of transducer. Resulting theoretical predictions of pressure waveforms and directivity patterns compare well to experimental results, and agree in the farfield limit with existing farfield analyses of thermoacoustic radiation. [ABSTRACT FROM AUTHOR]

Cellular polypropylene polymer foams, also known as ferroelectrets, are compelling candidates for air-coupled ultrasonic transducer materials because of their excellent acoustic impedance match to air and because they have a piezoelectric d33 coefficient superior to that of PVDF. This study investigates the performance of ferroelectret transducers in the generation and reception of ultrasonic waves in air. As previous studies have noted, the piezoelectric coupling coefficients of these foams depend on the number, size, and distribution of charged voids in the microstructure. The present work studies the influence of these parameters both theoretically and experimentally. First, a three-dimensional model is employed to explain the variation of piezoelectric coupling coefficients, elastic stiffness, and dielectric permittivity as a function of void fraction based on void-scale physics and void geometry. Laser Doppler vibrometer (LDV) measurements of the effective d33 coefficient of a specially fabricated prototype transmitting transducer are then shown which clearly indicate that the charged voids in the ferroelectret material are randomly distributed in the plane of the foam. The frequency-dependent dynamic d33 coefficient is then reported from 50 to 500 kHz for different excitation voltages and shown to be largely insensitive to drive voltage. Lastly, two ferroelectret transducers are operated in transmit-receive mode and the received signal is shown to accurately represent the corresponding signal generated by the transmitting transducer as measured using LDV. [ABSTRACT FROM PUBLISHER]

A foreword to the July 2006 issue of the journal is presented.

A theoretical and experimental study of moving thermoacoustic sources is presented in this paper. The inhomogeneous linear wave equation for a fluid containing heat sources [P. J. Westervelt and R. S. Larson, J. Acoust. Soc. Am. 54, 121-122 (1973)] is used to derive the acoustic response h(t) of a medium excited by an impulse of light. The pressure radiated by a laser source moving at constant speed over a water surface is then described in terms of a convolution between the impulse response h(t) and the optoacoustic source strength which is proportional to dI/dt, where I(t) represents the laser intensity. In some limiting cases of farfield radiation, the computed pressure waveforms can be compared with previous work [L. M. Lyamshev and L. V. Sedov, Sov. Phys. Acoust. 27(1), 4-18 (1981).] An experimental study is also in progress. The motion of the laser beam over the water surface at velocities up to Mach 2 is achieved by means of a rotating mirror. The laser is excited in the conventional non-Q-switched mode, with a pulse duration of 1 ms. [Work supported by ONR.] [ABSTRACT FROM AUTHOR]

Abstract: The aim of this paper is to characterize the static and dynamic response of a bistable–bidirectional microactuator. The bidirectional mechanism is generated from the interaction between the CoNiMnP magnet and the Au microcoil. The bistable mechanism is generated from the interaction between the CoNiMnP magnet and the soft magnetic (NiFe) base. The microactuator is entirely fabricated by surface micromachining on top of a single silicon wafer. The overall diameter (D overall) of the microactuator is 1600μm. The overall height (H overall) is approximately 600μm, including the thickness of the silicon wafer. A Laser Doppler Vibrometer (LDV) is used to test the microactuator. Test results show that the latching mechanism occurs at the membrane minimum displacement (d min), −21μm. An operational current (I coil) and power (P coil) as low as 13.9mA and 1.39mW are required to perform this latching mechanism. The membrane maximum displacement (d max) is found to be 29μm; it is achieved at I coil =0.13A and P coil =130mW. There are two different latching times observed from the experiments, a static latching time (t latch_st) and a dynamic latching time (t latch_dy). The t latch_st is found to be 2.5ms. The t latch_dy varies with I coil and the smallest observed value is 0.18ms. [Copyright &y& Elsevier]

Abstract: This paper presents our work in the bistable electromagnetic actuated microvalve. The microvalve is entirely fabricated by surface micromachining on top of a single Si (silicon) substrate. The microvalve has an overall diameter of 1600μm and the overall height of 600μm, including the wafer thickness. The bistable mechanism is achieved by integrating an electroplated micro CoNiMnP magnet on the membrane. The microvalve was tested in the flow of deionized (DI) water at 0–50μL/min. The latching/unlatching of the microvalve was performed to control the flow DI water at 30μL/min, it required the operational current of 0.38A and the power of 1.17W. The latching/unlatching response time of the microvalve is 10ms, with the leaking rate of 0.16–0.8μL/min. [Copyright &y& Elsevier]

Abstract: The dynamic response of two electromagnetically actuated microvalves operating in open-air conditions are measured experimentally and simulated using a three-dimensional fluid–structural finite element model. Open-air conditions mean that the fluid inlet and outlet are not pressurized. The dynamic response of the membrane is obtained experimentally by exciting the valve with a step voltage signal of 1kHz and measuring the membrane vertical displacement using a Polytec Laser Doppler Vibrometer (LDV) system. Vibration analysis of the experimental data provides dynamic parameters such as natural frequency, air damping coefficient, spring constant, and settling time of the membrane. An electromagnetic force model based on the reluctance method is constructed and validated from the experimental data. The validated electromagnetic force model and corrected material properties are then used in a three-dimensional, fluid–structural finite element model to simulate membrane dynamics. Pertinent dynamic parameters such as resonance frequency, spring constant, microvalve closing time, settling time of the membrane, and actuation energy of the microvalve are finally compared with the simulated results. The comparison of experimental and simulation results shows that the finite element model accurately reproduces the dynamics of the membrane in the slip-flow region. A valid simulation method can then be used to simulate microvalve dynamic response in pressurized flow conditions and evaluate new designs. Valve closing time of less than 150μs is demonstrated in one valve design. The energy required to close the microvalve is in the range of 300–678μJ. [Copyright &y& Elsevier]

This section discusses developments in the field of acoustics in 1988. The idea of using focused shock waves to destroy kidney stones was developed by scientists at the University of Saarbrucken in West Germany. The concept is called extracorporeal lithotripsy because no part of the apparatus is inside the patient's body. A more effective technique of radiation reduction is based on directing the waves radiated by the primary source into wave sinks created by secondary sources located in the close vicinity of the primary source. This principle can be understood by comparing the radiation of a monopole and dipole source. Two contemporary condensed matter problems which have been addressed with acoustic systems are inelastic effects in Anderson localization and the properties of quasicrystals. One possibility for communicating from above a water surface to distant points deep within the water involves the use of laser-generated sound. Laser light of time-varying intensity, while traversing and interacting with the water near the surface, can produce sound waves which then propagate within the water.