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The midfrequency enhancement phenomenon for tone burst backscattering by thin spherical shells in water has been discussed by several investigators. In their works, it is found that the earliest elastic tone burst echo is enhanced relative to the specular reflection, and this enhancement is mainly due to the lowest subsonic antisymmetric Lamb wave. In this paper, the tone burst backscattering obtained from the convolution integral of the incident tone burst and the impulse response by a submerged spherical shell is investigated to display the midfrequency enhancement. The modified ray approximations are used to calculate the echo contributions from different Lamb waves. The numerical results show that the ratio of a∕h and the dimensionless echo delay have nearly linear relationship with the frequency of greatest enhancement. Based on this property, two linear approximate equations are formulated to evaluate the radius and thickness of a thin spherical shell. A simple method is developed to estimate the frequency of greatest enhancement and the corresponding echo delay from a short tone burst echo with a higher carrier frequency. The evaluated results show that the present method is effective on determination of the radius and thickness of a thin spherical shell in water.

Inverse scattering problems for identifying underwater objects are of importance for all kinds of applications in seas that occupy more than two-thirds of the surface of the earth. The physical optics approximation has been applied to the acoustic scattering integral representation, yielding an inverse scattering equation which relates the profile function of a submerged object to the Fourier transform of the complex backscattered acoustic field. This technique is the so-called ramp response technique and usually requires the full scattering information for all frequencies to obtain accurate profile function of an object. In order to retrieve the shape information of an object from the band-limited backscattered frequency data, a Fredholm integral equation is formulated in this paper for calculating the ramp response of the submerged object. This Fredholm integral equation is for the unknown ramp response of an object in terms of the known incomplete scattering information. The kernel of this integral equation is the inverse Fourier transform of the known characteristic function of the scattering information aperture. In order to simulate the real experiment environment, a computer-generated Gauss noise is added directly to the simulated impulse response. This noise-contaminated impulse response is then used for the inverse identification of the underwater object. It is found that adding noise would lead to the ill-posedness of this inverse scattering problem. To deal with the ill-posedness, the regularization method by filtering is employed. Numerical examples have confirmed that the method presented in this paper can successfully reconstruct the shape of an underwater object using an incomplete set of backscattered frequency data with or without noise contamination. Meanwhile, the effect of bandwidth to the ramp response is also discussed in the numerical examples.

This paper presents time-domain analysis and synthesis of active low-frequency noise control systems in rigid-walled ducts. In contrast to frequency-domain analysis, time-domain modeling provides a direct physical insight of the noise cancellation mechanism, where noise signals are assumed to be arbitrary real functions of time instead of harmonics. The time-domain analysis is based on a state-space model which is derived from the governing wave equation using combined numerical (finite difference) and analytical techniques. Optimal feedback control laws are synthesized for a variety of active noise control systems with different sensor and source arrangements. The responses of closed-loop systems are simulated, and the results are compatible with those of transfer function synthesis. In addition, the active control systems so designed have the potential of providing global, broadband, and optimal noise reductions without running into the annoying feedback problems in most feedforward control systems.

The work presented in this paper regards modeling, synthesis, and simulation of an active acoustic field control system under state‐space feedback control approach. The active system is supposed to absorb an incident sound energy so that the reflection off an acoustic boundary is diminished. To illustrate the concept, a simple model describing transient sound reflection and transmission in an unbounded, one‐dimensional space is derived in a state‐space form. The distributed parameter system is approximated with a finite dimensional model using a standard discretization technique with special boundary treatments. Based on the state‐space model, a feasible feedback control algorithm is synthesized using optimal control theory (LQG). Numerical simulation of the closed‐loop response of an active control system including two sensors and one actuator is presented, which shows a broadband reflection (echo) reduction.

An active acoustic coating is presented for layering over objects in a fluid medium. The design of the active coating prevents an incident sound wave from reflecting off the acoustic boundary of a submerged object. The acoustic energy associated with the incident disturbance is absorbed out of the medium through a piezocomposite device in the coating and dissipated by internal electrical means. The components encapsulated inside the elastomer coating are a piezocomposite actuator and piezoelectric polymer sensors. The sensor arrangement detects and separates the acoustic field into a signal in terms of the incident acoustic pressure wave. The signal is manipulated through an electronic network to generate an appropriate driving voltage. This output drives a thickness extensional mode piezocomposite transducer (actuator) with a linear velocity profile over a broad frequency band. The actuator excitation results in a dynamic impedance matching with the incident disturbance such that the coating appears as a...

In an isotropic, noncentrosymmetric elastic solid, the displacement field u has to be supplemented by the independent microrotation field φ. In this mirror‐asymmetric (or chiral) medium, six different wave numbers are possible: Of these, two represent longitudinal fields, and the remaining four are (circularly) handed. This paper is concerned with the scattering of elastic waves by a chiral sphere, and the effect of the chiral parameter on its scattering response.

In an isotropic, noncentrosymmetric (chiral) elastic solid, the displacement field u has to be supplemented by the independent microrotation field φ. Since such a medium is mirror asymmetric or chiral, six different wavenumbers are possible: of these, two represent longitudinal fields, and the remaining four are circularly polarized. This paper is concerned with the scattering of elastic plane waves by a planar interface between an achiral elastic solid half‐space and a chiral elastic solid half‐space.

In determining the velocity and attenuation of suspensions of spherical particles, results obtained from the computationally simpler multiple scattering approach of Waterman and Truell (WTF) and from a multiple scattering formalism (MSF), which considers the pair correlation function, have been compared in order to justify the range and applicability of the former method. Although the difference between the results increases as the concentration of suspensions increases, the frequency‐dependent attenuation computed from the two approaches can be quite different particularly in the low‐frequency range even at lower concentrations. Due to the unavailability of experimental data for ultrasonic propagation, data from analogous optical experiments on suspensions of latex spheres are compared. The MSF shows excellent agreement, whereas the WTF shows large discrepancies from the experimental data.

To increase levels of sound attenuation, it is often necessary to utilize active acoustic control methods. A complete active acoustic control system typically consists of sensing devices, an electronic control circuit, and an actuating device. Each component is designed to cause destructive interference of the impressed acoustic pressure field. A piezocomposite actuator is presented for active underwater attenuation control of an impressed acoustic field. The actuator is composed of bilaminate plates for absorption of both the reflection and the transmission of normally incident acoustic waves independent of the backing impedance. An electromechanical model of the bilaminate piezocomposite design with an analytical discussion is presented. An illustrated numerical example and discussion of system implications are included.

The integration of surface acoustic waves (SAW), microelectromechanical systems (MEMS), and required microelectronics and conformal antennas to realize programmable microsensors suitable for many engineering and biomedical applications will be reviewed in this talk. This unique combination of technologies results in novel conformal sensors that can be remotely sensed by a wireless communication system with the advantage of no power requirements at the sensor site (passive sensor). The required features in many of these applications are high precision, wide dynamic range, and wide frequency range. The MEMS‐SAW‐based devices presented possess typical advantages of MEMS sensors including the additional benefits of robustness, excellent sensitivity, surface conformability, and durability. After a brief overview of SAW sensors and MEMS, the paper is focused on the design and fabrication of MEMS devices for engineering and medical applications, for example: accelerometer and gyroscopes for automobile, inertial navigation; sensors for pressure, torque, temperature, and strain; health monitoring of structures; ‘‘smart tongue and electronic nose;’’ micropump for drug delivery; humidity, temperature, and ethlyne oxide sensor for hospital sterilization system; microsensors for monitoring bone healing; and pollution and chemical sensing.

Smart structures incorporate sensors, actuators, and control electronics that permit the structure to tailor its response to changes in the environment in an optimal fashion. The sensors and actuators are constructed using functional materials such as piezoelectric materials. Finite‐element modeling has been successfully used to model these complex structures. More recently, closed‐loop numerical simulation of the tailored response of a smart structure has become possible by combining the finite‐element equations of the sensor response to applied dynamical loads to the input voltage or current to the actuators via a control algorithm. Optimization of sensor/actuator placement as well as optimization of the control gains will be discussed. Applications to control of cabin noise are studied. Constrained layer damping, incorporating active passive damping, can also be simulated. The talk will present an overview of the approach in relation to others in the extant literature.

Underwater acoustical imaging techniques and the inverse analysis of acoustic scattering problems have now found many important engineering applications. Most of the inverse techniques use the backscattering signals in the acoustical far field to retrieve the shape and size information of an underwater object, such as the ramp response technique. This paper addresses a modified ramp response technique, which could be used to reconstruct the 3D image of an object for the bistatic case. This technique shows that the bistatic ramp response is proportional to the profile function of an underwater object based on the small bistatic angle assumption. The numerical examples demonstrate that the bistatic ramp response technique is still valid to obtain an excellent profile function even for the bistatic case with a fairly large bistatic angle. The numerical results also suggest that, if the object is of a more slender shape, then its bistatic ramp response will be closer to the exact profile function for a larger bistatic angle. Finally, a 3D image of a spheroid with a/b = 5 has been reconstructed using the bistatic ramp response signals in three incident directions. This bistatic ramp response technique allows us to reconstruct the 3D image of an underwater object with only one receiver.

The inverse problems in the area of the acoustic scattering often concern the determination of the size, shape, and orientation of an object using the scattered field data. This paper presents a method to retrieve the shape information of an underwater object using illuminated lengths, which can be obtained from the ramp response signatures of the object. An ellipsoidal object submerged in water is considered. Both the low and high backscattered frequency data have been employed to calculate the illuminated lengths. The calculated results show that the illuminated lengths will be more accurate, if only the high-frequency-range data are employed. For ellipsoidal objects, any three illuminated lengths that are not of a same plane can in theory fully determine the shape of the ellipsoid. As the calculated illuminated lengths contain numerical errors, the calculated results of the three semiaxes of the ellipsoid will deteriorate and become unreliable, especially when the three incident directions of the illuminated lengths become close. The reason is that the condition number of the coefficient matrix becomes big in such situations, which leads to an increase of the relative error upper limit in the calculated results. To avoid such errors in close incident wave cases, it is found that the use of more than three incident waves works very well in the shape identification of an underwater object.

This paper presents the design and experimental evaluation of a gyroscope based on differential design of a surface acoustic wave (SAW) resonator and a sensor on a piezoelectric substrate. The resonator is divided into two halves, one with metallic dot arrays and other without any metallic dots. Standing waves are formed inside both cavities. The particles at the antinodes of the standing wave pattern experience large amplitude of vibration that serves as the reference vibrating motion for this gyroscope. The metallic dots strategically positioned at the antinode locations experience Coriolis force due to rotation and acoustically amplify the magnitude in the orthogonal direction. However, the other half of the resonator cannot generate any secondary SAW. A wideband SAW sensor arranged orthogonal to the SAW resonator picks up these two secondary SAWs and is fed to a differential amplifier. Any drift in the gyroscope signal can be completely eliminated by this novel design. The performance of this 74.2‐MHz gyroscope shows very high sensitivity and dynamic range, which is ideal for many commercial applications. Unlike other MEMS gyroscopes, this gyroscope has a planar configuration with no suspended resonating mechanical structures, thereby being inherently robust and shock resistant.

The design and experimental evaluation of a PVDF‐based MEMS hydrophone is presented in this paper. The basic structure of the hydrophone was fabricated on a silicon wafer using standard NMOS process technology. A MOSFET with extended gate electrode was designed as the interface circuit to the sensing material, which is a piezoelectric polymer, polyvinylidene difluoride (PVDF). Acoustic impedance possessed by this piezoelectric material provides a reasonable match to water, which makes it very attractive for underwater applications. The electrical signal generated by the PVDF film was directly coupled to the gate of the MOSFET. To minimize the parasitic capacitance underneath the PVDF film and hence improve the device sensitivity, a thick photoresist was first employed as the dielectric layer under the extended gate electrode. For underwater operation, a waterproof Rho‐C rubber encapsulated the hydrophone. A silicon nitride layer passivated the active device, which is a good barrier material to most mobile ions and solvents. The device after passivation also shows a lower noise level. The theoretical model developed to predict the sensitivity of the hydrophone shows a reasonable agreement between the theory and the experiment.

This paper presents the design and development of a simple and robust control system for structural acoustics based on state space control theory. The controller was developed using model Hankel singular value (MHSV)‐based model reduction, and analog controllers are fabricated. MHSV is developed for a model coordinate system describing vibrating structures, and analog controllers were designed from transfer functions of controllers, which have great advantage over digital controllers in terms of simplicity and cost. This controller design for structural acoustics is applied for controlling sound transmission and reflection. MHSV shows good model reduction of finite‐element‐based structure models. The feasibility of this controller for the underwater active reflection cancellation is also studied. 1‐3 piezocomposite material is used as a transducer that generates the acoustic signal. Its electroacoustic model is developed using the Mason circuit and the control parameters are directly derived. The controller has been designed accordingly and its performance is evaluated using pulse tube. The simulation and measurement show the possibility of achieving a very good reflection reduction in a desired frequency band.

The fabrication of low‐cost, high‐sensitivity, and miniaturized hydrophone array is highly desirable for application in sonar technology. However, as hydrophone size decreases, it becomes necessary to provide an amplifier or buffer in close proximity to avoid sensitivity loss due to interconnect capacitance. This suggests the concept of hydrpophone integration with microelectronics in an integrated circuit environment. A proptotype of an integrated transducer was thus realized in which a P(VDF‐TrFE) film was bonded to the extended gate of a metal–oxide–semiconductor field‐effect transistor (MOSFET) which was fabricated on a silicon wafer using standard NMOS process technology. This technique is compatible with IC fabrication processes and enables the integration of active elements with the transducer element on the same chip. The integrated structure eliminates long leads between the transducer and the amplifier and makes the net transducer–amplifier detectivity superior to the ceramic transducer–amplifier system. The devices are then encapsulated with Rho‐C rubber which results in good acoustic impedance to that of water. The acoustic sensing performance was carried out in a pulse tube over a frequency range from 4 to 10 kHz. The experimental results along with the theoretical prediction by Mason equivalent circuit will be presented.

Ice formation on an aircraft can cause serious safety problems, and hence, there is a real need for ice detection and monitoring on aircrafts. It is well known that acoustic wave devices offer a simple and inexpensive technique for a number of sensing applications. In this paper, we present a wireless ice detection and monitoring system using a surface acoustic wave (SAW) device utilizing shear horizontal (SH) waves as the sensing mechanism. The SAW device is designed and fabricated on a quartz substrate. Silicon dioxide is then deposited to serve as a guiding layer for SH waves. The acoustic wave concentrated in the guiding layer is highly sensitive to surface perturbations. The experimental setup used to measure the effect of ice formation is by an oscillator setup. The SAW device is designed to oscillate at a particular frequency and any fractional change in wave velocity due to surface perturbations leads to a change in oscillation frequency. It is observed that phase transformation from water to ice causes a shift in oscillator frequency of 2 MHz. The ice sensor can be operated wirelessly by connecting the SAW sensor to a stand‐alone oscillator and the oscillator can communicate with a remote monitoring system using a microstrip antenna.

Active underwater acoustic cancellation is investigated experimentally in a water‐filled pulse tube using a pressure sensor and a piezo‐composite actuator. The pressure sensor is made of thin 1‐3 piezo‐composite disk encapsulated in acoustic transparent rubber and put right in front of the actuator. A 20‐dB echo reduction to harmonic incident waves in 5–10 kHz is achieved experimentally by directly phase‐shifting the sensor signal and amplifying it to drive the canceling actuator. A robust analog controller is also developed based on the experimentally identified acoustic reflection coefficient of the piezo‐composite transducer with steel backing and the transfer function from piezo‐composite transducer to the pressure sensor. Issues such as robustness and stability of the analog controller are also discussed.

In applications such as ultrasonic immersion testing, damping of flow noise in pipes and ducts and coating of acoustic control surfaces, it is very useful to reduce the reflection of acoustic waves from surfaces. Significant echo reduction can be achieved by addition of an anechoic coating. According to some general design guidelines for a single layer of homogenous coating, a single layer of 15 dB coating must be at least one wavelength thick. For absorbing acoustic waves of low frequencies, inhomogeneities such as metallic fillers, cavities and irregular metallic helices can be introduced into the lossy matrix to substantially enhance the acoustic absorption by acoustic scattering. In this presentation, the acoustic scattering of a single metallic spring embedded in an infinite elastic medium has been investigated using a boundary element formulation. The cross sections of the scattered longitudinal and shear waves have been calculated for various spring sizes, orientations and wave numbers. It will be ...

The use of piezoelectric materials in controlling the vibration of continuous structures has grown significantly in recent years. A number of studies using Finite Element (FE) method [1, 2, 3, 4] have been made on noise transmission studies for rectangular enclosures through flexible smart panels. In these models, the host plate, actuators and sensors are modeled using 2D and 3D elements, which are later coupled, to the cavity in which the pressure is expressed in terms of rigid cavity modes. Although these earlier FE models predict the behavior of the structural panel and the fluid-structure interaction accurately at low frequencies, at high frequencies the size of the model increases resulting in very long computational time. Further, optimal sensor/actuator placement studies, involve repeated FE remeshing during the iterations, hence a simple model like the RR approach is preferred. The potential and kinetic energies of the panel with surface bonded discrete piezoelectric patches are estimated and the equations of motion for the smart panel are derived using Hamilton’s principle. The electric potential inside the piezoelectric patches are assumed to be a quadratic function of thickness coordinate. Classical laminated plate theory is used for modeling the host plate and the electroelastic theory is used to model the surface bonded patches. In electroelastic theory, reduced charge equation is satisfied inside both sensor and actuator patches. For the acoustic enclosure, the cavity pressure is expressed in terms of rigid cavity modes [5]. For the numerical study and to validate the RR approach, the frequencies obtained using RR approach are compared with the FE results for a smart aluminum plate backed cubic cavity.

Classical laminated plate theory (CLT) has been applied successfully in the past to laminates with discrete piezoelectric patches bonded to the surface or embedded within the layers (1–7). The basic assumptions made in the earlier models were that the strains inside the patches are assumed to be constant and hence the presence of the sensor and actuator patches were neglected while modeling the dynamic properties of the laminate. The validity of these assumptions, the effect of the size of the patches and these assumptions on the solutions obtained, has not been studied. In this paper, the CLT is applied to a laminate with surface‐bonded piezoelectric patches without the above‐mentioned assumptions. A detailed modeling of the patches is developed by expressing the electric potential inside the patch as a quadratic function of thickness coordinate. The equations of motion are derived for a generally orthotropic laminate and solution method for these equations. Analytical solutions are obtained for a plate bonded with one and five collocated piezoelectric actuator/sensor patches. The effect of the passive and active stiffness of the surface bonded actuator and sensor patches on the dynamic characteristics of host plate structure is studied.

This paper presents the design, fabrication, experimental results, and theoretical validation of a Love wave surface acoustic wave sensor for detecting the phase change from liquid water to solid ice. The sensing of this phase transition is due to the shear horizontal nature of Love waves which couple to a solid (ice) but not to a liquid (water). An SiO2 film of thickness 3.2 μm deposited on an ST cut quartz wafer via plasma‐enhanced chemical‐vapor deposition acts as the guiding layer for Love waves. Testing is carried out with the water or ice placed directly in the propagation path of Love waves. An oscillation frequency shift of 2 MHz is observed when water on the sensor is frozen and melted cyclically. The contribution to the frequency shift is explained in terms of the acousto‐electric effect (high permittivity and conductivity of water relative to ice), mass loading, and elastic film formation (solid ice). An arrangement for wireless interrogation of the sensor is proposed which is particularly attractive for aircraft and rotorcraft applications obviating the need for complex wiring and local power sources. t

Control of noise in an enclosure when the noise source is exterior to the enclosure has applications to rooms, automobiles, helicopters, airplanes, and other vehicles. Reducing the interior noise to acceptable levels is necessary for safety and comfort. One approach to control of interior noise is through vibration suppression of the trim panels that often line the interior of such enclosures. It is ultimately these trim panels that reradiate the noise to the interior. In order to actively control the vibrations of the trim panels, using discrete piezoelectric sensors and actuators, control electronics are needed. Adaptive digital control, while being efficient and powerful, also requires the presence of a computer and DSP boards. There may also be frequency limitations posed by the speed of available DSP boards. An alternative approach is to design compact and inexpensive analog controller units, which also offers the possibility of having such controllers designed as ASIC chips. To demonstrate the efficacy of this approach, results will be presented for the control of noise in a cabin with multiple trim panels when excited by broadband noise from the exterior using MEMS vibration sensors and PZT actuators.

This paper presents a discussion of multimode optical fibers used for acoustic wave (2–10 MHz) propagation. The optical fiber acts as a connecting rod between two piezoelectric transducers (PZT) that are attached at each end (dumbbell arrangement). This device is then completed by encapsulation of a solid epoxy or a strengthened composite. One piezoceramic is excited and generates an elastic wave to propagate along the optical fiber and into the other piezoceramic. An alteration of the steady‐state signal thus is indicative of an impact and/or random pressure disturbances upon the composite. Additionally, environmental temperature changes are also detectable. Experimental measurement and related theories are demonstrated.

This paper introduces a new method for three‐dimensional active sound‐field cancellation. Analytical considerations include the problem of an incident plane wave interacting upon a rigid sphere as the scattering object. An array with elements consisting of a set of rings perpendicular to the sphere's pole axis is arranged on the surface of the sphere. This discussion will present an analytical technique for cancellation of the scattered sound field to arbitrary extent over either the whole space or the partial area of interest, as long as the number of the array elements is sufficient. Further considerations include a fixed and a variable incident plane‐wave direction with respect to the pole axis of the sphere.

The displacement field u in an isotropic, noncentrosymmetric (chiral) elastic solid has to be supplemented by the independent microrotation field φ. Six different wavenumbers are possible in a chiral elastic solid: Of these, two represent longitudinal fields and the remaining four are circularly polarized. This paper is concerned with the reflection and refraction of elastic plane waves by planar achiral‐chiral interfaces, and the scattering of elastic waves by a chiral sphere.

For improved ultrasonic detection (nondestructive evaluation, NDE), an ultrasonic beam with weaker sidelobes and absence of amplitude nulls in the nearfield is desired. This goal can be accomplished by reconstructing the back electrode of a conventional transducer, such that the vibration amplitudes on the transducer surface exhibit a Gaussian distribution along the radial direction. The shape of the back electrode is computed based upon the Laplace equation of the electric field in the crystal with the corresponding boundary conditions of the Gaussian distribution. Experiments have been performed with a piezoelectric transducer (PZT) in water. Theoretical analysis supports experimental results.

Finite‐element modeling is used for the study of structures with piezoelectric actuators and sensors augmented by viscoelastic passive dampers. The structure is excited by a transient force; the response of the structure as determined by the computed voltage response of the sensor is used to construct a feedback loop that excites the actuator. A constant gain P–D (proportional and derivative) controller is used. The controller changes the effective stiffness of the structure and the derivative feedback also introduces the desired damping. The increased stiffness also lowers the vibration amplitude. Active vibration damping is compared and contrasted with passive damping provided by a viscoelastic dampers. A BMG model is used for the viscoelastic phase. A hybrid approach using modal superposition in conjunction with time integration is used for solving the matrix equations in space and time in an efficient manner. A parametric set of results is presented showing the contribution of active and passive damping for the transient response of a clamped plate structure.

Detecting the onset of ice formation on fixed wing and rotary wing aircraft before and during flight is very important. The experimental results of using a new type of piezoelectric sensor oscillator will be presented. The d15 coefficient of PZT (lead zirconate titanate) is exploited by poling a thin rod (1.3‐mm‐diam) specimen of PZT along its length direction, and the rod specimens reelectroded diametrically. Two rods were attached to an aluminum plate, and an oscillator circuit was set up using an amplifier and a frequency counter. The plate mode determines the oscillation frequency. Since this is a shear mode, the frequency is unchanged if there is a thin layer of water on the plate. The water is then frozen and a jump in the oscillation frequency is noted at the onset of freezing. Shear waves can couple to ice in solid form, and this changes the dispersion relation, and hence the plate mode frequency. Results are presented showing the sensitivity of the measurement, repeatability, effect of temperatur...

Active control of noise inside an enclosure from exterior noise sources is accomplished by the use of piezoelectric actuators and either piezoelectric or MEMS sensors placed on the enclosure wall. It is generally known that this approach provides very good results at the resonant frequencies of the cabin walls and if the excitation noise is at a single frequency. In the present study, broadband excitation is looked at from the exterior, in the time domain. A sensor mounted on the cabin wall provides the reference signal for the excitation; the error sensor can either be the piezoelectric sensor on the wall or a sound‐level meter placed at an appropriate position inside the cabin. Different types of closed loop control are attempted using both analog and digital strategies. The study examines the number and placement of actuators on the wall relative to bandwidth of the external source, as well as control of wall vibration as a means of controlling the radiated noise.

The performance of active sound isolation by using a piezoelectric sensor and an actuator on a thin plate is investigated experimentally. In the experiment, the aluminum plate covers an opening of an acoustic enclosure, where a sound source is located. The outside acoustic field is measured by an 11‐element microphone array, which scans on a surface of a hemisphere. The isolation performance of the original plate is poor at its resonance frequencies. The sound transmission from inside to outside of the enclosure is actively controlled by a one‐sensor one‐actuator control system which minimizes the output of the on‐plate sensor. Global reductions of 15–20 dB are achieved at the first three resonance frequencies.

An optimal design technique is used to find the optimal conditions for several parameters of an enclosure to reduce the noise radiated from an internal source. Design variables in the optimization procedure are the size and thickness of the enclosure, size and thickness of actuators, the location of actuators, and the applied voltage on the actuators. The objective function in optimal design is the total radiated sound power at far field. The practical limits of the design variables are taken as constraints. The acoustic field acts as a distributed load on the walls of the enclosure. A 3‐D finite element model is used to model the walls and the PZT actuators. With an applied voltage on the actuators and with acoustic pressure loading, the velocity distribution on the outside of the walls can be found by solving the finite element equations. These are used in the Helmholtz integral representation to compute the radiated sound field to the exterior of the enclosure. The design is performed at a single frequ...

Piezoelectric transducers are commonly used to measure the response in a structure to impressed acoustic fields and other forces. By definition, an ideal sensor must be nonintrusive and its output must be a close representation of the response of the structure at that location. Thus the design of the sensor—its shape and size, material properties relative to the structure, its location in the structure—all contribute to sensitivity and fidelity. In addition, material damping in the plate and the transducer must be taken into account. The sensor response involves computation of the voltage induced in the transducer by an incident acoustic wave in the fluid or a force to which the plate may be subjected to. A hybrid numerical method involving finite element modeling in the structure and transducer region and a plane wave representation in the fluid region will be used. Canonical structures such as plates will be considered for which the structural response can be obtained in the absence of the sensor for purposes of comparison. A parametric study will be presented.

In the modeling of structural acoustic problems involving infinite domains, finite element modeling is often used in regions involving complex geometry, material anisotropy, coupled fields, etc. These same problems often involve coupling of the structure with an infinite acoustic fluid. Several methods have been proposed involving infinite elements, Dtn boundary conditions, matching the FEM representation with analytical representations in the infinite domain, to name a few. In modeling the response of embedded sensors and actuators in a structure subject to fluid loading, we have found it convenient to match plane wave solutions in the acoustic fluid with the FEM representation. The matching can be done with and without slope constraints and the matching can also be done using the so‐called Dtn (Dirichlet to Neumann) boundary condition that results from the radiation conditions in the infinite domain. A parametric study of convergence and accuracy of numerical results using each of these methods will be ...

Control of noise radiated into an enclosure from an external source is of interest in several applications. The noise is transmitted into the enclosure by the induced structural vibrations of the walls of the enclosure. An active control experiment using discrete piezoelectric sensors and actuators is proposed to sense and control the structure‐borne sound. A sound level meter and error microphone are used to measure the acoustic field intensity inside the enclosure with and without control. The results of this experiment will define the feasibility of applying active control methods to larger enclosures such as office partitions, passenger compartments of vehicles, control rooms in factory floors, hospital rooms, and apartments in crowded urban areas.

Many scattering problems involve specification of different types of boundary conditions on different parts of the surface of the same scatterer or the geometry of the scatterer may involve concavities. Both these problems are difficult to model using available methods. The applications of such problems are however many—debonded fibers in fiber reinforced composites, partially coated inclusions in engineered coatings, to name just two. A modification of the usual T‐matrix approach is proposed by constructing a mathematical surface which partially includes the actual surface of the scatterer. The integral representations for the fields for the two regions—the actual scatterer volume and the volume enclosed by the extended surface are simultaneously solved. A general formulation is presented with numerical results for P‐ and S‐wave scattering by partially debonded fibers in an elastic matrix. a)Present address: Samsung Electronics, Suwon, Korea.

The use of a co‐located piezoelectric, disk‐shaped sensor/actuator is described for controlling several modes of vibration of an actual automobile fuel tank that is excited by a shaker. The shaker is attached to the tank where the fuel pump would normally be attached. Single mode control as well as multi‐mode vibration control was achieved for both the empty as well as partially filled tank. The radiated noise caused by the structural vibration can also be controlled. Experimental results and short video presentation of the experiment will be made.

It is important to model the sensor and actuator function of a finite disk‐shaped piezoelectric transducer in the environment that it will function in. This will involve taking into account the structure on which it is mounted, modeled here as a flat, solid, elastic plate and fluid loading to which the structure may be subject to. In addition, material damping in the plate and the transducer must be taken into account. The sensor response involves computation of the voltage induced in the transducer by an incident acoustic wave in the fluid or a force to which the plate may be subjected to. The actuator response involved computation of the acoustic field radiated into the fluid when the transducer is subject to a known voltage. A hybrid numerical method involving finite element modeling in the plate and transducer region and a plane wave representation in the fluid region will be used. [Work supported by ONR.]

A dual SAW sensor attached in cantilever fashion to a free‐floating membrane flush mounted on a structure exposed to turbulent fluid flow are described. Shear stress on the membrane due to the flow conditions, deforms and bends the cantilever beam. The SAW devices are mounted such that one device is in compression and the other in tension. The cantilever arrangement magnifies the deformation of the SAW sensors by many orders of magnitude relative to the membrane. The SAW devices are attached to oscillator circuits and the change in frequency of the SAW oscillators due to the deformation can be accurately measured. The dual arrangement compensates changes in temperatures, electrical, and mechanical noise and other changes and the difference in frequency measured by a mixer is directly related to the shear stress on the structure. Experiments conducted in a wind tunnel are described. [Work supported by ONR.]

Marine fouling in ships and biofouling in condenser tubes, etc. cause numerous problems and a large reduction of fuel efficiency. Conventional methods include toxic paints and hull vibration. A nontoxic solution would be to excite Love waves on the surface and prevent the very first bio‐attachment on the surface. A thin PZT film with IDTs (InterDigital Transducers) flush mounted to the surface is used to launch these waves. Experimental results of this technique, which seems viable and cost effective, and most importantly, nontoxic, will be discussed.

Wave propagation in a viscoelastic and porous medium, whose properties are temperature and frequency dependent, was investigated. The propagation characteristics of compressional waves in such a medium cannot be predicted using single scattering models when the porosity of the medium is considerable. Different from a previous study [Y. Ms, V. K. Varadan, and V. V. Varadan, ASME J. Heat Transfer 112, 402–407 (1990)], which was for electromagnetic waves, this investigation provided another proof of enhanced attenuation due to dependent scattering for the acoustic case. Most importantly, this is a problem of an elastic wave propagation in a lossy host medium with lossless scatterers which has seldom been considered. Numerical results of phase velocity as well as of attenuation of such a medium for different temperatures and different frequencies are presented. Differences in results between single and multiple scattering when the porosity is increased are also shown. Numerical results considering multiple sc...

Optical activity is exhibited by media whose molecular configurations are handed or chiral. Since the geometry is the basis for chirality, it can be probed by transverse waves, but not by longitudinal waves. Composite elastic media can also be made chiral because the elastic field in solids consist, in general, of both longitudinal and transverse components. The displacement field u in such isotropic, noncentrosymmetric (chiral) elastic solid has to be supplemented by the independent microrotation field φ. Six different wave numbers are possible in chiral solids. Of these, two represent longitudinal fields and the remaining four are circularly polarized. Composites can thus be tailor‐made by suitably designing the microstructure and volume fraction of chiral elements. In this talk, the theoretical foundation for wave propagation and scattering in chiral elastic solids will be outlined. Pertinent experimental results on scattering of acoustic waves by chiral elastic composites containing piezo‐chiral eleme...

A digital time delay system has recently been developed for control of an active underwater coating. The active coating contains piezoelectric polymer sensors and a piezocomposite actuator encapsulated within a host polymer. The coating is for layering on submerged objects to actively reduce underwater sound reflection. The control system incorporates time delay circuitry as opposed to the analog phase delays used in previous studies. The type of control is an open‐loop velocity feedback. This system prevents feedback instability because the acoustic reflection is decoupled from the actuator. The digital controller can characterize both the incident acoustic pressure and the reflected pressure through parallel multichannel processing. The reflected pressure is monitored at the sensors for determination of the effective sound reduction. Experimental results were obtained in a water‐filled acoustic pulse tube terminated at its boundary with the active coating. Echo reduction comparisons as functions of soun...

The “d15 ” constant of the piezoceramics was utilized to design shear mode transducers. These shear mode transducers were verified to be pure shear stress/strain sensors/actuators both theoretically and numerically (ANSYS). Active torsional vibration control experiments were conducted on various phenolic and pyrex tubes by using the shear mode sensors/actuators. The free and controlled frequency responses as transfer functions were measured and compared to each other. The finite element mode‐frequency analysis was performed to evaluate the structural characteristics for the tubes with and without transducers mounted. The dynamic coupling coefficients were derived for shear mode sensors/actuators based on the finite element mode shapes.

The prediction of acoustic pressure on the surface of baffles is interesting for designers of transducer arrays. A computer simulation program based on a finite/boundary element approach is developed for this purpose. In the simulation, the elastic baffle is modeled by finite elements and the infinite fluid surround the baffle by boundary elements. The two representations are coupled on the surface through eigenmode expansion. The composite program is able to predict the surface acoustic pressure of axisymmetric elastic baffles. The sensitivity and directivity of hydrophone or hydrophone arrays that are mounted on the buffer can then be obtained. A particular example of a baffle that is a cylindrical steel shell with truncated ends is computed. The convergence of the computation is discussed. The results are compared with experimental measurements.

Unlike an electric motor, the ultrasonic motor produces torque through ultrasonic vibrations. It has some unique advantages over electric motors such as light weight and small volume; no magnetic noise; and low‐rotation speed without gears. Also, because it is driven by the friction force between the stator and the rotor, it has a very quick reponse time which is a highly desired feature in a control system. The motor control characteristics including the response time for speed and position control are presented. Experiments have been performed on a simple robotic system with a rotating arm. The motor can be used to control the arm location as well as actively control the residual vibration of the ann tip.

A smart skin system for inclusion into composite structures is presented. The composite structure can be considered for both underwater and air‐borne applications. Individual component designs in the skin include a piezoelectric SAW sensor for the characterization of both the local and global boundary flow layer, piezocomposite sensors and actuators for noise and vibration control, and an array of piezocomposite shear transducers for local damage assessment. Recent research results of the components will be presented. Conceptual system designs will be discussed.

The often used terms “tailored material” or “engineered material” refer to the increased demands made by present day engineering innovations on material properties that are incompatible in a naturally occurring material. Composite materials provide a powerful solution. But, by definition they are an inhomogeneous mixture of one or more materials. The property of the material is determined not only by the types of constituents, but the actual customization of properties requires a good understanding of the inherent (micro or macro) structure‐property relationships. In this talk several theoretical models are reviewed that attempt such prediction of effective properties and response to applied fields. The strengths and flaws in such models are also examined by reviewing the comparison between such predictions and the results of experiments on controlled laboratory samples using acoustic and elastic wave propagation in such materials. Fully validated models provide the confidence needed to use them as materi...