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Piano tones have partials whose frequencies are sharp relative to harmonic values. A listening test was conducted to determine the effect of inharmonicity on pitch for piano tones in the lowest three octaves of a piano. Nine real tones from the lowest three octaves of a piano were analyzed to obtain frequencies, relative amplitudes, and decay rates of their partials. Synthetic inharmonic tones were produced from these results. Synthetic harmonic tones, each with a twelfth of a semitone increase in the fundamental, were also produced. A jury of 21 listeners matched the pitch of each synthetic inharmonic tone to one of the synthetic harmonic tones. The effect of the inharmonicity on pitch was determined from an average of the listeners' results. For the nine synthetic piano tones studied, pitch increase ranged from approximately two and a half semitones at low fundamental frequencies to an eighth of a semitone at higher fundamental frequencies.

Homodyne detection is often used for interferometers based on nonlinear optical gain media. For the configuration of a seeded, 'truncated SU(1,1)' interferometer Anderson et al. (Phys. Rev. A 95, 063843 (2017)) showed how to optimize the homodyne detection scheme and demonstrated theoretically that it can saturate the quantum Cramer-Rao bound for phase estimation. In this work we extend those results by taking into account loss in the truncated SU(1,1) interferometer and determining the optimized homodyne detection scheme for phase measurement. Further, we build a truncated SU(1,1) interferometer and experimentally demonstrate that this optimized scheme achieves a reduction in noise level, corresponding to an enhanced potential phase sensitivity, compared to a typical homodyne detection scheme for a two-mode squeezed state. In doing so, we also demonstrate an improvement in the degree to which we can beat the standard quantum limit with this device., 11 pages, 7 figures

This Letter presents a series of time reversal experiments conducted on the surface of a fused silica glass block. Four different time reversal techniques are compared using three different imaging conditions. The techniques include two classical time reversal experiments: one with a pulse waveform source and one with an impulse response generated from a chirp signal. The other two techniques utilize the deconvolution, or inverse filter, signal processing methods for obtaining the signals to back propagate using a pulse waveform and an impulse response from a chirp. The max-in-time, symmetry, and energy current imaging conditions are compared.

Four-wave mixing in atomic vapor allows for the generation of multi-spatial-mode states of light containing many pairs of two-mode entangled vacuum beams. This in principle can be used to send independent secure keys to multiple parties simultaneously using a single light source. In our experiment, we demonstrate this spatial multiplexing of information by selecting three independent pairs of entangled modes and performing continuous-variable measurements to verify the correlations between entangled partners. In this way, we generate three independent pairs of correlated random bit streams that could be used as secure keys. We then demonstrate a classical four-party secret sharing scheme as an example for how this spatially multiplexed source could be used., Comment: Accepted in Journal of Modern Optics, 7 pages, 3 figures, 2 tables

Spent nuclear fuel is often stored in stainless steel canisters in the United States. Stainless steel is susceptible to Stress Corrosion Cracking (SCC). This presentation will discuss progress on the use of the Time Reversed Elastic Nonlinearity Diagnostic (TREND) and Nonlinear Resonant Ultrasound Spectroscopy (NRUS) to determine whether SCC is present and attempt to quantify the depth of the cracking. NRUS is the measurement of the amplitude dependence of a sample’s resonance frequency, which occurs because of a softening of the elastic modulus in damaged media. NRUS provides a global indication of damage in a sample. TREND employs time reversal acoustics, which focuses wave energy at various points of interest to excite localized high amplitude. The amplitude dependence of this localized energy allows pointwise inspection of a sample. [This work was funded by the Nuclear Energy University Program of the U.S. Department of Energy through a subcontract from Los Alamos National Laboratory.]

An SU(1,1) interferometer, which replaces the beam splitters in a Mach–Zehnder interferometer with nonlinear interactions, offers the potential of achieving improved phase sensitivity in applications with low optical powers. We present a novel variation on the SU(1,1) interferometer in which the second nonlinear interaction is replaced with balanced homodyne detection. We show theoretically that this “truncated SU(1,1) interferometer” can achieve the same potential phase sensitivity as the conventional SU(1,1) interferometer. We build an experimental realization of this device using seeded four-wave mixing in Rb85 vapor as the nonlinear interaction, thus employing a bright two-mode squeezed state as the phase-sensing quantum state inside the interferometer. Measurements as a function of operating point show that even with ≈35% loss, this device can surpass the standard quantum limit by 4 dB. This device is simpler to build and operate than the conventional SU(1,1) interferometer, and also eliminates some sources of loss, thus making it useful for applications in precision metrology.

The time reversed elastic nonlinearity diagnostic (TREND) is a nondestructive inspection technique based on the principle of time reversal and used to scan the surface of a sample to identify and characterize cracks and other defects. TREND utilizes a series of individual time reversal experiments conducted on a grid of points in a region of interest to map out the spatial extent of surficial expressions of cracks and subsurface features that are less than a wavelength below the surface. The focal signatures from each of these experiments can be used to determine not only the location but also the orientation of the crack. We will discuss how this technique can be applied to a stainless steel sample with stress corrosion cracking (SCC) using stationary piezoelectric transducers broadcasting ultrasonic waves and a scanning laser vibrometer setup used to detect the three dimensional vibration of the sample surface. The orientation of the crack is critical to estimate how soon a crack might penetrate through...

This research optimizes a time reversal focus of energy in terms of amplitude and quality by exploring several signal processing techniques applied to the reversed impulse response. Techniques explored here include deconvolution or inverse filtering [M. Tanter et al., J. Acoust. Soc. Am. 108, 223-234 (2000)], one-bit time reversal [A. Derode et al., J. Appl. Phys. 85, 6343-6352 (1999)], and a new technique termed “clipping.” A parameterization study comparing the maximum focal amplitude and focal temporal quality of time reversal focal signals for these techniques with different settings will be presented. Focal signals are explored in two different systems. The first is a resonant system comprised of a steel sample with a piezoelectric transducer and a scanning laser Doppler vibrometer (SLDV). The second, nominally non-resonant system, comprises a studio monitor loudspeaker and a precision microphone in a reverberant space above the Schroeder frequency. Comparing the focal quality of various time reversa...

It has been shown experimentally that it is feasible to use time-reversal elastic nonlinearity diagnostics (TREND) to estimate the penetration depth of stress corrosion cracks (SCC) in steel samples. TREND is a method to focus elastic waves energy to a point in space in order to probe that point for damage. High frequencies are used to probe near the surface, while low frequencies are used to probe deeper into the material. Depth estimation of a crack is important to determine when a critical size of defect is reached. This paper aims at a parametric analysis of the time reversal focusing process by means of numerical simulations. The influence of excitation frequency and waveform on the focal spot size and penetration depth are analyzed. An in-house simulation framework based on a commercial Finite Element (FE) solver is used to perform a series of numerical simulations of time-reversal focusing in a steel sample with a curved crack. Results of this study could help to understand the relationship between...

International audience; The out-of-plane velocity component is focused on the flat surface of an isotropic solid sample using theprinciple of time reversal. This experiment is often reproduced in the context of nondestructive testingfor imaging features near the surface of the sample. However, it is not clear how deep the focus extendsinto the bulk of the sample and what its profile is. In this paper, this question is answered using bothnumerical simulations and experimental data. The profiles of the foci are expressed in terms of the wavelengthsof the dominant waves, based on the interpretation of the Lamb’s problem and the use of the diffractionlimit.

The emerging field of quantum engineering seeks to design and construct quantum devices for use in technological applications. To do so, one must learn to prepare a physical system in a well defined quantum state, drive it though a specified evolution, and access its final state through measurement. Historically, some of the most successful laboratory platforms with which to explore these challenges have originated in the field of quantum optics. This work reviews some of the recent advances in single- and many atom quantum control at the College of Optical Science, and their integration into a novel atom-light quantum interface.

Transmission loss and insertion loss measurements of building materials at audible frequencies are commonly made using plane wave tubes or as a panel between reverberant rooms. These measurements provide information for noise isolation control in architectural acoustics and in product development. Airborne ultrasonic sound transmission through common building materials has not been fully explored. Technologies and products that utilize ultrasonic frequencies are becoming increasingly more common, hence the need to conduct such measurements. This letter presents preliminary measurements of the ultrasonic insertion loss levels for common building materials over a frequency range of 28-90 kHz using continuous-wave excitation.

We present the first set of experiments in which air-coupled ultrasonic emission is used for exhaustive and rapid imaging of a Barely Visible Impact Damage (BVID) introduced by an impact test on a laminated composite. Such experiments have been limited thus far by the impedance mismatch of nearly five orders of magnitude between the air and an elastic solid. This limitation was overcome by designing an ultrasonic source capable of generating a focused elastic wave field in the sample with an amplitude sufficiently large to allow the use of nonlinear elastic wave spectroscopy. The images of the damaged area obtained with the proposed apparatus and signal processing technique reveal the same features, namely a delamination and a crack, as with vibro-thermography and more features than with a C-scan, but remove the need to be in direct contact with the composite, thus reducing the test time by orders of magnitude. [This work was supported by the U.S. Department of Energy through the LANL/LDRD Program and the...

The far-field radiation originating from a finite-length pipe is well studied, especially for steady-state conditions. However, because all physical systems do not begin in steady state, these radiation characteristics are only valid after the transient portion of the solution has decayed. Understanding transient radiation characteristics may be important (particularly for systems transmitting very short-duration signals), as they can differ quite significantly. A numerical complication to this problem involves dealing with a sharp corner in the domain of interest. While many numerical studies have attempted to couple solutions from the domains inside and outside a pipe, the analysis presented in this work treats the computational domain as a single region by expressing the entire physical domain as a map from a simple rectangular domain in generalized curvilinear coordinates. This method will be introduced in detail and general results of transient radiation will be presented for an infinitely baffled, finite-length pipe using the finite-difference method expressed in generalized curvilinear coordinates. Comparison will be made to previous results [P. Stepanishen and R. A. Tougas, J. Acoust. Soc. Am. 93, 3074–3084 (1993)] that used a semi-analytic approach with certain assumptions.

The sound radiation of the hammered dulcimer has been investigated. The dulcimer studied is a 16/15 fixed-top instrument of typical size with Baltic birch (laminated plywood) soundboard and back. To determine the instrument body resonances, the dulcimer was driven at the treble and bass bridges with a shaker. Accelerometers were used to obtain the resonance frequencies of the soundboard and back, and a microphone was placed inside the instrument to obtain the cavity resonances. The individual resonance peaks found were further investigated using scanning laser Doppler vibrometry and near-field acoustical holography. Preliminary results show that there is little modal response of the instrument at the fundamental frequencies of the lowest notes of the dulcimer. In addition, the vibration coupling to the back plate through the internal bracing causes it to serve as a second soundboard. Lastly, the holography results indicate significant radiation from the sound holes at some frequencies, which may contradict the commonly held notion that the dulcimer sound holes are largely decorative.

We implement arbitrary maps between pure states in the 16-dimensional Hilbert space associated with the ground electronic manifold of $^{133}\mathrm{Cs}$. This is accomplished by driving atoms with phase modulated radio-frequency and microwave fields, using modulation waveforms found via numerical optimization and designed to work robustly in the presence of imperfections. We evaluate the performance of a sample of randomly chosen state maps by randomized benchmarking, obtaining an average fidelity $g99%$. Our protocol advances state-of-the-art quantum control and has immediate applications in quantum metrology and tomography.

An air-coupled ultrasonic transducer is created by bonding a bulk piezoelectric element onto the surface of a thick plate with a wedge of power-law profile. The wedge is used to improve the ultrasonic radiation efficiency. The power-law profile provides a smooth, impedance-matching transition for the mechanical energy to be transferred from the thick plate to the air, through the large-amplitude flexural waves observed in the thinnest region of the wedge. The performance of the proposed transducer is examined numerically and compared to that of a design where the piezoelectric element is isolated and where it is affixed to a thin plate of uniform thickness. The numerical analysis is first focused on the free-field radiation of the transducers. Then, time-reversal experiments are simulated by placing the transducers inside a cavity of arbitrary shape with some perfectly reflecting boundaries. In addition to time-reversal mirrors, the proposed concept could be integrated in the design of phased arrays and parametric arrays.

The parameters of moving-coil loudspeaker drivers are typically determined using direct electrical excitation and measurement. However, as electro-mechano-acoustical devices, their parameters should also follow from suitable mechanical or acoustical evaluations. This paper presents the theory of an acoustical method of excitation and measurement using normal-incidence sound transmission through a baffled driver as a plane-wave tube partition. Analogous circuits enable key parameters to be extracted from measurement results in terms of open and closed-circuit driver conditions. Associated tools are presented that facilitate adjacent field decompositions and derivations of sound transmission coefficients (in terms of driver parameters) directly from the circuits. The paper also clarifies the impact of nonanechoic receiving tube terminations and the specific benefits of downstream field decompositions.

Time reversal (TR) utilizes an array of transducers, a time reversal mirror (TRM), to locate sources. Here TR is applied to simple sources using steady-state waveforms in a numerical, point source model in a half-space environment. It is found that TR can effectively localize a simple source broadcasting a continuous wave, depending on the angular spacing. Furthermore, the angular spacing and the aperture of the TRM are the most important parameters when creating a setup of receivers for imaging a source. This work optimizes a TRM when the source's location is known within a region of certainty.

This chapter provides a working knowledge of the basic acoustical or sound measurements and measurement techniques for the practicing engineer. It also provides a broad overview and introduction to more advanced acoustical measurement methods. The chapter discusses the acoustical quantities that are typically measured. It presents the definitions of the terms, measurement units, and weighting filters. The chapter also discusses the various types of microphones. It explains the hardware used to make the measurements along with principles of calibration and data acquisition. The chapter gives the detailed steps to the measurement of sound pressure. It also describes the acoustical measurements important in the areas of sound isolation and room acoustics. The chapter focuses on the measurements of sound intensity and sound power, which are the most advanced measures of a sound field. It concludes with a discussion of sound exposure and noise dosage measurements. Keywords: acoustical measurements; microphones; room acoustics measurements; sound exposure measurements; sound intensity measurements; sound isolation; sound power measurements

The time reversal acoustic noncontact source (TRANS) utilizes time reversal (TR) from several transducers arranged in a cavity to focus energy onto a solid surface to allow inspection of that surface. The advantage of TRANS is that multiple transducers may be used to increase the amplitude of the focused energy onto the surface and potentially allow interrogation of nonlinear surficial features such as cracks and delaminations. TR is known to be a matched signal process and therefore is limited in the temporal fidelity and spatial compression of the focused energy. Fortunately, using a deconvolution operation in conjunction with TR (or inverse filter similar to Tanter et al. [JASA, 108, 223-234 (2000)]) can greatly improve the quality of the spatial focusing as well as increasing the temporal fidelity. Visualizations of the impact of the deconvolution operation will be presented to provide insight into the increased quality of the spatial focusing along with results from using the deconvolution operation in conjunction with TRANS. [This work was supported by Institutional Support (LDRD) at the Los Alamos National Laboratory]

At Brigham Young University, two of the acoustics courses taught in the Physics and Astronomy Department focus on resonance topics. One of the undergraduate courses is a general education, descriptive acoustics course on musical instruments, speech, hearing, and audio. One of the graduate level courses using a differential equations approach to wave motion and resonances on strings, bars, membranes, and plates. In both of these courses, taught at very different levels, the concept of resonance is explored and analyzed. This presentation will discuss various demonstrations used in these classrooms and discuss how they are used in the learning development of the students. Demonstrations include wine glasses, strings, and bars.

Over the last decades, nonlinear acoustic techniques have been developed to detect mechanical damage in solids. They have been proven to be far more sensitive to early damage than standard linear acoustic techniques. Unfortunately, they often require high amplitude waves to propagate within the sample. To be practical in industrial applications, signals have to be generated without contact. Here, we will present a novel non contact acoustic source that can produce higher amplitude than standard transducers and how it can be used to image defects into a carbon fiber reinforce plastic plate damaged by an impact. We will show that we can not only image cracks but also separate two overlapping defects according to their orientation. [This work was supported by the US Department of Energy via the used fuel disposition campaign of the nuclear energy program.]

This presentation explores the effect of transducer directivity on the focusing properties of time reversal acoustics. Time reversal is a signal processing technique that can be used to focus sound to a location in space for source reconstruction, communication, and intentional sound focusing. One loudspeaker and one microphone are used to produce a time reversal focus in a reverberation chamber. The primary axis of the loudspeaker is oriented in various different directions to produce a time reversal focus for each orientation. Since source directivity depends on the frequency of the sound emitted, these experiments are conducted for a low frequency band and a high frequency band such that these two frequency bands have fairly different directivities. Properties of these foci are compared to determine the overall effect of directivity on the time reversal focus.

This presentation will review research conducted to determine the effect of inharmonic partials on the pitch of piano tones [JASA, 117, 3268–3272 (2005)]. Synthetic piano tones were created based on recordings of an upright piano. One set of tones were made to have inharmonic partials that matched the partial frequencies of the piano tone recording, whereas another set of tones consisted of harmonic partials. Listeners compared a synthetic, inharmonic piano tone to a set of synthetic, harmonic piano tone with varying fundamental frequencies. They were asked to select the harmonic tone whose pitch best matched that of the inharmonic tone. The pitch of the inharmonic tones was perceived to be sharp relative to the harmonic tones (sharp in terms of the comparison of their respective fundamental frequencies).

At Brigham Young University, a general education course introduces students to the basic descriptive acoustic principles of music, speech, and audio. A third of this course focuses on the physics of musical instrument families. Three of these families include bowed string, plucked string, and struck string instruments. The concepts of driven systems and freely vibrating systems are taught, including the consequences of these excitation conditions. A hands-on lab for the course enables students to explore how the length, density, and tension of the string change the fundamental frequency. An in-class demonstration highlights the role of inharmonicity on the partial frequency values for these string instrument families. The combination of hands-on activity and demonstration aids the students in comprehending the basic acoustic principles behind string instruments and the reason for varying levels of inharmonicity between these instrument families.

The purpose of this research is to develop a visual demonstration of time reversal focusing in a thin plate to be used as a teaching tool. This presentation will discuss the various plate materials tested to provide the optimal conditions for time reversal focusing. Specifically, the reverberation time in each plate and the sensitivity of the source of vibrations are quantified to provide the best spatially confined focus as well as the highest focal amplitude possible. A vibration speaker and a scanning laser Doppler vibrometer (SLDV) are used to provide the time reversal focusing. A series of objects are placed on the plate in an attempt to knock over objects near the focal location and not elsewhere. Spatial mapping of the plate vibration using the SLDV will be shown to illustrate why some objects fall and others do not.

Time reversal is a technique that may be used to achieve intentional sound focusing. The purpose of this presentation is to explore how intense the focus of airborne sound energy can be using time reversal. Experiments utilize loudspeakers and a single microphone placed in a reverberation chamber. Various frequency ranges and loudspeaker orientations are explored along with different types of loudspeakers in order to experimentally determine the optimum conditions for generating the loudest focus possible. The overall purpose of this study is to create an acoustic focus at nonlinear sound pressure levels in order to create a controllable source of nonlinear acoustic sound.

International audience; Time reversal is a proven technique for focusing wave energy in time and space. A known side effect of the process is the presence of temporal side lobes and spatial fringes, which can be undesirable depending upon the application. A deconvolution, also known as an inverse filter, technique has been developed to improve temporal compression of the focal signal. This presentation will explore the use of deconvolution in comparison to standard time reversal and provide insight into the process through visualizations of the wave-fields. Attention will also be given to the effect of the deconvolution technique upon nonlinear elasticity applications such as the time reversed nonlinear elasticity diagnostic (TREND).

The need to perform quantum state tomography on ever larger systems has spurred a search for methods that yield good estimates from incomplete data. We study the performance of compressed sensing (CS) and least squares (LS) estimators in a fast protocol based on continuous measurement on an ensemble of cesium atomic spins. Both efficiently reconstruct nearly pure states in the 16-dimensional ground manifold, reaching average fidelities FCS = 0.92 and FLS = 0.88 using similar amounts of incomplete data. Surprisingly, the main advantage of CS in our protocol is an increased robustness to experimental imperfections.

Nonlinear acoustic techniques have proven to be extremely sensitive to early stage damage detection. However, the main issue preventing widespread use of these techniques is that they require high amplitude waves to induce the nonlinear response. As an alternative to transducers bonded to the test material, a more practical realization consists of developing non-contact ultrasonic transducers capable of generating high amplitude waves. Time reversal (TR) techniques are particularly suited to concentrate sound energy at a chosen location in space and at a specific instance in time. In this paper, it is numerically demonstrated how TR and a "candy can" (i.e. a highly scattering, contained medium) can be used to create a focused, high amplitude non-contact transducer. We developed and investigated the results of a finite element model of the "candy can" concept. The model allows to demonstrate the concept in different situations and illustrates the ability to pre-calibrate the non-contact source in air. The results of the simulations allow a better insight in the new concept and will be used in a further stadium for optimization of the high amplitude non-contact source.

A nonlinear scatterer is simulated in the body of a sample and demonstrates a technique to locate and define the elastic nature of the scatterer. Using the principle of time reversal, elastic wave energy is focused at the interface between blocks of optical grade glass and aluminum. Focusing of energy at the interface creates nonlinear wave scattering that can be detected on the sample perimeter with time-reversal mirror elements. The nonlinearly generated scattered signal is bandpass filtered about the nonlinearly generated components, time reversed and broadcast from the same mirror elements, and the signal is focused at the scattering location on the interface.

Qualifying an anechoic chamber for frequencies that extend into the ultrasonic range is necessary for research work involving airborne ultrasonic sound. The ANSI S12.55/ISO 3745 standard which covers anechoic chamber qualification does not extend into the ultrasonic frequency range, nor have issues pertinent to this frequency range been fully discussed in the literature. An increasing number of technologies employ ultrasound; hence the need for an ultrasonic anechoic chamber. This paper will specifically discuss the need to account for atmospheric absorption and issues pertaining to source transducer directivity by presenting some results for qualification of a chamber at Brigham Young University.

Many experiments involving cold and ultracold atomic gases require very precise control of magnetic fields that couple to and drive the atomic spins. Examples include quantum control of atomic spins, quantum control and quantum simulation in optical lattices, and studies of spinor Bose condensates. This makes accurate cancellation of the (generally time dependent) background magnetic field a critical factor in such experiments. We describe a technique that uses the atomic spins themselves to measure DC and AC components of the background field independently along three orthogonal axes, with a resolution of a few tens of uG in a bandwidth of ~1 kHz. Once measured, the background field can be cancelled with three pairs of compensating coils driven by arbitrary waveform generators. In our laboratory, the magnetic field environment is sufficiently stable for the procedure to reduce the field along each axis to less than ~50 uG rms, corresponding to a suppression of the AC part by about one order of magnitude. This suggests our approach can provide access to a new low-field regime in cold-atom experiments., Comment: 7 pages, 8 Figures

Chaotic behaviour is ubiquitous and plays an important part in most fields of science. In classical physics, chaos is characterized by hypersensitivity of the time evolution of a system to initial conditions. Quantum mechanics does not permit a similar definition owing in part to the uncertainty principle, and in part to the Schrodinger equation, which preserves the overlap between quantum states. This fundamental disconnect poses a challenge to quantum-classical correspondence, and has motivated a long-standing search for quantum signatures of classical chaos. Here we present the experimental realization of a common paradigm for quantum chaos-the quantum kicked top- and the observation directly in quantum phase space of dynamics that have a chaotic classical counterpart. Our system is based on the combined electronic and nuclear spin of a single atom and is therefore deep in the quantum regime; nevertheless, we find good correspondence between the quantum dynamics and classical phase space structures. Because chaos is inherently a dynamical phenomenon, special significance attaches to dynamical signatures such as sensitivity to perturbation or the generation of entropy and entanglement, for which only indirect evidence has been available. We observe clear differences in the sensitivity to perturbation in chaotic versus regular, non-chaotic regimes, and present experimental evidence for dynamical entanglement as a signature of chaos.

The ability to direct sound energy through the flexural vibrations of a submerged plate at various angles of incidence using a near-field transducer array is investigated. An alumina bar is placed in front of a one-dimensional, eight-element transducer array, between the array and the water. Operating in a receive mode, data were taken as a function of angle of incidence and compared to data taken without the presence of the alumina bar. The array was also operated in transmit mode and results were compared to corresponding receive mode data, showing that reciprocity holds. Results show that in fact sound energy can be steered through a plate, and that the measurement method used provides a convenient method of measuring the angular dependence of transmission through a plate, including measurements at frequencies above the plate's critical frequency. Experimental results of sound transmission versus angle of incidence of finite sized plates agree qualitatively with theoretical results from an analysis of the transmission through an unbounded flexible partition.

This paper introduces a method to selectively reduce large time‐reversed focal events to provide the potential to illuminate smaller events which have been masked by the larger one. The method is demonstrated for two elastic wave pulses emitted simultaneously from two spatially separated sources. The method improves upon the spatial resolution abilities of time reversed acoustics in nondestructive evaluation applications to identify characteristics of a sample such as cracks (nonlinear elastic wave scattering sources).

The presence of strong sources of elastic waves often makes it impossible to localize weaker ones, which are sometimes the most meaningful, e.g. in the characterization of complexity of active Earth faults or of microdamage in a composite structural material. To address this problem, a selective source reduction method is proposed here which, applied in conjunction with time reversal acoustics (TRA), provides the means to selectively reduce the contribution of strong sources allowing full illumination of the weak ones. The method is complementary to other methods based on TRA which aim at the selective illumination of scatterers in the propagation medium. In this paper, a description of the method is given along with presentation of a few numerical results to demonstrate its usefulness for localization of sources. Validation and some experimental results are also presented.

We demonstrate the possibility of selectively imaging the features of a barely visible impact damage in a laminated composite plate by using an air-coupled time reversal mirror. The mirror consists of a number of piezoelectric transducers affixed to wedges of power law profiles, which act as unconventional matching layers. The transducers are enclosed in a hollow reverberant cavity with an opening to allow progressive emission of the ultrasonic wave field towards the composite plate. The principle of time reversal is used to focus elastic waves at each point of a scanning grid spanning the surface of the plate, thus allowing localized inspection at each of these points. The proposed device and signal processing removes the need to be in direct contact with the plate and reveals the same features as vibrothermography and more features than a C-scan. More importantly, this device can decouple the features of the defect according to their orientation, by selectively focusing vector components of motion into the object, through air. For instance, a delamination can be imaged in one experiment using out-of-plane focusing, whereas a crack can be imaged in a separate experiment using in-plane focusing. This capability, inherited from the principle of time reversal, cannot be found in conventional air-coupled transducers.

In some Non-Destructive Evaluation techniques, detecting a crack is only the first step. Some applications like monitoring of used nuclear fuel canister also requires the characterization of the orientation of the cracks if any exist. Here, we show experimental results that expand the Time Reversal Elasticity Nonlinear Diagnostic method (TREND) to obtain the crack depth and orientation information. TREND is based on using time reversal to focus energy at a point of interest and then quantifying the nonlinearity at this point. By varying the frequency of the focal signal, one can interrogate different depth of the material, therefore getting access to information about the penetration depth of a crack. We will also show how Time Reversal can be used to select the orientation of the focus wave to any desired direction and how this, in turn, allows measuring different nonlinear responses according to this orientation and how this could be used to determine the orientation of cracks. [This work was supported by the US Department of Energy via the used fuel disposition campaign of the nuclear energy program.]

Time reversal provides an optimal prefilter matched signal to apply to a communication signal before signal transmission. Time reversal allows compensation for wave speed dispersion and can function well in reverberant environments. Time reversal can be used to focus elastic energy to each of the three components of motion independently. A pipe encased in concrete was used to demonstrate the ability to conduct communications of information using three component time reversal. The ability of time reversal to compensate for multi-path distortion (overcoming reverberation) will be demonstrated and the rate of signal communication will be presented. [The U.S. Department of Energy, through the LANL/LDRD Program, is gratefully acknowledged for supporting this work.]

This paper presents experimental results comparing the detected acceleration values from sensors measuring American football helmet impacts. The sensors used in the study include a laser vibrometer, a research grade accelerometer, an Impakt Protective, Inc. Shockbox® Football Helmet Sensor, a Reebok CHECKLIGHT™ sensor, and a Pressure Analysis Company, Inc. prototype sensor. A human head model was placed into a Riddell helmet and this helmet was then affixed to a pendulum. The helmet was pulled back and allowed to collide with a large granite block to create a frontal, forehead area impact on the helmet. A comparison is made of the detected accelerations from these sensors with a wide range of impact velocities, along with a repeatability determination of several measurements made for two selected impact velocities.

The large impedance contrast between bulk piezoelectric disks and air does not allow for efficient coupling of sound radiation from the piezoelectric into air. Here, we present the idea of using wedges of power law profiles to more efficiently radiate sound into air. Wedges of power law profiles have been used to provide absorption of vibrational energy in plates, but their efficient radiation of sound into air has not been demonstrated. We present numerical modeling and experimental results to demonstrate the concept. The wedge shape provides a gradual impedance contrast as the wave travels down the tapering of the wedge, while the wave speed also continually slows down. For an ideal wedge that tapers down to zero thickness, the waves become trapped at the tip and the vibrational energy can only radiate into the surrounding air. [This work was supported by institutional support [Laboratory Directed Research and Development (LDRD)] at Los Alamos National Laboratory.]