Short Acoustic Pulses Research Articles

Precise Phase Measurements with the Acoustic Microscope

Abstmct-The measurement and the use of phase in acoustic microscopy are discussed. It is demonstrated that in many applications phase can be used to provide sensitivity and information unparalleled by amplitude-only measurement methods. A technique capable of highaccuracy measurement of the phase of short RF acoustic pulses is described. The power of this phase measurement technique is illustrated in a number of applications. Surface material property measurements such as the Rayleigh-wave velocity and the inversion of the complex V(z) to obtain the reflectance function of a liquid-solid interface are considered. Surface topography mapping based on phase measurement is examined. A Fourier transform approach for precision determination of linewidths comparable to the resolution spot size is also presented.

A statistical approach to determining the number density of random scatterers from backscattered pulses

This paper describes a method of using the statistical character of the waveform backscattered from random scatterers to estimate the scatterer number density. The moments of the probability density function of the backscattered intensity depend on the scatterer number density. Expressions exist for these moments in terms of the number of scatterers contributing to the echo signal. These expressions can give an estimate for the scatterer number density, provided the resolution cell size of the backscattering configuration is known, and provided the number of scatterers in the resolution cell is small. This latter constraint is equivalent to a requirement that the statistics of the return signal envelope deviate significantly from a Rayleigh distribution. The validity of these expressions is investigated by analyzing the acoustic waveform backscattered from a randomized volume distribution of polystyrene spheres suspended in water when insonified by a short acoustic pulse. Experimentally, the second-order moment of intensity is found to be given the most accurate predictions of number density. A specific application suggested for this work is acoustic fish-stock assessment. Other possible applications are ultrasonic tissue characterization and acoustic ocean bottom identification.

Cavitation from short acoustic pulses

We wish to present some preliminary results of a study of the generation of cavitation by short pulses of 1-MHz ultrasound. In order to avoid the problems associated with acoustic detection, we have developed a photometric cavitation detection mechanism that utilizes a photomultiplier and chemical additives that enhance the light output due to sonoluminescence and chemiluminescence. Light outputs that suggest the presence of cavitation have been observed for pulse lengths as short as 10 μs at moderate acoustic intensities (actual values are awaiting the arrival of a calibrated probe). A description of the apparatus as well as a report on our latest results will be given. [Work supported in part by the National Science Foundation and the Food and Drug Administration.]

Remote sensing of thermal profile in the lower atmosphere by a radio-acoustic system

A new radio-acoustic methodology, for the remote sensing of temperature profile in the lower troposphere, has been developed in the last few years. Since 1972 different types of radio-acoustic sounders have been designed and tested in the USA, Europe and Japan. The technique used for measurements in a Doppler tracking of a short acoustic pulse with a radar. Measurement of the sound speed as a function of delay from the start of the acoustic beam leads to the acquisition of the temperature vertical profile. After a brief review of the experimental radio-acoustic systems, the metric radio-acoustic sounder developed and tested at Istituto di Cosmogeofisica of the National Research Council (Italy) is analysed. Results of this sounder and its performance in fog-capping thermal inversion measurements are discussed. A comparison of the data measured with the radio-acoustic system and with traditional methods shows:a) sounder ability to produce vertical thermal profile with temperature accuracy and height discrimination comparable with conventional soundings;b) advantages typical of remote-sensing technique;c) applicability of the system both in assessing thermodynamic conditions in the planetary boundary layer (PBL) and in sensing conditions conductive to high concentrations of air pollutants at ground level. Preliminary results of the completely automatized system and application to air quality management are also presented.

Photoacoustic generation and detection of 10-ns acoustic pulses in solids

We describe the generation and detection of short acoustic pulses (∼10-ns widths) in opaque plates, using weak laser pulses (less than 1-mJ energy) for excitation. The longitudinal, shear, and surface acoustic waves generated by a single laser pulse can be time resolved by using a thin-film piezoelectric transducer, and acoustic mode conversions at surfaces are clearly observable. This new photoacoustic material testing technique is useful for fast ultrasonic measurements or for detecting deep subsurface flaws.

The effect of pulse length on transient cavitation inception

Most ultrasonic systems used for diagnostic purposes employ short acoustic pulse rather than cw operation in order to accomplish imaging or Doppler scanning. It is also commonly thought that these short‐pulse systems, although driven at high acoustic pressure amplitudes, do not induce cavitation effects due to the low duty cycle of the ultrasound. Although stable cavitation typically requires cw operation for its effects to be present, transient cavitation can theoretically occur in pulse lengths as short as one cycle. We have developed a detection scheme utilizing sonoluminescence to observe the cavitation produced by pulses of varying length and repetition rate. Normal detection methods that examine noise emitted by the collapsing cavity can not be used in short‐pulse excitation due to the high spectral content of the pulse itself. We have observed transient cavitation effects (sonoluminescence) in pulse lengths as low as a few tens of cycles and are modifying our apparatus to go to even shorter pulses. [Work supported in part by the ONR and the NSF.]

Aperture apodization to reduce the off-axis intensity of the pulsed-mode directivity function of linear arrays

There is considerable interest in aperture apodization for enhancing image quality. With continuous wave excitation, the influence of apodization can quite easily be predicted: the directivity function in the far-field corresponds to the Fourier transform of the aperture function. We, however, analyse a practical situation — short acoustic pulse emission focused in the near-field. Nine types of aperture function (for example, Tukey, Parzen, Hamming) are compared. It appears that with the pulsed mode a considerable reduction in off-axis intensity can be obtained by judicious choice of the aperture function. Side-lobes can be completely eliminated. As a tool, an approximate relationship is established between the Fourier transform of the aperture function and the main-lobe width of the directivity function. Further, an optimum balance is sought between the main-lobe width and the width at the −25 dB level of the directivity function.

Time-frequency-domain analysis: one-dimensional phantom studies

Two methods have been adopted to date for investigating the interference effects resulting from the scattering of ultrasound by soft mammalian tissues; one involves the conversion of time-domain signals into the frequency-domain, where the broad spectral characteristics of a short acoustic pulse are utilised, whilst the other varies the orientation of the specimen with reference to the incident beam and analyses the interference effects at a specific frequency. It is shown both theoretically and experimentally that the two techniques are equivalent though the former is more seriously restricted by the equipment features. Results are presented on simple linear arrays of nylon monofilaments which indicate the ability of the respective techniques to evaluate scatterer separation. Furthermore a detailed examination of the limitations of the techniques concludes that the monofrequency orientation method is to be preferred and allows for the more accurate determination of scatterer separation. The significance of these findings is discussed with reference to the usage of these techniques for tissue characterisation.

Method of and apparatus for audiometrically determining the acoustic impedance of a human ear

In order to determine the acoustic impedance of a human ear at various static pressures, a probe tightly fitted to the pinna of the ear transmits a short acoustic pulse with a wide frequency spectrum (such as white noise) from an electroacoustic transducer to the ear canal and receives back an acoustic response signal which a microphone converts into an electrical wave. The latter is digitized and transformed into terms of a Fourier series fed to a processor which, on the basis of similar terms stored in a memory and previously obtained with the same probe fitted to two different cylindrical calibrating cavities, derives therefrom the acoustic ear impedance at a particular air pressure. The processor also triggers an electric pulse generator, working into the electroacoustic transducer, and may progressively adjust a generator of static air pressure connected to the probe.

Scattering of acoustic waves by finite elastic cylinders immersed in water

Backscattering measurements using short acoustic pulses were made for finite elastic cylinders immersed in water. These solid aluminum cylinders were terminated with solid hemispherical caps. The overall length of the cylinders is four times the radius. For frequency scaling the factor 12kl (or 2ak) was used. Measurements were made perpendicular to the cylinder axis, end on, and at 45°. Results ranging from less than 4.0 to greater than 5.0 in 12kl were compared with the theoretical predictions using a T‐matrix formulation made by Varadan, Varadan, and Flax [V. V. Varadan, V. K. Varadan, and L. Flax, J. Acoust. Soc. Am. 60, S81 (1979)].

Backscattering of short ultrasonic pulses by solid elastic cylinders at large k a

Backscattering measurements from solid brass, aluminum, and lucite cylinders in water have been performed at a ka of approximately 240 using short acoustic pulses. Numerous discrete echoes are received due to multiple internal reflections of the acoustic pulse at the boundary of the cylinders. Excellent agreement between the measured echo arrival times and the echo arrival times predicted by the theory of Brill and Überall [J. Acoust. Soc. Am. 50, 921 (1979)] is obtained for all of the cylinders. The amplitudes of the backscattered echoes from the brass and aluminum cylinders are also compared with theory and fair agreement is obtained.

Determination of ultrasonic velocity in liquids using optical diffraction by short acoustic pulses

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Phonon echoes in a glass at low temperatures

Coherent acoustic echoes are generated in fused silica glass at very low temperatures by a sequence of short acoustic pulses that interact with the intrinsic two-level configurational systems of the glass. Coherent effects are observable below $T\ensuremath{\approx}100$ mK, where the two-level-system phase memory time ${{T}^{\ensuremath{'}}}_{2}$ is long compared to the acoustic pulse length, typically 100 nsec. The dependence of spontaneous (two-pulse) and stimulated (three-pulse) echo amplitude on the amplitude, width, frequency, and separation time of the generating pulses has been studied in the frequency range 0.7-1.5 GHz and at temperatures of 18-80 mK. The area of the generating pulses is shown to be the relevant descriptive parameter. The value of the area that produces maximum echo amplitude is used to deduce the average value of the deformation potential ${\ensuremath{\gamma}}_{L}$ which couples a two-level system and a longitudinal acoustic wave, with the result ${\ensuremath{\gamma}}_{L}=1.5\ifmmode\pm\else\textpm\fi{}0.4$ eV. The two-pulse echo decay time is 16 \ensuremath{\mu}sec at 18 mK and varies as ${T}^{\ensuremath{-}2}$. This behavior may be understood in terms of spectral diffusion arising from elastic dipolar interactions between resonant and nonresonant two-level systems. The three-pulse echo decay is not exponential, but may be characterized by an initial decay time of 100 \ensuremath{\mu}sec at 0.7 GHz and 18 mK. This is in reasonable agreement with a direct-process thermal equilibration time ${T}_{1}$ calculated from the present value of ${\ensuremath{\gamma}}_{L}$. At higher temperatures, the decay rate is faster than indicated by the direct process, and may be understood semiquantitatively in terms of spectral diffusion.

Surface acoustic wave image scanning using phase modulation

Optical image scanning utilizing the phase modulation of a surface acoustic wave in a separated medium configuration of silicon on lithium niobate has been experimentally demonstrated with two similar techniques. In on e technique the long-lived silicon surface states are charged and the image is scanned by a short counterpropagating acoustic pulse. In the second technique the surface states are charged by a short acoustic pulse but the image is scanned by a short pulse of uniform optical illumination.

An experimental investigation of the formation and development of a wave packet in a laminar boundary layer

The formation and development of a wave packet in the laminar boundary layer of a flat plate has been studied experimentally. The packet was arti­ficially generated by a short duration acoustic pulse, which was injected into the boundary layer flow through a small hole in the plate. The flow oscillations created by the passage of the packet were detected by a hot­wire anemometer which was positioned just outside the boundary layer. Repeated excitation of the disturbance enabled the noise content of the data to be significantly reduced by signal averaging which was carried out on­line by a small computer. From records generated at a number of probe locations over the plate it has been possible to show how the packet developed as it propagated downstream. Contours of the signal amplitude from each of the downstream stations showed that the patch of waves was initially roughly elliptic, but this evolved into a distinctly bowed shape as the patch spread out as it progressed downstream. The smoothly contoured wave packet, with peak amplitudes close to the centre, gradu­ally distorted and far downstream two amplitude maxima formed on either side of the centre line. The wave-like character of the fluctuations within the boundaries of the packet was revealed graphically through perspective projections of the signal records on the z, t plane ( z is across the span). These displays showed the gradual development from a small smooth packet close to the source to the final warped and distorted pattern. Spectral decomposition of the fluctuations at each streamwise measuring station provided information relating to the growth of narrow bands of waves. These showed that a particular band of oblique waves, which grew very rapidly at some stage, were linked to the observed distortions.

Determination of the Form Function for Rigid and Elastic Spheres from a Single Ping

A short acoustic pulse is used to determine significant portions of the form function vs ka curves for rigid and elastic spheres, from a single reflection. Excellent agreement between exact theory and experiment is obtained over limited ranges of ka for aluminum and tungsten carbide spheres measured in water and for metal spheres measured in air.

Reaction of nonlinear standing striations on a pulse disturbance

Short electric and acoustic pulses applied on plasma of a stratified positive column of a hydrogen glow discharge have induced in it transient phenomena, revealing some dynamic properties of nonlinear standing striations. A description of the experiments together with a qualitative interpretation of the results is given.

Schlieren Visualization of Radiation Caused by Illumination of Plates with Short Acoustical Pulses

Schlieren visualization of the radiated field in water above and below aluminum plates, illuminated by short acoustic pulses, is presented. At certain angles of incidence of the incoming pulse, radiated wavefronts are seen to progress along the plates with specific angles of inclination and associated speeds of progression. In particular, strong effects are observed near the longitudinal and shear critical angles. The measured angles of incidence, at which these effects were generated, are in good agreement with predictions of Lamb theory for plates whose thicknesses are 0.58 wavelengths and larger. For thinner plates, the effects of water loading appear to become significant. A path trace analysis gives good agreement with the group velocity of observed effects when obvious trapped propagation paths are suggested by the reflection coefficient curves for a water-aluminum boundary. Specific radiated wavefronts on plates are related to waves which were observed on cylindrical shells. Relationship of the shell results to Lamb modes is discussed. Plates with thicknesses from 0.002 to 0.241 cm, and frequencies of 5 and 7.2 MHz, are used.

Radiation from Thin Plates Illuminated by Short Acoustic Pulses in Water

Schlieren observations of effects which occur when thin aluminum plates are illuminated by short acoustic pulses, in water, are presented. At specific angles of incidence, the formation of wavefronts pro-progressing along the plate is visible. A sequence of photographs showing the incident pulse, and at later times its effects, is made possible by the pulsed schlieren system. Meaningful predictions are made by Lamb's theory for infinite plates with stress-free surfaces even though the experiments involve water-loaded plates. Comparisons are made with previous results on cylindrical shells.

Q-Switching and Cavity Dumping of Nd:YAlG Lasers

Experiments in Q-switching and cavity dumping of a Nd:YAlG laser have been carried out. The system in either case is composed of a Nd:YAlG folded cavity laser and an intracavity acousto-optic modulator made of fused silica and Brewster cut. Q-switching is achieved by first feeding the modulator with an acoustic pulse of sufficient duration and intensity to keep the laser below threshold (hold off pulse). Termination of the acoustic pulse allows the power inside the laser cavity to build up. A second short acoustic pulse, introduced to the modulator at the time when the internal energy reaches its maximum, couples the energy out of the cavity. This pulse may also be the beginning of the hold off pulse. Cavity dumping is accomplished by feeding the modulator with short acoustic pulses at fixed intervals. In the Q switching mode the system can operate with a maximum repetition rate of about 30 kHz. In the cavity dumping mode of operation the repetition rate of the light pulses can be controlled from 125 kHz to several MHz. The minimum pulse duration obtained in both modes of operation was about 80 nsec. The average power can reach 2 W, which is the maximum cw power capability of the laser used.