S. D. Bennett

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.

Reflection acoustic microscope for precision differential phase imaging

An acoustic microscope comprising a transducer for transmitting acoustic signals towards the surface to be studied, and means for receiving at least one reflected signal from the surface; in many embodiments of the invention, signals are received from two separate points. The signals received are passed to a synchronous phase detection system for analysis. The signals may be received at the same phase detector input and separated according to their expected time of receipt relative to their time of transmission, or they may be received at separated points on the transducer related to their separated points of transmission. The separated return signals are compared on the basis of phase (and in certain embodiments, magnitude) differential either to each other or to an internally generated reference signal to analyze the surface characteristics of the material.

Phase Measurements in Acoustic Microscopy

We have carried out experiments using a 50 MHz acoustic microscope to demonstrate the utility of phase information in acoustic microscopy. Phase perturbation was used to characterize surface r esidual stress on a glass disk sample. topography of different samples. inversion technique was used to obtain the reflectance function of a liquid/solid from the corresponding V (z) d ata measured in both amplitude and phase. Phase images were made to map the surface An exact

Precision phase measurement with short tone burst signals in acoustic microscopy

A new phase measurement system based on a sychronous detection scheme is described. It is capable of yielding high precision phase data with very short tone bursts. A theoretical analysis of the system is given and system performance characteristics are also discussed. This phase measurement technique is especially suited for acoustic microscopy applications. Examples in material characterization and surface topography mapping are presented.

Remote temperature measurement using an acoustic probe

We describe the preliminary results of a novel method for the noninvasive measurement of temperature distribution within a solid body using an acoustic beam. Applications of such a scheme could include the control and assessment of tumor treatment by hyperthermia in which small regions of tissue deep within the body are heated by either acoustic or microwave energy. A major practical difficulty in such a process is the accurate determination of the temperature distribution. The scheme we describe in this letter appears to be capable of detecting temperature differences with a sensitivity of a few tenths of a degree centigrade and with a spatial definition of the order of 1 cm3. The basic technique in which the phase differences between two colinear acoustic beams, one broad and collimated and the other focused, are compared electronically, is described, and results obtained with a tissue phantom are used to illustrate the viability of the approach.

Measurement of Surface Stresses Using Rayleigh Waves

We have developed a method for measuring surface s tresses on metal samples using the acousto-elastic effect of surface waves. A

A 50 MHz Synthetic Focus System

Last year at this conference we discussed a number of difficulties which can occur in synthetic aperture and other short pulse imaging systems. In this paper, we have carried out studies in both a linear and cylindrical format to understand some of these problems more clearly and to arrive at solutions for them. We have studied the effects of a finite number of digital phase samples on the sidelobe level, the effect of apodization on grating and sidelobe levels, the effect of a finite number of transducer elements, problems of imaging specular reflectors, and the use of selective back projection methods to improve images of specular reflectors. We show here that the use of this latter technique may be of great importance because it provides an alternative approach of imaging, which does not make use of the normal phase cancellation effects employed in conventional imaging systems.

Measurement of Stress

The Stanford acousto-elastic stress measurement program is reviewed in this paper. A computer-controlled scanned transducer system has been used with longitudinal wave and shear wave trnasducers. Recently an EMAT surface wave measurement has also been employed. Inhomogeneous stress states near cracks, residual stress states in aluminum, near surface stresses, and a 3D stress measurement have been made. A simple theory of texture effects has been developed, and has given good agreement with our experiments.

Reducing Sidelobe Levels of a Synthetic Aperture Digital Acoustic Imaging System

The sidelobes of an acoustic imaging system contribute to a general background clutter in the image, as well as speckle for coherent imaging systems. Therefore, it is desirable to keep sidelobe levels as low as possible. In this paper we will describe the effects of digital sampling (temporal quantization) and aperture sampling (aperture quantization) on the sidelobe levels of a synthetic aperture digital acoustic imaging system. Sidelobe levels can be diminished by both apodization of t he array and finer quantization accuracy. Improvement of image quality by these methods is demonstrated by a series of computer-simulated images which show a sidelobe level improvement from -20 dB to -40 dB .

Focused Acoustic Beams for Accurate Phase Measurements

There are many nondestructive evaluation tasks in which the ability to determine the state of stress in a component would be invaluable. Of the techniques potentially at our disposal, only the use of ultrasonic probes offers any real possibility of measuring stress below the immediate surface region. Acoustic measurements based on the acousto-elastic effect have been successfully demonstrated for determining the cross-sectional variation of stress for specimens in plane strain.1 Until now there has been little success in measuring stress distribution through the thickness of a specimen. In this paper we outline a theory and an initial experiment with a new technique which is capable of determining the distribution of stress in a solid body in three dimensions.