C.-H. Chou

A new design for air transducers

Air transducers in the 0.5-10-MHz frequency range are made with either one or two matching layers for broadband and efficient coupling of ultrasound into air. The authors present a design for the optimum value of the impedance of a second matching layer of maximum bandwidth and insertion loss. Theoretical designs predict a functional bandwidth of 30% and a two-way insertion loss of 35 dBs for a device operating at 1 MHz. Experimental results are in excellent agreement with theory. Silica aerogel samples are characterized, and designs that demonstrate their potential as matching layers for air transducer applications are shown. Also, results of transmission imaging through carbon-epoxy and Kevlar-epoxy samples are shown.<<ETX>>

Lens design for acoustic microscopy

Design criteria for acoustic microscope lenses are examined with respect to their intended application. Aside from buffer rod material and F-number, the factors influencing the lens design are the critical angle for surface wave excitation, lens illumination, and leak rate of the surface wave on the sample. It is found that the design criteria are different for surface and subsurface examination and that for different applications and materials, different lenses are required for optimum imaging performance. A formalism for evaluating the performance of an acoustic microscope by considering its response in the time domain, both theoretically and experimentally, is presented.<<ETX>>

Design and implementation of mixed-mode transducers

In order to design a mixed-mode transducer with high efficiency and broad bandwidth for both longitudinal and shear wave modes, a theory is developed to determine the properties of this transducer with arbitrary acoustic loads at both ends of the piezoelectric element. Several Y-cut lithium niobate (LiNbO/sub 3/) transducers were made on both [110] single-crystal bismuth germanium oxide (Bi/sub 12/GeO/sub 20/) and fused quartz. The piezoelectric plates were attached to indium bonding and later polished to operate in the 100-MHz frequency range. The experimental data of round-trip insertion loss for both longitudinal and shear modes showed an excellent agreement with theoretical predictions.<<ETX>>

A Three-Dimensional Synthetic Focus System

Synthetic focus imaging techniques suitable for reconstructing 3-D acoustic images of flaws inside silicon nitride are described. A 50 MHz imaging system consisting of a precision scanner, a microcomputer controller, and a minicomputer image processor has been developed for this purpose. A square synthetic aperture is used to image flaws in flat disc samples and a cylindrical synthetic aperture is used in the cylindrical rod case. We have developed the theory to predict the imaging performance of the two aperture geometries. The respective Point Spread Functions are simulated and agree well with theoretical results. Special attention is given to reconstructing images of specular reflectors. Computer simulations based on theoretical flaw models have been carried out.

Ultrasonic excitation and detection of capillary waves for the measurement of surface film properties

A 10 MHz focused ultrasonic transducer is used to excite capillary waves by placing its focus at the air‐water interface and using a tone burst to excite the ultrasonic wave. The radiation pressure associated with the upward propagating ultrasonic pulse lifts the surface of the water which then relaxes by exciting a radially propagating capillary wave. An amplitude and phase measuring acoustic microscope operating at 10 MHz is used to detect the amplitude and slope of the capillary wave as it propagates over the focused transducer of the acoustic microscope. This arrangement allows us to make a noncontacting measurement of surface tension and surface viscosity which will be used for characterizing surface films, such as the marine microlayer.

The Design of Broadband and Efficient Acoustic Wave Transducers

The basic design criteria for SAW transducers is typically a flat frequency response. design criteria for bulk wave t.ransducers in nondestructive evaluation and medical imaging is the compactness of the impulse response. This criteria is different from the usual flat frequency response criteria because a flat bandwidth does not necessarily imply a compact impulse response. Aniterative optimization program, based on a least mean square algorithm, has been developed and used to simultaneously optimize matching metworks and acoustic parameters to achieve either of the above design criteria. The optimization is first illustrated in t he frequency domain for an IDT transducer. Then the optimization is done in the time domain for a bulk wave transducer with the criterionof reducing the length of the impulse response. The impulse response is thus reduced from about 15 cycles to 3 cycles and has an almost Gaussian frequency response. The increase in t he round trip insertion loss of the transducer due to the tuning is of the order of a few dB. Transducers have been constructed at 5 and 35 Wz (Z = 3 kg/n?-sec) and no front matching layer. The agreement between theory and experiment is excellent. The basic

Defect characterization in the short-wavelength regime

The scattering of acoustic waves by different types of spherical defects in a silicon nitride matrix is calculated by using Ying and Truells scattering theory. The theoretical scattering results are interpreted using a ray tracing approach. Experiments were carried out at a high frequency (150–450 MHz) to characterize defects in silicon nitride. Time and space averaging, Wiener filtering, diffraction, and propagation loss corrections were used to remove the effect of the transducer response and propagation loss in the material from the scattered signal. Our experimental results indicate the presence of a new type of defect in silicon nitride. They give the type and size of voids, cracks, and Si inclusions in good agreement with measurements obtained after sectioning.

Near-field scanning acoustic microscope

A traditional scanning acoustic microscope (SAM) has been modified to operate in the near-field mode. A pinhole in a thin shim of brass defines the resolution of the instrument, which can be as small as 0.1 lambda . In an edge scan experiment, a 125- mu m-thick brass shim with a pinhole size of 125 mu m, a SAM operating at 3 MHz, and a transducer with an F-number of 0.7 are used. The improvement in resolution corresponds to using a transducer with an F-number of 0.2. The results of measurements of the line response of the system, using steel pinholes of several thicknesses and diameters at different linewidths and operating frequencies and showing the details of the design of the instrument, are presented.<<ETX>>

Ball bearing inspection with an acoustic microscope

The authors have developed an amplitude- and phase-measuring acoustic microscope to detect surface defects a few micrometers deep in ceramic ball bearings. The system is capable of measuring phase variation on the order of 1 degrees . The phase detection capability is necessary to detect shallow surface depressions when no crack is present at the bottom of the depression, and the detection of the defect, through V(z) variation, requires operating the system in the gigahertz frequency range. A C-scan system is operational and allows the detection of fissures and gouges, even when shallow. A novel B-scan system has been designed in order to detect smaller cracks and to get full coverage of the bearings.<<ETX>>

Quantitative Acoustic Microscopy Using Amplitude and Phase Imaging

We have developed a fast technique for locating and determining the depth of delaminations in composite materials. We use an acoustic microscope that measures amplitude and phase to scan the m aterial. By using both amplitude and phase, we can unambiguously determine delamination depth. We can also perform other signal processing, such as removing the effects o f surface roughness, that is not possible with microscopes that measure amplitude or phase a1 one.