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Measurements of the frequency dependence of ultrasonic attenuation can be used as the basis for the estimation of particle size distributions (PSDs) in solid-in-liquid suspensions. The method requires matching the attenuation simulated by a candidate PSD in combination with a wave propagation model to the measured function in a fitting procedure. Uncertainty in the type of candidate PSD, whether based on fractional volume or fractional number of the dispersed particles, can cause errors in the overall estimation process, particularly for the median particle size. These uncertainties are investigated in the first part of this paper. The second part deals with uncertainties associated with the values for the physical properties of the suspended particles, seven of which are required in the simulation stage. It is shown that the particle sizing exercise is relatively insensitive to all of the physical properties except density, for which values are necessary to an accuracy commensurable with that required for the two principal parameters associated with the PSD-median size and standard deviation. The discussion is limited to small (less than 1-μm) silica particles dispersed in water. The results will have more general application.

Ultrasonic transducer arrays are extensively used for the nondestructive evaluation of materials for aerospace and other applications. However, their use with composites requires some technique development because of reflections at the layer boundaries and the effects of attenuation. When used in full matrix capture mode, algorithms such as the total focusing method (TFM) must be applied to obtain the image. In composite materials, improvement to the algorithm is required to include the effects of material anisotropy (affecting wave speed) and optimum aperture limits to optimize the signal-to-noise ratio and location detection for a defect in the material. This paper presents simulations of the ultrasonic array signals in multilayer anisotropic materials with and without a simulated defect. A kernel model for plane wave propagation in the material is combined with an angular spectrum decomposition (for finite transducer elements) and transducer frequency response, to model the full array signals. Inclusion of the defect is through its far-field scattering response. The model facilitates the study of imaging algorithm development by identification of the effects of anisotropy, signal-to-noise ratio, and aperture limit. An analytical method for the calculation of the effective group velocity in the composite at low frequency is demonstrated, permitting rapid calculation of time delay laws in practice.

Proportionality between the ultrasonic wave attenuation coefficient in epoxies and other polymers and frequency is a commonly observed but little understood phenomenon. How it is ultimately explained will depend on the breadth of the frequency range over which it is significant. This paper presents results of experiments to measure loss in a single epoxy material over 5 decades of frequency using 4 complementary techniques--dynamic mechanical analysis, microwave excited low-frequency resonances, a novel guidedwave technique based on a metal-epoxy-metal sandwich, and a conventional pulse mode ultrasonic spectrometer. The results are confined to bulk shear waves in the epoxy. They confirm the linear relationship between attenuation and frequency, and it is shown that the broadband behavior of the attenuation and shear wave phase velocity is consistent with the Kramers- Kronig relationships.

This paper is concerned with modeling ultrasonic wave propagation in epoxy materials to better understand NDE procedures and to provide reliable input to more complex models of guided wave propagation in layered structures. Different physical models are considered in the context of how well they simulate the (known) linear relationship between bulk wave attenuation coefficients and frequency. The identified models are then extended to simulate wave propagation in materials with mechanical properties, which vary gradually in the spatial dimension. This is achieved using electric circuit transmission line analogs to the viscoelastic mechanical system. Verifying experimental results are included.

This paper is concerned with the design of magnetostrictive transducers for the excitation and detection of guided waves in metal pipes of small diameter (mm) and their application to the study of wave propagation in pipes filled with water or supercritical CO(2). Optimized system design is based on a simulation of the overall signal pathway which includes the electric circuit conditions at the transducers, mode excitability, and the wavenumber filtering effect of the spatial distribution of the exciting alternating magnetic field. A prototype system was built, and experimental observations on small diameter pipes indicated good agreement with expected results from simulations. The reassigned spectrogram has been used to compare expectation on the basis of guided wave dispersion curves for fluid-filled pipes with experimental data.

Ultrasonic measurements of attenuation versus frequency were made on model systems comprising olive oil, sucrose, and tripalmitin to represent the constituents of chocolate. Corresponding measurements also were made on chocolate flowing in a pipeline at a pilot plant where the temperature, pressure, and temper of the chocolate were precisely controlled and monitored. Experimental results combined with simulation studies indicate that the effect of ultrasonic scattering from tripalmitin crystals in olive oil is modified by the addition of sucrose crystals at a high concentration. It is proposed that the presence of seed crystals in chocolate (temper) cannot be detected ultrasonically in the practical measurement range 1-12 MHz due to a similar process.

This paper proposes a novel architecture for a data acquisition system intended to support the next generation of ultrasonic imaging instruments operating at or above 100 MHz. Existing systems have relatively poor signal-to-noise ratios and are limited in terms of their maximum data sampling rate, both of which are improved by a combination of embedded averaging and embedded interleaved sampling. "On-the-fly" pipelined operation minimizes control overheads for signal averaging. A two-clock sampling timing system provides for effective sampling rates that are a factor of 20 or more above the basic sampling rate of the analog-to-digital converter (ADC). The system uses commercial field-programmable gate array devices operated at clock frequencies commensurable with the ADC clock. Implementation is via the Xilinx Xtreme digital signal processing development kit, available at low cost. Sample rates of up to 2160 MHz have been achieved in combination with up to 16384 coherent averages using the above-mentioned off-the-shelf hardware.

Ultrasonic imaging of biofilms in water is difficult due to the very low contrast in acoustic impedance between the biofilm and water. In this paper, biofilms exposed to moist air are scanned through the substrate in order to obtain echoes from the biofilm/air interface. A 50 MHz scanning system was used to scan 1 mm x 1 mm areas of biofilms in a 10 microm grid pattern. Two fast Fourier transform (FFT) based methods for enhancement of the film thickness measurement resolution are compared. Using these techniques, the surface topography of biofilms with thickness less than the acoustic wavelength can be imaged.

This paper presents an analysis of the error generation mechanisms that affect the accuracy of measurements of ultrasonic wave attenuation coefficient and phase velocity as functions of frequency. In the first stage of the analysis we show that electronic system noise, expressed in the frequency domain, maps into errors in the attenuation and the phase velocity spectra in a highly nonlinear way; the condition for minimum error is when the total measured attenuation is around 1 Neper. The maximum measurable total attenuation has a practical limit of around 6 Nepers and the minimum measurable value is around 0.1 Neper. In the second part of the paper we consider electronic noise as the primary source of measurement error; errors in attenuation result from additive noise whereas errors in phase velocity result from both additive noise and system timing jitter. Quantization noise can be neglected if the amplitude of the additive noise is comparable with the quantization step, and coherent averaging is employed. Experimental results are presented which confirm the relationship between electronic noise and measurement errors. The analytical technique is applicable to the design of ultrasonic spectrometers, formal assessment of the accuracy of ultrasonic measurements, and the optimization of signal processing procedures to achieve a specified accuracy.

This paper presents an analysis of a new type of feedthrough recursive surface acoustic wave (SAW) device. The device combines a conventional SAW structure with positive feedback in a way that allows use of selective properties of the SAW structure, control of the central frequency and bandwidth, achieving significantly higher quality factors for given dimensions of the structure, and reduction of the sidelobe level. Several possible implementations are discussed from a simple one that uses external circuitry to the most advanced that includes digital supervisory control. Equations are presented that relate the central frequency, bandwidth, and sidelobe level to the parameters of the SAW structure and feedback loop. The simulation results were found to be in good agreement with experimental data. These data show the control of the central frequency within 1%, a 10-fold increase in the quality factor compared to the original SAW structure, and a reduction of the side-lobe level by 20 dB irrespective of the influence of second order effects and random manufacturing fluctuations.

Ultrasonic compression wave absorption is investigated as a means to monitor the seeded crystallization of copper (II) sulphate pentahydrate from aqueous solution. Simple models are applied to predict crystal yield, crystal size distribution, and the changing nature of the continuous phase. The Allegra-Hawley scattering formulation is used to simulate ultrasonic absorption as crystallization proceeds. Experiments confirm that simulated attenuation is in agreement with measured results.