High contrast air-coupled acoustic imaging with zero group velocity Lamb modes

The well known zero in the group velocity of the first-order symmetric (S1) plate wave mode has been exploited in air-coupled ultrasonic imaging to obtain significantly higher sensitivity than can be achieved in conventional air-coupled scanning. At the zero group velocity point at the frequency minimum of the S1mode, a broad range of wavenumbers couple into the first-order symmetric mode at nearly a constant frequency, greatly enhancing transmission at that frequency. Coupled energy remains localized near the coupling point because the group velocity is zero. We excite the mode with a broadband, focussing, air-coupled transducer at the frequency of the zero group velocity point in the S1 mode. By exploiting the efficient coupling at the zero group velocity frequency, we have easily imaged a single layer of Scotch tape attached to a 6.4-mm thick Plexiglas plate and 3.2-mm Teflon inserts in a composite laminate

High‐Sensitivity Air‐Coupled Ultrasonic Imaging with the First‐Order Symmetric Lamb Mode at Zero Group Velocity

A new method for high‐sensitivity non‐contact, through‐transmission, air‐coupled imaging of small material property changes or discontinuities in plates is demonstrated. Our approach exploits the excitation of the first‐order symmetric Lamb wave mode at its minimum frequency point of zero group velocity. Because this Lamb wave resonance couples energy extremely efficiently with the air and does not propagate energy in the plane of the plate, it is the dominant mode of transmission of an airborne focussed‐beam broadband impulse through the plate. We take advantage of the sensitivity of this mode by performing C‐scans at the frequency of the group‐velocity zero to image spatial discontinuities and property changes. Our results show that images measured at this frequency are more sensitive and more consistent than those measured elsewhere in the plate‐wave spectrum

Air-coupled acoustic imaging with zero-group-velocity Lamb modes

A Lamb wave resonance has been found that allows unusually efficient transmission of airborne sound waves through plates. This occurs at the zero-group-velocity point at the frequency minimum of the first-order symmetric (S1 ) Lamb mode. At this frequency, plane waves with a range of incident angles can couple between the air and the Lamb mode in the solid plate, dominating the spectrum of transmitted focused sound beams by 10 dB or more. We use this frequency for C-scan imaging, and demonstrate the detection of both a 3.2-mm-diameter buried flaw and a subwavelength thickness changes of .005l ~1%!

Towards Intelligent Databases

This article is a presentation of the objectives and techniques of deductive databases. The deductive approach to databases aims at extending with intensional definitions other database paradigms that describe applications extensionaUy. We first show how constructive specifications can be expressed with deduction rules, and how normative conditions can be defined using integrity constraints. We outline the principles of bottom-up and top-down query answering procedures and present the techniques used for integrity checking. We then argue that it is often desirable to manage with a database system not only database applications, but also specifications of system components. We present such meta-level specifications and discuss their advantages over conventional approaches

Spherically focused capacitive-film, air-coupled ultrasonic transducer

A spherically focused no mirrors capacitive-film, air-coupled ultrasonic transducer, constructed using a spherically deformed backplate and metalized polymer film, has been designed, fabricated, and its performance characterized. A 1 cm diameter device has a center frequency of 805 kHz and a 6 dB bandwidth of 760 kHz. Comparisons of field strength in the focal zone with theoretical calculations for a spherically focused piston show that the device achieves diffraction-limited focusing. The nominal focal point of 25 mm lies within 0.01 mm of the calculated value for this device

Residual Bias Phenomenon in Air‐Coupled Ultrasonic Capacitive Film Transducers

We discuss in this paper the underlying physics of a residual bias phenomenon, whereby the metalized Mylar films of air‐coupled film transducers accept and retain a residual electrostatic charge. Experimental measurements to demonstrate and quantify this effect are reported here, along with a hypothesis of the mechanism of charge transfer and embedding. The measurements show the amplitude performance of the capacitive film transducers as a function of applied bias voltage and frequency. Factors such as humidity and decay time also play roles in the acquisition and holding of charge on a film. We hypothesize that charge transfers from the conductive backplate and collects on the non‐metalized side of the film. The charged films therefore are electrostatically attracted to the transducer backplate even with no applied voltage bias. Typically, an externally applied bias voltage is needed to charge the capacitor. With a persistent residual bias effect, these air‐coupled capacitive film transducers could be used like conventional piezoelectric transducers with no biasing required. This effect has substantial implications for the operation of air‐coupled film transducers

Multimode Dispersion Compensated Pulse-Echo Guided Wave Inspection

Obtaining temporal/spatial resolution in guided wave measurements comparable to that of bulk wave measurements is impeded by the complicating effects of multimode dispersion. Transport of signals by multiple dispersive modes of propagation can transform an initially compact transient into an extended, visually unintelligible wavetrain. Guided mode inspections therefore tend to restrict measurements to regimes for which a single mode can be generated with minimal dispersion, consequently restricting the achievable temporal/spatial resolution of the measurement. The objective of the work reported here is to remove such restrictions, through implementation of wavefield measurements which accommodate the total complexity of multimode dispersed signals. Temporal and spatial Fourier analysis of guided wave fields enables a full identification of individual mode contributions. It is therefore conceivable that, given measurements taken over an appropriate spatial array, processing could be implemented to effectively remove the effects of multimode dispersion, enabling operation in frequency regimes currently associated with bulk wave measurements. This paper will report on work which is exploring this possibility. Results will be presented demonstrating array-based MHz regime plate wave measurements, generating 10 or more modes, in which individual modes are isolated, effects of dispersion are removed, and temporal resolution of the original transmitted pulse is restored. Examples of application will be presented, and factors determining the effectiveness of temporal resolution restoration will be discussed

Evaluation of Clinical and Ultrasonographic Parameters in Psoriatic Arthritis Patients Treated with Adalimumab: A Retrospective Study

Objectives. The aim of this study was to evaluate clinical and US-PD parameters in PsA during adalimumab treatment. Methods. A retrospective study has been conducted in forty patients affected by moderate-to-severe peripheral PsA. Clinical, laboratory, and US-PD evaluations were performed at baseline, after 4, 12, and 24 weeks of treatment. They included erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), visual analogue scale (VAS), Health Assessment Questionnaire (HAQ) modified for Spondyloarthritis, Psoriasis Area Severity Index (PASI) score, the 28-joint Disease Activity Score (DAS 28), and US-PD assessment. US-PD findings were scored according to a semiquantitative scale (ranging 0–3) for synovial proliferation (SP), joint effusion (SE), bone erosions (BE), and PD. Results. Data obtained for clinical, laboratory findings and US-PD evaluation showed statistical significant improvement in all the measures performed except for BE. A significant parallel decrease in SE, SP, and PD values were demonstrated. Conclusion. This study demonstrated that US-PD is a valid technique in monitoring the response to adalimumab in moderate-to-severe PsA

Leak detection using structure-borne noise

A method for detection and location of air leaks in a pressure vessel, such as a spacecraft, includes sensing structure-borne ultrasound waveforms associated with turbulence caused by a leak from a plurality of sensors and cross correlating the waveforms to determine existence and location of the leak. Different configurations of sensors and corresponding methods can be used. An apparatus for performing the methods is also provided

Air-coupled, focused ultrasonic dispersion spectrum reconstruction in plates

This paper presents and demonstrates a noncontact method for measuring the Lamb wave dispersion spectrum of a plate. Noncontact air-coupled source and receive transducers are used with line-focus mirrors and 50–700 kHz broadband apparatus for simultaneous measurement over a broad spectrum of refractive angles and multiple guided modes. Broadband, wide-angle wave forms are measured as a function of position. The Fourier transform of these wave forms from the t – x domain to the v – k domain gives an approximate spectrum of the dispersion relation. We measure the dispersion spectra of Lucite™, aluminum, balsa wood, and a carbon fiber epoxy laminate, and show that the measured spectra agree well with the dispersion relation calculated from Lamb wave theory