Biomedical subjects

Stephen D Holland

Publications and source records attributed to Stephen D Holland.

4 recordsLinked to original sources

PubMed2006

An ultrasonic array sensor for spacecraft leak direction finding.

We have developed an ultrasonic array sensor useable for locating air leaks in manned spacecraft and have found that this sensor locates leaks in a 1-m(2) plate to within 2 cm. The sensor consists of a 63-element multiplexed array plus a reference element, all constructed from a single PZT disc and a printed circuit board. Cross-correlations of signals from the array elements with signals from the single reference element provide a measurement of the leak noise passing through the spacecraft skin under the array. A spatial Fourier transform reveals the dominant direction of propagation. Triangulation from multiple sensor locations can be used to find the source of the leak.

Equipment Design↗

PubMed2006

Automatic determination of acoustic plate source-detector separation from one waveform.

We discuss an automated procedure for determining the separation between a transient acoustic source and a detector on a plate. We use a time-estimation algorithm based on the assumption that the detected signal is represented by a small, finite number of discrete band-limited impulses. This is carried out using the MUSIC (MUltiple SIgnal Classification) algorithm in time-estimation mode to automatically estimate both the first arrival time of the lowest order antisymmetric (Ao) mode and the arrival time of the Rayleigh wave. Using the material and geometric properties of the plate and these two arrival times, we calculated the distance to the source. This technique allows the automatic determination of source-receiver separation from a single transient waveform.

Journal Article↗

PubMed2004

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 S1 mode, 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.

Journal Article↗

PubMed2002

A time-resolved method for nonlinear ultrasonic measurements.

We describe a time-resolved method for measuring nonlinear ultrasonic phenomena. Current approaches rely on a narrowband measurement of harmonic generation to identify and characterize nonlinearity. Concomitant with these techniques is poor time resolution. We address this limitation with a hybrid narrowband/broadband approach that provides simultaneous time resolution and harmonic isolation for the measurement of weak nonlinearites. We discuss applications and present demonstrative results showing harmonic generation both in water and at a dry contact aluminum-aluminum interface.

Journal Article↗