Combined laser-Doppler vibrometer and needle-hydrophone detection of interface waves

Karel van Dalen , Guy Drijkoningen , Christ Glorieux

At the fluid/elastic-solid interface the well-known leaky Rayleigh and Stoneley waves can propagate. The impedance associated with such an interface wave interrelates the corresponding waveforms that can be observed in different components, i.e., the particle motion and fluid pressure. We show that the interface-wave impedance of the leaky-Rayleigh wave, excited at a water/aluminum interface, can be successfully extracted using an experiment in which we simultaneously measure both components at ultrasonic frequencies. In particular, the normal interface displacement is measured using a laser Doppler vibrometer and the pressure with a needle hydrophone. The impedance of the leaky Rayleigh wave can be clearly distinguished in the wavenumber-frequency domain. Any measurement that employs a laser Doppler vibrometer is perturbed by refractive-index changes along its beam once it interferes with acoustic waves. Using a model that accounts for these perturbations, we predict a significant impedance decrease with respect to the plane-wave impedance of the leaky Rayleigh wave. Although this deviation is different for the experimentally extracted impedance, there is very good agreement between the observed and predicted waveforms in both the displacement and fluid pressure. This shows that laser ultrasonic devices and small pressure detectors can be successfully combined in laboratory-scale experiments.

Mots clés: Laser ultrasonics; Needle hydrophone; Interface waves; Wave impedance

Domaines: Acoustique [physics.class-ph]

Karel van Dalen, Guy Drijkoningen, Christ Glorieux. Combined laser-Doppler vibrometer and needle-hydrophone detection of interface waves. Acoustics 2012, Apr 2012, Nantes, France. ⟨hal-00810809⟩