Waveshape distortion of high frequency acoustic waves in gas media.

In molecular dynamics simulations of an acoustic domain excited by a sinusoidally oscillating plane acoustic source in the frequency range of hundreds of megahertz, the density and velocity perturbations adjacent to the source are observed to be non-sinusoidal in shape. This distortion in the shape of the waves is investigated using a number of simulations of frequencies in the hundred of megahertz range and velocities up to 0.50 Å/ps. The relative distortion of the wave shape is characterised by a developed nested trigonometric function. The distortion is shown to be a function of the Mach number of the acoustic source rather than the source velocity amplitude. Trends in the distortion with source amplitude and frequency indicate that distortion of the velocity and density are independent of frequency. It is shown that the density and velocity perturbation can be approximated for any sound source Mach number within the range examined using the parametrised developed equation. The developed approximation could be used to accurately simulate the influence of an oscillating plane using a stationary analytical source. This could be used to develop a hybrid molecular/continuum model that will allow lower frequency simulations. The improved understanding of the causes of the distorted high frequency waveshape could also improve the fidelity of parametric arrays.

Adaptive velocity-based six degree of freedom load control for real-time unconstrained biomechanical testing.

Robotic biomechanics is a powerful tool for further developing our understanding of biological joints, tissues and their repair. Both velocity-based and hybrid force control methods have been applied to biomechanics but the complex and non-linear properties of joints have limited these to slow or stepwise loading, which may not capture the real-time behaviour of joints. This paper presents a novel force control scheme combining stiffness and velocity based methods aimed at achieving six degree of freedom unconstrained force control at physiological loading rates.

Active noise control in a pure tone diffuse sound field using virtual sensing.

Local active noise control systems generate a zone of quiet at the physical error sensor using one or more secondary sources to cancel acoustic pressure and its spatial derivatives at the sensor location. The resulting zone of quiet is generally limited in size and as such, placement of the error sensor at the location of desired attenuation is required, which is often inconvenient. Virtual acoustic sensors overcome this by projecting the zone of quiet away from the physical sensor to a remote location. The work described here investigates the effectiveness of using virtual sensors in a pure tone diffuse sound field. Stochastically optimal virtual microphones and virtual energy density sensors are developed for use in diffuse sound fields. Analytical expressions for the controlled sound field generated with a number of control strategies are presented. These expressions allow the optimal control performance to be predicted. Results of numerical simulations and experimental measurements made in a reverberation chamber are also presented and compared.

Active noise control in a free field with virtual sensors.

The zone of local control around a "virtual energy density sensor" is compared with that offered by an actual energy density sensor, a single microphone, and a virtual microphone. Intended as an introduction to the concept of forward difference prediction and a precursor to evaluating the virtual sensor control algorithms in damped enclosures, this paper investigates an idealized scenario of a single primary sound source in a free-field environment. An analytical model is used to predict the performance of the virtual error sensors and compare their control performance with their physical counterparts. The model is then experimentally validated. The model shows that in general the virtual energy density sensor outperforms the actual energy density sensor, the actual microphone, and the virtual microphone in terms of centering a practically sized zone of local control around an observer who is remotely located from any physical sensors. However, in practice, the virtual sensor algorithms are shown to be sensitive (by varying degrees) to short wavelength spatial pressure variations of the primary and secondary sound fields.