1.6 Gigabit/second, 25-85 kHz acoustic imaging array-novel mechanical and electronics design aspects

Abstract

This paper addresses some novel mechanical and electronic design aspects of a high frequency Ambient Noise Imaging (ANI) system being built at the Acoustic Research Laboratory of the National University of Singapore. Modularity in construction of the array permits upgrading and servicing of the array at sensor module level. A neoprene acoustic window without any coupling medium is used on ceramic faces in place of conventional potting. This greatly simplifies the design and maintenance of the array. Test results showed that sensors with dry-coupled neoprene perform as well as ones with conventional polyurethane-based encapsulation. A hierarchical noise reduction strategy is employed in the array design to reduce performance degradation due to rear-propagating noise and structural noise transmission. The whole array has been engineered so as to keep the cabling to a minimum. Different units, called modules, of the array can be simply plugged in or taken out without the need for soldering or using interconnecting cables. The mechanical design of the array has been optimised using FEM analysis. Signal acquisition and conditioning electronics are placed well close to the sensors to give a compact module design and it provides a digital output. Another novel part of the design is the use of 54 Pentium processors networked in two loop topologies employing the Fibre Channel protocol for the wet-end processing. Each of the loops carries 0.8 Gbit/S of data. Results of the sensitivity and beam pattern measurements carried out on a prototype array, a scaled down version of the main array, are also presented.