Modern tactics for carrying out military and antiterrorist operations calls for the development of a new generation of enhanced portable infrared imagers. The high performance of these imagers relies on the focal plane arrays, which are maintained at cryogenic temperatures using rotary Stirling cryogenic engines. These engines are known as powerful sources of wideband vibration export. For the sake of weight and compactness, the enclosure of the above imager is usually designed in the form of a light metal thin-walled shell, accommodating a directly mounted Infrared Detector Dewar Cooler Assembly. The operation of the device typically leads to an excitation of the inherently lightly damped structural resonances and therefore, to a radiation of the specific acoustic signature capable of compromising the aural stealth of the IR imager. Such a noisy IR imager may be detected from quite a long distance using enhanced sniper detection equipment or even aurally spotted when used in a close proximity to the target. Numerous efforts were taken towards achieving the desired inaudibility level, apparently becoming one of a crucial figure of merit characterizing the portable IR imager. However, even the best examples of modern should-be silent imagers are quite audible from as far as 50 meters. The presented research intends to improve the aural stealth of the portable IR imager by using three different approaches: First, by compliantly mounting the Infrared Detector Dewar Cooler Assembly where the stiffness and damping of the vibration protective pad are optimized for the best acoustical performance without developing excessive line of sight jitter. Secondly, by using the concept of the weak radiator to reshape the enclosure mode shapes, and finally developing a multi-modal distributed dynamic absorber (MMDA) to enhance the absorption of the vibrating structure. The multi-modal characteristic of such a dynamic absorber makes it highly dynamically reactive through a wide frequency range (20 kHz) of excitation. It will be shown that incorporating a MMDA into the vibrating structure will result in ultra range vibration attenuation, making the IR aurally silent.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.