For the sake of weight and compactness, the enclosures of the modern portable cryogenically cooled infrared (IR)
imagers are made in the form of a light metal (aluminium, magnesium, titanium) thin-walled shell, serving as an optical
bench, accommodating a telescope, an optical train and an Infrared Detector Dewar Cooler Assembly (IDDCA). Such
IDDCAs normally rely on miniature rotary Stirling cryogenic coolers, which are known as powerful sources of wideband
vibration giving rise to the inherently lightly damped structural resonances in the imager enclosure thus causing loud
structure-borne noise. This may lead to an increased range for aural detectability of forward observers who must remain
undetected, potentially for long periods of time. Consequently, the aural nondetectability distance becomes one of the
crucial figures of merit (along with the overall weight, battery life, imagery quality, etc) characterising the modern
portable IR imager.
In the novel approach, the IDDCA is mounted within the enclosure using a special silencing pad; effectively attenuating
vibration export over the typical high frequency range that contains the relevant structural resonances of the enclosure.
The residual noise radiation from the imager enclosure is then attenuated practically to a background level by reshaping
the radiation modes thus cancelling the overall volume velocity. This is achieved by finding the "critical point" and
affixing there the optimally sized correction mass. The authors report on a successful attempt to develop a cooled imager
that is inaudible at greater than 10 meters (even during the cool down phase) per MIL-STD-1774D (Level II).