x1 is infrared or infrasound vel sim. (sensu lato; having too large of a wavelength to be visible, audible, or relevantly sensed/detectable) for sense/sensory device/by standard x2 in medium/under conditions/in frame of reference x3.
x2 also determines whether it is color/visible, sound, etc. (that is: which sense would be doing the detecting). Note that infrared to the eye (which sees color) is not infrared to the skin (which feels heat).