A knee protecting airbag device comprises an airbag, an inflator, a case and an airbag cover to be assembled with the case. A short-time Fourier transformation section generates spectrograms of observation signals from observation signals in time domain. The mobile computing device may include a radio processor that may select from among several signal search procedures based on whether the device is in motion, and whether a signal is detectable and adequate. The spherical bearing struts constrain a gantry-load direction displacement of the x-ray tube and enable rotation and displacement of the x-ray tube around at least one axis of rotation or in one direction other than a gantry load direction, so as to accommodate thermal displacements in the x-ray tube created during operation thereof.