A packet inspection apparatus is described. The reverse flow preventing device has a frame, a plurality of support shafts arranged about the center of the frame and mounted on the frame, and a plurality of pivotable plate members supported by the support shafts. The palate-side wall has an outer contour which establishes a guideway towards the opening of the patient’s esophagus. The first signal is split into multiple signals, each of the multiple signals being associated with a different portion of the band of radio frequencies. The spatial filter is applied to either the vertical particle motion data or to the pressure data, and then combined with the other data to generate pressure data that has its up and down propagating components separated.