Because the evanescent wave into the solid sample is improved with a more intimate contact, solid samples are usually firmly clamped against the ATR crystal, so that trapped air is not the medium through which the evanescent wave travels, as that would distort the results. The signal to noise ratio obtained depends on the number of reflections but also on the total length of the optical light path which dampens the intensity. Therefore, a general claim that more reflections give better sensitivity cannot be made.