The invention relates to a method for verifying thickness modulations, in
particular watermarks, in or on sheet material. To diminish the influence
of disturbances from the surroundings and reduce the adjustment effort
during production and maintenance of corresponding measuring systems, the
sheet material is subjected to sound, in particular ultrasound, the sound
transmitted through the sheet material and/or reflected on the sheet
material is measured at a plurality of places on the sheet material, at
least one value characteristic of the transmitted and/or reflected sound
is generated for each of the places, and the thickness modulations are
verified by comparing the values with predetermined reference values.
