Patent ID: 8181958

Claim:
A stiffness detector comprising: a bending portion configured to bend a paper sheet to be carried; a transmission module configured to transmit acoustic waves to an incidence point on an inner surface of the paper sheet which is bent by the bending portion and is carried, thereby exciting Lamb waves, wherein the transmission module is disposed such that the acoustic waves enters the incident point on the inner surface of the paper sheet at a predetermined angle; a reception module configured to receive leaky waves of the Lamb waves emitted from a detection point on the inner surface of the paper sheet, wherein the reception module is disposed such that an output direction of the acoustic waves is parallel with an incidence direction of the leaky waves entering the reception module; and a first judgment module configured to specify a maximum crest value based on an output from the reception module and judge whether the paper sheet is an unimpaired sheet based on the specified maximum crest value.