| 摘要 |
In a machine for processing mail articles, the method of distinguishing between mail articles (P1, P2, P3) contained in envelopes made out of paper and mail articles contained in envelopes made out of plastics material, consists in displacing each mail article in front of a pressure member (5) so as to produce an acoustic signal that results from a suction effect of the pressure member on one face of said article while it moves in front of said pressure member, and in comparing said acoustic signal with a reference signal in order to detect whether the envelope of said article is made out of paper or out of plastics material.
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