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3,928,777 A 12/1975 Massa 4.329, 838 A 5, 1982 Zerle et al. 4,330,909 A 5, 1982 Peschke et al. 4,366,406 A 12/1982 Smith et al. 4,523,122 A 6, 1985 Tone et al. 4,532,167 A 7, 1985 Mohr 4,606,486 A 8, 1986 Brunner et al. 4,727,260 A 2f1988 Krauth 4,760,626 A 8, 1988 Lonner et al. 4,780,631 A 10/1988 Groninger 4,848,632 A 7, 1989 Macket al. least a portion of the at least one web material traveling along a predetermined travel path interferes with at least a portion of the ultrasonic signals transmitted by the at least one transmit terblock. The receiver output signals generated by the at least one receiver block is indicative of the position of the web of material as the same is moved along the travel path between the transmitter block and the receiver block. In an alternative embodiment, the functions of the at least one transmitter block and the at least one receiver block are performed essen tially equivalently by at …