Abstract: The web in a high speed printing press passes between a transmitter and receiver, the former of which produces ultrasonic sound which the latter receives, after attenuation by the web, and the latter of which converts the sound into an oscillating electrical signal of corresponding frequency. This electrical signal, after being amplified and filtered, is impressed upon a peak detector and a comparator. The peak detector produces a constant potential equal to the amplitude of the peaks in the amplified signal, and this constant potential is reduced a predetermined proportion and is applied to the comparator as a reference potential. The comparator compares the reference potential with the amplified oscillating signal. The reference potential remains undisturbed even though the oscillating signal may momentarily drop due to the further attenuation of the sound caused by a splice moving through it.