What is "ultrasonic multifeed" and how is it useful?
Ultrasonic multifeed detection is the best way to prevent multiple documents feeding through the scanner at the same time. When these "multifeeds occur," the page on the bottom is not scanned and the data on that page is lost. Ultrasonic multifeed detection consists of two sensors: one above the paper transport and one below. The top ultrasonic sensor emits an ultrasonic wave through the document. Normally, only one document is being fed through at a time, causing this wave to pass through the paper and directly into the bottom sensor. If multiple documents feed through, small gaps of air get trapped between the pages. When the ultrasonic wave hits these air pockets, the wave bounces off in multiple directions and is not picked up by the bottom sensor. If the bottom ultrasonic sensor does not detect the wave, it stops the scanner's paper transport and indicates that a multifeed has occurred to the user. Thus, ultrasonic multifeed ensures that multiple documents don't feed through simultaneously and that no data is lost. Other technologies like length and width detection and infrared fall offer basic multifeed detection, but can be easily fooled. Multifeed detection can also be turned off, if the user prefers. But a better method, is to ignore the multifeed by length. The “ignore by length” can be set up to a maximum of 10 inches. That will accommodate must documents that have things taped to them, stickers, or envelopes. Using the multifeed detector in that mode will still detect true multifed documents.