An Auditory Filter is a device used by Starfleet Intelligence.
This device was the size and shape of a Medical Feinberger. Sometimes called a white-noise generator, or 'masker', it can filter out selected sounds from other background noise to detect one specific sound, voice, or body function (such as heartbeat). It must be used with a science tricorder or ship's computer. The auditory filter uses one standard small-equipment energy cell, good for twelve hours of continuous use. (FASA RPG module: Star Fleet Intelligence Manual: Agent's Orientation Sourcebook)