A data acquisition low pass filter sounds like a technological mouthful. While it is a lot to say, what it does is nothing short of miraculous. Typically, it is used in criminal investigations, and was originally designed for use in field operative/spy work. To get a better understanding how this technology applies to police work, the following is provided.
Removing "White Noise" to Clarify Sounds
When a caller calls the police and asks for help or makes a threat or abduction demands, there is almost always some noise in the background. While the police conduct their investigation, a forensic technology expert uses the low pass filter technology to sort through the recording to find background sounds, such as a church bell, a train whistle, or some other sound that would indicate a location at the time of the call. Such filtered sounds become data and digital leads on which the detectives can follow up. Similarly, the process can be used on voices.
Getting Unique Vocal Patterns
Even people who are experts at mimicking voices do not realize that every voice has its own vocal "fingerprint." When the voices are run through low pass filter technology, it becomes clear who the mimicker is, and who the real voice is. Patterns in shifting tone and emphasis turn into a digitized readout, clearly identifying the speaker. Ergo, police are able to use this technology to sort a real voice out from a voice belonging to someone that is attempting to disguise his/her voice. A clear recording of the actual voice is produced, which further assists officers with finding the caller(s).
Criminals are unaware of how police use such technology. However, hostages and kidnapped victims can use this to their advantage, if they know a few things. For example, if the victim knows Morse code, he/she can tap out valuable information in secret while still in the presence of his/her abductors simply by tapping the code out on the wall, on the hard arm or part of the chair to which they are tied, etc., whenever the abductors are on the phone with the police. Singing a song with words in it where the person applies a loud emphasis to specific words may also work, if the expert applying the low pass filter can recognize what the victim is doing and trying to communicate. Both of these tactics can help lead the police right to the victim's location, when the filter technology is applied and the signals are deciphered.