Forensic audio experts began studying recordings during World War II. While first used to identify enemy voices in radio broadcasts, forensic experts created a need for the invention of the spectrograph, a device that graphs the frequencies of voice patterns. The sound waves are analyzed after they are represented in a graph of time, intensity and frequency. Spectrograph evidence is widely admissible in a court of law, according to Wire.com. Spectrograph technology is now integrated into most digital audio forensic software to create a unique voice print of a person, and is used to investigate audio tapes of terrorists.
The advent of technological advances in digital audio gave forensic audio experts opportunities to use more powerful tools. Computer programs such as DC Live/Forensics reduce background noise in order to make voices more discernable, where the sole need of the forensics expert is to eliminate all ambient noise. Programs have been designed to help reduce background noise during audio restoration. In connection with close listening and the use of a computerized spectrograph, audio forensic experts examine syntax and vocal context for identification. Many software programs allow the audio examiner to identify sounds that are considered "noise" and should therefore be removed from the recording. They then create a new recording after eliminating specific groups of background noises.
In order to authenticate tape recordings, which means to determine whether they have been edited or altered in any way, an audio forensics expert may use a microscope to physically look at the tape using a magnetic developing solution, according to Digital Forensic Investigator News. The recording is separated from its electric network frequency, which helps determine where and when a recording was made by comparing it to data from the National Grid, a database in use in the United States and the United Kingdom. The database stores frequency fluctuations during any given time across any location.