The Noise Floor: A Forensic Aid for Audio Authentication and Voice Identification

noise floorAudio authentication and voice identification requires that a forensic expert examine three critical aspects of an audio recording before beginning any forensic process. Whether it be analogue or digital audio recording, an audio forensic expert should inspect the consistent characteristics of the sound wave formations; listen critically to various tones present in the recording, background noise (noise floor) of the audio recording; and examine the electronic spectrograph measurement. These three critical aspects of an audio recording must be consistent throughout the recording to determine authenticity.

An audio forensic expert has been trained by examining hundreds, if not thousands, of hours of audio recordings. This experience helps the forensic examiner to develop a critical listening skill far more precise than the average persons. That keen sense of sound perception is very important for audio authentication and voice identification.

During the examination process, regardless of analogue or digital audio examination, it is advantageous that the original recording, and recorder, as well as other recording equipment (wireless transmitter, microphone) also be examined. That way, the forensic examiner can recreate the characteristics of the audio recording including signatures (stop-start) and noise floor.

The noise floor is a critical aspect in audio authentication as well as audio identification because it provides the forensic examiner a second dimension of sound to examine and authenticate other than the main recorded signals (speech, gunshot and voice mail).

Alterations in an audio recording, analogue or digital, most likely will be first detected by a change in the noise floor of the audio recording followed by an anomaly that can be heard auditorily, measured electronically and viewed on the computer screen by examining the wave form.

Part of this noise floor is the background noise of the recording. It is the sounds present on the audio recording that the author had not necessarily intended to have recorded but is still part of the recording that is helpful to a forensic examiner.

Both analogue and digital audio recordings have background ambient noise, the noise floor, when the speech or other audio recorded is not present. This background noise speaks volumes on whether the audio recording being examined is original, authentic or has been altered or edited in addition to the examination outcome of the main recorded signals.

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