Digital audio recording analysis: the Electric Network Frequency (ENF) Criterion

This article reports on the Electronic Network Frequency Criterion as a means of assessing the integrity of digital audio evidence. A brief description is given of phenomena that determine ENF varaiations. In most situations,to reach a non-authenticity option, the visual inspection of spectrograms and comparison with an ENF database are enough. A more detailed investigation, in the time domaine, requires short time windows measurements and analyses. The stability of the ENF over geographical distances has been established by comparison of synchronized recordings made at different locations on the same network. A real case is presented, in which the ENF Criterios was used to investigate an audio file created with a secret surveillance system. By applying the ENF Criterion in forensic audio analysis, one can determine whether and where a digital recording has been edited, establish whether it was made at the time claimed, and identify the time and date of the registering operation.