Forensic audio authenticity has developed remarkably over the last few years due to advances in technology of digital recording processing. The ENF (Electric Network Frequency) Criterion is one of the possible tools and has shown very promising results in forensic authentication of digital recordings. However, currently there are very few experiments and papers on studying the source of ENF signals existing in digital recordings. In addition, it is unclear whether or not there are detectable ENF traces in battery-powered digital audio recordings. In this paper the study of ENF source in battery-powered digital recordings is presented, and it shows that ENF in these recordings may not be mainly caused by low frequency electromagnetic field induction but by low frequency audible hum. This paper includes a number of experiments to explore the possible sources of ENF in battery-powered digital recordings. In these experiments, the electric and magnetic field strength in different locations is measured and the results of corresponding ENF extraction are analyzed. Understanding this underlying phenomenon is critical to verify the validity of ENF techniques.
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