Authentication of digital recordings by tracing the electric network frequency (the ENF criterion) has shown promising results. This methodology is possible due to the presence of electromagnetic fields radiated from all kinds of electric equipment and connected power lines. In the ENF range - around the 50/60 Hz - the magnetic component is dominating. In this paper the results of magnetic field strength measurements carried out in different relevant environments are presented. Also presented are the results of an experiment performed in order to quantify the thresholds by which the magnetic fields leave traceable ENF in various battery powered digital audio/video recording equipment. In this experiment no battery powered digital recording device equipped with electret microphones were susceptible to the magnetic field.
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