For medium coercivity iron oxide pigments, as typically used for analogue open reel audio recordings (coercivity around 400 Oe (Oersted)) the maximum permissible field strengths have been determined to be:
5 Oe (= 400 A/m) AC
25 Oe (= 2000 A/m) DC
These thresholds have been set at 50% of those levels where influences of magnetic fields on pre-recorded audio tape have become measureable. Chromium and metal tapes have a higher coercivity.