One of the big advantages of magnetic tape over any other recording medium is that the stored information can be erased and the tape used over and over again. If, however, tape is to be used as a permanent store of information, as in a Sound Archive, this property is a very big disadvantage indeed. It would be fine if one could - by some still unknown process similar to that of fixing a photographic picture - freeze the once recorded sound for good.
Aside from complete erasure, which can be considered the extreme case, the magnetic inscription on the tape can be influenced by a number of factors including:
In addition, and perhaps most dangerously, the magnetic inscription can be influenced by external magnetic fields.
G.A. Knight and others have thoroughly investigated the influence of AC and DC magnetic fields on stored tape. In particular, they studied print-through, partial erasure, increase in distortion and noise, etc., caused by external fields originating from electric machines, lightning strikes and the like.
In his paper G.A. Knight suggests that a value of 25 Oe (not distinguishing between AC and DC) may be accepted as a level “below which very little degradation is likely to occur”. No information is given, however, on the degree of degradation that is considered to be acceptable. Also, there appears to be no literature on what happens to tape being moved through a stray magnetic field.
At the Phonogrammarchiv, we came across this question when we discovered that many technical devices used in a studio exhibit quite strong stray magnetic fields - in fact, even tape recorders of prominent manufacture show field strengths which, at first, shocked us and then prompted a closer study. The question is, what happens to a pre-recorded tape travelling through a magnetic field, AC or DC, of, say, 5 Oe, or 50 Oe, or more.