Does Magnetism Affect Tape Recordings?

If you have a collection of tape recordings, you may be concerned about their long-term prospects. While heat and humidity will affect their viability, you should also be aware of the effects magnetic fields have on them. The tapes, once recorded, continue to be sensitive to magnetism. This means they can be erased and reused many times, though it also means they'll be vulnerable to a variety of magnetic effects. Though a stray magnetic field probably won't erase them completely, it may degrade their quality over time.

Distance

Distance plays an important role in protecting tapes from magnetic fields. The force exerted by a magnet falls off rapidly, as the inverse cube of the distance. A few inches apart, two magnets barely influence each other. Slightly closer, they will suddenly attract and snap together. This means even strong conventional magnets have influence for only short distances. Keeping external magnetic sources at least a foot or two away from your tapes will keep them in good condition.

Degaussing

A common way to re-use tapes is to bulk degauss or demagnetize them. This involves placing the tapes in a device with a powerful AC magnetic field, or "wiping" the reels with a hand-held degaussing tool. This removes all the recorded information from the tape, returning them to a clean state. The tapes should be reviewed carefully beforehand. Once erased, the information can't be recovered.

Stray Magnetism

Refrigerator magnets, toys containing magnets, and other similar items must be kept away from tape recordings. If they come within a few inches of the tape, they will degrade or erase the material, whether it's music, data or video. You need to be careful with neodymium magnets in particular, as their fields can be quite strong. They usually have a bright nickel finish. Ceramic and plastic magnets are weaker but still able to affect a recording if they come close enough.

Other everyday items that have magnetic fields include AC wall adapters, speakers, electric motors, and stereo amplifiers. Power cables and extension cords will have a small field around them if they're carrying current.

Print-Through

Given enough time, magnetic patterns on tightly-wound tape will bleed onto adjacent layers. This is called "print-through" and can be heard on audio recordings as pre- and post-echoes of loud sounds during quieter passages. For example, at the end of a quiet part of music, you may hear a faint echo of loud percussion or brass instruments that immediately follow. The thickness of the tape and its age affect the chances for print-through.

Residual Magnetism

Magnetism builds up on tape deck parts that come into contact with the tape. This includes both metal parts and rubber pinch rollers. This buildup will slowly degrade tapes played on the machine. It comes from metal parts that become magnetized and from accumulations of tape oxide.

To keep this from being a problem, regular tape-deck maintenance includes degaussing all the parts that touch the tape, combined with careful cleaning with alcohol swabs.

References

• Inverse Cube Law
• Care and Handling of Magnetic Tape