Unlike other kinds of permanent recordable media (such as photographic film), only magnetic tape has the unique ability to be used and reused virtually thousands of times. The process that makes this possible is erasure.

Erasure is a fundamental step in achieving high quality recordings. Although the process of erasing tape is a simple, everyday procedure . . . usually performed with manual bulk degaussers or automatically with recording systems . . . the scientific process itself is not commonly understood. This issue of Sound Advice is intended to help promote understanding of that process.

The first step in understanding tape erasure is to know the composition and behavior of the magnetic material used in tape. The most commonly used material is the gamma form of iron oxide, or gamma Fe203. This material exists as tiny, needle shaped particles. Each particle is a permanent magnet which cannot be demagnetized.

When subjected to an external magnetic field of sufficient intensity, these particles will accept the polarity of the magnetic field they encounter; the induced magnetism will not be greater or less in magnitude than the original field.

A magnetic tape contains thousands of magnetic particles throughout the width and depth of its recording tracks. During recording these particles are uniquely magnetized. The magnetic field emanating from a recorded area, and translated through the reproducing head, results in sound.

In the erased condition, half of the magnetic particles on a tape have one polarity and half retain the opposite polarity. These particles cancel each other out, producing zero external field (and zero sound) from the tape.

This polarity cancellation, of course, is performed by the erase heads of a recording machine or by a bulk degausser.

All magnetic materials have an intrinsic property called coercivity, usually measured in units designated as oersteds.

Coercivity determines the intensity of a magnetic field that is required to erase magnetic material. Because the coercivity of all the particles on a magnetic tape is not exactly equal, a magnetic field approximately three times the coercivity of the tape is required to adequately erase all materials. A low field of intensity can erase particles with a low coercivity, but a much higher magnetic field is required to erase all of the particles

Since standard audio tape has a coercivity of approximately 300 oersteds, adequate erasure of information occurs if a field approaching 900 oersteds (3 times the tape coercivity) penetrates the reel .

When bulk degaussers areused to erase tape, the magnetic field generally penetrates a reel or the wall of a cassette from the side . . perpendicular to the tape. On the other hand, erase heads on a recording system generate intense parallel as well as perpendicular erasure fields. As a result, they are some what more effective than bulk degaussers.