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MEASUREMENTS of noise spectra on a conventional unequalized tape recorder give results such as those shown in Fig. 1, where the root-mean-square (rms) noise was measured on a 1/4-inch full-track unequalized recorder at 15 inches per second. Three types of noise can be distinguished: the noise from bulk-erased tape; noise, similar in spectrum shape but some 15 db (decibels) higher in level, from uniformly saturated tape; and noise, with a pronounced 20-db peak at long wavelengths, from tape magnetized non-uniformly by feeding direct current into the record head. The first type of noise can be attributed to the magnetic particles themselves, the second to imperfect particle dispersion (bulk inhomogeneity), and the third to variations in contact between the surfaces of the tape and the record head.1
Eric D. Daniel (Fri,) studied this question.