Sub-audible noise spectra measurement.

An apparatus has been constructed to extend measurements of electrical noise spectra to sub-audible frequencies. The system of measurement incorporates a process of recording the noise magnetically and later playing the noise back for analysis at a much higher speed. The process makes low frequency noise measurable by conventional apparatus in the audio frequency range. The apparatus has been tested and found to be linear and independent of frequency in its range of use. The noise power spectra of a 6SJ7 pentode has been measured. Above 10 kc/s shot noise predominated, at lower frequencies flicker noise became important. The l/f-spectrum flicker noise was observed to exist down to 0.02 c/s, the lower frequency limit of the apparatus