SOAE levels fluctuate widely while their frequencies stay remarkably stable

Evidence

Some SOAEs are not very stable in level. An emission appearing before a weekend may not be found afterwards, and vice versa. Even during the same day, spontaneous emission levels can vary distinctly. The frequencies of the spontaneous emissions, however, are very stable. Although emissions disappear, they show up again at almost the same frequency. The variation in frequency is less than 1 % and in most cases it is only a few parts per thousand. Emissions with larger levels seem to be somewhat more stable than emissions with low levels. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 259