。
The mean frequency decreases as the amplitude increases.
Literal
Frequency [attributive-の] average-value [topic-は] amplitude [attributive-の] increase [in-accordance-with-にしたがって] decreases.
Technical/scientific (physics). ~にしたがって ('in accordance with, in proportion to, as X') is a formal pattern for correlation — used in science to state 'as X changes, Y changes correspondingly.' 減じる ('to decrease') is a formal/literary verb, more so than 減る or 下がる. 周波数 ('frequency'), 振幅 ('amplitude'), 平均値 ('mean value') are all physics/engineering kango compounds.