monotone; monotonous; dull; lacking variety
彼の話し方は一本調子で退屈だ。
His way of speaking is monotonous and boring.
一本調子な演奏に聴衆は飽きた。
The audience grew tired of the monotonous performance.
文章が一本調子にならないよう工夫する。
I take care so the writing doesn't become monotonous.