Her voice became perceptibly louder as she spoke.

perceptibly 是形容词 perceptible（可察觉的）的副词形式，由词根 percept（感知）和后缀 -ible（能够...的）构成，再加上副词后缀 -ly。perceptibly 意思是"可察觉地、明显地"，表示变化或差异能够被明显感知到。