Subject to both environmental (location, situation) and internal (emotional mood) factors, the human voice frequently changes in terms of tone and pitch.
語音處理也有類似問題。每個人、甚至同一個人在不同地方、不同狀況、不同情緒時，說話的語氣、音調都會不一樣，是電腦辨識上的一大難題。