https://blog.gongquiz.com/2021/01/12/40phon-%ec%9d%b4-1sone-%ec%9d%bc-%eb%95%8c-60phon-%ec%9d%80-%eb%aa%87-sone-%ec%9d%b8%ea%b0%80-%ec%82%b0%ec%97%85%ec%95%88%ec%a0%84%ec%82%b0%ec%97%85%ea%b8%b0%ec%82%ac-2013%eb%85%84-3%ec%9b%94-10/