IU possesses a soprano vocal range, with acoustician and professor Myung-jin Bae expressing her range also has mezzo-soprano qualities when writing an article comparing her voice to Hyun Bin. Her vocal ability is often met with positive reception. Monique Danao of South China Morning Post noted that IU could quickly transition from a "sweet" voice to "hit powerful notes". On his 2011 academic paper co-written by Doo-heon Kyon at Soongsil University, Bae used "Good Day" as the basis for his vocal analysis, stating that when IU reached the highest note (F♯5) in the three-note increment, her stability in frequency showed a lengthy duration with abundant lung capacity, and commenting that she "expresses well beyond the highest range required by the soprano [voice type]"