An appropriate acoustic environment is necessary in music practice rooms, and the preferred room acoustic conditions vary depending on various factors such as proficiency level, motivation, and contents to be practiced. In this study, we focused on practice elements that vary during the process of mastering a piece and investigated the preferred room acoustic conditions of solo singers for practice. To study the room acoustic condition "Sound Cask", a 3D sound field simulation system based on the boundary surface control principle, was used. First, an interview survey was conducted to understand the structure of the practice elements in the singers' process of mastering a piece. As a result, a three-step process was identified. The first is score reading, second is exploration of expression, and third is the ensemble and stage performance. Subsequently, a subjective experiment was conducted to evaluate three different room acoustic conditions (short, moderate, and long reverberation time) by asking the singer to play an assigned phrase of a piece considering the three-step process in each acoustic condition. Consequently, it was confirmed that the preferred acoustic conditions were changed during the process of mastering a piece.