每个场景结束时录一段没有对白的环境声。剪辑时用它填补对白之间的空隙，能避免背景噪声突然断掉；同时记录麦克风位置和增益，方便补录。 Record a clean room-tone pass at the end of every scene. Use it under dialogue edits to prevent the background from dropping out, and note microphone placement and gain for pickups.