《汉书·志·五行志中之下》
当国君的严猛寡恩，就使下边的人闭口不敢说话，使臣子们提心吊胆，也就等于塞上自己的耳朵，于是胡听妄间之气变成声音发出来，所以有鼓妖。
君严猛而闭下，臣战栗而塞耳，则妄闻之气发于音声，故有鼓妖。
When the monarch is cruel and not gracious, it makes the lower class dare not speak out, making the courtiers panic, which is equivalent to blocking one' ears, so the naive and absurd air turns into a voice and comes out, so there are drum spirits.