拍（音） are a 形（曲）式 of 振幅 调制 As two 频率 are brought closer together the 拍（音） will gradually slow down and disappear when they become identical If two 音调 are about 15 赫兹 or less apart interference will result from their 类似 though 不 exactly identical 频率 Gradually they will move out of 相位 until at 180° 破坏性 interference results producing diminished 响度 When they move back into 相位 constructive interference will produce 增加 响度 拍（音） arising from the mistuned unison are called first-order 拍（音） and are both an acoustic and psychoacoustic 现象 拍（音） also may be 听 between 纯 音调 that are nearly an 倍频程（八度） fifth or fourth apart These are called secondary or second-order 拍（音） 源 Barry Truax - Handbook for Acoustic 生态学 CD-ROM Edition Cambridge Street 出版 1999 - CSR-CDR 9901