When two 音 are 感知（认为） simultaneously other 音 often appear because of 失真 效果 in the 耳 Relatively 高 强度 级 are required for 组合 音 to be 听 and strong differences 存在 between individuals as to how many are 听 If two 正弦 音 are played at a sufficient 强度 级 one 高 x and the other 低 y the 组合 音 通常 听 are the difference 音 at 频率 相等 to x - y 2y - x and 3y - 2x The 总和 音 of 频率 x + y and 2x + y are seldom if ever 听 even when in the 可听 范围（排列） possibly because of 掩蔽 效果 源 Barry Truax - Handbook for Acoustic 生态学 CD-ROM Edition Cambridge Street 出版 1999 - CSR-CDR 9901