Two nearby frequencies alternate between reinforcement and cancellation. Their sum has a rapidly oscillating carrier and a slowly varying amplitude; successive loudness peaks are separated by the reciprocal of the frequency difference.两个相近频率的振动交替相长、相消。叠加结果包含快速振荡的载波和缓慢变化的振幅；相邻响度峰值的时间间隔是两者频率差的倒数。
The green trace adds angular frequencies 10 and 11. The gold boundaries show ; the amplitude vanishes where those boundaries meet.绿色曲线叠加角频率为 10 和 11 的振动。金色边界为 ，边界相遇时振幅降为零。