How Loudspeakers Work 37 2.7 Modulation Distortion With any crossover the drive units have to handle a range of frequencies, and this leads to the problem known as modulation distortion. What has been called "total modulation distortion" [4] is made up of two components: amplitude modulation distortion and frequency modulation distortion. The first is intermodulation distortion caused by non- linearities in the drive unit. The second is a result of the Doppler effect, a low-frequency movement of the cone causing frequency modulation of the higher frequencies present. Both modulation distortion components are reduced by limiting the range of frequencies that the drive units handle and by using steep crossover slopes. Further Reading If you want to get deeper into loudspeaker design issues, two excellent books are Loudspeakers by Newell and Holland, [5] and The Loudspeaker Design Cookbook by Vance Dickason [6]. References http://en.wikipedia.org/wiki/Thiele/Small (accessed Dec 2010). [1]