Oh, yes, it's the resonance frequency and the frequency of whatever, like horses or other things in unison. Think of a child on a playground swing. At the right time a small push and the amplitude is greater if that push is right after the extreme limit of travel. If the push is just before the limit, amplitude is reduced. John Gardner wrote: > 20-odd hoss & riders that different from 10-ton truck? > > Jack > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist