Straight from the horse's mouth

Move over Mister Ed. Scientists working on the University of Rhode Island's Equine Vocalization Project are compiling an acoustical analysis of horse vocalizations to determine how stress is communicated when horses whinny. The analysis determined that a horse's whinny consists of two elements: a constant tone with varied harmonics that increase as the horse becomes more agitated and a variation in frequency that may be associated with communication or expression.

Scientists found that all equine vocalizations, including those of donkeys and zebras, have complex spectrums, wide bandwidths, and varying frequencies. And while a donkey's bray is acoustically rich, they have little control of the content of their sounds. Still, all equine sounds are much more complex than those of most domestic animals. Under high-stress conditions, a horse's whinny can sound like a high-pitched scream, but in calm conditions, especially when a horse's vision is restricted, scientists say that the whinnies have a rich and variable sound.