Despite what many theories, Researchers at the Georgia Institute of Technology recently discovered the real reason for the great acoustics at the ancient Greek amphitheater, Epidaurus. The reason that you can hear an actor's voice all the way in the back of the massive arena was not because of the slope or wind patterns as some suspected, but rather the rows of limestone seats. The limestone created a highly efficient acoustics filter that deadened the lower frequency chatter of the crowd and reflected the higher frequency voices of the actors on stage all the way up the rigid structure to the top of the nosebleed section. Their experiments revealed that frequencies below 500 Hz were hushed yet frequencies above 500 Hz were being echoed throughout the amphitheater. Audience members were able to hear the lower frequency sounds of the actors' voices through a phenomenon called "virtual pitch", where our brains are capable of reconstructing frequencies and incomplete sounds in order to interpret noises that aren't really there. This is just one interesting example of how the audience was able to hear.
.Source: www.newswise.com