Scientists have now finally solved the mystery:
Now, researchers at the Georgia Institute of Technology have discovered that the limestone material of the seats provide a filtering effect, suppressing low frequencies of voices, thus minimizing background crowd noise. Further, the rows of limestone seats reflect high-frequencies back towards the audience, enhancing the effect.The funny thing is that the Greek builders of this theater didn't themselves understand why the audibility of sound was so great in this amphitheater. Attempts to recreate the Epidaurus design never matched the original.
Researcher Nico Declercq, a mechanical engineer, initially suspected that the slope of the theater had something to do with the effect.
“When I first tackled this problem, I thought that the effect of the splendid acoustics was due to surface waves climbing the theater with almost no damping,” Declercq said. “While the voices of the performers were being carried, I didn’t anticipate that the low frequencies of speech were also filtered out to some extent.”
However, experiments with ultrasonic waves and numerical models indicated that frequencies up to 500 hertz (cycles per second) were lowered, and frequencies higher than 500 hertz went undiminished, he said.