The ancient Greeks were placing the last few stones on the magnificent theater at Epidaurus in the fourth century B.C. In the ancient Greece, it was one of the wonders of its time.

Unwittingly, the ancient builders made a sophisticated acoustic filter that enabled audiences in the back row to hear music and voices with amazing clarity, thousands of years before modern acoustic could do this.

The ancient Greeks tried to replicate the Epidaurus' design, but without much success.

A team at the Georgia Institute of Technology have found the factor missed by the ancients while looking for reproducing of this acoustic marvel: its seats, not its slope, or the wind.

The rows of limestone seats at Epidaurus made an efficient acoustics filter that eliminated low-frequency background noises like the murmur of the public and reflected the high-frequency noises of the actors off the seats and back toward the seated spectators.

This way, the performer's voice was reaching the back rows of the theater.

Some theories assumed that site's wind, which blows primarily from the stage to the audience, would have been the cause, while others thought that masks could have acted as primitive loudspeakers or that the rhythm of Greek speech had something to do with it.

There were also assumptions that the slope of the seat rows could have been implied in the acoustic.

But acoustician and ultrasonics expert Nico Declercq suspected that the theater's corrugated limestone structure was filtering the sound waves at certain frequencies.

"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."

But when his team tested with ultrasonic waves and numerical simulations of the theater's acoustics, the researchers discovered that frequencies up to 500 Hz were retained while frequencies above 500 Hz were reflected, a similar effect to the ridged acoustics padding on walls of a parking garage.

"So, how did the audience hear the lower frequencies of an actor's voice if they were being suppressed with other background low frequencies? There's a simple answer," said Declercq.

The human brain can figure out the missing frequencies through a phenomenon called virtual pitch which enables us to appreciate the incomplete sound coming from small loudspeakers (in a laptop or a telephone), even if the low (bass) frequencies are not emitted by a small speaker.

"The Greeks' misunderstanding about the role the limestone seats played in Epidaurus' acoustics likely kept them from being able to duplicate the effect. Later theaters included different bench and seat materials, including wood, which may have played a large role in the gradual abandonment of Epidaurus' design over the years by the Greeks and Romans," said Declercq.