The mystery of the amazing sound of the Greek amphitheater is solved
Stop Talking! The ancient mystery of the magnificent acoustics of the theater at Epidaurus in Greece has been solved.
The theater, dating back to the 4th century B.C. and arranged in 55 semicircular rows, remains the great masterpiece of Polycletus the Younger. Audiences of up to 14,000 have long been able to hear the actors and musicians without amplification, even from the back row of the architectural masterpiece.
How this sound quality was achieved has been the source of academic and amateur speculation, with some theories suggesting that prevailing winds carry the sounds or mask the amplified voices.
Echoes in the seats
Now researchers at the Georgia Institute of Technology have discovered that the limestone material of the seats provides a filtering effect by suppressing the low frequencies of voices, thereby minimizing the background noise of the crowd. In addition, rows of limestone seats reflect high frequencies back to the audience, amplifying the effect.
Researcher Nico Declerk, a mechanical engineer, initially suspected that the tilt of the theater had something to do with the effect.
“When I first encountered this problem, I thought the great acoustic effect had to do with the surface waves rising through the theater with virtually no attenuation,” Declerk said. “While the performers’ voices were sounding, I didn’t expect that the low frequencies of speech would also be filtered out to some degree.”
However, ultrasonic wave experiments and numerical models showed that frequencies up to 500 hertz (cycles per second) were reduced and frequencies above 500 hertz were not reduced, he said.
Acoustic traps.
The corrugations on the surface of the seats act as natural acoustic traps. Although this effect would also seem to remove low frequencies from actors’ voices, listeners actually fill in the missing part of the sound spectrum through a phenomenon known as virtual pitch. The human brain reconstructs the missing frequencies, creating a virtual pitch phenomenon, as when listening to someone speaking on the phone without low frequencies.
Surprisingly, the Greek theater builders themselves did not understand the principles that led to the exceptional audibility of sound from the stage.
Attempts to recreate the design of the Epidaurus never matched the original. Later seating used other materials, such as wood for benches, which may have ultimately thwarted efforts to duplicate the design.