Greece is a place where myths and history melt together in a tale that still enchants people of all races and age, although many centuries have gone by. In a small city of the Argolis, Epidaurus, Aslepius (also known as Hepius), son of Apollo and god of Medicine was born around the 5th century BC, or at least his cult was born there.
Below: Propylaea of the sanctuary of Aslepius in Epidaurus, Greece. Picture by Janmad shared via Wikipedia – licence CC BY-SA 4.0
The sanctuary of Aslepius was the most famous health centre of the classical society, where the ill people used to go looking for a cure. The patients would sleep over for a night and during the sleep the god himself, in their dreams, would indicate the right cure. The great fame of the sanctuary brought prosperity to the city, where many civil buildings started to bloom, one of which being the renown theater. To this day, the sanctuary of Aslepius is one of the better preserved monuments of the classical Greece.
Below: picture by Carole Raddato shared via Wikipedia – licence CC BY-SA 2.0
Its beauty and perfect symmetry are signs of perfection which fascinated the many people who passed through. From Pausanian, Greek traveller and geographer from the 2nd century AD to the American Henry Miller, who wrote:
“At Epidaurus, in the stillness, in the great peace that came over me, I heard the heart of the world beat”.
For many centuries, the mystery of the exceptional acoustics of the theatre gave rise to hypotheses and explanations amongst academics as well as amateurs on how this result had been achieved. The theatre, which dates back to the 4th century BC is the main work of the architect Polykleitos the younger. In its 55 steps (21 were added by the Romans), the theatre still can contain up to 14,000 people and, like any other Greek theatre, the beauty of the landscape behind the skēnē (the scene at the back of those who were acting on the stage) becomes integral part of the theatre itself.
Below: the theatre, picture by Andreas Trepte shared via Wikipedia – licence CC BY-SA 2.5
But leaving its aesthetic aside, it is its acoustic capability what makes of it a jewel of engineering from the past: any noise, even the one of a dropping coin, can be perceived with no amplifier whatsoever, from the proscenium all the way to the last row on the upper side.
Some theories suggested that it was due to the winds that sound and voices were able to be carried on each stair amplified.
Below: the theater, picture by Vislupus shared via Wikipedia – licence CC BY-SA 3.0
Thanks to the investigation of the researchers of the Institute of Technology of Georgia, it has been found out that the slabs of limestone that have been employed for the construction of the stairs function as filter. This allows to remove the low frequency waves of the sound. Furthermore the rows of the higher seats amplify the high frequency sounds which arrives from the stage.
Researcher Nico Declercq, mechanical engineer involved in the discussion, initially thought that it was the incline of the theatre that was delivering the incredible acoustics for the “sound waves that rise as if they were not deadened”. The surprise arrived later with the filer effect on the low frequency waves.
Below: the museum of Epidaurus, picture by Angela Monika Arnold shared via Wikipedia – licence CC BY-SA 3.0
The coarse stone of the stairs acts as a natural acoustic trap, absorbing le low frequencies from the voices of the actors. The audience compensated for the missing part of the audio spectrum with a phenomenon that take the name of “virtual pitch”: the human brain reconstructs the missing frequencies, such as during disturbed phone conversations. It is believed that the constructors of the Epidaurus theater were not aware of this exceptional acoustic qualities that the limestone possessed, considering that later on no one else in the Classical era obtained such a result.