Situated across the Saronic Gulf from Athens, the ancient greek city of Epidaurus is home to a 15,000 seat theatre that was once used for lavish dramas and banquets. It is well known for having remarkable acoustic properties, allowing voices from the stage to travel clearly to the very top row of seats, without amplification.
Researchers from Georgia’s Institute of Technology have discovered the science behind the Greek theatre’s incredible acoustics, carrying out in-situ measurements and using computer modelling to determine what effect the steep incline and unique layout have on the propagation of sound.
Previously, the cause of the remarkable acoustics found at Epidaurus were unknown. There were many theories to explain how well sound travelled in the theatre, ranging from wind amplification to specially shaped masks worn by the performers that were able to direct the sound up into the crowd. Others suggested that the steep incline of the seats meant that there was always a clear path between the performers and spectators. Due to GIT’s research it has been proven that the seating had the greatest impact on the acoustics of the theatre.
The researchers found that for low frequencies (those below around 530Hz) the seating did not diffract sound, while for higher frequencies the seating created reflections that constructively interfered with one another, effectively amplifying that area of the spectrum. The amplification was caused by negative second-order diffractions, sound that passes the listener is diffracted by the surface of the seat behind them, and then received a second time by the spectator. This meant that unwanted noise caused by the rustling of peoples movement, as well as the sound of the wind, was unaffected; the lack of diffraction at low frequencies meant that these sounds were at a much lower level than that of speech.
The spectators were then able to fill in the missing low frequency content through “virtual pitch”, when the higher frequency harmonics of the fundamental tone were present. In humans the fundamental frequencies of speech are between 85-155Hz for men and 165-255Hz for women. This is the same technique that allows small speakers (such as those found in telephones and radios) to create the illusion of bass.
Up until this research attempts to recreate this acoustic phenomenon have been unsuccessful. Although the incline and shape of the theatre has been mimicked in the past the stone seating which is now known to have provided the high frequency diffraction has not, instead cheaper and lighter wooden seating has been used that cannot provide the same effect.
For more detailed information refer to the original paper from Georgia’s Institute of Technology, found here:
Declercq, N. 2007. Acoustic diffraction effects at the Hellenistic amphitheater of Epidaurus: Seat rows responsible for the marvelous acoustics. Acoustical Society of America.
