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The Acoustic Trap at Epidaurus


Audicus is taking a little trip back in time to the year 340 BCE when the Greek architect Polykleitos the Younger built the Great Theatre of Epidaurus. Carved into the side of a Mount Kynortio at a 26 degree slope, the theater is considered to be the most beautiful of the ancient theaters as well as the best preserved.

Originally built as a religious healing center, Epidaurus was later used for entertainment. However, unlike the silent films of the early 1900s, the theater was not only the place to go see but to go listen as it is renowned for its exceptional acoustics. The theater can hold up to 14,000 spectators who can all hear speakers on the stage perfectly without a single microphone.

Watch the below YouTube video to see a firsthand demonstration of what it would sound like to be in the theater.

How do the acoustics work in this amphitheater so that ancient Greeks sitting in the last row could hear unamplified sound just as clearly as those sitting in the first? This architectural masterpiece has been subject to many theories and speculation, ranging from selective hearing, to ancient actors wearing sound amplifying masks, to the slope of the theater allowing for sound to echo. Assumptions went back as long ago as the 1st Century BCE, when the Roman architect Vitruvius said:

“By the rules of mathematics and the method of music, they sought to make the voices from the stage rise more clearly and sweetly to the spectators’ ears… by the arrangement of theatres in accordance with the science of harmony, the ancients increased the power of the voice.”

Researchers at the Georgia Institute of Technology found the answer and it was none of the above. By conducting a series of experiments with ultrasonic waves they discovered that the seats made of limestone and bearing corrugations, allow for a filtering effect on background noise by suppressing low frequencies such as wind and the movement of people. At the same time, the seats also reflect high-frequency sounds back towards the audience, thus magnifying the effect.

This is known as an acoustic trap and creates a higher clarity of sound. If you’re questioning the effect of filtering out low frequency sounds coming from the actors on stage, worry not because of a phenomenon known as virtual pitch. The virtual pitch phenomenon refers to how the human brain can reconstruct missing frequencies. A good example of this effect would be speaking to someone on a phone with no low end.

Interestingly enough, it seems as if the architect who designed the Epidaurus Theatre didn’t realize he was creating a sonic masterpiece. The effect has been attempted in different replications over the years but has not been successful since other theaters used wood or other materials for their seating; the natural acoustics don’t sound nearly as good.

It is true that modern theaters like IMAX can depend on other innovations to increase audibility but Nico Declercq, the lead researcher involved in the study done by GIT, says that the acoustic trap might still be a good idea: “In certain situations such as sports stadiums or open-air theaters, I believe the right choice of the seat row periodicity or of the steps underneath the chairs may be important.”

by Esther Shasho

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