If you’ve ever been to an ancient Greek amphitheater, you have no doubt been amazing at how sound carries throughout the entire section of seating. How could the Greeks from over 2500 years ago have such a modern understanding of acoustics? Well, researchers from the Georgia Institute of Technology may have figured out why you can hear perfectly well even on the back row of these amazing feats of engineering.
Studying one amphitheater in particular, the researchers have pinpointed what makes the theater at Epidaurus on the Peloponnese in Greece such an acoustic marvel. It’s neither the slope of the seats nor the wind that consistently blows through the theater. It’s the rows and rows of seats. The seats made of limestone create a natural acoustic filter, hushing low frequency noises such as the murmur of a crowd, while amplifying high frequency sounds like the performers on stage. In fact, the sounds from the stage are reflected off of the backs of the seats, sending the sound waves directly at the viewer, which helps carry the sounds all the way to the back row of the theater.
Many previous theories have been posed as to why the amphitheater at Epidaurus has such great sound-carrying qualities. Some have speculated that the wind, which typically blows for the stage toward the audience, may have aided in the acoustics. Other claims that the Greek rhythms of speech made it easy for sound waves to travel long distances. Some more absurd speculations in the idea that the Greeks wore special masks that acted as primitive loudspeakers, amplifying the sounds that came from the stage. Other technical theories suggested that the actual slope of the seats created the acoustic perfection.
The team from Georgia Tech that set out to tackle the problem had somewhat of a wrong idea as well, although they seemed to be the closest of anyone. They thought that the actual composition of limestone, such as the ridged and porous nature of it, allowed sound waves to travel up the seats almost completely unhindered. However, the team was completely surprised when the seats seemed to stifle low frequency sounds as well. In fact, the team experimented with ultrasonic waves and numerical simulations that showed that the frequencies under 500Hz were suppressed while those above 500 Hz were amplified. In essence, the grooved nature of the seats was acting much like the ridged acoustic padding on walls, or the insulation in a parking garage.
So how did the audience hear the lower frequencies of the actors’ voices? According to the researchers, the brain can reconstruct lower frequencies without actually hearing them, which is a phenomenon called virtual pitch. In fact, your brain employs virtual pitch every time you listen to music through laptop speakers or talk to someone on the telephone, despite the fact that the lower frequencies are produced by the speaker.
Source: Ashton Hendricks, “Ancient Greek Amphitheater: Why You Can Hear From Back Row.” ScienceDaily. URL: (http://www.sciencedaily.com/releases/2007/04/070404162237)
