In 1982 I toured Europe with the Harding University A Cappella Chorus. Among the many interesting places we sang was the ancient Greek theatre at Epidaurus. Carved out of the side of a hill in the fourth century A.D., it seats 14,000 and boasts such perfect acoustics that any one of those 14,000 can hear a whisper from anywhere on stage.
Maybe the Greeks’ ability to create such theatres is why the word “acoustics” comes from a Greek word, “akouein,” meaning “to hear.” “Acoustics” can mean the overall science of sound, but it’s generally used to refer to architectural acoustics: the science of constructing enclosed spaces that enhance the hearing of speech or music.
At Epidaurus, the semi-circular stone bleachers enable the maximum number of people to sit as close to the stage as possible, and the slope of the hill creates a natural resonance chamber, trapping the sound waves so they bounce back into the spectators’ ears rather than just zipping uselessly off into the distance.
Things get a lot more complicated in an enclosed structure, where the sound bounces and re-bounces off the floor, the ceiling, the walls, the chairs, the audience members themselves, and every little protuberance from door-handles to light fixtures to heating ducts.
Science wasn’t really applied to architectural acoustics until the late 19th century. In 1856 the American physicist Joseph Henry produced the first thorough scientific treatment of the subject; since then, many first-rate concert halls have been built. Unfortunately, so have many very bad ones. The acoustics of any large space are so complex that even with modern technology and knowledge, there’s no guarantee the finished product will live up to expectations.
There are three main elements to a room’s acoustics: size, shape and building materials.
Size effects reverberation time, the time it takes a loud sound to die away to nothing. In a large room, it takes sound longer to travel the source to the walls and ceiling and back again than in a small room, so the echoes go on longer. This is why large cathedrals have such long reverberation times–usually around 4.5 seconds. Although ideal for organ music, this is far from ideal for singers or speakers: their words tend to get lost in the echoes.
Opera houses aim for a reverberation time of around one second, while symphony orchestras prefer a reverberation time of 1.5 to two seconds. This means the upper size limit for a top-notch concert hall is around 2,000 seats. Beyond that, acoustic clarity diminishes.
A room’s shape also affects its acoustics. The classic shape of a concert hall is a “shoebox,” with the stage at one narrow end. The close-in side walls reflect the sound quickly back into the audience, amplifying it while maintaining clarity. Horseshoe shapes are also popular; short balconies on the sides serve the same function as the close-in walls of the shoebox.
Finally, the hall’s building materials affect the sound. Carpet and other soft, porous materials absorb sound; hard materials like stone, wood and glass reflect it.
Beyond these broad elements of design there are countless details that also affect acoustics, from the slope of the floor to the placement and composition of the seats to protrubances of various sorts on the ceiling and walls.
Acousticians’ biggest problem has long been that they could never be certain what a space would sound like until the building was finished. Today, computers can help; a designer can input the design of a new concert hall and the building materials being used into a new device called the Auditioner, for example, and it will create a virtual model of that hall that allows the designer to hear exactly what it sounds like from any location within it.
Some halls are even using an array of microphones and speakers to create artificial reverberation. An organist is playing? Crank up the reverberation to cathedral levels. Someone is speaking? Turn it off entirely. The Elgin Theatre in Toronto has just such a system.
Perhaps someday electronic systems will become so effective that your own living room can be made to sound like Carnegie Hall. Until then, though, acoustic engineering remains as much an art as a science. Science can inform the designer, but in the end it’s his own experience and intuition that counts…and his dream that, centuries from now, someone will stand in the hall he designed and marvel at the perfect acoustics, just as I did at Epidaurus.