Engines of Our Ingenuity

No. 1221:
ACOUSTICS

by John H. Lienhard

Click here for audio of Episode 1221.

Today, we try to hear the music in a large hall. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Houston's Jones symphony hall has gradually been altered over the years -- sound-reflecting surfaces added and taken away. In 1980 we had a good orchestra and a dubious hall. Now we have a fine orchestra in a much better hall, and the sound is glorious to hear!

Of course, the problem of providing good acoustics was far more severe before the 19th century -- when musical instruments had softer voices, and before we could enhance sound electronically.

Aristotle knew that air carries sound, though that fact wasn't verified in a way we'd call scientific until the 17th century. In the first century AD, the Roman architect Vitruvius showed how to enhance sound in halls by placing pots along the walls or in the ceiling. The pots, of various shapes and sizes, each resonated to different frequencies. Better to make the pots of brass, but clay worked too. You could still find acoustical pots as late as the 17th century. In fact we use variations on the idea even today.

When Christopher Wren designed churches during the 17th century, he had his own acoustical formula. He figured a modest voice could be heard only 50 feet in front and 30 feet on the sides, so he set pulpits far out toward the nave. A century later, the architect of Drury Lane Theater used 92 feet in front and 75 feet on the sides. (But that was for trained actors, not the average vicar.)

Eighteenth-century architects began using rules of light reflection to predict how sound would travel in closed spaces. They began putting parabolic reflectors behind altars and orchestras.

All the while new knowledge begat new problems. For if sound waves travel like light waves, they too move out spherically, reflecting off every object in their path. As mathematicians took up acoustics, the vertical horseshoe of boxes surrounding a concert stage gave way to today's wedge-shaped halls flanked by acoustical reflectors and resonators. And we're back to Vitruvius's pots.

In the 1960s, London's Royal Festival Hall turned the matter of hall acoustics on its ear. After years of tinkering, engineers hid microphones about the rear of the hall and softly played the sound back through speakers in front. The tiny delay gave an illusion of rich reverberation. That was kept secret until the new sound got rave reviews. When people learned what'd been done, they questioned the moral implications of doing it! Now with electronic boosting fairly common, we wonder what can be next.

Well, last week I spoke in a large hall where I was both miked and projected on a vast screen by the podium. What the audience heard was not me. What they looked at was not me. On the one hand, we seem to be moving toward a kind of virtual live performance.

And yet, the direct acoustic hall will survive. The simultaneous presence of corporeal listeners and performers is an essential rite. It is a ceremony we will not be content to lay aside.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


Elliott, C. D., Technics and Architecture. Cambridge, MA: MIT Press, 1992, Chapter 16.

Lewcock, R., Acoustics. The New Grove Dictionary of Music and Musicians (Stanley Sadie, ed.). New York: MacMillan Publishers, Ltd., 1980, Vol 1, pp. 43-67.

Beranek, L., Concert and Opera Halls: How They Sound. Woodbury, NY: Acoustical Society of America, 1996.


The Engines of Our Ingenuity is Copyright © 1988-1997 by John H. Lienhard.

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