Have you ever wondered how many people in the audience actually heard the Gettysburg Address? How about the Sermon on the Mount or Moses at Sinai?
The answer to those questions, according to two New York University researchers, is more than you think.
People gave famous speeches for millennia without amplification. But how many people in the crowd could understand what was being said? In Monty Python's film, "Life of Brian," a large crowd shows up to hear Jesus' sermon, but by the time Jesus' words made it to the fringes of the crowd some clarity was lost. ("Blessed are the cheese-makers.")
The two researchers, Braxton Boren and Agnieszka Roginska at NYU's Music and Audio Research Lab, replicated a famous experiment by Benjamin Franklin using modern scientific techniques, and reported their findings at a meeting of the Acoustical Society of America in San Francisco last week.
The story begins in the late 1730s. A preacher, George Whitefield, was drawing immense crowds at his outdoor sermons in London. At one sermon, held in Mayfair, Whitefield claimed to have addressed 80,000 people, all of whom he presumed could understand him. Franklin didn't believe it and took an opportunity to test the claim in 1739 when Whitefield spoke in Philadelphia.
"I had the curiosity to learn how far he could be heard," Franklin wrote.
Whitefield was described as handsome, slim, and slightly cross-eyed, with a voice "like a lion," according to contemporary reports. He was one of the founders of Methodism in America and was among the most famous preachers of his time.
In Philadelphia, he spoke to an audience of perhaps 6,000 people stretched between Front Street and the Delaware River, a good crowd considering that Philadelphia's population at the time was only about 13,000. Franklin couldn't resist. He walked from the front of the crowd toward the river, listening for the point at which the preacher was no longer intelligible.
Then, he went home and did the calculations.
According to Boren, Franklin ran two numbers, the area over which Whitefield's voice was intelligible, and a guess at how many people could fit in that area. The area, he decided, was about 23,000 square meters.
Using modern modeling technology and archeological records of what Market Street looked like then, the NYU researchers thought Franklin nailed it.
Franklin also estimated each person took up 2 square feet, or about 0.2 square yards. His conclusion then was that Whitefield could have been heard by as many as 125,000 people under perfect conditions, but, Boren said, being a modest New Englander, Franklin concluded that figure was wildly off and settled on "more than 30,000," Boren said.
"There were certain things he didn't account for and certain things he over-accounted for, but by the end his estimate was a pretty good estimate," Boren said.
The original calculation was off, Boren said, because the kind of density Franklin was using is what modern acoustical researchers call "mosh-pit conditions." The crowd more likely was "solid," which is about half a square meter per person, little more than half a square yard, and much less dense.
Using those calculations Boren and Roginska concluded Whitefield could be heard by 20,000-30,000 people on a good day, a perfectly still crowd, no wind or carriages clattering by, just as Franklin said.
Still, how could someone be heard that far? Whitefield was speaking in an area of dense construction and the buildings and streets probably acted as sound reflectors.
But, it was probably more than just the acoustics of his environment.
The NYU computer models, based on Franklin's description, give one answer: Whitefield was very loud. They estimate that if you stood three yards in front of him, his voice would have registered 90 decibels, matching the loudest average sound levels from voices ever measured in a modern lab.
"The international standard for loud speech is 74 decibels," Boren said.
According to Jack Randorff, an acoustical consultant and engineer from Ransom Canyon, Texas, the NYU model assumed people could hear about 30 percent of what was being said. Since the audience knew the context of his material, their minds filled in the rest.
Additionally, the style of public speaking was different in the past. Randorff said the speakers stood up straight and might have raised and stretched their arms so their diaphragms were extended.
Speeches of the past also had an entirely different cadence. Orators often spoke in bursts of four or five words with considerable emphasis instead of long phrases. Lincoln might have said, "Four score...and seven years ago….our fathers...brought forth..." It also allowed them to take deep breaths so they could keep going.
The buildings around Whitefield would have added six decibels to his voice, which would have doubled the effective area his voice would have carried, Randorff said.
Reprinted with permission from Inside Science, an editorially independent news outlet of the American Institute of Physics, a nonprofit organization supporting the dissemination of science news and information to society.