George Whitefield was the most spectacular preacher of the First Great Awakening in Britain and America, drawing revival audiences reported in the tens of thousands. News accounts of these meetings drew the attention of many, including Whitefield's friend and publisher, Benjamin Franklin of Philadelphia.

I recently interviewed Braxton Boren, a PhD candidate at the Music and Audio Research Laboratory at New York University, about his new study of Whitefield's preaching and the science of sound. Boren specializes in the physics of sound and computational acoustic simulation techniques. 

How did you get interested in studying George Whitefield's revival audiences? 

My dad was a history teacher at my high school, and he had me read Benjamin Franklin's autobiography for one of his classes. After I had done some research into acoustical simulation of music in the Venetian Renaissance, my dad read my thesis and reminded me about the autobiography's story of Franklin performing an experiment to estimate the range of Whitefield's voice. I realized that modern acoustical simulation techniques could test Franklin's data to estimate Whitefield's overall loudness, and to project his intelligible range at the actual sites where he was preaching.

The reported size of Whitefield's crowds was controversial during his lifetime and remains so among historians today. How many people do you think might reasonably have heard him speak at one time?

It is important to note that Whitefield's largest crowds were reported during the summer of 1739 when he was a young man, after which his novelty wore off. When he was older and perhaps wiser, he revised his earlier journal entries and removed passages he considered to be “justly exceptionable.” This included changing estimates of crowds larger than 20,000 to say “so many thousands that many went away because they could not hear.” It may be that Franklin's acoustical method is the best estimate we have for the effective size of Whitefield's largest outdoor assemblies.

Using speech intelligibility, distance, geometry, and our best information about the Market Street area during Franklin's experiment, we worked backward to estimate the average sound pressure level (SPL) of Whitefield's speaking voice. This was interesting because the computer model yielded a best guess of about 90 decibels at a distance of 1 meter from his mouth. This is incredibly loud. The international standard for “loud” speech is only 74 decibels, so it was unclear initially whether such a high SPL could be achieved by any human voice.

To test it, we measured SPLs in a lab for several trained actors and opera singers, and surprisingly the highest levels anyone could produce were right around 90 decibels. This suggests that there is a higher loudness level available to trained vocalists, which may have been more common in the days before electronic amplification. So the first component of this research projected that Whitefield was probably as loud as anyone for whom we have experimental evidence in history.

Based on this assumption, we then set up a computer model of the sites of Whitefield's largest reported congregations in London, using a virtual George Whitefield preaching to a crowd filling the entire area of the space. We had to account for different levels of background noise, as Whitefield made it clear that some crowds were quiet while others were boisterous or unruly. For different sites, our models project that Whitefield had a maximum intelligible area of 25,000 to 30,000 square meters under optimal conditions. A solid crowd over that area would constitute about two people per square meter, leading to an overall crowd of 50,000 to 60,000. However, if the crowd was slightly noisier, or if Whitefield was a little hoarse, the intelligible crowd area could decrease quickly.

The ideal acoustic conditions probably were fragile with any crowd of such a large size, but it seems possible that on certain occasions he may have been able to reach 50,000 people, at least for short periods of time. However, the majority of his large crowds were reported at 20,000 to 30,000, and these were the sizes Franklin was trying to validate. From our simulations we can say that these sizes of crowds were certainly possible.

How did Franklin do his experiment, and how reliable do you think his calculations were?

Franklin walked backwards from Whitefield's crowd in Philadelphia and recorded the distance at which Whitefield's voice ceased to be intelligible. His experiment was based on the idea of a uniform intelligible distance, which he then used to calculate the intelligible area by assuming a semicircular acoustic radiation pattern. However, this method does not account for many factors. For instance, the human voice does not radiate in a perfect semicircle. Even the ancient Greeks knew this, as they put people of lower social standing (like foreigners and pregnant women) on the sides of their semicircular amphitheaters because of the poorer speech intelligibility there. So Franklin's model adds a little area on the sides but misses out on some behind Whitefield where people could have also heard him. In addition, Franklin's method doesn't account for the reinforcing reflections at Philadelphia from the courthouse. Finally, his method does not anticipate changes in background noise.

Despite the omission of these factors, Franklin's estimate gave an area of about 23,000 square meters, which is just slightly lower than our largest projections. His largest error was his assumption of the crowd density, which he calculated as one person per two square feet—what modern crowd statisticians refer to as “mosh pit conditions.” This experiment gave him an overall crowd estimate of about 125,000 people, which is much larger than Whitefield's largest reported audience of 80,000. However Franklin, the modest New Englander, merely reported that he believed Whitefield could be heard by “more than thirty thousand,” since that was the number he was trying to validate.

All in all, Franklin's back-of-the-envelope approach was pretty good for estimating intelligible area quickly. Given the constraints of 18th century science, I doubt anyone else in that period could have done much better.

Do you think that people in a time before electric amplification had different ideas of what it meant to “hear” a speech or sermon?

Whitefield's outdoor assemblies were a novelty in Britain and the colonies because sermons had been traditionally confined to churches. There were some European traditions of itinerant preachers, but none attracted crowds of the size of Whitefield's. Certainly today we have clear ideas of speech intelligibility necessary in different spaces, and the minimum speech intelligibility values used in this research (about 30 percent of words understood for an average hearer) would not be up to code for a public address system in a train station or public building.

However, when given the context of the message, the hearers' minds can usually fill in the rest of the details by hearing this much. The problems of “hearing” in churches would have been primarily related to reverberation. In the fields, the open air would take all sound away quickly, leaving background noise as the main interference in hearing a speaker outdoors. Hearers today would be pickier, but back then people might have been more accepting of poor acoustic conditions in exchange for the chance to see a celebrity like Whitefield.

Even if it wasn't “mosh pit conditions,” what do you think it looked and sounded like at a Whitefield assembly: more like a traditional church meeting or a modern rock concert?

Probably a little of both! Closer to Whitefield the most devoted followers gathered and would sometimes shout out or faint. However, on many occasions he specifically noted the silence that his devoted listeners observed.

On the other end of the spectrum, some people—including skeptic David Hume—came to hear Whitefield solely for the spectacle of the occasion, without necessarily believing in his revival message. The famed Shakespearean actor David Garrick admired Whitefield's oratorical skills but apparently did not embrace Whitefield's gospel. (Garrick reportedly said that Whitefield could “make men weep or tremble by his varied utterances of the word 'Mesopotamia.'”) The success of the Methodist revivals in Britain and America certainly had roots in many convergent historical factors, including the advent of print advertising, increasing ease of transcontinental communication and travel, and not least the singular booming voice of George Whitefield.