Benjamin Franklin, one of the famous Founding Fathers of the United States of America once listened to the famous preacher George Whitefield. He calculated that 30,000 people could hear him. He did the calculation because he doubted stories of generals in ancient times speaking to his entire armies. He says this in his autobiography. Here's an excerpt.
He had a loud and clear Voice, and articulated his Words and Sentences so perfectly that he might be heard and understood at a great Distance, especially as his Auditors [audience], however numerous, observ’d the most exact Silence. He preach’d one Evening from the Top of the Court House Steps, which are in the middle of Market Street, and on the West Side of Second Street which crosses it at right angles. Both Streets were fill’d with his Hearers to a considerable Distance. Being among the hindmost in Market Street, I had the Curiosity to learn how far he could be heard, by retiring backwards down the Street towards the River; and I found his Voice distinct till I came near Front Street, when some Noise in that Street, obscur’d it. Imagining then a Semicircle, of which my Distance should be the Radius, and that it were fill’d with Auditors, to each of whom I allow’d two square feet, I computed that he might well be heard by more than Thirty Thousand. This reconcil’d me to the Newspaper Accounts of his having preach’d to 25,000 People in the Fields, and to the ancient Histories of Generals haranguing whole Armies, of which I had sometimes doubted.
http://nationalhumanitiescenter.org/pds/becomingamer/ideas/text2/franklinwhitefield.pdf
