Computer simulation of Benjamin Franklin's acoustic experiment on George Whitefield's oratory

The Anglican preacher George Whitefield was renowned for his loud voice and the huge crowds he drew during the transatlantic revivals of the 18th century. Benjamin Franklin was skeptical of the accounts of crowds of 30,000 gathering in London, and when Whitefield came to Philadelphia in 1739, Franklin performed one of the earliest recorded ‘archeoacoustic’ experiments: walking backwards down Market Street, Franklin continued listening to Whitefield speak from the old courthouse until his sermon became unintelligible. Using this maximum intelligible distance, Franklin calculated that Whitefield probably could have been heard by more than 30,000 listeners. Using Franklin’s account and period maps and prints of the colonial city, we have built a virtual CAD model of Philadelphia as it would have existed during Whitefield’s visit. This paper discusses techniques employed using geometric acoustic simulation software to approximate the loudness of Whitefield’s voice based on the STI at Franklin’s position. To determine the STI, the background noise at Franklin’s position is simulated according to his account of a noise source on Front Street. Given a specific noise source and a minimum intelligible STI, this system yields a loudness value in dB-SPL for an acoustic source at Whitefield’s position.