I believe the answer is 20 feet. According to McBurney and Collings, the average threshold of human hearing is the tick of a watch from 20 feet in very quiet condition.
The human hearing range is a description of the pitches and loudness levels a person can hear before feeling discomfort. There is a variety of sounds in our environment, ranging from faint sounds like birdsong and rustling leaves to louder sounds like music, yelling, and industrial noise.
