The physicist Wallace Sabine pioneered the scientific study of architectural acoustics when he was asked in 1895 to fix a university lecture hall in which the echo of a speaker's words rendered them unintelligible. He found that the length of time it takes a sound's echo to decay is determined by the absorption of the sound's original energy by surrounding material. By hanging panels of sound-absorbing felt on the walls, Sabine reduced the echo enough to make the hall usable. And the data he compiled yielded a mathematical formula for the relationship between a room's echo duration, its quantity and quality of sound-absorbing materials, and its spatial volume.