Question

In 1895, Harvard University completed the construction of the Fogg Art Museum, which housed an auditorium notorious almost immediately for its catastrophic acoustics. Speeches delivered in the lecture hall reverberated so excessively that spoken words overlapped into an unintelligible din. Tasked with rectifying this design failure, Wallace Clement Sabine, an assistant professor of physics, initiated an investigation that would establish architectural acoustics as a quantitative physical science.

At the time, prevailing architectural practice treated acoustics as an unpredictable art, relying on vague rules of thumb, such as stringing wires across high ceilings or copying the rough proportions of halls reputed to possess good sound. Sabine rejected these intuitive methods, hypothesizing instead that reverberation—the persistence of sound in an enclosed space after the source has stopped—depended directly on room volume and the sound-absorbing qualities of surrounding boundary materials. To test this systematically, Sabine and his assistants conducted exhaustive late-night experiments in the empty Fogg auditorium, when ambient city noise subsided.

Sabine’s experimental setup relied on an organ pipe powered by a constant-pressure air reservoir as a standardized sound source and a stopwatch to measure reverberation time: specifically, the duration required for a sound to decay to one-millionth of its initial intensity (a drop of 60 decibels). To measure absorption, Sabine progressively brought into the hall varying quantities of uniform cushions borrowed from the nearby Sanders Theatre. By meticulously plotting the reduction in reverberation time against the total length of cushions introduced, Sabine discovered an exact inverse hyperbolic relationship: reverberation time multiplied by total absorption equaled a constant directly proportional to the volume of the room.

Crucially, Sabine demonstrated that different materials exhibited distinct absorption coefficients per unit area. While hard plaster, glass, and wood reflected nearly all acoustic energy, porous substances such as felt, heavy fabrics, and human clothing absorbed significant acoustic energy. In solving the Fogg auditorium's defect, Sabine did not alter the building's physical geometry; rather, he lined specific reflective wall surfaces with thick hair-felt covered with decorative canvas. This intervention dramatically reduced reverberation time to an acceptable range, validating his mathematical formulation and transforming acoustic design from speculative guesswork into an empirical engineering discipline.

According to the passage, what specific physical modification did Sabine implement to resolve the acoustic deficiency of the Fogg Art Museum auditorium?