Question

In the late nineteenth century, architectural acoustics was largely an empirical art governed by vague rules of thumb rather than rigorous physical principles. When Harvard University opened the Fogg Art Museum in 1895, its main lecture hall suffered from such severe echo that spoken lectures were virtually unintelligible. The university tasked a young assistant professor of physics, Wallace Clement Sabine, with resolving the acoustic deficiencies of the hall.

Sabine began his investigation by systematically examining how sound persisted in the room after its source ceased. Utilizing an organ pipe tuned to a standard pitch, a stopwatch, and his own trained hearing, he measured the duration required for a sound to decay below the threshold of audibility. Sabine discovered that the duration of sound reverberation depended directly upon the total volume of the lecture hall and inversely upon the aggregate sound-absorbing capacity of the surfaces within it—a relationship now formalized in Sabine’s reverberation equation.

Crucially, Sabine realized that different materials absorbed sound at vastly different rates. To quantify these differences, he introduced a standardized metric of sound absorption, using a single square meter of open window—which absorbs all incident sound without reflection—as the unit of complete absorption. By introducing cushions brought from the nearby Sanders Theatre into the Fogg lecture hall, Sabine meticulously measured how each added cushion incrementally reduced the reverberation time. He determined that cushions covered in heavy horsehair fabric provided significantly higher sound absorption per unit area than the bare wooden pews or hard plaster walls of the original hall construction. Through these exhaustive trials, Sabine transformed architectural acoustics from an unpredictable guesswork into a precise, quantitative science.

According to the passage, Wallace Clement Sabine established which of the following as the benchmark unit for complete sound absorption?