Sound insights about the birth of air conditioning
On July 17, 1902, engineer Willis Carrier produced drawings for the first practical air-conditioning system. The invention thoroughly transformed patterns of work and leisure — not least of all, concertgoing. But that transformation reached the concert hall later than one might think.
Boston’s Symphony Hall, for example, was not fully air-conditioned until 1973. Before then in warmer weather, the heating system’s fans would be turned on, circulating air but not cooling it. Doing both was an acoustical challenge; those in charge of the job, John Curtis, Robert Hoover, and Robert Jones of Cambridge-based Bolt, Beranek and Newman (BBN), presented their findings to annual meetings of both the Acoustical Society of America and the American Society of Heating, Refrigerating, and Air-Conditioning Engineers.
Symphony Hall’s first air conditioning was installed for the benefit of a different audience: television. Hot lights for WGBH-TV’s “Evening at Pops” broadcasts, which began in 1970, necessitated air conditioning for the stage area, installed in 1971. That system was pretty quiet, but not quiet enough; the BBN technicians noted that, while patrons hadn’t complained, “Evening at Pops” recording engineers certainly had.
At the time, acousticians measured noise against a set of “Noise Criterion” (NC) curves, a method invented by Leo Beranek in the 1950s. NC curves were more sophisticated than simple decibel levels, taking into account the fact that the ear is more sensitive to some frequencies than others. (Low-frequency sounds, for instance, can rumble along with more energy than high-frequency sounds without being noticed.) With the stage AC on, readings at the hall’s center approached NC-35: the high end of what would be considered acceptable in a residential situation, and far too high for a concert hall.
An NC-20 curve became the goal. (Outside traffic noise was already slightly higher than that, anyway.) For the onstage system, less was more. Its numerous dampers and deflectors, intended to limit fan noise, were too noisy themselves; without them, the fan could run slower, eliminating sound at the source. The rest of the hall was cooled by three 30-horsepower fans in the basement — two to supply air, one to pull it back — along with the pumps, condenser, cooling tower, and a 230-ton steam absorption unit. Mufflers and padding in the ductwork cushioned the racket; discrepancies in airflow speed in different parts of the hall (a main source of noise from the old ventilation system) were eliminated. Excepting some (largely unnoticeable) low-frequency sound, decibel levels actually ended up lower than before.
Boston being conservative Boston, however, orchestra manager Thomas D. Perry Jr. felt compelled to justify the change, telling Globe reporter Alison Arnold, “You have to spend money to make money.”