In 1973 Boston Symphony Hall was provided with its first air-conditioning system, and similarly in 1968 an air-conditioning system was installed in the Philadelphia Academy of Music.

Due to the high reputation of these two halls for good concert hall acoustics, considerable effort was taken in designing and laying out the air-conditioning systems to avoid any impairment to the acoustics or the appearance of the halls in any way. In this paper we wish to describe both the systems that were installed and some of the noise control measures, and to report on the present and former sound level measurements in the unoccupied halls.