This paper reports on the sound pressure levels of an open-air stage performance held at a 2011 college campus festival in Toyosu, Koto-ku, in Tokyo, Japan. This festival was open to the general public, and attracted many students and residents from the surrounding neighborhoods. The festival was hosted by an executive committee who constructed an open-air stage on the campus grounds. The members of various student groups performed live music and danced to recorded music on this stage throughout the day. With loudspeakers set on the stage, music at high volume was diffused to campus neighborhoods. We utilized the A-weighted sound pressure level, and the equivalent continuous A-weighted sound pressure level (LAeq), as metrics to evaluate the loudness of the festival. The purpose of this evaluation was to inform the festival staff if the volume reached a level dangerous to the participants and campus neighbors. We therefore proposed a volume control method for the performance sound to be employed by the stage staff. This method controlled the volume when the noise level at the campus boundary exceeded the reference value of LAeq,12h = 60 dB decreed by the Tokyo Metropolitan Government. The A-weighted sound pressure level before implementation of control measures was 61.4 dB, but through the proposed method was reduced to 58.2 dB, slightly less than the reference value.