“The Renée and Henry Segerstrom Concert Hall at the Orange County Performing Arts Center is slated to open Sept. 15, but builders are racing to finish at least a month earlier — the leeway the hall needs to have a fighting chance of sounding right in its debut. Its resident orchestra, the Pacific Symphony, needs the time to become acclimated to the 2,000-seat venue in Costa Mesa, which is considerably more intimate and acoustically sensitive than its previous home of nearly 20 years.”