The sound strength parameter G is one of the most important objective parameters related to the acoustical quality of concert halls, opera houses, and chamber music halls. G at mid frequencies is highly correlated with the acoustic quality reported by music conductors and subjects with musical training. Measuring this parameter requires a calibrated sound source or the direct sound isolated using a suitable time window as in [B. Katz, In situ calibration of the sound strength parameter G, JASA 138 (2) (2015) EL167–EL173]. In this article, the performance of several window functions and lengths used to isolate the direct sound is investigated. Although the errors obtained for low frequencies are large, an error in the order of a just-noticea...