The acoustic measure G (Strength) represents the total level of the acoustic response, in other words how much an acoustic space gains the sound level within an acoustic space. The reference level for G is the direct sound level at 10 m from the source. Measuring G requires that the measurement system is calibrated and this is probably the major reason for why this measure often is not included when acousticians investigate acoustic conditions. The calibration is normally done by testing the measuring equipment in use within an anechoic chamber, where the direct sound level at 10 m is obtained. It can alternatively be done in-situ (at the location), but this puts some restriction on the frequency range where correct values of G are obtained. A brief article on how to calibrate or confirm G in-situ (that will result in sufficiently valid values of G from the
250 Hz octave band and above, dependent on your source) is available on the Articles page.
So why bother with calibrating the system for being able to measure G? From measured G and C80 the level of the early and the late (reverberant) acoustic response from an acoustic environment can be obtained – namely Ge and Gl, the level G for the early (e) acoustic response arriving before 80 ms and the late (l) acoustic response arriving after 80 ms (relative to the direct sound arrival). The reverberation time T and the level of the reverberation appears to be essential attributes of an acoustic space. By knowing the level of the reverberation (as well as the early acoustic response) we can tell for instance if an acoustic space will contribute to an excessively loud environment or if the reverberant response will be inaudible. If knowing the reverberation time T and the volume of the acoustic space V, the level of the acoustic response can be estimated. But the estimate can often be too far off from the actual situation, so by actually measuring the acoustic level we will get rid of some uncertainty factors.
Recent articles where the relevance and benefits of measuring G are discussed includes Barron (2008) , Buen (2010) and Beranek (2011), the latter is unfortunately not freely available (abstract only). Regarding the relevance of G and Gl for stage acoustics, see the articles on suggested objective assessment of concert hall stages, Dammerud et al (2010) and Dammerud (2011).
Also see the Spreadsheets page for spreadsheets on how to calculate Ge and Gl based on measured G and C80 or estimate Gl based on measured T and hall volume V.