The "speech transmission index" is the most-used value describing speech intelligibility.
The STI measures the speech intelligibility within a room. For a long time, it wasn’t possible to measure this value in an objective way. In former times, a group of test persons were asked to pronounce the word “logatomen“ (an artificial word without meaning) in the respective room. When 50% of the sounds were identified directly, the STI equaled 0.5.
The new measuring method using the STI was developed by the TNO (Dutch Institute for Applied Sciences) and the Technical University of Delft. It uses an artificial voice signal, which consists of a modulating sound in the spectrum of a male and a female voice. The speech intelligibility is influenced by (disturbing) background noises and reverberations.
On the website of the TU Delft you will find more information on the subject: "STI voor beginners"