Speech Transmission Index
Speech intelligibility can also be determined by measuring the speech transmission index , in addition to and , This index can be obtained by using special modulated noise signals or post-processing the impulse response.
The obtained in Treble is calculated following the indirect method as proposed by the standard IEC 60268-16 . In this method, applicable only to linear, time-invariant systems, the modulation transfer function MTF is calculated using the Schroeder method  for each filtered octave band and modulation frequencies set by the standard .
The calculations for the are performed assuming the source as omnidirectional, using a male speech spectrum with speech levels of 60 dB as defined in the standard  and using predefined background noise NC curves ranging from NC-15 to NC-75 .
 IEC 60268-16, Sound system equipment — Part 16: Objective rating of speech intelligibility by speech transmission index.
 M. Schroeder. Modulation Transfer Functions: Definition and Measurement. Acustica, 49:179-182, 1981.
 ANSI/ASA S12.2-2008, Criteria for Evaluating Room Noise