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Speech Transmission Index

Speech intelligibility can also be determined by measuring the speech transmission index STI{STI}, in addition to C50{C_{50}} and D50{D_{50}}, This index can be obtained by using special modulated noise signals or post-processing the impulse response.

The STI{STI} obtained in Treble is calculated following the indirect method as proposed by the standard IEC 60268-16 [1]. In this method, applicable only to linear, time-invariant systems, the modulation transfer function MTF is calculated using the Schroeder method [2] for each filtered octave band and modulation frequencies set by the standard [1].

The calculations for the STI{STI} are performed assuming the source as omnidirectional, using a male speech spectrum with speech levels of 60 dB LA{L_{A}} as defined in the standard [1] and using predefined background noise NC curves ranging from NC-15 to NC-75 [3].

References​

[1] IEC 60268-16, Sound system equipment — Part 16: Objective rating of speech intelligibility by speech transmission index.

[2] M. Schroeder. Modulation Transfer Functions: Definition and Measurement. Acustica, 49:179-182, 1981.

[3] ANSI/ASA S12.2-2008, Criteria for Evaluating Room Noise