User`s manual

15.6 STI CALCULATION
The Speech Intelligibility Index are calculated from a single measured MLS
response.
Some care should be followed while executing the measurement of the impulse
response to be used for the STI calculation:
- the procedure is valid only for an MLS stimulus as the signal to noise ratio is
collected in a single measurement.
- the impulse length must be at least 1.6 seconds to correctly calculate the lowest
modulation frequency needed for the MTF matrix.
The STI is a single number index that takes into account different effects that are
decreasing the speech intelligibility in a room as the background noise and the
reverberation. the index is calculated starting from a set of MTF (modulation
transfer functions) calculated for 7 octave bands and 14 modulation frequencies as
defined in the ISO IEC 60268-16:2003 standard. The 7 x 14 matrix is then
reduced to a vector of 7 MTI modulation transfer indexes and combined into a
single number index.
The text file is divided in three sections:
STI index- “classic” STI index calculation as defined in IEC 60268-16:1998. The
STI index is rated accordingly to the following table:
0 < STI < 0.3 BAD
0.3 < STI < 0.45 POOR
0.45 < STI < 0.6 FAIR
0.6 < STI < 0.75 GOOD
0.75 < STI < 1 EXCELLENT
STIr index - “revised” STI index calculation as defined in IEC 60268-16:2003, two
different MTI weighting are present hence two STI indexes: STImale and
STIfemale.
RaSTI index - Rapid Speech Transmission index, is a simpler version of the STI
calculated from only two frequency bands and 9 modulation frequencies
15 ACOUSTICAL PARAMETERS 207