13 STI CALCULATIONS – STIPA
The Speech Transmission Index (STI) is applied in the deterioration of the speech intelligibility induced by the
transmission channel. The STI method involves the use of a test signal applied to the transmission channel,
and an analysis of the received test signal. The speech transmission quality of the channel is determined and
expressed in values from 0 to 1 in the form of STI. Using the obtained STI value, it is possible to determine
the potential speech intelligibility.
The applications of STI include evaluation of sound reinforcement and emergency systems, communication
channels, speech intelligibility, communication in rooms and auditoria, etc.
STIPA (speech transmission index for public address systems) is a STI derivative approach that was
developed for fast measurements of electro-acoustic and acoustic environments effects that affect the
speech intelligibility in the room acoustics and/or public address systems.
STIPA calculations are realized in SV 971A in accordance with IEC 60268-16:2011.
Note: In addition to the fact that measurements can be made using only SV 971A, Svantek
also provides an application (BA Assistant) for mobile devices (smartphone, tablet) with
Android 7.0 platform or higher, which performs STIPA measurements using SV 971A but has
a more convenient user interface (see BA Assistant User Manual).
13.1 STI METHOD BACKGROUND
The STI method is based on measurements of the STI test signal comprising seven octave band noise
signals corresponding with the octave bands from 125 Hz up to 8 kHz. Each noise carrier is modulated
with one or more modulation frequencies at one-third octave intervals ranging from 0,63 Hz up to and
including 12,5 Hz. Such test signal which simulates speech fluctuations is reproduced in a room or
through a communication channel and received at a listener position (superimposed by distortions
presented in the room like echoes or noise) by means of omnidirectional microphone.
The STI method determines the modulation transfer function m(F) – reduction in the modulation depth
for modulation frequency - of the transmission channel for the seven octave bands. The RMS level of
each octave-band carrier matches the relative level of the average, long term spectrum of speech
material. Each octave band has a contribution to speech intelligibility which is weighted according to that
band. Using the weighted sum of these transmission index values, the overall STI value for the
transmission channel is determined.
STIPA method uses two modulation frequencies per octave band that are generated simultaneously
giving a total of 14 modulation indices.
13.2 SELECTING STIPA FUNCTION
To activate the STIPA function, enter the Function
section, select the Meas. Funct position and press the
<Enter> key. In the Meas. Funct screen, select the
STIPA function and press the <Enter> key.
Note: The STIPA function is optional and should be unlocked by entering the activation code
in the text editor screen, which is opened after first attempt to select this function. Once
unlocked this option will be available permanently.
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