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SV 971A User Manual

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 S

ELECTING

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.

<ENT>

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.