Norsonic nor140, Instructional Manual, Page: 179 / 220

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Norsonic Nor140
Instruction Manual
In order to fully take care of the effects of non-linearity,
it is important that the basic signal being modulated is
a noise signal with a high crest-factor, a spectral distri-
bution similar to the long-term speech spectrum, and
that the main modulation frequency is selected one
by one. The measurement of the full STI therefore has
to be performed as a sequence of measurements. If
each of the 98 combinations is measured for 10 sec-
onds, the total measurement time will be about a quar-
ter of an hour. The need for such a long measurement
time in order to obtain the STI-value in one position of a
room limits the applicability of the full STI-method.
The STI-method may be modified in different ways
to reduce the time needed for the measurement. If the
system to be measured is regarded as linear, then a
number of solutions exist. The excitation signal may
be modulated with all modulation frequencies simul-
taneously and the components may be separated af-
ter reception by the use of filters or Fourier analysis.
A more common method is to calculate the complex
modulation transfer function from the impulse response
of the room.
If the impulse response can be regarded as a well-
behaved room response with an exponential decay-
ing envelope characterised by the reverberation time,
the modulation transfer function at frequency, F, may
be calculated directly from the value of the reverbera-
tion time, T, and the effective signal-to-noise ratio S/N
in dB. A simplified formula, not taking the effects of
masking and the threshold of hearing into considera-
tion, indicated the following relationship:
As seen from this formula, a limited signal-to-noise
ratio reduces the modulation transfer function for all
frequencies. A long reverberation time reduces the
modulation most for the highest modulation frequen-
cies.
The STI-value is a weighted average of the differ-
ent modulation indexes. The last revision of the method
(IEC-standard) also considers masking effects and the
absolute threshold of hearing.
RASTI and STIPA
In order to simplify the direct measurement, the RASTI-
method (Room Acoustic Speech Transmission Index)
was developed at TNO in 1979. Different instruments
were developed for the measurement according to this
standard. A typical measurement time was 10 to 15
seconds. The RASTI method only considers two oc-
tave bands 500 Hz and 2 kHz.
Due to the simplicity in use, the RASTI-instruments
were used also for applications beyond the main de-
sign goal – room acoustics. The RASTI-value is often
used for assessing the quality of public address sys-
tems, but comparisons with subjective measurements
have shown that the deterioration of speech intelligibil-
ity is not handled correctly if the PA-system is strongly
non-linear or suffers from limited bandwidth.
In order to improve the accuracy in the intelligibil-
ity assessment, the STIPA-method was developed. It
handles effects due to reverberation in the room and
distortions commonly found in public address sys-
tems. It also performs well for room acoustics and can
therefore in nearly all cases replace the RASTI-method
and deliver results more closely to the values obtained
by the full STI-method. The measurement time for a
STIPA-measurement is similar to the RASTI-method:
10 – 15 sec.
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