The STI Method
1/F
Energy Envelope
1/F
Acoustic
system
time
time
The STI method is based on the MTF concept: a carrier
signal (one-octave-band-filtered noise) is amplitude
modulated at a given modulation frequency with 100%
modulation depth. At the receiver, the modulation depth
is reduced, due to noise, reverb, echoes, etc.
11 novembre 2010
Speech Transmission Index
1
STI method
• Le riflessioni
ed il rumore
di fondo
riducono la
profondita’
della
modulazione
del segnale
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Speech Transmission Index
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MTF from Impulse Response
• It is possible to derive the MTF values from
a single impulse response measurement:
To compute each value of m(F) from the impulse response
h(t), an octave-band filter is first applied to the impulse
response, in order to select the carrier’s frequency band f. Then
m(F) is obtained with the formula

 h f    exp  j  2    F    d
2
m( F ) 
0

 h f    d
2
0
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Background noise
• If the background noise is superposed to the impulse
response, the previous method already takes care of it, and
the MTF values are measured correctly
• However, in some cases, it is advisable to perform a noisefree measurement of the IR, and then insert the effect of
the noise with the following expression:
1
m( F )  m' ( F ) 
1  10

 Lnoise Lsignal 


10


This makes it possible to measure a “clean” impulse
response, and then to perform separately the signl an noise
recordigs
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Post processing of impulse responses
• A special plugin has been
developed for the computation
of STI according to IEC-EN
60268-16:2003
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The STI Method – MTF matrix
Overall weight STI (male)
Octave-band STI value, computed by the
average of MTF values
The STI method is based on the MTF concept: a carrier
signal (one-octave-band-filtered noise) is amplitude
modulated at a given modulation frequency with 100%
modulation depth. At the receiver, the modulation depth
is reduced, due to noise, reverb, echoes, etc.
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Transducers: mouth simulator
• The mouth simulator was
built inside a wooden
dummy head, employing
low-cost parts. Its
compliance with the ITU
recommendation was
confirmed by means of
anechoic directivity tests.
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Directivity measurements
• In both cases, the anechoic
directivity measurements were
performed employing a
rotating table, directly
synchronized with the sound
board employed for measuring
the impulse response. The
Aurora software generates the
required pulses on the right
channel, which cause the
rotating board to advance.
Continuous MLS signal
11 novembre 2010
Pulse every 8 MLS periods
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Directivity of the mouth simulator


11 novembre 2010
The amplitude varies
smoothly, and respects the
directivity mandated by
ITU recommendation.
Nevertheless, the sound is
heavily coloured, as shown
by the horizontal stripes in
the lower plot.
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Directivity of the mouth simulator
+0°
-15°
5
+0°
125 Hz
+15°
-30°
+30°
-5
-45°
+45°
-75°
+75°
-25
-35
-90°
-105°
-120°
-60°
-90°
+105°
-105°
-120°
-135°
+0°
-30°
+15°
-5
-15°
+45°
-75°
+60°
+75°
-25
-35
-90°
-45°
+90°
-105°
-60°
-120°
+120°
+135°
-150°
+150°
+165°
-165°
±180°
11 novembre 2010
-35
-90°
+105°
+120°
+135°
+135°
-150°
±180°
+0°
5
+15°
+0°
8000 Hz
-15°
-30°
+30°
-5
-45°
-15
-60°
-45°
+45°
+60°
-60°
-25
+75°
-35
-120°
+135°
+150°
+165°
-75°
+90°
-90°
+105°
-105°
+120°
-165°
+150°
+165°
-165°
+165°
-30°
+60°
-150°
+90°
-105°
+150°
-15°
+45°
-135°
+75°
-25
±180°
+105° -105°
-135°
-135°
-150°
-25
-90°
-75°
+60°
-135°
-165°
-15
-75°
+90°
+45°
-15
-60°
-120°
+30°
-5
+30°
+120°
+15°
-30°
+75°
-120°
4000 Hz
5
+60°
+105°
+165°
+0°
2000 Hz
-15
-60°
-105°
+15°
-5
-45°
-35
±180°
+30°
-45°
+105°
+45°
-25
+150°
-165°
+165°
5
-90°
1000 Hz
+0°
5
-15°
-30°
-15
-75°
+135°
-150°
-5
-60°
+90°
+120°
±180°
-15°
+75°
-35
+150°
-165°
+60°
500 Hz
+30°
-45°
+45°
-25
+135°
-150°
+15°
-30°
-15
-75°
+90°
+120°
-135°
-5
5
-15°
+30°
-45°
+60°
+0°
250 Hz
+15°
-30°
-15
-60°
5
-15°
+75°
-35
-45
+90°
-75°
-90°
5
-5
-15
-25
-35
-45
-55
-65
16000 Hz
+15°
+30°
+45°
+60°
+75°
+90°
+105° -105°
-120°
+120°
-135°
+135°
-150°
+150°
+165°
-165°
±180°
Speech Transmission Index
±180°
+105°
-120°
+120°
-135°
+135°
-150°
+150°
+165°
-165°
±180°
10
Directivity of a real human (I. Bork, PTB)
125 Hz
0
330
320
310
300
340 350 5
10 20
330
320
310
30
40
-5
50
290
70
-25
280
-35
270
260
250
240
230
220
210
280
90
270
100
260
120
200 190
170 160
300
-25
-35
240
1000 Hz
0
10 20
340 350 5
330
320
310
30
40
-5
50
60
-15
-35
260
250
300
240
280
90
270
100
260
120
170 160
180
11 novembre 2010
90
270
100
260
200 190
170 160
130
140
150
50
70
80
-35
90
100
110
240
120
200 190
170 160
130
140
150
180
2000 Hz
0
10 20
340 350 5
330
320
310
30
40
-5
50
300
60
-15
-25
-35
250
240
280
90
270
100
260
120
170 160
130
140
150
180
Speech Transmission Index
4000 Hz
0
10 20
340 350 5
30
40
-5
50
60
-15
290
80
110
200 190
60
-15
-25
230
220
210
70
230
220
210
40
250
130
140
150
290
80
110
200 190
280
120
30
-5
290
80
110
500 Hz
0
10 20
340 350 5
180
-25
230
220
210
300
70
230
220
210
70
270
60
250
130
140
150
290
280
50
-15
180
330
320
310
40
290
80
110
330
320
310
30
-5
300
60
-15
250 Hz
0
10 20
340 350 5
70
-25
80
-35
90
100
250
110
240
230
220
210
120
200 190
170 160
130
140
150
180
11
Equalization of the mouth simulator
• The spectrum of the
emitted test signal
should correspond to the
prescriptions of ITU TP50 Recommendation.
• The overall SPL should
be 60 dB(A) at 1m, on
axis, for measurements
compliant with IEC
60268-16:2003 standard
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Equalization of the mouth simulator
• The MLS signal is
prefiltered, so that the
frequency response,
measured at 1m in front of
the mouth, complies with
the IEC spectrum.
• The filtering is performed
by means of the grahic
equalizer incorporated in
Adobe Audition.
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