Speech Transmission Index (STI) Speech Transmission Index (STI)
IEC 60268/16
IEC 60268/16
The STI Method
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.
Energy Envelope
Acoustic system
1/F 1/F
time time
The STI Method
MTFtestsignal (modulatedbandpassnoise)
125 Hz 250 Hz
500 Hz 1000 Hz
2000 Hz 4000 Hz
8000 Hz 0
0.2 0.4 0.6 0.8 1
0 0.25
0.5 0.75
1 1.25
1.5 1.75
2 2.25 Time(s)
125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz
STI method
• Reflections and background
noise reduce the depth of
modulation
MTF from Impulse Response
• It is possible to derive the MTF values from a single impulse response measurement:
2 0
2 exp 2
) (
d h
d F
j h
F m
f
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
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 noise-free measurement of the IR, and then insert the effect of the noise with the following expression:
10
101 ) 1 (
' )
( F m F
Lnoise Lsignalm
This makes it possible to measure a “clean” impulse response, and then to perform separately the signal and noise recordings
Post processing of impulse responses
• A special plugin has been
developed for the computation of STI according to IEC-EN 60268-16:2003
The STI Method – MTF matrix
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.
Octave-band STI value, computed by the average of MTF values
Overall weight STI (male)
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.
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 Pulse every 8 MLS periods
Directivity of the mouth simulator
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.
125 Hz
-35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
+135°
+150°
+165°
±180°
-165°
-150°
-135°
-120°
-105°
-90°
-75°
-60°
-45°
-30°
-15°
Directivity of the mouth simulator
250 Hz
-35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
+135°
+150°
+165°
±180°
-165°
-150°
-135°
-120°
-105°
-90°
-75°
-60°
-45°
-30° -15° 500 Hz
-35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
+135°
+150°
+165°
±180°
-165°
-150°
-135°
-120°
-105°
-90°
-75°
-60°
-45°
-30°
-15° 1000 Hz
-35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
+135°
+150°
+165°
±180°
-165°
-150°
-135°
-120°
-105°
-90°
-75°
-60°
-45°
-30°-15°
2000 Hz
-35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
+135°
+150°
+165°
±180°
-165°
-150°
-135°
-120°
-105°
-90°
-75°
-60°
-45°
-30°-15° 4000 Hz
-35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
+135°
+150°
+165°
±180°
-165°
-150°
-135°
-120°
-105°
-90°
-75°
-60°
-45°
-30°-15° 8000 Hz
-45 -35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
+135°
+150°
+165°
±180°
-165°
-150°
-135°
-120°
-105°
-90°
-75°
-60°
-45°
-30°-15° 16000 Hz
-65-55 -45 -35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
+135°
+150°
+165°
±180°
-165°
-150°
-135°
-120°
-105°
-90°
-75°
-60°
-45°
-30°-15°
Directivity of a real human (I. Bork, PTB)
125 Hz
-35 -25 -15 -5 5 0
10 20 30
40 50
60 70
80 90 100 110 120 130 140 160150 170 180 200190 210 220 230 240 250 260 270 280 290
300 310
320 330340 350
250 Hz
-35 -25 -15 -5 5 0
10 20 30
40 50
60 70
80 90 100 110 120 130 140 160150 170 180 200190 210 220 230 240 250 260 270 280 290
300 310
320 330340 350
500 Hz
-35 -25 -15 -5 5 0
10 20 30
40 50
60 70
80 90 100 110 120 130 140 160150 170 180 200190 210 220 230 240 250 260 270 280 290
300 310
320 330340 350
1000 Hz
-35 -25 -15 -5 5 0
10 20 30
40 50
60 70
80 90 100 110 120 130 230
240 250 260 270 280 290
300 310
320 330340 350
2000 Hz
-35 -25 -15 -5 5 0
10 20 30
40 50
60 70
80 90 100 110 120 130 230
240 250 260 270 280 290
300 310
320 330340 350
4000 Hz
-35 -25 -15 -5 5 0
10 20 30
40 50
60 70
80 90 100 110 120 130 230
240 250 260 270 280 290
300 310
320 330340 350
Equalization of the mouth simulator
• The spectrum of the emitted test signal
should correspond to the prescriptions of ITU T- P50 Recommendation.
• The overall SPL should be 60 dB(A) at 1m, on axis, for measurements compliant with IEC
60268-16 standard
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.
Measurement example (no noise)
• The measured IR is
saved as a TIM file, and
processed with MLSSA
Measurement example (noise)
• The measured IR is
saved as a TIM file, and
Verification of noise simulation
• The values of m(F) obtained by the measurement without noise were corrected for the S/N ratio, and compared with the m(F) values measured with noise
MTF values at F=2Hz
0.4 0.5 0.6 0.7 0.8 0.9 1
125 250 500 1000 2000 4000 8000
Carrier Frequency (Hz)
MTF
No Noise Noise (real) noise (simul.)
Noise-free IR method (Aurora)
Performing the STI measurement with Aurora requires these 4 steps:
•Step 1: Calibration
•Step 2: Background Noise spectrum
•Step 3: Signal spectrum
•Step 4: Impulse Response (noise-free, ESS method)
Step 1: calibration
Click
Click Click
Step 2: background noise spectrum
Click
Click Click
Step 3: signal spectrum
Click
Click Click
Step 4: impulse response
Click
Click
