11 novembre 2010Speech Transmission Index 1 The STI Method The STI method is based on the MTF concept: a carrier signal (one-octave-band-filtered noise)

11 novembre 2010 Speech Transmission Index 1

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.

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rgy

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Acousticsystem

1/F 1/F

time time

11 novembre 2010 Speech Transmission Index 2

STI method• Le riflessioni

ed il rumore di fondo riducono la profondita’ della modulazione del segnale

11 novembre 2010 Speech Transmission Index 3

MTF from Impulse Response

• It is possible to derive the MTF values from a single impulse response measurement:

0

2

0

2 2exp

)(

dh

dFjh

Fm

f

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

11 novembre 2010 Speech Transmission Index 4

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:

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1)(')(

signalnoise LLFmFm

This makes it possible to measure a “clean” impulse response, and then to perform separately the signl an noise recordigs

11 novembre 2010 Speech Transmission Index 5

Post processing of impulse responses

• A special plugin has been developed for the computation of STI according to IEC-EN 60268-16:2003

11 novembre 2010 Speech Transmission Index 6

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)

11 novembre 2010 Speech Transmission Index 7

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.

11 novembre 2010 Speech Transmission Index 8

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

11 novembre 2010 Speech Transmission Index 9

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.

11 novembre 2010 Speech Transmission Index 10

125 Hz

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-150°

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Directivity of the mouth simulator

250 Hz

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500 Hz

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-30°-15° 1000 Hz

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2000 Hz

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-30°-15° 4000 Hz

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-30°-15° 8000 Hz

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-30°-15° 16000 Hz

-65-55-45-35-25-15-55

+0°+15°

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±180°-165°

-150°

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11 novembre 2010 Speech Transmission Index 11

Directivity of a real human (I. Bork, PTB)125 Hz

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10 2030

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210220

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250 Hz

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210220

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500 Hz

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10 2030

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130140

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1000 Hz

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10 2030

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2000 Hz

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10 2030

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130140

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4000 Hz

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11 novembre 2010 Speech Transmission Index 12

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 60268-16:2003 standard

11 novembre 2010 Speech Transmission Index 13

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.