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Evolution of Speech Transmission Index (STI) in Acoustics: Past, Present, Future Innovations

Explore the historical evolution of STI from its inception in 1971 to present standards like STIPA and forecasted improvements like binaural testing. Learn about objective and subjective assessment methods, speech signal complexities, and the significance of STI in predicting speech intelligibility under various distortions.

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Evolution of Speech Transmission Index (STI) in Acoustics: Past, Present, Future Innovations

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  1. Past, Present andFuture of STI Herman J. M. Steeneken (www.steeneken.com)

  2. Past: STI (1971, 1980), Envelope Spectrum (1972), MTF (1975), RASTI (1979), prediction by ray-tracing(1981), IEC 60268-16 (1988, 1998)Present: Revised STI (1992, 1999, 2002), STIPA (2001), IEC 60268-16 (2003), ISO 9921 (2003)New and Future: Binaural STI, improvement measurement STI from speech signal, non-native speech, vocoders

  3. Signal-to-Noise ratio !!!

  4. Assessment Methods • Subjective assessment with subjects (speakers and listeners): representative, limited reproduction, no diagnostics, laborious • Objective assessment based on physical properties (measurements): reproducible, diagnostic, fast • Objective methods allow prediction of system performance: design tool

  5. Speech and Noise

  6. Speech envelope function and envelope spectrum

  7. Spatial line frequency equivalent to MTF

  8. Distortion of Speech Envelope

  9. Dynamic measurement of SNR

  10. Matrix for seven MTF’s 1 Hz 3 Hz 10 Hz X

  11. CALCULATION of STI

  12. Calculation scheme

  13. Frequency weightings (various studies)

  14. Redundancy with image detection

  15. Frequency weightings (CVC words)

  16. Relation Noise and Band-pass limiting S.d.= 4.4% Male speech

  17. Listening test with four subjects

  18. Embedded CVC words: versta des over en nu fijs uit het woord zek einde noteer lal punt “Semi random” combination of: 17 initial consonants 15 vowels 11 final consonants

  19. Relation AGC and echoes S.d.= 6.9 % Male speech

  20. Qualification of STI (Acustica, 1984)

  21. Performance cabin public address

  22. Iso-STI contours

  23. Effective gain of a PA-system

  24. Field measurement

  25. Full STI (STI-14 and limited modulation frequency range STI-3) STI-14 • Seven octave bands • 14 Modulation frequencies • Random envelope for octaves not under test STI-3 • Seven octaves • 3 Modulation frequencies (1.0, 3.15, and 10 Hz) • Random envelope for octaves not under test

  26. RASTI (Room Acoustics STI,1979) Only for person-to-person communication assessment Two octave bands (500Hz and 2 kHz) Complex envelope (4 and 5 mod. freq.) see Acustica 1984, IEC 60268-16

  27. STI-PA (STI Public Address, 2001) Assessment of Public Address including: acoustic environment, band-pass limiting, overload dis- tortion Seven octave bands (125 Hz and 250 Hz merged) Complex envelope (2 simultaneous modulation frequencies per band) see IEC 60268-16

  28. Future • Binaural STI • Improvement using Speech as test signal • Non-native talkers and listeners

  29. Future: Binaural STI (I) Use artificial head and perform simultaneous measurement on both ears Select highest performance (best ear selection) Use cross correlation approach for 500, 1000, and 2000 Hz.

  30. Future: Binaural STI (II)

  31. Future: improvement Speech Signal as test signal

  32. Future: non-native talkers and listeners

  33. Conclusions • STI predicts the speech intelligibility for many types of distortion: noise, band-pass limiting, non-linearity's, temporal, vocoding, non-native speech, and binaural hearing. • Improvements (measuring methods or scope) of STI will not change the qualification ranges. • STI is an international standardized method (ISO9921, IEC 60268-16) and used for many national standards.

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