Speech Transmission Index

The Speech Transmission Index, short STI ( Speech Transmission Index of English ), is a measure of speech transmission quality of a transmission path from the speaker to the listener. A transmission path is understood as an acoustic or electro-acoustic speech signal transmission. The STI measure describes the expected speech intelligibility for the listener.

The speech transmission index depends on

  • The volume of the speech signal at the listener
  • The background noise in the speaker and listener
  • Echoes (reflections with delays > 100 ms)
  • The reverberation time of the audience - room
  • Psychoacoustic masking effects
  • The distortion and the frequency response of the transmission path (eg electro-acoustic system)
  • The quality and directional characteristic of the sound source.

Applications

Typical applications are the measurement of speech intelligibility in

  • Rooms without electro-acoustic system
  • PA and Emergency Sound
  • Telecommunications.

Scale

The speech transmission index reflects the transmission quality of speech transmission line used as a numeric value ranging from 0 ( inaudible) ( = excellent ) to 1 at. The following scales are distinguished

  • STI ( Speech Transmission Index )
  • CIS (Common Intelligibility Scale) by Barnett (1995, 1999)

Mathematical relation of the two scales: CIS = 1 log (STI). The typical minimum requirements for public address and evacuation systems are 0.45 or 0.5 STI STI.

STIPA

STIPA ( Speech Transmission Index for Public Address Systems) is a simplified rapid method for measuring the intelligibility of a transmission path, taking into account the electro-acoustic and acoustic effects. A typical STI - PA application is the measurement of speech intelligibility for public address and emergency sound systems. This verifies whether emergency calls are understandable in an emergency for the people present.

Language can be noise in the frequency range 125 Hz - 10 kHz with an amplitude modulation of low -frequency sinusoidal signals ( 0.63 Hz - 12.5 Hz) are described. Such a language- like signal is used as STIPA test signal and thus simulates the human speaker. The meter measures the modulation of the received signal and compares it to the original test signal in the octave bands 125 Hz - 10 kHz. It is concluded from a loss of modulation depth on a reduced speech intelligibility.

History of STIPA measurement

Initially, the speech intelligibility STIPA 12 individual octave-band modulation frequency combinations was determined, covering the language as much as possible. The measurement of low -frequency speech components of the 125 -Hz and 250 -Hz frequency band were pooled and thus simplified.

In 2003 NTI developed the Advanced STIPA measurement with individual coverage of all the modulation frequencies of the language. This development further improved the correlation of subjective speech intelligibility of people with STI - reading and was standardized in IEC 60268-16, edition 2011.

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