Speech transmission index

Speech transmission index

Speech Transmission Index, short STI is a measure of speech transmission quality. The absolute measurement of speech intelligibility is a complex science. The STI measures some physical characteristics of a transmission channel (a room, electro-acoustic equipment, telephone line, etc.), and expresses the ability of the channel to carry across the characteristics of a speech signal. STI is a well-established objective measurement predictor of how the characteristics of the transmission channel affects speech intelligibility.

The influence[1] that a transmission channel has on speech intelligibility is dependent on:



STI was introduced by Tammo Houtgast and Herman Steeneken in 1971,[2] and was accepted by Acoustical Society of America in 1980.[3]


STI is a numeric representation measure of communication channel characteristics whose value varies from 0 = bad to 1 = excellent.[4] On this scale, an STI of at least .5 is desirable for most applications.

Barnett (1995,[5] 1999[6]) proposed to use a reference scale, the Common Intelligibility Scale (CIS), based on a mathematical relation with STI (CIS = 1 + log (STI)).

STI CIS Scale.
Speech Intelligibility may be expressed by a single number value. Two scales are most commonly used: STI and CIS

STI predicts the likelihood of syllables, words and sentences being comprehended. As an example, for native speakers, this likelihood is given by:

STI Value Quality according to IEC 60268-16 Intelligibility of Syllables in % Intelligibility of Words in % Intelligibility of Sentences in %
0 - 0.3 bad 0 - 34 0 - 67 0 - 89
0.3 - 0.45 poor 34 - 48 67 - 78 89 - 92
0.45 - 0.6 fair 48 - 67 78 - 87 92 - 95
0.6 - 0.75 good 67 - 90 87 - 94 95 - 96
0.75 - 1 excellent 90 - 96 94 - 96 96 - 100

If non-native speakers, people with speech disorders or hard-of-hearing people are involved, other probabilities hold.

It is interesting but not astonishing that STI prediction is independent of the language spoken - not astonishing, as the ability of the channel to transport patterns of physical speech is measured.

Another method is defined for computing a physical measure that is highly correlated with the intelligibility of speech as evaluated by speech perception tests given a group of talkers and listeners. This measure is called the Speech Intelligibility Index, or SII.[7]


STI has gained international acceptance as the quantifier of channel influence on speech intelligibility. The International Electrotechnical Commission Objective rating of speech intelligibility by speech transmission index,[8] as prepared by the TC 100 Technical Committee, defines the international standard.

Further the following standards have, as part of the requirements to be fulfilled, integrated testing the STI and realisation of a minimal speech transmission index:

  • International Organization for Standardization (ISO) standard for sound system loudspeakers in Fire detection and fire alarm systems[9]
  • National Fire Protection Association Alarm Code[10]
  • British Standards Institution Fire detection and alarm systems for buildings[11]
  • German Institute for Standardization Sound Systems for Emergency Purposes[12]


STIPA (Speech Transmission Index for Public Address Systems) is a simplified version of STI designed for practical application in specific situations (amongst others measuring PA Systems in airports and railway stations).

In STIPA testing, the preferred test signal employed is a signal with speech-like characteristics.

Speech can be described as noise that is modulated by low-frequency signals. Therefore STIPA employs a special amplitude modulation scheme to generate its test signal. At the receiving end of the communication system, the depth of modulation of the received signal is measured and compared with that of the test signal in each of a number of frequency bands. Reductions in the modulation depth are associated with loss of intelligibility.

An alternative Impulse response method assumes that the channel is linear and requires stricter synchronization of the sound source to the measurement instrument.


Historically (for example, at the time when aviation authorities first demanded sufficiently high STI measurements of electro-acoustic systems) the RASTI (Room Acoustics Speech Transmission Index) method was a practical solution, because full STI measurements were technologically not feasible. RASTI does not take the whole signal-degrading effects of electro-acoustic components into account and was declared obsolete by the IEC in June 2011. At this time, this simplified STI derivative was still stipulated as a standard method in some industries. In this case, inferior-quality electro-acoustic components may pass testing, which would be avoided by applying the STIPA method.

See also


Jacob, K., McManus, S., Verhave, J.A., and Steeneken, H., (2002) "Development of an Accurate, Handheld, Simple-to-use Meter for the Prediction of Speech Intelligibility", Past,Present, and Future of the Speech Transmission Index, International Symposium on STI

External links

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