Speech transmission index
Speech Transmission Index (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 that a transmission channel has on speech intelligibility is dependent on:
- the speech level
- frequency response of the channel
- non-linear distortions
- background noise level
- quality of the sound reproduction equipment
- echos (reflections with delay > 100ms)
- the reverberation time
- psychoacoustic effects (masking effects)
The STI was introduced by Tammo Houtgast and Herman Steeneken in 1971, and was accepted by Acoustical Society of America in 1980. Steeneken and Houtgast decided to develop the Speech Transmission Index because they were tasked to carry out a very lengthy series of dull speech intelligibility measurements for the Netherlands Armed Forces. Instead, they spent the time developing a much quicker objective method (which was actually the predecessor to the STI).
Houtgast and Steeneken developed the Speech Transmission Index while working at The Netherlands Organisation of Applied Scientific Research TNO. Their team at TNO kept supporting and developing the STI, improving the model and developing hardware and software for measuring the STI, until 2010. In that year, the TNO research group responsible for the STI spun out of TNO and continued its work as a privately owned company named Embedded Acoustics. Embedded Acoustics now continues to support development of the STI, with Herman Steeneken (now formally retired from TNO) still acting as a senior consultant.
In the early years (until approx. 1985) the use of the STI was largely limited to a relatively small international community of speech researchers. The introduction of the RASTI ("Room Acoustical STI") made the STI method available to a larger population of engineers and consultants, especially when Bruel & Kjaer introduced their RASTI measuring device (which was based on the earlier RASTI system developed by Steeneken and Houtgast at TNO). RASTI was designed to be much faster than the original ("full") STI, taking less than 30 seconds instead of 15 minutes for a measuring point. However, RASTI was only intended (as the name says) for pure room acoustics, not electro-acoustics. Application of RASTI to transmission chains featuring electro-acoustic components (such as loudspeakers and microphones) became fairly common, and led to complaints about inaccurate results. The use of RASTI was even specified by some application standards (such as CAA specification 15 for aircraft cabin PA systems) for applications featuring electro-acoustics, simply because it was the only feasible method at the time. The inadequacies of RASTI were sometimes simply accepted for lack of a better alternative. TNO did produce and sell instruments for measuring full STI and various other STI derivatives, but these devices were relatively expensive, large and heavy.
Around the year 2000, the need for an alternative to RASTI that could also be applied safely to Public Address (PA) systems had become fully apparent. At TNO, Jan Verhave and Herman Steeneken started work on a new STI method, that would later become known as STIPA (STI for Public Address systems). The first device to include STIPA measurements available for sale to the general public was made by Gold-Line. At this time, STIPA measuring instruments are available from various manufacturers.
RASTI was standardized internationally in 1988, in IEC-60268-16. Since then, IEC-60268-16 was revised three times, the latest revisions (rev.4) appearing in 2011. Each revision included updates of the STI methodology that had become accepted in the STI research community over time, such as the inclusion of redundancy between adjacent octave bands (rev.2), level-dependent auditory masking (rev.3) and various methods for applying the STI to specific populations such as non-natives and the hearing impaired (rev.4). An IEC maintenance team is currently working on rev. 5.
RASTI was declared obsolete by the IEC in June 2011, with the appearance of rev. 4 of IEC-602682-16. At this time, this simplified STI derivative was still stipulated as a standard method in some industries. STIPA is now seen as the successor to RASTI for almost every application.
STI is a numeric representation measure of communication channel characteristics whose value varies from 0 = bad to 1 = excellent. On this scale, an STI of at least .5 is desirable for most applications.
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.
Nominal qualification bands for STI
The IEC 60268-16 ed4 2011 Standard defines a qualification scale in order to provide flexibility for different applications. The values of this alpha-scale run from "U" to "A+".
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, 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
- National Fire Protection Association Alarm Code
- British Standards Institution Fire detection and alarm systems for buildings
- German Institute for Standardization Sound Systems for Emergency Purposes
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).
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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.
List of manufacturers of STI measuring instruments
STI measuring instruments are (and have been) made by various manufactures. Below is a list of brands under which STI measuring instruments have been sold, in alphabetical order.
- Audio Precision . Offers an STI Plug-in option for use with APx500 Series audio analyzers.
- Bedrock Audio . This is the brand under which Embedded Acoustics sells their STIPA hardware, such as the SM50.
- Gold Line . First to offer STIPA measuring solutions (DSP2 and DSP30), but currently not offering any tools that comply with the latest standards (IEC-60268-16 rev. 4).
- HEAD acoustics . Offers STI options (including STIPA, STITEL, and RASTI) for both the Artemis Suite  and ACQUA  test systems.
- Ivie . Offers STIPA-capable acoustic measuring tools such as the IE-45.
- Norsonic . Norsonic was early to adopt STIPA and offer STIPA modules on their instruments (Nor-140). Appears not to be sold in the US.
- NTi Audio . Offers STIPA modules with their AL1 and XL2 line of acoustic measuring instruments as well as a Talkbox and other peripherals. Apparent market leader at this moment (2013).
- Quest . Now part of 3M, Quest produces tools such as the Quest Verifier.
- TNO. Not currently marketing any products, but sold (among others) the STIDAS series of measuring instruments before.
- Svantek 
The market for STI measuring solution is still developing, so the above list is subject to change as manufacturers enter or leave the market. The list does not include software producers that produce STI-capable acoustic measuring and simulation software. Mobile apps for STIPA measurements (such as the ones sold by Studio Six Digital  and Embedded Acoustics ) are also excluded from the list.
- Speech Intelligibility Measurement Methods
- Houtgast, T. and Steeneken, H.J.M. (1971), "Evaluation of Speech Transmission Channels by Using Artificial Signals", Acustica 25, 355-367.
- Steeneken, H.J.M. and Houtgast, T. and (1980), "A physical method for measuring speech-transmission quality", J. Acoust. Soc. Am 67, 318-326.
- Sander van Wijngaarden, Jan Verhave and Herman Steeneken (2012). The Speech Transmission Index after four decades of development.
- THE MEASUREMENT OF SPEECH INTELLIGIBILITY Herman J.M. Steeneken TNO Human Factors, Soesterberg, the Netherlands
- Barnett, P. W. and Knight, R.D. (1995). "The Common Intelligibility Scale", Proc. I.O.A. Vol 17, part 7.
- Barnett, P. W. (1999). "Overview of speech intelligibility" Proc. I.O.A Vol 21 Part 5.
- Speech Intelligibility Index site created by the Acoustical Society of America (ASA) Working Group S3-79
- International Electrotechnical Commission IEC 60268-16: Sound system equipment – Part 16: Objective rating of speech intelligibility by speech transmission index Fourth edition 2011-06
- ISO 7240-24:2010 Fire detection and fire alarm systems -- Part 24: Sound-system loudspeakers
- NFPA 72 National Fire Alarm Code (2010 edition)
- BS 5839-8 Fire detection and alarm systems for buildings. Code of practice for the design, installation and servicing of voice alarm systems
- Deutsches Institut für Normung DIN 60849 System regulation with application regulation DIN VDE 0833-4
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