Speech Transmission Index (STI) measurements

Speech intelligibility is often of vital importance to public safety. Numerous national and international standards now require STI measurements to be carried out to deliver proof that speech intelligibility is at an acceptable level. For instance, Voice Evacuation Systems are required in many countries  to be evaluated in terms of speech intelligiblity. The Speech Tranmission Index is often the preferred (or only) method that is accepted as capable of delivering "proof of performance".

Embedded Acoustics knows the STI like no other. Not only were we the developers of many versions of IEC-compliant hardware and software measuring solutions; our team also played a major role in developing the core technology behind the Speech Transmission Index and contributed significantly to the various standards that define the STI. Nobody knows the STI like we do!

STI measurements provide more than just a single number...

The index itself (the number between zero and one) is of course important, but a wealth of diagnostic information also becomes available through STI measurements. Our experienced consultants will not only be able to tell you whether your systems complies with standards, but also how to make it perform even beter. Also, they will be able to pinpoint separate sources of signal degradation. We offer more than just a measuring services: we offer a thorough analysis.

Embedded Acoustics most frequently carries out STI-related evaluation assignments in our own geographic region: the Netherlands, Belgium and northern Germany. However, our services have been proven competitive on a world-wide scale. Contact us to discuss your measuring needs or to request a quotation.


Principles behind the Speech Transmission Index

Speech is by nature a modulated signal. It contains noisy and tonal parts, covering the frequency spectrum between (roughly) 100 Hz and 10,000 Hz. This means that the so-called "phone band" between 300-3400 Hz covers only part of the relevant spectral range; this puts the upper STI limit of classical telephony systems (including GSM) at a value around 0.70.

Since the signal is indeed modulated, speech also has an associated modulation spectrum: the range of amplitude modulation frequencies applied by the human vocal system. Again roughly, this stretches from 0.5 to 30 Hz. The key fact on which the STI is based is this one:

In almost every case, loss of modulations (decrease of modulation depth) is equivalent to loss of intelligibility.

The same holds true in the optical domain. Optical systems are limited in their ability to transfer (spatial) modulation frequencies, in turn limiting the visibility of the signal. This is expressed through the Modulation Transfer Function, which became popular in optics in the 1970s. This inspired Steeneken and Houtgast to apply the same principle to speech - which turned out to produce amazingly accurate predictions of intelligibility.

The Speech Transmission Index (STI)

The Speech Transmission Index is an objective, physical measure of speech transmission quality. The STI is a 0 to 1 index, indicating the degree to which a transmission channel degrades speech intelligibility. This means that perfectly intelligible speech, when transferred through a channel with an associated STI of 1, will remain perfectly intelligible. The closer the STI value approaches zero, the more information is lost. There are standardized ratings linking certain ranges of the STI to subjectively experienced intelligibility. Relations between the STI and various subjective intelligibility tests (such as CVC wordscore and phonetically balanced word lists) are also well established. These relations are shown in the table below.

Part of the appeal of the STI is it wide range of application areas. The STI is extensively used in room acoustics, for instance to assess intelligibility in auditoria, churches and conference rooms. But the STI is also applied to telecommunication channels, such (mobile) telephone lines and radio transmissions.

The scientific principle on which the STI is based, is that information in speech is represented acoustically in the form of modulations. A speech utterance is essentially nothing more than a sequence of modulated tonal and noisy sounds. Loss of these modulations translates into loss of intelligibility. The Modulation Transfer Function, which can be computed or measured, expresses loss and preservation of modulations. The STI is calculated directly from the Modulation Transfer Function.

The STI was first invented in the 1970's by Herman Steeneken and Tammo Houtgast, both then working at TNO in Soesterberg, the Netherlands. The STI has developed gradually since then, into the mature and widely recognized and applied measuring tool it is today. An estimated population of over 10,000 engineers, scientists and consultants have the STI in their professional toolbox, in some form or another.

An overview of the STI and its possibilities is given in the book "Past, present and future of the STI." Registered users can download a PDF version of this book through the downloads section of this website, free of charge.

Application areas of the Speech Transmission Index

There are several application areas for the STI.

Telecommunications engineering

Telecommunication channels introduced various types of signal distortion, including electronic noise, nonlinear distortions such as peak clipping, effects of ambient acoustics, and frequency transfer limitations of transducers. The STI reflects the effects of all of those sources of signal distortion, and has been thoroughly validated to accurately predict intelligibility for a wide range of communication channels.

"Pure" Room Acoustics

The STI accurately reflects the effects of reverberation, echoes and ambient noise. Since measurements can be carried out quickly and robustly, the STI has been a popular tool to measure speech intelligibility in rooms ever since the introduction of RASTI measuring devices. However, RASTI is not equipped to deal with nonlinear distortion. In pure room acoustics (i..e, no electro-acoustic devices are used), this is not an issue: the physics of sound propagation in rooms is strictly linear. This is not the case when equipment such as public address systems, microphones or loudspeakers come into play - these do introduce nonlinearities. Since the introduction of STIPA (which is equally suitable for situations with or without electro-acoustic devices), RASTI is considered obsolete.

Electro-acoustic applications and room acoustics

The typical example of this type of application would be evaluation of intelligibility in churches and auditory, where microphones, loudspeakers and amplifiers are relied upon to ensure adequate sound levels and intelligibility. However, more complex situations can also be dealt with, such as large, noisy and open environments, airports and stations. Extremely reverberant (as well as noisy) environments the STI is especially suited for include sports stadiums and tunnels.

Basic research

The STI has been used as a research tool in many scientific studies, including the following topics:

  • Effect of speaking style on intelligibility
  • Foreign accents and non-native listening
  • Numerous studies related to hearing impairment and development of hearing aids

Some of these studies have widened the scope of engineering applications for the STI. Others have their own scientific merits, but are of little or no relevance to acoustic consultants and engineers.

Suitability of STI versions and test signals

The table below indicates which version of IEC-60268 and which test signal are optimal (++), adequate (+) or unsuitable (-) for various applications.


History of the STI

The STI was first invented in the 1970's by Herman Steeneken and Tammo Houtgast, both then working at TNO in Soesterberg, the Netherlands. They were inspired by the application of the so-called modulation transfer function as used in optics to express degradation of visual signals in optical pathways. The STI also utilizes modulation transfer functions, but in the acoustic domain. Their landmark 1980 paper in the Journal of the Acoustical Society of America first introduced the STI to a wider audience, although still consisting exclusively of scientists.

From 1980 onwards, the STI also made its way to consultants and engineers. The first practical measuring device sold commercially was the RASTI (Room Acoustics STI) brought to the market by B&K around 1985. RASTI is basically a limited version of the "full" STI method, specifically tuned for applications in room acoustics. Widespread use of the STI method was further promoted by the appearance of an IEC standard (IEC 60268-16). This standard has been regularly updated to incorporate recent additions to the STI framework. Its 5th edition is expect to be published in 2018. A table listing suitability of the different versions of the STI standard (and of different test signals) is found in the section of this website on the different STI application areas.

At the International STI symposium in 2002 (organised by the founders of Embedded Acoustics to honor the lifetime achievements of Steeneken and Houtgast), it was estimated that the worldwide user base of the STI had grown to about 10,000 professional users. This number has grown further since, mostly because of the introduction of STIPA (STI for Public Address systems). This method covers the same scope as RASTI, but also takes signal-degrading effects of electo-acoustic components into account. It has effectively rendered RASTI obsolete.

While the user base of the STI grew, scientists and engineers from TNO (who later on were to found Embedded Acoustics) continued adding features to the STI framework. They developed a way to incorporate between-band synergistic effects, a shortcoming of the original STI that was corrected in the 2nd version of the STI standard. They also developed a scheme to include effects of level-dependent masking, that made its way into the 3rd edition of IEC-60268-16. In fact, they developed and validated just about every change to the core STI model since its invention.

From 2002 onwards, STIPA became the dominant implementation of the STI. The more univerally applicable "Full STI" went all but obsolete; the equipment, which had not been for sale commercially since the early 1990s, was complex and expensive, and a single measurement took 15 minutes. In 2017, Embedded Acoustics re-introduced Full STI, based on a novel implementation equivalent to the original, but only taking 70 seconds. The dramatically shortened measurement time, and the availability on Bedrock SMxx measuring instruments, makes Full STI a viable (and more universally reliable) alternative to STIPA.

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