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.