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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〔(Speech Intelligibility Measurement Methods )〕 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) == History == The STI was introduced by Tammo Houtgast and Herman Steeneken in 1971,〔Houtgast, T. and Steeneken, H.J.M. (1971), "Evaluation of Speech Transmission Channels by Using Artificial Signals", Acustica 25, 355-367.〕 and was accepted by Acoustical Society of America in 1980.〔Steeneken, H.J.M. and Houtgast, T. and (1980), "A physical method for measuring speech-transmission quality", J. Acoust. Soc. Am 67, 318-326.〕 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).〔Sander van Wijngaarden, Jan Verhave and Herman Steeneken (2012). The Speech Transmission Index after four decades of development.()〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Speech transmission index」の詳細全文を読む スポンサード リンク
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