Speech transmission index
The STI measures some physical characteristics of a transmission channel (a room, electro-acoustic equipment, telephone line, etc.
STI is a well-established objective measurement predictor of how the characteristics of the transmission channel affect speech intelligibility.
The influence[1] that a transmission channel has on speech intelligibility is dependent on: The STI was introduced by Tammo Houtgast and Herman Steeneken in 1971,[2] and was accepted by Acoustical Society of America in 1980.
[4] Houtgast and Steeneken developed the Speech Transmission Index while working at The Netherlands Organisation of Applied Scientific Research TNO.
Embedded Acoustics now continues to support development of the STI, with Herman Steeneken (now formally retired from TNO) still acting as a senior consultant.
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.
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.
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.
STI is a numeric representation measure of communication channel characteristics whose value varies from 0 = bad to 1 = excellent.
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: STIPA (Speech Transmission Index for Public Address Systems) is a version of the STI using a simplified method and test signal.
A single STIPA measurement generally takes between 15 and 25 seconds, combining the speed of RASTI with (nearly) the wide scope of applicability and reliability of full STI.
In general, the indirect method is often the best option when studying speech intelligibility based on "pure room acoustics," when no electro-acoustic components are present within the transmission path.
4 does not disallow the indirect method for such applications, but issues the following words of warning: "Critical analysis is therefore required of how the impulse response is obtained and potentially influenced by non-linearities in the transmission system, particularly as in practice, system components can be operated at the limits of their performance range."
Although many measuring tools based on the indirect method offer STIPA as well as "full STI" options, the sparse Modulation Transfer Function matrix inherent to STIPA offers no advantages when using the indirect method.
Mobile apps for STIPA measurements (such as the ones sold by Studio Six Digital [15] and Embedded Acoustics [16]) are also excluded from the list.
