Application areas of the Speech Transmission Index
There are several application areas for the STI.
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.
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.