Speech Transmission Index (STI)

Professional Definition

A value in the range of 0 to 1 representing the quality of speech transmission in terms of its understandability through the speech transmission channel.

The STI (Speech Transmission Index) is an objective measure based on the modulation transfer function. In engineering practice, a value of 1.0 indicates perfect understandability (like a face-to-face conversation in a quiet room), while a value of 0.0 indicates a complete lack of understanding of the content. For safety systems (e.g., DSO), standards often require a minimum value of 0.5, which corresponds to "sufficient" understandability.

Acoustics in Simple Terms

Imagine you are looking at text through a window. If the window is perfectly clean (STI = 1.0), you read without any problem. If the window is slightly fogged or frosted (STI = 0.5), you have to guess some letters, but the meaning of the sentence is readable. If the window is made of frosted glass (STI close to 0), you only see patches of light, but you cannot read any content. In acoustics, this "frosting" is reverberation and background noise, which blur the sharp contours of consonants, making speech unintelligible.

Summary

STI is a standard parameter for assessing the quality of sound systems and room acoustics regarding verbal communication. It is essential when designing lecture halls, train stations, and warning systems, where the precision of communication determines safety.