We often do not hear speech or announcement clearly at churches, gymnasiums, stadiums or factories. This happens because the original sound is interfered by its reflected sound and noise.

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The human ears judged the easiness of speech listening in the past. Such judgment is ambiguous and prone to be influenced by individual variations among listeners and the state of voice.

The Speech Transmission Index (STI) provides the sound transmission with an objective value, focusing the physical phenomenon of sound mixing.

In short, the easiness or difficulty of speech listening can be assessed by the resemblance of the original sound wave and transmitted sound wave to ears.

The STI values range from 0 to 1. If the transmitted sound to ears includes much more reflected sound than the original, in other words, the transmitted sound badly differs the original, the STI value decreases toward 0. (poor intelligibility)
Contrarily the transmitted sound resembles the original sound, containing less reflected sound, the STI value increases up to 1. (good intelligibility)

The STI value is generally required more than 0.6 for human speech to be intelligible.
STI

Let’s try listening to some actual sounds.

Animation : STI value
  • Minimize the reflected sound and noise at the architectural design stage by adopting the sound absorption and isolation materials in walls, ceilings, and other areas.
  • Position Line Array speakers, which are characterized by a high level of directionality and the ability to transmit sound energy directly with little attenuation over long distances, so that they are facing listeners in order to increase the amount of sound reaching the ear directly and limit reflected sound. (Link: Line Array website)

    Select the Line Array speakers, which are characterized by the sharp dispersion angle and little energy attenuation over long distance. Then install them toward the listeners in order to increase the direct and decrease the reflected sound.
  • Use a digital mixer or processor which can suppress the reverberation or and resonance. (Link: M-633D Web Flash, DP-K1)
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