Knowledge
What is STI?
The speech transmission index from 0 to 1: reverberation, noise, how STI differs from RT60, and why “clear sound” is not a specification.
STI is a number from zero to one describing how much speech modulation reaches the listener. Reverberation smears syllables. Noise covers them. Fixing only one may not be enough.
Definition
The index is described in IEC 60268-16. Speech is intelligible because its level changes quickly. The room and the noise reduce the depth of that change. The index measures how much modulation remains across bands that matter to speech.
Short field versions such as STIPA are common in commissioning, but they are not the whole standard. When you compare two proposals, name the method. Do not write only the word “clarity”.
Why it matters
A lecture hall with an acceptable decay can still fail because an air-handling unit sits over the last row. A quiet theatre can fail because the tail is long. STI forces the team to see both causes.
Principle
Common labels on the 0–1 scale: below 0.30 bad, 0.30–0.45 poor, 0.45–0.60 fair, 0.60–0.75 good, above 0.75 excellent. These are qualitative labels associated with the standards family, not a promise that your seats will reach them.
A microphone close to the mouth raises the speech-to-reverberation ratio. An undirected loudspeaker layout can raise level and lower clarity at the same time. The index punishes both if they are badly designed.
Measurement
Test signal — A standard signal is played from the talker position or through the system and read at the seats. Aim and height change the figure.
Condition — Empty or occupied, HVAC on or off, door open or shut. A report that omits the condition is not usable at handover.
Coverage, not only the average — A handsome average with a weak patch under the balcony means the balcony failed.
Units
Unitless, 0 to 1 — STI is not a decibel. It is not added the way levels are added.
Do not turn it into a casual percentage in a contract — If the figure is 0.60, write 0.60 and name the method. “60% clarity” is advertising copy.
Design
Lower the noise — Air path, grille velocity, machine isolation, and the corridor door. This is architectural and mechanical work before a loudspeaker is bought.
Shorten the tail where speech is the job — Calculated absorption, a suitable volume, then a system that does not pour energy onto distant reflective surfaces.
Bring the source closer — A speech loudspeaker aimed at the row, or a decent microphone, usually beats turning up a distant system.
Use
Lecture and meeting rooms — Success is a sentence understood from the last seat, including remote participants if the use is hybrid.
Stadia and evacuation speech — Intelligibility here is a safety matter, not a taste. The local code is what names the target.
Spoken theatre — Light reinforcement may be required so the room does not become a concert. STI helps set that limit.
Limits
It does not measure musical quality — It is a speech index. A room can pass it and fail as a recital room. The reverse is less likely but the point stands.
The listener’s language and fatigue — The index is not a full human comprehension test. A rushed talker and a poor microphone remain outside the model.
A prediction is not a measurement — A computer model is labelled as a prediction. The handover figure comes from a field measurement by the named method.
Common mistakes
Turning it up until the system shouts — Excess level increases excitation and reverberation. Clarity can fall.
Forgetting metal near a hearing loop — Metal loss reduces the loop field. That is another intelligibility path, with its own reference in IEC 60118-4.
Accepting the front row only — The front row succeeds in failed rooms. The requirement is written for the worst seat you still call acceptable.
Example
Two rooms with the same level at the seat: one quiet with a longer decay, one noisy with a shorter decay. Their STI values may converge or diverge. You cannot pick the winner from RT60 alone, nor from one decibel reading on a phone.
Questions
What figure do I put in a contract?
The figure the brief agrees, with the method: for example a minimum at a percentage of seats, by a named method from IEC 60268-16, in a named room condition. Do not copy the label “good” from an article.
Does an expensive loudspeaker raise STI?
Only if its aim, delay, and level suit the room. An expensive box aimed at a reflective wall remains an expensive box.
Does the index work for Arabic?
The signal is standard, not an Arabic sentence. The index describes the transmission channel. Human comprehension has extra factors, but a bad channel fails in any language.
STIPA or full STI?
Write what you will actually measure. Full STI is slower and richer. The short method is common at commissioning. Mixing them in one table ruins the comparison.
Does sound isolation raise STI?
If the noise comes from outside or from the next room, yes, indirectly. Isolation does not shorten the reverberation of the room itself.