What is STI?

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.

Technical 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 considerations

• 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.

Typical applications

• 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.

Limitations

• 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.

Worked illustration

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.

Related parameters

• RT60

  It explains the tail. It does not replace a speech index.

• Signal-to-noise ratio

  The level difference between speech and noise in the speech bands. A component inside the index, not a permanent substitute measure.

• C50

  Early-energy clarity with a 50 ms limit. Useful in discussion, different in definition from STI.

Using the number

Calculators and teaching numbers on this site are preliminary. They do not replace an acoustic model, a measurement made to the relevant standard, or the designer's responsibility.