IEC 60268-16 Speech Transmission Index

IEC 60268-16 defines the Speech Transmission Index (STI), an objective measurement of how well speech is transmitted through a space or audio system. STI quantifies speech intelligibility on a scale of 0–1, where:

• 0.0–0.3: Poor (many words unintelligible)
• 0.3–0.45: Fair (occasional words missed)
• 0.45–0.6: Good (minor difficulty understanding)
• 0.6–0.75: Excellent (clear understanding)
• 0.75–1.0: Perfect (all words intelligible)

STIPA (STI for Public Address) is the rapid assessment method using speech-weighted noise analysis, useful for quick commissioning and troubleshooting.

For AV integrators, STI is the universal standard for validating speech audio system design in venues where intelligibility is mission-critical: courtrooms, emergency alert systems, boardrooms, classrooms, theaters, and public address installations. Specifiers increasingly mandate STI targets; integrators who understand STI design and measurement gain competitive advantage and ensure system acceptance.

Key Requirements & Specifications

Measurement Methodology

STI assessment involves:

1. Impulse response of the space (how the room's reflections and absorption affect sound)
2. Frequency-dependent modulation transfer function (how the space attenuates speech-frequency fluctuations)
3. Calculation of speech-intelligibility metrics across 14 frequency bands (125 Hz–8 kHz)

Practical measurement instruments:

• STI sound level meter (dedicated hardware; expensive but field-deployable)
• STIPA measurement using a speech-weighted noise analyzer or audio interface + software (faster, less expensive, acceptable for commissioning)
• Lab analysis with full impulse-response measurement (most accurate; used for design validation)

Design Targets by Application

Factors Affecting STI

Reverberation time (RT): Excessive reverberation (>2–3 seconds in small rooms) destroys intelligibility by overlapping syllables. RT must be controlled via absorption. STI degrades rapidly above optimal RT for the space.

Background noise: Noise floor (HVAC, traffic, audience) masks quiet phonemes. Target noise floor is typically 40–50 dBA for indoor spaces. Louder environments require more SPL from AV systems, increasing risk of feedback and fatigue.

Frequency response: Speech intelligibility relies on 2–4 kHz energy (consonant clarity). Overemphasizing bass or treble reduces intelligibility. System EQ should maintain neutral to slightly brightened response (1–3 dB boost at 2–4 kHz).

Loudspeaker directivity and placement: Poor speaker placement (hidden behind structural elements, excessive distance from listener, misaligned with sight lines) reduces effective SPL and intelligibility. Coverage patterns must be verified during design.

Sound reflection and flutter echo: Parallel hard surfaces (glass, tile, concrete) cause flutter echoes (rapid reflections) that interfere with speech. Diffusion or absorption is needed.

Practical Application for AV Integrators

STI design considerations appear in:

• Courtrooms and legal facilities where intelligibility is a compliance requirement
• Emergency alert system commissioning (fire alarm, mass notification)
• University classrooms and lecture halls with large rooms or complex acoustics
• Corporate training and conference facilities requiring crisp audio for video conferencing
• Theaters and performing arts centers with paging and accessibility requirements
• Hospitals and urgent care with overhead paging and emergency notification
• Airports, transit hubs, and large public facilities with public address systems

Integrator responsibilities:

1. Design phase: Specify speaker types, placement, and EQ to meet STI targets. Use acoustic modeling software (SIM4, EASE, or equivalent) to predict STI performance.
2. Installation: Verify speaker coverage, angle, and alignment matches design intent.
3. Commissioning: Measure STIPA with a sound level meter or dedicated STI analyzer. Document results. Identify problem areas (high reverberation, inadequate coverage) for corrective action.
4. Tuning: Adjust EQ, speaker levels, and delay alignment based on measured STI. Minor tuning often improves STI by 0.05–0.10 points.

Common Pitfalls

Ignoring reverberation during design. A beautiful cathedral or marble-clad courtroom may have 3–4 second RT, making speech intelligibility nearly impossible regardless of loudspeaker power. Reverberation control (absorption, curtains, soft furnishings) must be addressed upstream; AV system alone cannot compensate.

Assuming "loud = intelligible." Cranking amplifier gain increases noise floor and causes feedback, worsening intelligibility. STI is degraded by excessive SPL (listener fatigue, clipping distortion). Target moderate SPL (typically 75–85 dBA) with proper room acoustics.

Neglecting high-frequency content. Cheap loudspeakers with weak treble response (<2 kHz) fail to deliver consonant clarity. Voice systems require speakers with usable response to 4–6 kHz minimum. Specification review prevents this.

Overlooking measurement variability. STI varies significantly with position and environmental factors (audience presence, open/closed windows, HVAC state). Measure at multiple locations (front, middle, back, sides) to understand coverage uniformity. A single measurement at the podium does not validate facility-wide intelligibility.

Confusing STI with hearing loop technology. Hearing loop systems (induction coil receivers for hearing aids) improve speech delivery to assisted listeners but do not inherently improve STI. Both are valuable for accessibility; neither replaces good room acoustics and loudspeaker design.

Failing to document and monitor STI after commissioning. As facilities age or change (furniture added, windows replaced), room acoustics degrade. Periodic STI measurement (annually for critical spaces) identifies problems early.