IEC 60268-16:2011

Overview

IEC 60268-16:2011 - Sound system equipment, Part 16: Objective rating of speech intelligibility by Speech Transmission Index (STI) - is the 4th edition (2011) international standard that defines objective methods for rating speech intelligibility. It provides standardized procedures, test signals and post‑processing guidance for the Speech Transmission Index family of methods (STI, STIPA and STITEL) used across audio, communications and acoustics applications. This edition refines STI methodology, introduces a formal treatment of speech level and test‑signal level setting, and adds a prediction function for auditory masking effects.

Key topics and technical requirements

• STI family of methods: Describes three closely related objective methods - STI, STIPA (Practical A), and STITEL - and their intended scope (with STIPA and STI for general systems and STITEL for restricted uses).
• Measurement approaches: Covers direct (modulated test signals) and indirect (impulse‑response based) measurement methods, including when to apply each approach.
• Test signal and calibration: Specifies the concept of speech level, calibration and the setting of the STI test‑signal level to ensure repeatable, comparable results.
• Post‑processing and MTF: Defines how to compute the Modulation Transfer Function (MTF) from measurements and convert it to an STI score, including binaural measurements and corrections for occupancy/noise.
• Limitations and error sources: Documents applicability limits and common distortions that affect STI (e.g., echoes, non‑linear distortion, fluctuating noise, frequency shifts, compression/expansion, vocoders and hearing impairment considerations).
• Prediction and design: Includes methods for statistical prediction, simulation from impulse responses and design guidance for predicting intelligibility during system planning.
• Normative and informative annexes: Provide detailed procedures (Annexes A–C, J, L) and supporting guidance (qualification bands, reporting examples, adjustments for non‑native or hearing‑impaired listeners).

Practical applications and primary users

IEC 60268-16:2011 is used by:

• Audio and acoustical engineers for commissioning and tuning PA systems, conference rooms, auditoria and voice evacuation systems.
• Communications engineers assessing intelligibility of telephony, VoIP, intercom and wireless voice links.
• System integrators and consultants performing objective acceptance tests and compliance checks.
• Safety and building services professionals validating voice alarm and emergency communication intelligibility.
• Researchers modeling and predicting speech intelligibility in design simulations.

Practical uses include performance acceptance testing, design prediction of room/system intelligibility, troubleshooting intelligibility complaints, and comparing alternative system designs.

Related standards

• Other parts of the IEC 60268 series (Sound system equipment) cover complementary topics in audio equipment and measurement. National regulations and local building/safety codes may reference IEC 60268-16 for intelligibility requirements.

Keywords: IEC 60268-16:2011, Speech Transmission Index, STI, STIPA, speech intelligibility, objective rating, sound system, acoustics, PA systems, impulse response, modulation transfer function.