What IEC 60268-16 Covers

IEC 60268-16 "Sound system equipment — Part 16: Objective rating of speech intelligibility by speech transmission index" provides a standardized method for predicting how well speech can be understood through an electroacoustic system or in a room. The Speech Transmission Index (STI) is the primary metric, ranging from 0 (completely unintelligible) to 1 (perfect intelligibility).

The standard applies to any situation where speech must be transmitted to listeners, including public address systems, emergency voice alarm systems, conference rooms, classrooms, courtrooms, and transportation facilities.

Edition History

The current Edition 5 (2020) introduced several refinements over earlier versions:

Updated auditory masking model for better accuracy with non-flat noise spectra
Refined STIPA test signal specification (Annex F, Table F.1)
Clarified measurement procedures and reporting requirements
Improved documentation of uncertainty and limitations

Editions 1-4 used slightly different computational methods; results from older editions may not be directly comparable.

Key Applications

Emergency voice alarm: EN 54-16 requires STI ≥ 0.50 (Fair) in all areas served by the voice alarm system. Many authorities require STIPA verification during commissioning.
Classrooms: Standards like ANSI S12.60 and BB93 (UK) specify minimum STI values for learning environments, typically ≥ 0.60.
Transport hubs: Airports, train stations, and metro systems need intelligible announcements for passenger safety and wayfinding.
Courtrooms: Legal proceedings require clear speech transmission from bench to gallery.

What STI Accounts For

STI captures three main degradation mechanisms: reverberation (late reflections smear temporal modulations), background noise (masks the speech signal), and nonlinear distortion (intermodulation in the transmission chain). It does not directly account for talker characteristics, listener hearing ability, or language complexity.

Measurement Methods

IEC 60268-16 defines several measurement approaches:

Direct STI: Full 14-modulation-frequency measurement per band (7 bands × 14 frequencies = 98 MTF values). See MTF details.
STIPA: Simplified method using a special test signal with 2 modulation frequencies per band (14 total).
STI from impulse response: Computed directly from measured impulse response with known noise spectrum.

SonaVyx STI Tools

The SonaVyx STI measurement tool implements STIPA with the standard test signal, STI from impulse response, and simplified STI from RT60 + SNR. Results include per-band MTF values, the composite STI score, CIS (Common Intelligibility Scale), and the qualitative rating. For reporting and documentation, see the reporting section.