IEC 60268-16: Sound system equipment — Part 16: Objective rating of speech intelligibility by speech transmission index

Defines the STI method for objectively measuring speech intelligibility, including STIPA for simplified field use.

IEC 60268-16: Objective Rating of Speech Intelligibility

IEC 60268-16 Edition 5 is the international standard that defines the Speech Transmission Index (STI) as an objective method for rating speech intelligibility. Published by the International Electrotechnical Commission, this standard is used worldwide by acoustic consultants, system designers, and building inspectors to verify that speech communication systems perform adequately in venues ranging from classrooms to airports.

The STI Method

STI is based on the Modulation Transfer Function (MTF), which measures how well amplitude modulations in speech are preserved as sound travels from a speaker to a listener. The method examines 7 octave bands (125 Hz to 8 kHz) at 14 modulation frequencies (0.63 to 12.5 Hz). Each combination produces a modulation reduction factor. These factors are converted to apparent signal-to-noise ratios, averaged with speech-weighted coefficients, and corrected for inter-band redundancy to produce a single STI value between 0 and 1.

STIPA: The Practical Method

Full STI measurement requires 98 modulation index measurements, which is time-consuming. The standard defines STIPA (STI for Public Address) as a faster alternative using a specially designed test signal with two modulation frequencies per octave band (14 total). STIPA produces results within 0.02 of the full STI value in most practical situations. SonaVyx implements STIPA with both Quick (15-second) and Pro (3x averaged) measurement modes.

Rating Scale

IEC 60268-16 defines five qualitative categories: Bad (STI below 0.30), Poor (0.30 to 0.45), Fair (0.45 to 0.60), Good (0.60 to 0.75), and Excellent (above 0.75). Building codes and standards typically require minimum STI of 0.50 for general paging and 0.60 or higher for emergency voice alarm systems (EN 54-16) and courtrooms.

How SonaVyx Implements IEC 60268-16

SonaVyx generates the STIPA test signal per Table F.1 of the standard, measures the modulation transfer function from the captured signal, applies male speech weighting factors from Table 4, and computes the STI with redundancy corrections. The tool also supports STI calculation from impulse response data and from simplified RT60 plus SNR models for design estimation.