Speech Transmission Index (STI)
Definition
Speech Transmission Index (STI)
The Speech Transmission Index is a metric ranging from 0 to 1 that quantifies how well a transmission channel preserves the modulation patterns essential for speech intelligibility. Defined by IEC 60268-16 Edition 5, STI accounts for reverberation, noise, and nonlinear distortion. SonaVyx measures STI directly in your browser.
How STI Is Measured
STI is measured by transmitting a modulated test signal through the acoustic path and comparing the received modulation depth to the original. IEC 60268-16 Clause 4 specifies the Modulation Transfer Function (MTF) across 7 octave bands (125 Hz to 8 kHz) at 14 modulation frequencies (0.63 to 12.5 Hz). The STIPA method uses a simplified signal with 14 simultaneous modulation frequencies, reducing measurement time to 15 seconds.
Practical Example
An airport terminal PA system measures STI of 0.38, rated "Poor" — passengers cannot understand departure gate announcements. After adding distributed ceiling speakers to reduce source-to-listener distance and installing acoustic baffles to lower RT60 from 3.2 to 1.4 seconds, STI improves to 0.62 ("Good"), meeting EN 50849 requirements for emergency voice evacuation.
STI Rating Scale
IEC 60268-16 Table 1 defines five qualitative categories: Bad (0.00 to 0.30), Poor (0.30 to 0.45), Fair (0.45 to 0.60), Good (0.60 to 0.75), and Excellent (0.75 to 1.00). Most building codes and fire safety standards require a minimum STI of 0.50 for public address systems. Emergency voice evacuation systems per EN 50849 typically mandate STI above 0.50 in all zones, with some jurisdictions requiring 0.55 or higher.
The Modulation Transfer Function
Speech carries information through amplitude modulations at rates between 0.5 and 16 Hz. The MTF measures how faithfully these modulations survive transmission through a room or PA system. Reverberation smears modulation peaks by filling temporal gaps between syllables. Background noise masks modulation troughs. The MTF value at each combination of octave band and modulation frequency produces a 7-by-14 matrix that captures the complete intelligibility profile.
STI from Impulse Response
IEC 60268-16 Annex A describes calculating MTF directly from the room impulse response using the relationship between modulation reduction and the energy decay. This method avoids the need for a dedicated STIPA test signal and can derive STI from any calibrated impulse response measurement. SonaVyx supports both direct STIPA measurement and impulse-response-derived STI calculation per Clause A.4.
Factors Affecting STI
Three primary factors degrade STI: excessive reverberation time (RT60 above 1.5 seconds significantly impacts speech clarity), insufficient signal-to-noise ratio (SNR below 15 dB reduces intelligibility), and nonlinear distortion from overdriven amplifiers or speakers. The direct-to-reverberant ratio is equally critical — increasing speaker directivity or reducing listener distance to the source improves STI more efficiently than acoustic treatment alone.
Common Intelligibility Scale (CIS)
The Common Intelligibility Scale provides a percentage-like mapping of STI values, making results more intuitive for non-technical stakeholders. CIS ranges from 0 to 1 and compresses the extremes while expanding the mid-range where small STI changes have the greatest perceptual impact. CIS is derived from STI using the formula in IEC 60268-16 Annex F and is particularly useful for client-facing reports.
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Measure Speech Intelligibility (STI) — free in your browser with SonaVyx