You know what STI is — a number from 0 to 1 that rates how clearly speech can be understood in a space. But how do you actually measure it? The answer involves modulation transfer functions, octave bands, and a surprisingly elegant piece of signal processing. Understanding the measurement method helps you interpret STI results correctly, choose the right measurement tool, and spot when measurements might be unreliable.
TLDR
STI is measured by quantifying how well a room preserves the amplitude modulations that carry speech information. Per IEC 60268-16:2020, there are two primary methods. The indirect (impulse response) method measures the room's impulse response and mathematically derives the modulation transfer function (MTF) across 7 octave bands (125 Hz to 8 kHz) and 14 modulation frequencies (0.63 Hz to 12.5 Hz), yielding 98 modulation transfer values that are combined into a single STI number. The direct (STIPA) method uses a standardised test signal containing amplitude modulations at specific frequencies, transmitted through a loudspeaker and analyzed at the receiver. STIPA uses a subset of the full STI's modulation frequencies (2 per octave band) and produces a result that approximates full STI within plus or minus 0.03. Both methods account for reverberation decay and background noise, the two factors that degrade speech intelligibility.
Real-World Analogy
Imagine sending a message in Morse code through a long tunnel. If the tunnel is short and quiet, the dots and dashes arrive clearly — the listener can decode every letter. But if the tunnel is long and reverberant, each dot bleeds into the next, the dashes overlap, and background noise fills the gaps. The message becomes garbled. STI measurement is essentially testing how well a room preserves "Morse code" — it sends known amplitude patterns (modulations) through the room and measures how much those patterns are blurred by reverberation and masked by noise.
Technical Definition
The Modulation Transfer Function (MTF)
Speech carries information through amplitude modulations — the rhythm of syllables, the envelope of vowels and consonants. These modulations occur at rates from about 0.5 Hz to 16 Hz. The room's ability to preserve these modulations determines intelligibility.
The MTF describes, for each octave band and each modulation frequency, how much the modulation depth is reduced between source and receiver. The modulation reduction factor m(F,f) for modulation frequency F in octave band f is:
m(F,f) = |∫(from 0 to ∞) h²(t,f) × e^(-j2πFt) dt| / ∫(from 0 to ∞) h²(t,f) dt
Where h(t,f) is the octave-band-filtered impulse response. This is the magnitude of the Fourier transform of the squared impulse response at modulation frequency F, normalised by the total energy.
In practice, reverberation and background noise both reduce m:
m_effective = m_reverb × (1 / (1 + 10^(-SNR/10)))
Where SNR is the signal-to-noise ratio in that octave band. High reverberation reduces m_reverb. High background noise reduces the noise correction factor.
Full STI Calculation (IEC 60268-16 Section 4)
- Measure or calculate the impulse response at the receiver position.
- Filter into 7 octave bands: 125, 250, 500, 1000, 2000, 4000, 8000 Hz.
- Calculate m(F,f) for 14 modulation frequencies (0.63 to 12.5 Hz) in each band (98 values total).
- Convert each m to an apparent signal-to-noise ratio: SNR_app = 10 × log₁₀(m / (1 - m)), limited to the range -15 to +15 dB.
- Average the 14 SNR_app values within each octave band to get the Modulation Transfer Index (MTI) per band.
- Apply octave-band weighting factors (IEC 60268-16 Table 1) and redundancy corrections between adjacent bands.
- The weighted, corrected sum is the STI value.
STIPA Method (IEC 60268-16 Section 5)
STIPA is a practical field measurement method that uses a standardised test signal instead of a swept sine or impulse. The signal contains amplitude modulations at 2 specific frequencies per octave band (14 modulation frequencies total, chosen to avoid harmonics of each other). A STIPA analyser (e.g., NTi Audio XL2, Bedrock SM90) plays the signal through a loudspeaker and analyses the received signal to extract modulation depths.
STIPA advantages:
- Real-time measurement (15 seconds per reading)
- Works through live PA systems (tests the complete signal chain)
- No impulse response extraction needed
- Accounts for all real-world degradation including loudspeaker distortion and time-variant effects
- Requires a dedicated test signal source
- Approximation of full STI (accuracy plus or minus 0.03)
- Does not provide the full 98-point MTF matrix
Why It Matters for Design
Knowing how STI is measured helps designers understand what it captures and what it misses:
- STI accounts for reverberation and noise — the two dominant factors in most spaces. If RT60 is correct and background noise is known, calculated STI will be accurate.
- STI does not account for distortion, equalization, or temporal effects in the calculation method (these are captured by STIPA measurement through a live system).
- The signal-to-noise ratio at each octave band is critical. A room with excellent RT60 but high HVAC noise at low frequencies will have a poor STI. This is why background noise surveys are essential inputs to STI prediction.
How AcousPlan Uses This
AcousPlan calculates STI using the indirect method from the room model's predicted impulse response parameters. The platform computes the modulation transfer function across all 7 octave bands and 14 modulation frequencies, applies the background noise level you specify (or the default for the room type), and reports the full STI value per IEC 60268-16. The results page shows both the overall STI and the per-band MTI breakdown, so you can identify which octave bands are limiting intelligibility.
Related Concepts
- What is STI? — The fundamentals of Speech Transmission Index
- What is an Impulse Response? — The measurement STI is derived from
- What is a Background Noise Survey? — Essential input to STI calculation
- What is RT60? — The primary room factor affecting STI
- What is an NC Rating? — Noise criteria linked to STI
Calculate Now
Calculate STI for your room design with AcousPlan. The platform shows per-band modulation transfer so you can optimise both reverberation and background noise.