Speech Intelligibility
Speech Intelligibility Assessment and Prediction · Also known as: intelligibility metrics, STI, Speech Transmission Index, clarity index
Speech intelligibility is a quantitative measure of how well listeners understand spoken content in acoustic environments. Formalized by Steeneken and Houtgast in 1980 with the Speech Transmission Index (STI), intelligibility metrics combine room acoustic parameters (RT60, noise, clarity) to predict listener comprehension. Understanding speech intelligibility is essential for designing classrooms, offices, hearing aids, and public address systems where clear communication is critical.
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When to use it
Use speech intelligibility metrics when designing spaces where communication quality is critical: classrooms, lecture halls, courtrooms, hospitals, hearing aid design. Intelligibility assessment is valuable for validating acoustic improvements before and after renovation. For hearing-impaired or multilingual audiences, conduct listening tests with representative listeners; metrics alone may underestimate difficulty.
Strengths & limitations
- Objective, repeatable measurement; no need for listener panels once calibration is established.
- Integrated framework combining multiple acoustic factors (reverberation, noise, clarity) into a single score.
- Frequency and modulation bandwidth dependence captured; accounts for how ears integrate temporal and spectral cues.
- Correlation with subjective intelligibility is well-established; STI predictions correlate ~0.9 with listening test results.
- Practical: applicable to recorded signals, real-time room measurements, and hearing aid characterization.
- Assumes linear acoustics; nonlinear effects (distortion, loudness adaptation) are not fully captured.
- STI and variants based on speech production assumptions; intelligibility of music, non-native speech, or heavily accented speech may differ.
- Listener-dependent factors (hearing acuity, prior knowledge, attention) modulate intelligibility; metrics alone cannot predict individual performance.
- Environmental variability: reverberation, noise, and source directivity vary with position in a room; single-point measurements may not represent overall performance.
- Computational complexity: STI involves multiple frequency and modulation bands, requiring specialized equipment or software.
Frequently asked
What does an STI of 0.6 mean in practical terms?
STI 0.6 corresponds to 'fair' speech intelligibility: roughly 80–85% word recognition in listening tests. Listeners can understand most content but miss some words; conversation is possible but requires some repetition. STI < 0.3 (poor) implies significant misunderstanding; STI > 0.8 (good) implies near-perfect comprehension.
How do reverberation and noise independently affect speech intelligibility?
Reverberation (late reflections) blurs phonetic differences (consonant confusion); noise masks weak consonants and modulation details. STI separately weights these: reverberation primarily affects high modulation frequencies; noise dominates at low modulation frequencies. Both degrade intelligibility, but through different mechanisms—one affecting temporal clarity, the other spectral detail.
Can STI metric predict intelligibility for non-native speakers or hearing-impaired listeners?
STI provides a baseline prediction valid for native, normal-hearing listeners. Non-native speakers typically need 5–10 dB higher SNR (roughly 0.1–0.2 lower STI) for equivalent comprehension. Hearing-impaired listeners' intelligibility depends on auditory profile (frequency-specific loss); custom hearing aid fitting may improve or worsen STI-predicted performance.
How do I measure STI in an existing room?
Use an STI analyzer (specialist acoustic meter) or software with a calibrated microphone. Emit an STI test signal (pink noise or speech-weighted noise) through a speaker; measure the impulse response. Alternatively, analyze recorded speech in the environment to compute MOD metrics. Measure at multiple positions for complete room characterization.
What are typical STI targets for different room types?
Classrooms/lecture halls: STI ≥ 0.75. Offices and conference rooms: STI ≥ 0.70. Hearing aid fitting: STI ≥ 0.65 (with hearing aid). Noisy industrial environments: STI ≥ 0.50 with speech enhancement. These targets guide acoustic design; actual comfort and comprehension vary with listener and context.
Sources
- Steeneken, H. J., & Houtgast, T. (1980). A physical method for measuring speech-transmission quality. Journal of the Acoustical Society of America, 67(1), 318–326. DOI: 10.1121/1.384464 ↗
- Houtgast, T., & Steeneken, H. J. (1985). A review of the MTF concept in room acoustics and its use for estimating speech intelligibility in auditoria. Journal of the Acoustical Society of America, 77(3), 1069–1077. DOI: 10.1121/1.392224 ↗
- ASTM E2638-19 (2019). Standard Guide for Workplace Acoustics and Noise Reduction. American Society for Testing and Materials. link ↗
How to cite this page
ScholarGate. (2026, June 3). Speech Intelligibility Assessment and Prediction. ScholarGate. https://scholargate.app/en/acoustics/speech-intelligibility
Which method?
Set this method beside its closest kin and read them side by side — the library lays the books on the table; the choice is yours.
- BeamformingAcoustics↔ compare
- FxLMS Active Noise ControlAcoustics↔ compare
- Psychoacoustic MaskingAcoustics↔ compare
- Room Impulse ResponseAcoustics↔ compare
- RT60 Reverberation TimeAcoustics↔ compare