IEC 60268-16 STI Rating Scale: From Bad to Excellent
TL;DR
IEC 60268-16 Table 6 defines five STI quality categories. Bad (STI < 0.30): less than 34% sentence intelligibility, speech essentially unintelligible. Poor (0.30–0.45): 34-70% intelligibility, significant effort required. Fair (0.45–0.60): 70-90% intelligibility, acceptable for PA announcements. Good (0.60–0.75): 90-96% intelligibility, suitable for most speech communication. Excellent (> 0.75): >96% intelligibility, full clarity. The CIS (Common Intelligibility Scale) maps STI to a 0.0–1.0 scale logarithmically: CIS = 1 + log10(STI).
Qualitative Categories (Table 6)
IEC 60268-16 maps the continuous STI value to five descriptive categories:
| Category | STI range | Approx. sentence score | Typical scenario |
|---|---|---|---|
| Bad | 0.00 – 0.30 | <34% | Reverberant cathedral, unmixed open-air event |
| Poor | 0.30 – 0.45 | 34 – 70% | Poorly treated gym, noisy industrial PA |
| Fair | 0.45 – 0.60 | 70 – 90% | Average train station, basic classroom |
| Good | 0.60 – 0.75 | 90 – 96% | Well-designed conference room, modern PA |
| Excellent | 0.75 – 1.00 | >96% | Studio, close-distance direct speech |
These categories are guideline descriptors — the appropriate minimum STI depends on the application. Emergency announcements where lives depend on comprehension require higher STI than background music announcements.
Regulatory Thresholds
Various standards specify minimum STI values:
- EN 54-16 (voice alarm): minimum STI 0.50 (Fair)
- ANSI S12.60 (classrooms): effectively requires STI ≥ 0.60 (Good) via RT and noise criteria
- BB93 UK (school acoustics): STI ≥ 0.60 in furnished unoccupied classrooms
- ISO 7240-19 (mass notification): minimum STI 0.50, preferred 0.55
- NFPA 72 (fire alarm): references intelligibility without specifying a fixed STI value
CIS — Common Intelligibility Scale
The CIS provides an alternative representation: CIS = 1 + log₁₀(STI). This logarithmic transformation spreads the upper range: STI 0.50 → CIS 0.70, STI 0.75 → CIS 0.88. CIS is sometimes preferred for reporting because the differences between Good and Excellent are perceptually significant but compressed on the linear STI scale.
Percentage Intelligibility
The sentence intelligibility scores in the table above are approximate and depend on the specific speech material used. The Phonetically Balanced (PB) word score is typically lower than the sentence score for the same STI because sentence context provides redundancy. For critical applications, both STI and subjective listening tests may be required.
Design Targets
When designing a PA system, aim for at least one category above the minimum requirement to provide a safety margin for equipment aging, maintenance issues, and ambient noise variability. If the minimum is Fair (0.50), design for Good (0.60+). See measurement procedures for commissioning verification.
SonaVyx Rating Display
The STI tool displays the STI value with color-coded category badge (red/orange/amber/green/blue), the CIS equivalent, and the applicable pass/fail threshold if you select a target standard. Per-band MTF charts show which bands are limiting overall intelligibility, guiding optimization efforts.
Try It Now
Open this measurement tool in your browser — free, no download required.
Last updated: March 19, 2026