Field Story
The Church Where Nobody Could Understand the Pastor
A 400-seat church had invested $80,000 in a new PA system, but congregants complained they still couldn't understand the sermon. The installer claimed the system was 'perfectly tuned.' An STI measurement using SonaVyx's STIPA tool revealed a score of 0.38 — rated 'Poor' per IEC 60268-16. The culprit: RT60 was 2.8 seconds, and the system delay was set 15ms too late, creating destructive interference with the direct sound. After correcting the delay alignment and adding absorption panels, STI improved to 0.68 ('Good').
House of WorshipSTI/STIPA Measurement per IEC 60268-16
TL;DR
IEC 60268-16 Edition 5 defines the Speech Transmission Index method for objectively measuring speech intelligibility. SonaVyx implements both direct STIPA measurement (using the standardized test signal from Table F.1) and STI derivation from impulse response. This guide covers the complete measurement workflow.
Understanding Speech Transmission Index
STI quantifies how well a transmission channel preserves the temporal modulation patterns essential for speech understanding. The method analyzes 14 modulation frequencies across 7 octave bands (125 Hz to 8 kHz).
STIPA Field Measurement
STIPA (STI for Public Address systems) uses a standardized test signal containing modulated noise in 7 octave bands, each modulated at 2 specific frequencies per Table F.1 of the standard:
- 125 Hz band: 1.0 Hz and 4.0 Hz modulation
- 250 Hz band: 0.63 Hz and 3.15 Hz modulation
- 500 Hz band: 2.0 Hz and 5.0 Hz modulation
- And so on through 8 kHz
Rating Scale
| STI Range | Rating | Typical Application |
|---|---|---|
| < 0.30 | Bad | Unacceptable for any speech application |
| 0.30 – 0.45 | Poor | May be acceptable for background announcements |
| 0.45 – 0.60 | Fair | Minimum for PA systems in reverberant spaces |
| 0.60 – 0.75 | Good | Target for classrooms and conference rooms |
| > 0.75 | Excellent | High-quality speech reproduction |
Standard Reference
IEC 60268-16:
Try It Now
Open this measurement tool in your browser — free, no download required.
Last updated: March 19, 2026