How to Fix an Unintelligible PA System

Symptoms

Announcements over the public address system cannot be understood by listeners. People ask "What did they say?" after every announcement. In transportation hubs, passengers miss critical boarding and safety information. In retail environments, promotional announcements fail to communicate. The problem is usually worst at positions far from speakers, in reverberant corridors, and in areas with high ambient noise from HVAC, crowds, or equipment.

Common Causes

The four factors that determine speech intelligibility in PA systems are speaker coverage (signal level), reverberation (room decay), background noise (ambient interference), and frequency response (spectral balance). Any one of these factors below acceptable thresholds degrades intelligibility regardless of the others. Typical PA problems include ceiling speakers spaced too far apart, creating coverage gaps. High-reverb corridors and atriums with hard surfaces. Background noise from HVAC, crowds, and adjacent spaces masking speech. Band-limited amplifiers or speakers that lack the 2-4kHz consonant region critical for intelligibility. Over-compressed audio that reduces the natural dynamic range speech needs for clarity.

Measurement Procedure

1. Open SonaVyx STI measurement tool.
2. Play the STIPA test signal through the PA system at normal announcement level.
3. Measure STI at the worst-case listener position (farthest from speakers, highest noise).
4. Record the overall STI value and per-band MTF values.
5. The MTF per-band breakdown shows which frequency bands contribute least to intelligibility.
6. Measure background noise level (LAeq) to check if ambient noise is the dominant factor.
7. Measure RT60 to check if reverberation is the dominant factor.

Interpretation

PA systems require STI of 0.50 minimum ("Fair") for general announcements and 0.50+ for safety-critical announcements per IEC 60268-16. STI below 0.45 means most listeners cannot reliably understand announcements. The per-band MTF reveals which factor dominates: low MTF across all bands suggests reverberation. Low MTF only in low-frequency bands suggests background noise masking. High MTF at the microphone position but low at distant positions suggests coverage problems.

Solutions

For coverage: reduce speaker spacing so that the direct sound level exceeds the reverberant level by at least 10dB at every listener position. Use directional speakers aimed at listener areas rather than omnidirectional ceiling speakers that excite room surfaces. For reverberation: add absorption to the dominant reflective surface (usually ceiling in corridors). For background noise: reduce HVAC noise, add sound masking in adjacent areas, or increase PA signal level. For frequency response: ensure the system reproduces 500Hz to 4kHz with adequate level, and apply a gentle high-frequency boost (2-4kHz) to emphasize consonants. Avoid excessive compression that flattens speech dynamics.

Verification

After corrections, remeasure STI at the same positions. The STI should meet or exceed 0.50 for general PA and 0.50+ for safety-critical announcements. Conduct a subjective listening test with representative announcement content to confirm the objective improvement translates to real-world intelligibility.

Measure PA intelligibility with SonaVyx STI