Transfer Function Measurement per IEC 60268-16

TL;DR

Transfer function measurement directly supports IEC 60268-16 speech intelligibility by revealing how the PA system and room modify the speech signal spectrum. A PA system with poor frequency response will degrade STI even in a room with acceptable reverberation. The transfer function shows where the system response deviates from flat, which frequencies are attenuated or boosted, and where equalization is needed to maximize speech clarity. SonaVyx transfer function analyzer combined with the STI tool creates a complete workflow for voice alarm and PA system commissioning per IEC 60268-16.

Transfer Function and Speech Intelligibility

IEC 60268-16 measures intelligibility through the modulation transfer function, but the overall frequency response of the PA system determines the speech spectrum reaching the listener. Transfer function measurement optimizes this spectral delivery.

Critical Frequency Bands for Speech

The STI calculation weights seven octave bands differently. Transfer function optimization should focus on:

500 Hz to 4 kHz: highest weighting for male speech intelligibility
2 kHz band: most critical for consonant recognition
125 Hz band: lowest weight but still contributes to overall STI
8 kHz band: contributes to sibilance and presence

PA System Response Targets

Measure the transfer function from the PA input to each listening position
Target a flat response within plus or minus 3 dB from 200 Hz to 6 kHz
Roll off below 150 Hz to reduce room mode excitation and improve clarity
Gentle HF rolloff above 6 kHz is acceptable if the room is reverberant
Avoid sharp peaks that indicate resonances or feedback-prone frequencies

Coherence and Intelligibility

Transfer function coherence correlates with intelligibility. Frequencies where coherence drops below 0.5 will have poor STI contribution. Common causes include reverberation, competing reflections, and poor signal-to-noise ratio.

Common Mistakes

Optimizing the transfer function for music reproduction instead of speech clarity
Not measuring at the worst-case listener positions (farthest, off-axis)
Applying EQ corrections without checking coherence first
Ignoring the pre-delay and early reflections visible in the impulse response

SonaVyx Integration

Measure system response with the SonaVyx transfer function analyzer. Run the STI measurement to quantify intelligibility. Check SPL coverage with the SPL meter. Measure room RT60 with the RT60 tool for the reverberation component. Use the AI diagnostic for automated EQ recommendations. See the PA tuning workflow for the complete process.

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