SPL Meter Measurement per IEC 60268-16

TL;DR

IEC 60268-16 defines the Speech Transmission Index (STI) for evaluating speech intelligibility. While STI is a modulation-based metric, SPL measurement is critical for two reasons: the STIPA test signal must be reproduced at a calibrated level representing typical speech or PA output, and ambient noise must be measured to assess its masking effect on the modulation transfer function. SonaVyx SPL meter helps you set the correct STIPA signal level (typically 60-65 dBA for normal speech, or the PA system operating level) and documents the ambient noise condition. Without proper level calibration, your STI measurement may not reflect real-world intelligibility conditions.

SPL and Speech Intelligibility

IEC 60268-16 calculates STI from the modulation transfer function (MTF) across 7 octave bands from 125 Hz to 8 kHz. The modulation depth is affected by both room reverberation and ambient noise. Your SPL meter quantifies the noise component so the STI result accurately reflects real conditions.

STIPA Signal Level Calibration

The STIPA test signal must be reproduced at a level representing actual use conditions:

For direct speech: 60 dBA at 1 meter (normal vocal effort per ISO 9921)
For raised speech: 66 dBA at 1 meter
For PA systems: the normal operating level of the system
Measure the STIPA signal level with A-weighting at the source position

Ambient Noise Measurement

IEC 60268-16 requires ambient noise to be measured and documented:

Measure LAeq at each receiver position with the PA/source off
Record octave band levels from 125 Hz to 8 kHz
The standard allows noise correction when SNR is between 5 and 25 dB per band
If SNR is below 5 dB in any band, that band cannot be reliably measured

Signal-to-Noise Ratio Requirements

For meaningful STI measurement, the STIPA signal must exceed ambient noise by at least 15 dB in each octave band. Use your SPL meter to verify this condition before running the full STI test.

Common Mistakes

Running STIPA at maximum system volume instead of normal operating level
Not measuring ambient noise separately from the test signal
Ignoring band-specific SNR, leading to corrupted low-frequency MTF values
Measuring ambient noise with doors closed when they are normally open

Workflow with SonaVyx

Use the SonaVyx SPL meter to measure and document ambient noise levels. Then calibrate your STIPA signal level before running the STI measurement tool. Review room reverberation with the RT60 calculator for context. The impulse response tool provides an alternative IR-based STI method. Check overall system response with the RTA analyzer. See our learning modules for step-by-step tutorials.

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