Equipment Requirements

The measurement chain for STI comprises:

Test signal source: STIPA signal generator or full STI modulated noise. The signal must be injected into the PA system at line level or played from a reference speaker.
Measurement microphone: Omnidirectional, Class 1 preferred. The microphone frequency response should be flat within ±1 dB from 100 Hz to 8 kHz.
Analyzer: Dedicated STI meter or software with octave-band filtering and modulation depth extraction per the standard algorithm.
Calibrator: Sound calibrator for verifying the measurement chain sensitivity.

Signal Level

The test signal must be played at the same level the PA system uses for speech during normal operation. This is critical because STI depends on the signal-to-noise ratio at the receiver — if the test signal is louder than normal speech, the measured STI will be optimistically high. For emergency voice alarm testing, use the alarm signal level.

If the PA system has automatic gain control (AGC) or compression, ensure these are active during measurement, as they affect the transmitted modulation depth.

Measurement Positions

Positions should be representative of actual listener locations:

Cover the entire served area, including positions at maximum distance from speakers
Include positions near walls and in corners where reflections may differ
Microphone height: 1.2 m (seated) or 1.5 m (standing), matching the listener scenario
Minimum 3 positions per zone for EN 54-16 compliance; more for large zones

See ISO 3382-1 positioning for general spatial sampling guidance.

Ambient Noise

Ambient noise during measurement must be representative of the worst-case normal operating condition. For a train station, measure during peak hours. For a factory, measure during production. If measuring in unoccupied conditions, the masking model can apply a measured noise correction to predict occupied STI.

Measurement Duration

For STIPA, a minimum 15-second capture is required. The lowest modulation frequency is 0.63 Hz (period 1.6 s), so 15 seconds provides approximately 9 complete cycles. Longer measurements (30-60 s) reduce random fluctuation. For full STI, each modulation frequency requires sufficient integration time.

Repeatability

Well-controlled STIPA measurements show a standard deviation of approximately ±0.02 STI at a single position. Position-to-position variation in a well-designed system is typically ±0.05. If results vary by more than ±0.05 at the same position, check for time-varying noise sources or PA system instability.

SonaVyx Workflow

The STI tool guides you through the measurement procedure. Play the generated STIPA signal through the system, position your device at the measurement point, and capture 15 seconds. Pro mode repeats three times and reports mean ± standard deviation. Results feed into the compliance report.