Impulse Response Measurement per IEC 60268-16

IR-Based STI Calculation

IEC 60268-16 Annex A specifies how to compute the modulation transfer function directly from the impulse response in each octave band. The method assumes a linear, time-invariant system.

Mathematical Method

• Filter the impulse response into 7 octave bands (125 Hz to 8 kHz)
• Square the filtered impulse response to get the energy envelope
• Calculate the Fourier transform of the squared IR
• Normalize by the DC component to get the MTF at each modulation frequency
• Apply ambient noise correction if noise levels are known

Advantages Over STIPA

1. Single measurement yields both room acoustic parameters and STI
2. Complete 7x14 MTF matrix (STIPA only measures 14 of 98 cells)
3. Not affected by nonlinearities in the reproduction chain
4. Can be applied retrospectively to archived impulse response data

Limitations

The indirect method assumes that the system is linear and time-invariant. In systems with nonlinear processing (compressors, limiters, AGC), the STIPA method may give different results because it captures the actual signal path including nonlinearities.

Noise Correction

Ambient noise affects the IR-derived STI. IEC 60268-16 allows noise correction by measuring the ambient noise spectrum separately and applying the SNR-based MTF reduction per band.

Common Mistakes

• Not applying ambient noise correction, overestimating STI in noisy environments
• Using an IR captured at unrealistic source levels
• Truncating the impulse response before the noise floor, biasing the MTF
• Comparing indirect STI with STIPA results without accounting for nonlinearities

SonaVyx Workflow

Capture the impulse response with the SonaVyx IR tool. The STI calculator automatically derives intelligibility from the IR. Measure RT60 from the same IR with the RT60 tool. Verify noise levels with the SPL meter. Check system response with the transfer function. See the learning hub for detailed tutorials.