RT60 Measurement per IEC 60268-16

TL;DR

IEC 60268-16 defines the relationship between reverberation time and speech intelligibility through the modulation transfer function. RT60 is one of the two primary degradation factors for STI, alongside ambient noise. The standard provides a formula to estimate STI directly from RT60 and signal-to-noise ratio without requiring a full STIPA measurement. SonaVyx RT60 tool feeds directly into the STI prediction model, letting you estimate speech intelligibility from a quick impulse response measurement. This is particularly useful during the design phase or when a full STIPA measurement is not practical.

RT60 and Speech Intelligibility

Reverberation degrades speech intelligibility by smearing temporal modulations in the speech signal. IEC 60268-16 quantifies this through the modulation transfer function (MTF), where longer RT60 reduces modulation depth at all frequencies.

The RT60-STI Relationship

IEC 60268-16 Annex A provides the indirect method for calculating STI from RT60 and SNR:

MTF(F,f) = 1 / (1 + (2 * pi * F * T / 13.8)) * 1 / (1 + 10^(-SNR/10))
Where F is modulation frequency, f is octave band center frequency, T is RT60
This allows STI prediction without acoustic measurement
Useful for design verification and quick assessments

Target RT60 Values for Speech

Based on IEC 60268-16 requirements for STI above 0.50 (Fair intelligibility):

Small rooms (under 200 cubic meters): RT60 below 0.8 seconds
Medium rooms (200-1000 cubic meters): RT60 below 1.2 seconds
Large rooms (above 1000 cubic meters): RT60 below 1.5 seconds with PA support
Emergency announcement systems: RT60 below 1.0 seconds or increased speaker density

Octave Band Considerations

STI weights the seven octave bands (125 Hz to 8 kHz) according to male speech importance. RT60 in the 500 Hz to 2 kHz range has the most impact on intelligibility. Focus your acoustic treatment on controlling mid-frequency reverberation.

Common Mistakes

Using a single broadband RT60 value instead of per-band values for STI prediction
Ignoring the noise term, which can dominate in industrial environments
Assuming that reducing RT60 always improves STI (in very quiet rooms, it may already be reflection-limited)

SonaVyx Workflow

Measure RT60 with the SonaVyx RT60 tool across all octave bands. Feed the results into the STI predictor for intelligibility estimation. Measure ambient noise with the SPL meter for the noise correction term. Use the impulse response tool for the most accurate RT60 extraction. Predict treatment needs with the treatment calculator. Design-stage calculations are available at AcousPlan.

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