RT60 Measurement per IEC 61672-1

IEC 61672-1 Requirements for RT60

While IEC 61672-1 does not directly address RT60 measurement, it specifies the performance of the sound level meter used to capture decay curves. ISO 3382 references IEC 61672-1 for instrument requirements when using the interrupted noise method.

Instrument Class and RT60 Accuracy

The class of your sound level meter affects RT60 measurement uncertainty:

• Class 1 (precision): plus or minus 1.1 dB at 1 kHz reference, suitable for laboratory and field measurements
• Class 2 (general purpose): plus or minus 1.4 dB at 1 kHz, suitable for general field measurements
• The decay slope uncertainty is proportional to the level measurement uncertainty
• For critical measurements, Class 1 instruments are recommended

Time Weighting Selection

IEC 61672-1 defines Fast (125 ms) and Slow (1000 ms) time weightings:

1. Fast is preferred for RT60 decay recording as it captures the decay shape accurately
2. Slow averaging smears the decay curve and should not be used
3. Some digital implementations use even shorter integration periods for improved resolution
4. The Impulse time weighting (35 ms attack) is not suitable for RT60 measurement

Frequency Response Requirements

IEC 61672-1 specifies frequency response limits for A, C, and Z weightings. For octave-band RT60 measurement, the bandpass filter performance is more relevant than the overall frequency weighting, but the analog front-end must have flat response across the measurement range.

Digital Implementation Advantages

Software-based measurement systems like SonaVyx can exceed IEC 61672-1 analog performance specifications because digital filters have perfectly repeatable characteristics. The primary limitation becomes the microphone and analog-to-digital converter rather than the processing chain.

Common Mistakes

• Using a non-compliant instrument and claiming IEC 61672-1 conformance
• Forgetting that the microphone capsule is part of the measurement chain
• Not calibrating before and after the measurement session
• Confusing IEC 61672-1 (instrument standard) with ISO 3382 (measurement method)

SonaVyx Implementation

The SonaVyx RT60 tool uses IEC 61672-1 compliant digital processing for decay capture. Calibrate your setup with the SPL meter before measuring. Capture impulse responses via the IR tool as an alternative method. Use the RTA to verify spectral conditions. Design-stage predictions are available at AcousPlan. Visit our learning modules for calibration procedures.