What minimum INR is needed for T30?

ISO 3382-1 clause C.6 requires 45 dB minimum INR for T30. For T20, 35 dB minimum. Insufficient INR contaminates the decay curve before the evaluation point. Use longer sweeps or higher source levels to increase INR when measurements fall short.

When should I report T20 instead of T30?

Report T20 when background noise prevents 45 dB INR but 35 dB is met. Always state which metric was used. T20 and T30 can differ 10-20% in rooms with non-linear decay or coupled volumes, so the choice affects reported values significantly.

Why does EDT differ from T30?

EDT measures the first 10 dB dominated by early reflections. T30 evaluates the established reverberant field. Strong early reflections make EDT shorter than T30. The EDT/T30 ratio reveals whether the room sounds more or less reverberant than the decay curve suggests.

How many positions does ISO 3382-1 need?

At least three source and three receiver positions per source for engineering-grade measurements. Source-receiver distance must exceed twice the critical distance. Survey-grade allows fewer. More positions improve spatial averaging reliability.

What minimum INR is needed for T30?

ISO 3382-1 clause C.6 requires 45 dB minimum INR for T30. For T20, 35 dB minimum. Insufficient INR contaminates the decay curve before the evaluation point. Use longer sweeps or higher source levels to increase INR when measurements fall short.

When should I report T20 instead of T30?

Report T20 when background noise prevents 45 dB INR but 35 dB is met. Always state which metric was used. T20 and T30 can differ 10-20% in rooms with non-linear decay or coupled volumes, so the choice affects reported values significantly.

Why does EDT differ from T30?

EDT measures the first 10 dB dominated by early reflections. T30 evaluates the established reverberant field. Strong early reflections make EDT shorter than T30. The EDT/T30 ratio reveals whether the room sounds more or less reverberant than the decay curve suggests.

How many positions does ISO 3382-1 need?

At least three source and three receiver positions per source for engineering-grade measurements. Source-receiver distance must exceed twice the critical distance. Survey-grade allows fewer. More positions improve spatial averaging reliability.

RT60 Measurement — T20, T30 & EDT per ISO 3382-1

Schroeder Backward Integration

ISO 3382-1 uses backward integration of the squared impulse response for a smooth decay curve equivalent to the ensemble average of many noise decays. This enables reliable extraction from a single measurement.

T20

Evaluates -5 to -25 dB decay, requires 35 dB INR per clause C.6. Useful when noise limits the usable range. The extrapolation assumes constant rate below -25 dB, which may not hold in coupled volumes.

T30

Uses -5 to -35 dB, requires 45 dB INR. Preferred metric: evaluates more of the decay. R-squared of linear regression must exceed 0.995 per Annex A for valid extraction.

EDT

Measures first 10 dB (0 to -10 dB). Correlates better with perceived reverberance. When EDT differs from T30, early reflections create a different impression than the physical decay rate indicates.

Octave Bands

RT60 varies across frequency. Low frequencies typically decay longer. Report in bands 125 Hz to 4 kHz. The mid-frequency average (500 + 1000 Hz) is the common single-number descriptor.

RT60 Measurement — T20, T30 & EDT per ISO 3382-1

Schroeder Backward Integration

T20

T30

EDT

Octave Bands

Frequently Asked Questions

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→Complete Guide to Room Acoustics Measurement — ISO 3382

→Impulse Response — Measurement & Analysis

→ISO 3382 Parameters — Complete Reference