How to Measure RT60 Reverberation Time
Quick Answer
Measuring RT60 means capturing the room's impulse response and calculating the time it takes for sound energy to decay by 60 dB after the source stops. This fundamental acoustic parameter determines speech intelligibility, music clarity, and the suitability of a space for its intended purpose.
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Equipment Needed
- ✓Omnidirectional measurement microphone
- ✓Loudspeaker source (ideally omnidirectional or dodecahedron)
- ✓SonaVyx RT60 measurement tool (browser-based)
- ✓Audio interface for high-quality capture
- ✓Mic stand and cables
Step-by-Step Guide
Choose Excitation Method
Select your impulse response capture method based on the environment. Swept sine (logarithmic chirp) offers the best signal-to-noise ratio, typically 40 to 60 dB above the noise floor, making it ideal for occupied or noisy spaces. MLS (Maximum Length Sequence) provides faster measurements but requires a quieter environment. For quick field estimates, a balloon pop or starter pistol works but offers limited low-frequency data and poor repeatability. SonaVyx supports all three methods in the IR measurement tool.
Position Source and Microphone
Place the loudspeaker source on stage or at the primary sound source location. ISO 3382-1 requires the source to be omnidirectional, so use a dodecahedron speaker if available, or a single loudspeaker elevated 1.5 meters above the floor. Position the measurement microphone at least 2 meters from the source and away from reflecting surfaces. Take measurements at a minimum of 3 receiver positions distributed across the seating area. For ISO compliance, use 6 or more positions.
Verify Background Noise Level
Measure the background noise level using SonaVyx's SPL meter. The noise floor must be at least 35 dB below the initial sound level for a valid T20 measurement, or 45 dB below for T30. If HVAC noise is too high, request temporary shutdown of air handling systems during measurement. Record the background noise level as it affects the valid evaluation range of the decay curve. Check the INR (Impulse-to-Noise Ratio) after capturing each impulse response.
Capture Impulse Response
Start the measurement in SonaVyx's IR tool. For swept sine, the tool generates a logarithmic sweep from 20 Hz to 20 kHz over 5 to 20 seconds, then deconvolves the captured signal to extract the impulse response. Allow complete silence during the sweep and decay. For larger rooms with longer RT60, extend the sweep length to ensure the full decay is captured. The resulting impulse response shows the direct sound, early reflections, and reverberant tail.
Apply Schroeder Integration
SonaVyx automatically applies Schroeder backward integration to convert the impulse response into an Energy Decay Curve (EDC). This method, defined in ISO 3382-1, integrates the squared impulse response from the end of the signal backward, producing a smooth monotonically decreasing curve. The EDC is more reliable than individual decay events because it represents an ensemble average of all possible decay processes in the room.
Extract RT60 Parameters
From the EDC, extract T20 (decay from -5 dB to -25 dB, extrapolated to 60 dB), T30 (decay from -5 dB to -35 dB, extrapolated), and EDT (decay from 0 dB to -10 dB, extrapolated). SonaVyx calculates all three automatically per octave band from 125 Hz to 4 kHz. Check the linear regression correlation coefficient: r-squared should be above 0.95 for a valid measurement. T20 and T30 should agree within 10 percent if the room has a simple exponential decay.
Compare Against Target Values
Evaluate the measured RT60 against target values for the room's intended use. Concert halls typically target 1.8 to 2.2 seconds, lecture rooms 0.6 to 0.8 seconds, recording studios 0.2 to 0.4 seconds, and open offices 0.4 to 0.6 seconds. SonaVyx displays room type targets alongside your measurements. If RT60 is too long, absorption treatment is needed. If too short, the room may benefit from diffusive surfaces to maintain liveliness without excessive reverb.
Understanding RT60 and Its Variants
RT60, or reverberation time, was first defined by Wallace Clement Sabine in 1898 as the time for sound to decay by 60 dB after the source stops. In practice, measuring a full 60 dB decay requires extremely quiet conditions, so ISO 3382-1 defines two practical alternatives: T20 and T30, which measure smaller portions of the decay and extrapolate to 60 dB.
T20, T30, and EDT
T20 evaluates the decay from -5 dB to -25 dB below the initial level and multiplies by 3, requiring at least 35 dB of dynamic range above the noise floor. T30 evaluates from -5 dB to -35 dB and multiplies by 2, requiring 45 dB of dynamic range. EDT (Early Decay Time) evaluates only the first 10 dB of decay and multiplies by 6. EDT correlates more strongly with perceived reverberance because human hearing weighs early reflections more heavily than late reverberation.
Octave Band Analysis
RT60 varies significantly with frequency. Low frequencies below 250 Hz typically have longer reverberation times because most common absorptive materials (carpet, curtains, acoustic foam) are ineffective at absorbing long wavelengths. High frequencies above 2 kHz decay faster due to air absorption, which becomes significant in large rooms. Reporting RT60 per octave band from 125 Hz to 4 kHz provides a complete picture of the room's acoustic behavior and guides targeted treatment decisions.
Measurement Validity: INR
The Impulse-to-Noise Ratio (INR) quantifies the usable dynamic range of your impulse response measurement. ISO 3382-1 requires INR of at least 35 dB for T20 and 45 dB for T30. If INR falls below these thresholds, the computed RT60 values are contaminated by noise and should not be relied upon. Increasing sweep length, raising source level, or reducing background noise all improve INR. SonaVyx calculates and displays INR for each measurement with color-coded quality indicators.
Common Mistakes to Avoid
Using a sweep that is too short for the room, causing the reverberant tail to be truncated before complete decay
Measuring with HVAC running, which raises the noise floor and invalidates T30 extraction
Placing the microphone too close to a wall, where boundary effects distort the measured decay
Reporting a single-number RT60 without specifying octave band or averaging method
Confusing EDT with T30, which measure different aspects of the decay and serve different purposes
Applicable Standards
| Standard | Clause | Relevance |
|---|---|---|
| ISO 3382-1:2009 | Clause 7 | Defines T20, T30, and EDT extraction methods from the energy decay curve |
| ISO 3382-2:2008 | Clause 5 | Reverberation time measurement procedures for ordinary rooms |
| IEC 60268-16 | Annex A.4 | Relationship between RT60 and speech intelligibility (STI) |
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Related Guides
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