RT60 Measurement Quick Reference — ISO 3382 Procedures

Reverberation Time: The Most Important Room Acoustic Parameter

Reverberation time (RT60) — the time for sound to decay by 60 dB after the source stops — is the single most important metric for evaluating room acoustic quality. It affects speech intelligibility, music clarity, listener comfort, and sound system performance. ISO 3382-1 (performance spaces) and ISO 3382-2 (ordinary rooms) define the standard measurement procedures that this guide summarizes.

Target RT60 Values by Room Type

The guide provides a comprehensive table of recommended reverberation times at 500-1000 Hz:

• Recording studio control room: 0.2-0.4s — extremely controlled environment for critical monitoring
• Broadcast studio: 0.3-0.5s — speech clarity with natural feel
• Classroom (ANSI S12.60): 0.4-0.6s — mandatory for learning environments, maximum 0.6s for rooms under 283 m³
• Conference room: 0.4-0.7s — balance between speech clarity and naturalness
• Worship space (speech-focused): 0.8-1.4s — compromise between speech and congregational singing
• Multipurpose hall: 1.0-1.5s — typically with variable acoustics
• Concert hall (orchestral): 1.8-2.2s — the golden range for classical music
• Cathedral/organ music: 2.5-4.0s+ — long reverb enhances organ and choral music

ISO 3382 Measurement Procedure Summary

The standard specifies requirements for excitation signals, microphone positions, and data extraction:

1. Excitation signal: Use an omnidirectional source. Acceptable signals include log sine sweep (recommended, best SNR), MLS (fast but sensitive to time variance), balloon pop (quick survey, limited frequency range), or starter pistol (high energy, safety concerns).
2. Source positions: Minimum 2 positions, placed where the actual sound source would be (stage, lectern, speaker location).
3. Receiver positions: Minimum 3 per source position, at least 1m apart, at least 1m from walls, at seated ear height (1.2m).
4. Signal quality: Impulse-to-Noise Ratio must exceed 35 dB for T20 and 45 dB for T30 measurements.
5. Extraction: Compute the Schroeder backward integration (energy decay curve) and fit linear regression to the -5 to -25 dB range (T20) or -5 to -35 dB range (T30). Check r² > 0.99.

Prediction Formulas

Before measuring, you can estimate RT60 to verify your results are in the right ballpark:

• Sabine equation: T60 = 0.161V / A, where V is room volume (m³) and A is total absorption (m² Sabins). Best for rooms with low, uniform absorption (alpha below 0.2).
• Eyring equation: T60 = 0.161V / (-S × ln(1 - alpha_avg)), where S is total surface area and alpha_avg is mean absorption coefficient. More accurate for higher absorption.
• Fitzroy equation: Accounts for non-uniform absorption distribution across the three room axis pairs. Best for rooms with very different surface treatments (e.g., absorptive ceiling, reflective walls).

Common RT60 Measurement Mistakes

• Measuring with HVAC running (the background noise reduces your usable decay range)
• Too few measurement positions (spatial averaging is essential)
• Ignoring octave-band analysis (a single broadband T60 hides frequency-dependent problems)
• Not checking INR before extracting T20 or T30

Measure RT60 in Your Browser Right Now

Open the SonaVyx RT60 Tool to measure reverberation time with log sine sweep or MLS, automatic Schroeder integration, and T20/T30/EDT extraction per ISO 3382. Use the Treatment Calculator to find out how much absorption you need and predict the post-treatment RT60.