T20 (Reverberation Time)

T20 evaluates a 20 dB portion of the energy decay curve (from -5 dB to -25 dB) and linearly extrapolates the result to represent the time for a full 60 dB decay. The evaluation starts at -5 dB rather than 0 dB to avoid the direct sound and early reflections that would skew the regression. The -25 dB lower limit keeps the evaluation above the noise floor in most practical measurement scenarios. According to ISO 3382-1:2009, valid T20 measurements require an INR (impulse-to-noise ratio) of at least 35 dB. The INR represents the difference between the peak level of the impulse response and the background noise floor. When the INR is insufficient, the decay curve bends upward as it approaches the noise floor, producing an artificially long reverberation time. T20 is the most commonly reported reverberation time metric because it is achievable in most real-world spaces with modest excitation signals. T30, which evaluates a 30 dB decay range, requires a higher INR (45 dB) and is harder to achieve without powerful source signals or very quiet conditions. For occupied rooms with HVAC running, T20 is often the only reliable metric. In acoustically well-behaved spaces (smooth, linear decays), T20 and T30 agree closely. A significant discrepancy — typically where T20 is shorter than T30 — indicates a non-linear decay (coupled rooms, flutter echoes, or strong early reflections). ISO 3382 recommends reporting both T20 and T30 when possible, as their ratio serves as a decay linearity diagnostic. SonaVyx computes T20 from the Schroeder backward-integrated energy decay curve with linear regression and reports the r² correlation coefficient as a quality indicator.