Early Decay Time (EDT)
Definition
Early Decay Time (EDT)
Early Decay Time (EDT) measures the time for sound to decay by 10 dB from its initial level, extrapolated to a 60 dB decay for comparison with RT60. Defined by ISO 3382-1:2009 Clause 4.2, EDT correlates more closely with the subjective perception of reverberance in occupied rooms than T20 or T30 because early reflections dominate the listener experience. SonaVyx measures EDT from the energy decay curve.
EDT = 6 × t₁₀, where t₁₀ = time for initial 10 dB decay
How It Is Measured
EDT is calculated from the energy decay curve (EDC) derived by Schroeder backward integration of the impulse response. SonaVyx fits a linear regression to the EDC between 0 dB and -10 dB, computes the slope, and extrapolates to 60 dB. The r² value of the linear fit indicates measurement quality — values above 0.99 confirm a clean exponential decay in the initial region.
Practical Example
A concert hall measures T30 of 2.0 seconds (computed from -5 to -35 dB) but EDT of only 1.4 seconds. The shorter EDT indicates strong early reflections that steepen the initial decay, making the hall sound less reverberant than T30 would suggest. The EDT/T30 ratio of 0.7 indicates effective early reflection management — close to the ideal ratio of 1.0 for uniform decay.
EDT vs T20 vs T30
T20 evaluates decay from -5 to -25 dB, representing the middle portion of the decay curve. T30 spans -5 to -35 dB, providing a more robust estimate less affected by background noise. EDT spans 0 to -10 dB, capturing the very beginning of the decay. In a perfectly diffuse room, EDT equals T20 equals T30. In real rooms, EDT is often shorter because early reflections create a steeper initial decay, while late reverberation establishes a longer tail.
Subjective Correlation
Research by Barron and others has shown that EDT correlates more strongly with subjective judgments of reverberance than T30. This is because human perception weights the first 50 to 80 ms of decay most heavily. A room with short EDT and long T30 sounds drier and clearer than a room where EDT and T30 are equal at the same T30 value. This makes EDT particularly valuable for spaces prioritizing speech clarity.
EDT in Occupied vs Unoccupied Rooms
EDT changes significantly between occupied and unoccupied conditions because the audience absorbs early reflections from the floor and nearby surfaces. T30 changes less because it primarily reflects the room's gross absorption. SonaVyx recommends measuring both EDT and T30 to predict how the room will sound when occupied based on unoccupied measurements.
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