Early Decay Time (EDT)

EDT measures how quickly the first 10 dB of sound energy decays after a source stops. Because human perception of reverberation is most strongly influenced by the initial decay — the first 100-200 milliseconds — EDT correlates better with subjective assessments of reverberance than T20 or T30, which evaluate later portions of the decay. In a perfectly diffuse sound field with a single exponential decay, EDT equals T30. In practice, EDT is almost always shorter than T30 because early reflections from nearby surfaces (floor, ceiling, stage shell) accelerate the initial decay. A short EDT with a long T30 is actually desirable in many performance spaces: the short EDT provides clarity for speech and musical detail, while the longer late reverberation provides warmth and envelopment. The EDT/T30 ratio is a key design diagnostic. Values near 1.0 indicate a uniform, diffuse decay. Values significantly below 1.0 suggest strong early reflections or absorption concentrated near the source. Values above 1.0 (EDT > T30) are unusual and may indicate focused reflections or flutter echoes in the early decay. EDT is position-dependent to a much greater degree than T30. Near reflective surfaces, EDT shortens; in the center of a large room, EDT lengthens. This spatial variation makes EDT useful for evaluating seat-to-seat consistency in performance venues. ISO 3382-1 recommends measuring at multiple positions and reporting both the spatial average and the standard deviation. For speech intelligibility, a low EDT is beneficial because rapid initial decay reduces the masking of subsequent syllables. Concert halls aim for EDT values between 1.8 and 2.2 seconds, while lecture rooms target 0.4-0.6 seconds. SonaVyx computes EDT alongside T20 and T30 from the same impulse response measurement, enabling direct comparison.