Early Decay Time (EDT) — Why It Matters More Than RT60 for Listeners | AcousPlan

In acoustic consulting practice, clients ask about RT60. Researchers study EDT. The gap between these two conversations matters because of a fundamental fact about human hearing: the auditory system does not treat all parts of a sound's decay equally. It weights the beginning of the decay most heavily — and it is precisely this early portion that EDT measures.

Understanding EDT, and why it diverges from RT60, is what separates competent acoustic analysis from a superficial calculation.

What EDT Actually Measures

ISO 3382-1:2009 (Room Acoustics — Measurement of Room Acoustic Parameters — Part 1: Performance Spaces) defines Early Decay Time as the time derived from the first 10 dB of the room's impulse response decay.

In practice:

1. A room impulse response is recorded — typically using a log-swept sine signal or MLS sequence through a calibrated omnidirectional loudspeaker, captured with a calibrated microphone
2. The backward-integrated Schroeder curve is computed from the squared impulse response: the integral is computed from the end of the signal back to the beginning, giving a curve that starts at 0 dB and decays
3. A linear regression is fit through the –0 dB to –10 dB portion of this Schroeder curve
4. The slope of this regression is extrapolated to find the time for a 60 dB decay

The Subjective Correlation

The case for EDT as the perceptually primary metric comes from a body of research culminating in the work of Leo Beranek (Concert Halls and Opera Houses, 2nd ed., 2004). Across Beranek's survey of 100 auditoria rated by professional musicians and experienced concert-goers:

• Subjective reverberance ratings correlated with EDT more strongly than with RT60, with correlation coefficients of r = 0.79 for EDT vs. r = 0.65 for RT60 in multi-position averages
• Halls where EDT and RT60 were nearly equal received the highest overall ratings
• Halls where EDT was substantially shorter than RT60 were rated as "dry" and "unsupporting" — even when RT60 was in the nominally correct range

Why EDT Diverges from RT60

In a perfectly diffuse room (uniform absorption on all surfaces, no strong directional reflections), EDT should equal RT60. Real rooms deviate from this ideal in systematic ways:

Cause 1: Non-Uniform Absorption

Audience seating is the dominant absorber at mid-frequencies in a concert hall. Upholstered seats absorb 0.30–0.50 m² per seat at 500 Hz. In a 2,000-seat hall with 1,000 m² of reflective side walls (stone), the absorption is concentrated in the audience area — not distributed uniformly across all surfaces as the Sabine equation assumes.

When absorption is concentrated at the audience, sound energy arriving at an audience-seat microphone position encounters the absorber quickly (via the floor-level early reflections). The early decay therefore proceeds faster than the overall room average — EDT < RT60. Ratio: 0.70–0.85 in halls with over-absorbing seating relative to room boundary reflections.

Cause 2: Proximity to Reflective Surfaces

A listener sitting near a reflective side wall receives strong lateral reflections early — within 20–30 ms. These reflections arrive during the EDT integration window (0–10 dB of decay) and add energy to the early field, slowing the early decay. EDT > RT60 at positions with strong early reflections: EDT/RT60 = 1.1–1.3.

This is not necessarily detrimental — strong early lateral reflections are associated with high Lateral Energy Fraction (LF) and high perceived envelopment, which are desirable properties in concert halls. The elevated EDT reflects the hall doing its job of distributing lateral energy to the listeners.

Cause 3: Room Geometry

Fan-shaped auditoria with diverging side walls direct lateral reflections away from the audience, rather than toward it. The early field is impoverished — very little energy arrives within the 0–80 ms window from lateral directions. EDT < RT60 in these rooms because the early decay is dominated by ceiling and direct sound reflections, which die quickly, while late reverberation persists from rear-wall and balcony reflections. Envelopment is poor and EDT/RT60 ratios below 0.80 are common in wide-fan halls.

Cause 4: Coupled Rooms

A room with a large balcony overhang creates an acoustically coupled sub-space: the under-balcony area has a different effective volume and RT60 from the main floor. A listener under the balcony hears the under-balcony reverberant decay first (fast, short), then the main hall reverberation couples in later. The decay curve has two distinct slopes — EDT (from the fast under-balcony phase) is shorter than RT60 (influenced by the main hall's longer tail). Ratios of 0.6–0.75 are common under deep balconies.

EDT in Different Room Types

Concert Halls

Target: EDT closely matching RT60 (ratio 0.90–1.10). The metric EDT/RT60 is used as a uniformity indicator — high uniformity across seating positions indicates good diffusion and consistent listener experience.

Design strategy to achieve high EDT:

• Avoid over-absorbing seating — specify seat absorption coefficients carefully, balance against room boundary reflectivity
• Maximise reflective side wall area (shoebox geometry)
• Diffusing surface treatment on rear and upper walls (QRD panels, irregular profiling) broadens reflection arrival angles and fills the early decay with scattered energy

Theatres and Drama Spaces

Target: EDT = 0.5–0.8 s at 500 Hz (shorter than RT60 of 0.7–1.0 s is acceptable). In spoken-word spaces, a relatively short EDT improves speech clarity because the early decay period is clear of reverberation — the direct-to-reverberant ratio is high in the Haas window (0–35 ms). EDT/RT60 ratio of 0.7–0.9 is acceptable and may be slightly preferred.

Classrooms and Conference Rooms

EDT is not typically specified explicitly for small rooms. In rooms below Schroeder frequency, the decay is irregular and EDT measurement is not meaningful in the same way as for large auditoria. The relevant metric for small rooms is the full RT60 at 500 Hz and 1000 Hz, and STI at representative positions.

Offices and Open Plan Spaces

EDT is used in ISO 3382-3 (Open Plan Offices) indirectly through the EDT's relationship to early-arriving sound levels. The standard's D₂,S metric (spatial decay of sound level per doubling of distance) depends on the ratio of direct to reverberant energy, which is closely related to EDT in the 125–500 Hz range.

Practical Design Implications

If EDT is significantly below RT60 (ratio < 0.80):

• Excessive floor-level absorption (over-specified seating, thick carpet in side aisles)
• Insufficient reflective boundary area relative to audience absorption
• Design response: increase reflective surface area (raise reflective wall panel heights), reduce seating absorption by specifying thinner upholstery or solid-panel seat bottoms that reduce exposed absorptive area when unoccupied

• Strong specular early reflections from one or two dominant surfaces
• Design response: apply diffusion (not absorption) to the offending surfaces — a QRD panel with design frequency 500 Hz scatters the reflection over ±35° without reducing energy

• Inhomogeneous sound field — some positions receive much more early energy than others
• Common in wide-fan halls, halls with strong central gallery reflectors, or halls with large overhanging balconies
• Design response: ceiling diffusion to broaden reflection distribution, or under-balcony loudspeakers to supplement energy in acoustically shadowed positions

EDT and Frequency Dependence

One of the most practically useful aspects of EDT analysis is that it is reported per octave band, not as a single-number summary. The frequency profile of EDT reveals problems that a single-number RT60 hides.

Low-frequency EDT (125 Hz, 250 Hz): In most rooms, low-frequency RT60 and EDT are longer than mid-frequency due to the lower absorption efficiency of most materials at long wavelengths. For concert halls, low-frequency EDT slightly longer than mid-frequency is beneficial — it contributes to bass warmth and fullness. Bass Ratio = (EDT₁₂₅ + EDT₂₅₀) / (EDT₅₀₀ + EDT₁₀₀₀) should be in the range 1.0–1.3 for orchestral music.

Mid-frequency EDT (500 Hz, 1000 Hz): This is the reference range. EDT values here represent the subjectively dominant reverberance. Concert hall targets: 1.7–2.1 s. Theatre: 0.6–0.9 s. Conference room: 0.3–0.5 s.

High-frequency EDT (2000 Hz, 4000 Hz): Usually shorter than mid-frequency because air absorption increases with frequency — approximately 0.5 dB per 100 m of air path at 2000 Hz, 1.5 dB per 100 m at 4000 Hz. In a large concert hall (25 m long), high-frequency EDT may be 10–20% shorter than mid-frequency EDT. This is expected and appropriate. If high-frequency EDT is more than 30–40% shorter than mid-frequency, the room has excessive high-frequency absorption — often over-specified acoustic treatment or dense audience upholstery that absorbs too much in the treble range.

Diagnostic matrix example (concert hall, problem diagnosis):

This pattern — EDT/RT60 ratio declining consistently from low to high frequency — indicates that the audience seating is absorbing too broadly. The fix: reduce seating absorption at high frequencies by specifying thinner seat cushioning or solid armrests rather than fully upholstered sides.

Measurement Checklist

When using EDT as a design verification metric:

1. Measure EDT at all ISO 3382-1 specified receiver positions (minimum 8–12 in a concert hall)
2. Report EDT as mean ± standard deviation across positions — spatial variance is as important as the average
3. Report EDT for each ISO octave band (125 Hz to 4000 Hz)
4. Calculate EDT/RT60 ratio per band to identify frequency-specific divergence
5. Cross-reference with C80 — a room with low EDT/RT60 AND high C80 (> +2 dB) is consistently under-reverberant; a room with high EDT/RT60 AND low C80 (< –4 dB) has poorly distributed early reflections
6. Repeat measurements occupied and unoccupied — EDT typically decreases by 0.2–0.4 s when the hall fills with an audience