How to Calculate Sound Early Decay Time Between Rooms

By James Hartley · February 16, 2026

How to Calculate Sound Early Decay Time Between Rooms

Early Decay Time (EDT) tells you how quickly sound energy begins to die away right after it’s produced. It’s often more closely tied to perceived “liveness” than RT60 because it focuses on the first part of the decay, where our hearing is most sensitive to changes in clarity and ambience. When you’re working between rooms—control room to live room, booth to corridor, apartment to hallway—EDT becomes a practical way to quantify what a listener experiences across a boundary: does the adjacent room sound “short and tight,” “ringy,” or “washed out” once sound leaks or passes through an opening?

This tutorial shows a reliable, repeatable method to calculate EDT in one room and compare it to EDT in another room so you can understand how sound decays on each side of a partition or doorway. You’ll learn measurement setup, signal choices, how to derive EDT from a decay curve (including the math), and how to interpret results in real studio and field scenarios.

Prerequisites / Setup Requirements

Measurement software: REW (Room EQ Wizard), ARTA, Smaart, or similar. The steps below reference REW terminology, but the concept is the same everywhere.
Measurement microphone: Calibrated omni (e.g., UMIK-1, Earthworks M23, iSEMcon EMX-7150). Use the calibration file if available.
Audio interface: Two channels recommended (output to speaker, input from mic). If using UMIK-1 you can skip interface input.
Speaker / source: A studio monitor or small PA speaker capable of clean output from 100 Hz–10 kHz. For small rooms, a single monitor is fine.
SPL meter (optional but helpful): For level consistency between rooms.
Quiet time window: Background noise should ideally be at least 25 dB below the decay tail you need to measure. If HVAC is loud, note it and expect low-frequency EDT to be less reliable.
Basic room notes: Room dimensions, door states (open/closed), and key surfaces (glass, drywall, carpet). You’ll use these to interpret the “why” behind the results.

Step-by-Step Instructions

Define the “between rooms” condition you’re testing

Action: Write down exactly what configuration you want to evaluate: door closed, door ajar 10 cm, door open; vocal booth window open/closed; corridor door latched vs unlatched.

Why: EDT is extremely sensitive to early reflections and coupling. A door cracked open can introduce a strong early energy path and change the first 10 dB of decay dramatically—often more than it changes RT60.

Specifics: Choose one configuration and keep it fixed for the full measurement set. Photograph the door gap if it’s partially open, and note any movable objects (gobos, rolling racks).

Common pitfalls: Changing door position between measurements, or measuring one room with furniture arranged differently than normal use. EDT is about what users experience, so measure the “real” setup.
Place the source to represent a real-world emitter

Action: Put the speaker in the “sending room” where sound normally originates (e.g., live room, neighbor apartment, hallway). Aim it toward the boundary or toward the typical listener area depending on your scenario.

Why: EDT depends on early energy arriving at the microphone. The source position controls which reflections dominate in the first milliseconds and therefore controls the early slope of the decay curve.

Specifics: Typical starting positions:
- Studio scenario: Source at 1.2–1.5 m height, at least 1 m from large reflective surfaces, 1–2 m from the separating wall/door.
- Speech/vocal booth scenario: Source at mouth height (~1.5 m) and 0.5–1 m from the booth door/window.
- Apartment/hallway scenario: Source at 1.2 m height, 1 m from the apartment door, aimed at the door.
Common pitfalls: Putting the speaker in a corner (inflates low-frequency energy), changing speaker aim between takes, or placing the speaker so close to the door that near-field effects dominate.
Set the measurement level for usable decay without distortion

Action: Calibrate the playback level so the impulse response has strong signal-to-noise while keeping the speaker and mic preamp clean.

Why: EDT is derived from the first 10 dB of decay. If noise masks that region or if the signal clips, the estimated decay slope becomes wrong.

Specific targets:
- At the mic in the receiving room, aim for 75–85 dB SPL during the sweep (C-weighted, slow), depending on noise limits.
- In REW, keep input peaks around -12 dBFS and never above -3 dBFS.
- Sweep length: 256k or 512k (roughly 5–11 seconds depending on sample rate) for better SNR in small rooms.
Common pitfalls: Running too quietly and getting a noisy decay tail (especially in the receiving room), or running too hot and producing harmonic distortion that contaminates the decay curve.
Measure the impulse response in the receiving room (Room B)

Action: Place the mic at a typical listening position in Room B (the receiving room), run a sweep, and store the measurement. Repeat for multiple positions if you need a room-average EDT.

Why: “Between rooms” questions usually care about what the listener experiences in the receiving space: a control room hearing the live room, a bedroom hearing a corridor, etc.

Specific technique:
- Mic height: 1.2 m seated ear height for control rooms; 1.5 m standing for speech.
- Mic-to-boundary distance: don’t pin it to a wall; keep at least 0.6 m from large surfaces to avoid skewing early energy.
- Do 3–6 mic positions in Room B, spaced ~0.5–1 m apart, then average EDT by octave band.
Common pitfalls: Measuring only one position and drawing broad conclusions. Early reflections can vary dramatically across a room, especially near doors and glass.
Measure the impulse response in the sending room (Room A) for context

Action: Without moving the speaker, measure the impulse response in Room A at a representative location (where someone would be present, or near the boundary).

Why: Comparing Room A and Room B EDT helps you identify whether the “problem” is that Room A is too lively (driving energy into the boundary), Room B is too reflective (holding onto what comes in), or the coupling path creates strong early reflections in Room B.

Specific technique: Use the same sweep settings and similar mic height as in Room B. If you’re specifically evaluating transmission through a doorway, also measure Room A with the mic about 1 m from the doorway on the source side.

Common pitfalls: Changing sweep length or levels between rooms, which makes comparisons less meaningful. Keep settings consistent.
Extract EDT from the decay curve (the actual calculation)

Action: In your software, view the Energy Time Curve (ETC) or Schroeder-integrated decay curve and calculate EDT from the first 10 dB of decay.

Why: EDT is defined as the time it would take to decay 60 dB, assuming the initial decay rate (0 to -10 dB) continues linearly. This is why EDT is often different from RT60; real rooms don’t decay as a perfect straight line.

The math:
- Find the time when the decay curve drops from 0 dB (reference at the start of decay) to -10 dB. Call this time T10.
- Compute EDT = 6 × T10.
Practical REW-style method (typical):
- Open the measurement > go to RT60 or Decay view.
- Select EDT as the metric (if available). If not available, note the T10 and multiply by 6.
- Use octave bands: 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz. These are the most actionable for real rooms.
Specific settings that improve reliability:
- Set the impulse response window long enough to include the decay: often 500 ms to 2 s for small rooms.
- Use 1/1 octave or 1/3 octave smoothing only for display; calculations should be band-limited, not heavily smoothed.
Common pitfalls:
- Noise floor too high: If the decay curve hits the noise floor before -10 dB, EDT becomes meaningless. Increase sweep level or reduce noise.
- Wrong reference point: Ensure “0 dB” is aligned to the start of decay after the direct sound peak, not random graph scaling.
- Strong early reflection bias: A big early reflection (like a glass panel in Room B) can flatten the first 10 dB and inflate EDT even if later decay is short.
Compare Room A and Room B EDT by frequency and interpret the coupling

Action: Create a small table (or spreadsheet) listing EDT per octave band for Room A and Room B, then compute the difference: ΔEDT = EDT(B) − EDT(A).

Why: The “between rooms” story lives in frequency dependence. Doors leak lows; glass throws highs; hallways create midrange flutter. A single broadband number hides the cause.

How to read the results:
- If EDT(B) is much higher than EDT(A) from 1–4 kHz, Room B likely has strong early reflections (bare drywall, glass, desk surface) that make incoming sound feel splashy and intelligibility worse.
- If EDT(B) spikes at 125–250 Hz but not at higher bands, Room B may have modal ringing or poor low-frequency absorption, and the door/wall is coupling low end efficiently.
- If EDT(B) is lower than EDT(A), Room B may be well damped; what you’re hearing as “leak” is more about isolation/transmission level than decay character.
Common pitfalls: Concluding “isolation is bad” when the real issue is “decay is long.” EDT is not a transmission-loss metric; it’s a decay metric. Use it to describe character, not loudness.
Validate with a real-world listening check (speech and music)

Action: Play a short dry spoken-word recording and a percussive music excerpt through the same speaker position and listen in Room B. Toggle the door state if that’s your test variable.

Why: EDT correlates strongly with perceived immediacy and clarity. A quick listen often confirms whether your measured EDT differences match what you hear (and flags measurement mistakes).

Specifics: Use speech with plosives and sibilants. For music, use dry drums or muted guitar. Keep playback level consistent within ±1 dB if possible.

Common pitfalls: Judging from memory. Switch A/B quickly, and keep the source and listener positions fixed.

Before and After: Expected Results

When your setup and calculations are correct, you should see:

Consistent EDT trends across mic positions: Not identical, but similar shapes by frequency. If one position is wildly different, it’s likely near a strong reflector or doorway path.
Door state effects most visible in early decay: For example, a door opening can reduce EDT in Room B at mid/high frequencies (because energy escapes and early reflections change), while low frequencies may change less or even worsen due to modal shifts.
Realistic small-room EDT values: Many treated control rooms land around 0.10–0.25 s from 500 Hz–2 kHz. A lively live room might be 0.30–0.60 s. A hard hallway can show 0.40–0.90 s midband due to flutter-like early energy.

If you apply treatment or change configurations, your “after” is usually a reduction in Room B’s mid/high EDT (clearer, less splash) and/or a smoother low-frequency EDT (less “one-note” resonance). Expect improvements on the order of 0.05–0.20 s in targeted bands when you make meaningful changes (adding absorption at reflection points, thick bass trapping, closing/ sealing a door properly).

Troubleshooting When Things Go Wrong

EDT is erratic or negative: Your decay curve is corrupted by noise, gating, or an incorrect reference point. Increase sweep level, extend the IR window, and re-check that the decay trace is derived from the integrated impulse response.
EDT is unrealistically high (e.g., >1.5 s) in a small room: Often caused by a strong early reflection dominating the first 10 dB or by measuring too close to a reflective surface. Move the mic 0.5–1 m and repeat.
Low-frequency EDT won’t stabilize: Background noise and HVAC dominate below 125–250 Hz. Try longer sweeps (512k), higher playback level, and measure at a quieter time. Also consider reporting only 250 Hz and above if the LF data can’t be trusted.
Room A and B results seem identical: You may have accidentally measured in the same room twice, moved the speaker, or left the door open/closed inconsistently. Confirm configuration notes and re-run a single verification sweep.

Pro Tips for Taking the Technique Further

Map EDT along the boundary path: In Room B, measure positions at 0.5 m, 1 m, and 2 m from the door/wall. This shows whether the “wash” is localized near the coupling point or a whole-room issue.
Use two source positions: One near the boundary (worst case) and one realistic performance position. Many isolation complaints come from a singer standing too close to a door or window.
Compare EDT with clarity metrics: If your software provides C50 (speech) or C80 (music), correlate them with EDT changes. A lower EDT in 1–4 kHz often tracks improved clarity.
Document the ETC, not just the EDT number: The Energy Time Curve reveals the culprit reflections (desk bounce, glass slap, hallway ping). If you see a strong spike at 20–40 ms in Room B, treat that surface before you chase broadband decay.
Standardize your reporting: Always note: door state, mic height, source position, sweep level, and whether EDT is octave-band or broadband. This makes your measurements usable weeks later.

Wrap-Up

Calculating Early Decay Time between rooms is less about finding a single magic number and more about building a dependable comparison: how the initial decay behaves on the source side versus the receiving side, and how that behavior changes with doors, openings, and surface treatments. Run the measurement sequence a few times, keep your configurations carefully documented, and you’ll start predicting outcomes before you even look at the plots—exactly the skill that separates casual measurement from engineering practice.