How to Calculate Decay Time for Your Room

By Sarah Okonkwo · February 16, 2026

How to Calculate Decay Time for Your Room

1) What you’ll learn (and why it matters)

Room decay time—usually described as RT60 (the time it takes sound to drop by 60 dB)—is one of the fastest ways to predict whether your room will feel tight and controlled or washy and smeared. It directly affects:

Mix translation: long decay masks detail, especially in the low-mids (150–400 Hz).
Vocal and instrument recording: too much decay makes close-mic tracks sound “roomy” and harder to place.
Speech clarity: excessive decay reduces intelligibility, especially around 1–4 kHz.

This tutorial shows two practical ways to calculate decay time:

Prediction using room size and absorption (Sabine/Eyring), helpful for planning treatment.
Measurement using a test signal and analysis (RT60/EDT), which tells you what the room is actually doing.

2) Prerequisites and setup

Basic tools: tape measure (or laser measure), calculator/spreadsheet.
For measured RT: an omnidirectional measurement mic (e.g., UMIK-1 or ECM8000 + interface), audio interface, and analysis software (REW is common and free).
Quiet time: HVAC off if possible; avoid traffic noise. You want a stable noise floor.
Room state defined: Decide what “the room” means—door open/closed, curtains drawn, furniture in place. Decay time changes with these choices.

Target ranges (typical, not absolute):

Small mixing room (home studio): ~0.20–0.35 s above 500 Hz, with controlled low-frequency decay (often longer below 200 Hz).
Vocal booth: ~0.10–0.20 s above 500 Hz.
Podcast/speech room: ~0.20–0.40 s depending on tone desired, but aim for consistent decay and strong early-reflection control.

3) Step-by-step: calculate decay time (prediction + measurement)

Step 1 — Measure the room volume accurately

Action: Measure length, width, and height in meters (or feet, but be consistent). Calculate volume: V = L × W × H.

Why: RT calculations scale with volume. A 10% error in dimensions is a 10% error in the predicted decay time.

Technique and values: Use meters if you can. Example: L=4.2 m, W=3.4 m, H=2.5 m → V = 35.7 m³.

Common pitfalls:
- Measuring to baseboards/trim inconsistently.
- Ignoring ceiling height changes (soffits, beams). If height varies a lot, break the room into sections and sum volumes.
- For open doorways, deciding volume incorrectly. If a door stays open during work, include the connected space as part of the acoustic system.
Step 2 — List major surfaces and estimate absorption by frequency

Action: Create a table of surface areas: floor, ceiling, each wall, large windows, doors, and any big absorption (thick curtains, couches, acoustic panels).

Why: Decay is dominated by total absorption area (A, in sabins). Different materials absorb differently by frequency; a carpet might help at 2 kHz but do almost nothing at 125 Hz.

Technique and values:
- Compute surface area in m². Example wall: 4.2 m × 2.5 m = 10.5 m².
- Use absorption coefficients (α) per octave band if possible: 125, 250, 500, 1k, 2k, 4k Hz.
- Then compute absorption per surface: A_surface = S × α.
Typical absorption coefficients (rough, for planning):
- Painted drywall on studs: α ≈ 0.02 (250 Hz), 0.04 (500 Hz), 0.05 (1 kHz)
- Concrete: α ≈ 0.01–0.02 midband
- Carpet on concrete: α ≈ 0.02 (125 Hz), 0.08 (250 Hz), 0.20 (500 Hz), 0.40 (1 kHz)
- Heavy curtain (folded, 2:1 fullness): α ≈ 0.10 (250 Hz), 0.35 (500 Hz), 0.55 (1 kHz)
- 2" (50 mm) fiberglass panel with air gap: α ≈ 0.30 (250 Hz), 0.80 (500 Hz), 1.00 (1 kHz)
Common pitfalls:
- Using one “average” α for everything. At minimum, work at 500 Hz and 1 kHz, then sanity-check low end separately.
- Double-counting: don’t add a panel’s absorption on top of the wall absorption for the same area—replace the wall area with the panel area.
- Assuming foam fixes bass decay. Most thin foam is largely a high-frequency absorber.
Step 3 — Calculate predicted RT60 using Sabine (and know when it’s valid)

Action: Sum total absorption: A = Σ(S × α) in sabins. Then compute:

Sabine RT60 (metric): RT60 = 0.161 × V / A

Why: Sabine gives a fast prediction and helps you compare “before vs after” when adding absorption. It’s most accurate in diffuse rooms with relatively even absorption, and less accurate in small, very reflective, or highly treated rooms.

Example (midband): If V = 35.7 m³ and total A at 500 Hz is 18 sabins, RT60 ≈ 0.161 × 35.7 / 18 = 0.32 s.

Common pitfalls:
- Getting an unrealistically low RT because you used α=1.0 for too much surface area.
- Using Sabine in a very dead room where average α is high. If average α is > 0.20–0.30, consider Eyring (next step) for a better estimate.
- Trying to predict 63–125 Hz RT accurately with Sabine alone. Low-frequency decay is dominated by modes and boundary conditions, not diffuse-field assumptions.
Step 4 — Refine the prediction with Eyring for higher absorption rooms

Action: Calculate average absorption coefficient: ᾱ = A / S_total, where S_total is total room surface area (all walls + floor + ceiling). Then compute:

Eyring RT60: RT60 = 0.161 × V / ( -S_total × ln(1 - ᾱ) )

Why: Sabine assumes absorption is small; Eyring handles higher average absorption more realistically and often predicts slightly longer RT than Sabine in very treated rooms.

Specific technique: If your ᾱ is around 0.05–0.15, Sabine and Eyring will be close. If ᾱ is 0.25+, Eyring is usually safer.

Common pitfalls:
- Forgetting that ln is natural log (base e), not log10.
- Getting ᾱ ≥ 1.0 due to incorrect coefficients or double-counting. ᾱ must be between 0 and 1.
Step 5 — Measure real decay time with REW (preferred for actual decisions)

Action: Use a measurement mic and run a sine sweep in REW to compute RT (T20/T30) by octave bands.

Why: Prediction gets you in the ballpark. Measurement tells you what the room actually does—including uneven decay caused by modes, treatment placement, and furnishings.

Specific settings and technique:
- Mic position: At the listening position for mixing (ear height), or where the performer’s head would be for recording. Point an omni mic straight up (common practice) to reduce HF directionality errors.
- Speaker setup: Use one speaker at a time for diagnostic clarity. Start with left only, then right only. Subwoofer on if it’s part of your system.
- Sweep level: Aim for peaks around 75–85 dB SPL at the mic position if possible. In REW, adjust output so the recorded sweep has a healthy level without clipping.
- Sample rate: 48 kHz is fine; 96 kHz not required for RT analysis.
- Length: 256k sweep length is a good default in REW for better low-frequency resolution in typical rooms.
- RT metric: Use T20 or T30 (REW derives RT60 from a 20 dB or 30 dB decay segment). In small rooms, true 60 dB decay may be buried in noise; T20 is often more stable.
Common pitfalls:
- Noise floor too high: HVAC rumble makes low-frequency decay look longer or makes T30 fail entirely. If REW warns about noise, reduce noise or use shorter metric (T20).
- Clipping: Any clipping ruins decay calculations. Keep at least 6 dB headroom on recording.
- Mic too close to a boundary: Don’t place the mic within ~0.5 m of a wall if you can avoid it; boundary interference can skew results.
- Interpreting comb filtering as decay: RT is a time-domain decay metric; don’t confuse frequency response nulls with decay problems (though both matter).
Troubleshooting tips:
- If RT graphs look jagged and inconsistent between runs, increase sweep level slightly and ensure the room is quieter.
- If low-frequency RT seems absurdly long (e.g., > 1.5 s at 63 Hz in a bedroom), confirm you’re not measuring with a door open into a long hallway, and check for HVAC noise contamination.
- If L and R speaker measurements differ a lot, you likely have asymmetry (desk placement, side wall distance, window/curtain differences).
Step 6 — Interpret the results by frequency band (what “good” looks like)

Action: Look at octave-band decay times and compare them to your room’s purpose. Focus on consistency rather than chasing a single number.

Why: A room that measures 0.30 s at 1 kHz but 0.80 s at 125 Hz will still feel boomy and unclear on kick/bass decisions. Conversely, a room that is 0.15 s at 4 kHz but 0.35 s at 1 kHz can feel dull yet still muddy.

Practical interpretation targets (small-room reality):
- 500 Hz–4 kHz: aim for roughly 0.20–0.35 s in a mix room; tighter for vocal booths.
- 125–250 Hz: expect longer. The goal is not “as short as possible,” but controlled and smooth (no huge spikes at single bands).
- EDT (Early Decay Time): if EDT is much higher than RT60, the room has strong early reflections—often untreated side walls/ceiling.
Common pitfalls:
- Over-treating highs while lows remain uncontrolled (a room that feels dead but still boomy).
- Chasing perfectly flat RT across all bands in a tiny room—modes will limit what’s possible below ~200 Hz.
Step 7 — Recalculate “after” treatment and confirm with a second measurement

Action: If you add treatment, update your absorption table and recalculate predicted RT (Sabine/Eyring), then re-measure with the same mic/speaker positions and sweep settings.

Why: Predictions guide quantity; measurements verify placement effectiveness. Two rooms can have the same amount of absorption but different decay behavior depending on where it’s installed.

Specific technique: Treat one variable at a time when possible. Example: add two 2" panels at first reflection points, re-measure; then add bass trapping, re-measure.

Common pitfalls:
- Changing mic position between “before” and “after” and attributing differences to treatment.
- Moving furniture (especially couches) and forgetting it’s a major absorber in mid/high bands.

4) Before/after comparison (expected results)

In a typical spare-bedroom studio (around 30–45 m³), an untreated room often measures roughly:

500 Hz–2 kHz: ~0.4–0.7 s (fluttery, splashy imaging)
125–250 Hz: ~0.6–1.2 s with peaks (boomy notes, kick/bass hard to judge)

After adding practical treatment—example: 6–10 panels of 50–100 mm mineral wool placed at first reflections plus 2–4 corner bass traps—a realistic improvement is:

500 Hz–2 kHz: ~0.2–0.35 s (clearer stereo image, less “room tone” in close mics)
125–250 Hz: reduced peaks and smoother decay; maybe ~0.4–0.8 s depending on trap depth and coverage

In real-world mixing terms, you should notice:

Reverbs and delays are easier to set because the room adds less “free ambience.”
EQ decisions in the 200–500 Hz range stop feeling like guesswork.
Vocal compression sounds more consistent because room reflections aren’t triggering the compressor unpredictably.

5) Pro tips to take it further

Measure multiple positions: RT can vary across a room. Measure at the listening position plus 2–3 nearby points (±30–50 cm). If results swing wildly, you’re seeing modal behavior; bass trapping and speaker/listener placement matter as much as absorption quantity.
Separate early reflections from late decay: A room can have an acceptable RT but still image poorly due to strong early reflections. Use side-wall/ceiling first-reflection treatment to bring EDT closer to the RT curve.
Use one speaker at a time: It’s easier to diagnose asymmetry and reflection issues when L and R are measured separately.
Check decay vs frequency response: Pair RT with a waterfall/spectrogram. If you see a long “ridge” at, say, 63 Hz, that’s a mode ringing. Treating only highs won’t fix it.
Plan treatment with numbers: If your RT at 1 kHz is 0.55 s and you want 0.30 s, you need roughly about 1.8× the midband absorption (because RT is inversely proportional to A in Sabine terms). That doesn’t mean randomly doubling panels—place them where they reduce reflections and improve uniformity.

6) Wrap-up (practice beats guessing)

Calculating decay time gives you a working model of how your room behaves, and measuring it confirms what your ears already suspect. Do one careful “before” measurement, make a simple prediction using your room dimensions and absorption, then change one thing at a time and measure again. After a few cycles, you’ll stop buying treatment based on hype and start making decisions based on repeatable results—the same habit that separates casual setups from rooms you can trust.