RT60 Reverberation Guide | FreeAcoustics.tools

🎯 What Is RT60?

RT60 (Reverberation Time 60) is the time it takes for sound to decay by 60 decibels after the source stops. It's the standard measure of how "live" or "dead" a room sounds.

Long RT60 (>1s): Room sounds reverberant, "echoey" — sound lingers
Short RT60 (<0.4s): Room sounds "dead," controlled — sound stops quickly
Optimal RT60: Depends on room use — speech needs shorter, music often needs longer

                    Why it matters: Wrong RT60 causes speech intelligibility problems (words blur together), listening fatigue, or mixes that don't translate to other rooms. Getting RT60 right is fundamental to any acoustic design.
                

📊 Target RT60 by Room Type

Room Type	Target RT60	Why
Recording studio (voice)	0.2-0.4s	Clean, dry recordings without room coloration
Control room / mixing	0.25-0.35s	Accurate monitoring, hear details clearly
Podcast / streaming	0.3-0.5s	Clear speech, professional sound
Home theater	0.4-0.6s	Balance between dialogue clarity and immersion
Classroom	0.5-0.7s	Speech intelligibility for all students
Conference room	0.4-0.6s	Clear communication, video calls
Worship space	1.0-2.5s	Warmth for music, but speech can suffer
Concert hall	1.5-2.5s	Musical richness and blend

📏 How to Use the Calculator

Step 1: Enter Room Dimensions

Measure the interior length, width, and height of your room. The calculator uses these to compute total volume and estimate surface areas.

Step 2: Add Surfaces and Materials

For each surface in your room, specify:

Surface name: What it is (floor, ceiling, front wall, etc.)
Material: Select from the dropdown (absorption coefficients built in)
Area: Surface area in square feet or meters

Step 3: Interpret Results

The calculator shows:

Calculated RT60: Your estimated reverberation time
Target comparison: How you compare to the selected room type
Treatment needed: How many additional sabins of absorption to reach target

                    Tip: Start with the major surfaces (floor, ceiling, walls) then add furniture and treatment. Even carpet, couches, and bookshelves contribute absorption.
                

🔬 Sabine vs Eyring Formula

Sabine Formula (Default)

RT60 = 0.049 × V / A (imperial) or 0.161 × V / A (metric)

Works well for rooms with low absorption (average α < 0.25). This includes most untreated rooms.

Eyring Formula

More accurate for rooms with high absorption (heavily treated studios, anechoic spaces). Accounts for the fact that with high absorption, sound doesn't bounce as many times before decaying.

Rule of thumb: Use Sabine for initial estimates. Switch to Eyring if your average absorption coefficient exceeds 0.3 or if you're designing a heavily treated space.

🛠️ Common Absorption Materials

Material	α (500 Hz)	Best For
2" acoustic panel (rigid fiberglass)	0.80-0.95	Mid/high frequency absorption
4" acoustic panel	1.00+	Broadband absorption including some low-mid
Heavy carpet + pad	0.35-0.50	High frequency only (doesn't help bass)
Acoustic ceiling tile	0.55-0.75	General treatment, common in offices
Heavy drapes	0.40-0.60	Mid/high frequencies, variable with pleating
Upholstered furniture	0.50-0.70	Free absorption! People absorb too (~4 sabins each)
Concrete/brick	0.01-0.05	Reflective — adds reverb
Drywall (painted)	0.05-0.10	Reflective — standard walls

🔧 What To Do With Your Results

If RT60 Is Too Long (Too Reverberant)

Add absorption: acoustic panels, ceiling clouds, heavy curtains
Cover hard surfaces: area rugs, upholstered furniture
Priority: treat large surfaces first (ceiling often has most impact)

If RT60 Is Too Short (Too Dead)

Remove or reduce absorption
Replace absorptive materials with diffusers
This is rare in untreated rooms — usually only happens in over-treated studios

How Much Treatment?

The calculator shows "Treatment Needed" in sabins. To convert:

Sabins needed ÷ material α = area required
Example: Need 50 sabins, using α=0.85 panels → 50÷0.85 = 59 sq ft of panels

                    25% coverage rule: For most studios and listening rooms, treating about 25-35% of wall/ceiling surface area with proper absorbers gets you close to target RT60.
                

📱 Measuring RT60 in the Real World

To verify calculations or measure existing spaces:

Apps: REW (Room EQ Wizard - free), Studio Six Digital apps, miniDSP UMIK
Method: Play a burst of noise, measure the decay curve
Best practice: Measure at multiple positions and average

Calculated RT60 is an estimate. Real measurements account for furniture, openings, and actual material performance.