RT60 and Reverberation Time: The Essential Guide

Education · 12 min read · ResonAia Editorial

Understand RT60: what it measures, how to interpret it, target values for different rooms, and how treatment affects reverberation time.

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 one of the most important acoustic metrics for characterizing a room, though not a complete diagnosis on its own — see "RT60 vs. Other Metrics" below.

The Measurement

Generate a loud sound (impulse, sweep, or noise burst)
Stop the source
Measure how long until sound level drops 60 dB
This duration is RT60

Example: If a room's RT60 is 0.5 seconds, the sound pressure level drops from, say, 100 dB to 40 dB in half a second.

Why 60 dB?

60 dB represents approximately the dynamic range of typical audio. When sound has decayed 60 dB, it's essentially "gone", masked by background noise.

In practice, we often measure RT30 or RT20 (30 or 20 dB decay) and extrapolate to RT60, since achieving 60 dB of decay above the noise floor is difficult in real rooms.

Frequency Dependence

RT60 varies with frequency. A room might have:

0.8s at 125 Hz (boomy bass)
0.5s at 1000 Hz (well-controlled mids)
0.3s at 4000 Hz (absorbed high frequencies)

This common pattern (longer RT at low frequencies) occurs because bass is harder to absorb.

Measuring at Multiple Frequencies

Standard practice measures RT60 at octave bands:

125, 250, 500, 1000, 2000, 4000 Hz

Full-range absorbers reduce RT60 across all bands. Thin foam reduces high-frequency RT60 while leaving bass unaffected.

Target RT60 Values

Different room types have different target RT60 ranges, used as starting points for planning rather than fixed requirements:

Room Type	Target RT60	Notes
Recording/Control Room	0.3-0.4s	Tight, controlled sound
Podcast Studio	0.3-0.5s	Clear speech intelligibility
Music Practice Room	0.5-0.7s	Some liveliness for instruments
Small Home Theater	0.4-0.5s	Clarity without deadness
Living Room	0.5-0.8s	Natural, comfortable
Classroom	0.5-0.7s	Speech intelligibility balance
Concert Hall	1.5-2.5s	Sustain and envelopment
Cathedral	4-8s	Grand, reverberant character

Room Volume Affects Targets

Larger rooms need slightly longer RT60 to sound natural:

Small room (< 50m³): 0.3-0.4s Medium room (50-100m³): 0.4-0.5s Large room (100-500m³): 0.5-0.8s

The Sabine Equation

Wallace Sabine developed the first formula for predicting RT60 in the 1890s:

RT60 = 0.161 × V / A

Where:

V = room volume (m³)
A = total absorption (m² Sabins)

Calculating Total Absorption

A = Σ(surface area × absorption coefficient)

For example:

Floor (20m², α=0.1): 2.0 Sabins
Ceiling (20m², α=0.2): 4.0 Sabins
Walls (40m², α=0.05): 2.0 Sabins
Furniture, people, etc.: 3.0 Sabins
Total: 11.0 Sabins

For a 50m³ room: RT60 = 0.161 × 50 / 11 = 0.73s

Adding Treatment

Add a 2m² absorber with α=0.9: New absorption: 11.0 + (2 × 0.9) = 12.8 Sabins New RT60: 0.161 × 50 / 12.8 = 0.63s

This shows why small rooms are hard to treat. Percentage change in absorption required for noticeable RT60 reduction is large.

Improving RT60 Problems

RT60 Too Long (Echoey, Reverberant)

Solution: Add absorption

Where to add:

Ceiling: Often the largest untreated surface
First reflection points: Side walls, rear wall
Corners: Especially effective for low frequencies

How much: Use Sabine equation to estimate. Typically 15-30% of surface area needs treatment in small rooms.

RT60 Too Short (Dead, Lifeless)

Solution: Remove absorption or add diffusion

Options:

Replace absorption with diffusion at rear wall
Remove excessive soft furnishings
Add reflective surfaces strategically

Uneven RT60 (Bass Too Long)

Solution: Targeted bass absorption

Thick absorbers (4"+) or bass traps in corners
Membrane/panel absorbers tuned to problem frequencies
May need significant corner treatment

Measuring RT60 Yourself

Equipment Needed

Measurement microphone (or calibrated smartphone)
Audio interface or adapter
Measurement software (Room EQ Wizard is free and excellent)
Sound source (speaker or balloon pop)

Basic Procedure

Set up microphone at listening position
Connect to measurement software
Play frequency sweep or impulse
Software calculates RT60 from decay

Tips for Accurate Measurement

Multiple positions: RT60 varies somewhat by position. Average 3-5 measurements.

Low background noise: Turn off HVAC if possible. Measure when quiet.

Appropriate levels: Sound source should be 45+ dB above background noise.

Check decay curve: The decay should be linear on a dB scale. Non-linear decay indicates problems with measurement or room geometry.

RT60 vs. Other Metrics

RT60 is central to room decay, but it isn't a complete diagnosis on its own — especially in small rooms. Pair it with frequency response, waterfall/decay data, and early-reflection timing, covered below.

Early Decay Time (EDT)

Measured from 0 to -10 dB decay, extrapolated to 60 dB. Often correlates better with subjective perception than RT60.

Clarity (C80, C50)

Ratio of early to late energy. High clarity means more energy arrives early (better speech intelligibility).

Definition (D50)

Percentage of energy arriving in first 50ms. Related to speech intelligibility.

For studio work, RT60 and EDT are most important. For speech spaces, C50 and D50 matter more.

Common Mistakes

Over-treating

Aggressive absorption can make RT60 too short. The room sounds dead, fatiguing, and unnatural. Target the appropriate RT60, don't just "add absorption."

Treating Only High Frequencies

Thin foam lowers high-frequency RT60 while leaving bass reverb unchanged. The room sounds dark and boomy. Use thick, broadband absorbers.

Ignoring Frequency Response

RT60 doesn't tell you about modal problems. A room can have perfect RT60 and still have 20 dB peaks from room modes. Both matter.

Practical Workflow

Measure current RT60 across frequency range
Identify problems: Too long? Uneven? Bass buildup?
Calculate needed absorption using Sabine equation
Plan treatment: Prioritize bass trapping and first reflections
Install incrementally: Measure after each addition
Stop when target reached: Don't over-treat

Our room analysis tools help with steps 1-4, providing predictions before you build or buy.

RT60, Reverberation, Acoustics, Measurement

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