Room Acoustic Reverberation Time Calculator

What Is Reverberation Time and Why It Matters

Reverberation time (RT60) is the duration required for sound energy in a room to decay by 60 decibels after the sound source stops—equivalent to reducing intensity to one-millionth of its original value. This single number encapsulates how "live" or "dead" an acoustic space feels. A cathedral with stone walls and minimal absorption has an RT60 of 5–10 seconds or more; sound reflects off hard surfaces repeatedly, creating a rich, extended tail that is desirable for religious music and choral performances. A recording studio or office, conversely, should have an RT60 of 0.4–0.8 seconds to allow speech clarity and prevent overwhelming echo. A concert hall's RT60 typically ranges from 1.5–2.5 seconds to support orchestra and solo music.

Reverberation time affects not only aesthetics but also intelligibility, safety, and function. Excessive reverberation makes speech unintelligible—a problem in classrooms, hospitals, and offices where clarity is critical. Insufficient reverberation can make spaces sound dead and unpleasant. HVAC noise, impact from footsteps, and other sources of sound are all influenced by reverberation characteristics. Architects and audio engineers use RT60 calculations during design to predict acoustic behavior and specify acoustic treatments—absorptive panels, diffusers, resonators—needed to achieve target reverberation times.

The Sabine Formula for Reverberation Time

In 1898, American physicist Wallace Clement Sabine conducted experiments in Harvard's Fogg Art Museum and derived a formula that predicts reverberation time based on room volume and acoustic absorption. The Sabine formula is:

where is the room volume in cubic meters and is the total acoustic absorption in the room, also called the equivalent absorption area (measured in square meters of sabine). The constant 0.161 (or sometimes 0.16) incorporates unit conversions and the physics of exponential decay.

The equivalent absorption area is calculated from the room's surface area and the average absorption coefficient of the materials:

where is the total surface area (m²) and (alpha) is the average absorption coefficient (a value between 0 and 1, where 0 means perfectly reflective and 1 means perfectly absorptive). Combining these formulas:

This elegant formula reveals a fundamental principle: larger rooms (higher ) have longer reverberation times, while more absorptive surfaces (higher ) reduce reverberation time. By manipulating absorption coefficients through material selection and placement, acoustic designers can achieve target RT60 values.

Worked Example: Estimating RT60 for a Recording Studio

An engineer is designing a small vocal recording booth with dimensions 4 m (length) × 3 m (width) × 2.8 m (height). The walls and ceiling will be lined with acoustic foam panels (α ≈ 0.25 at 500 Hz). Calculate the expected RT60:

Step 1: Calculate volume – V = 4 × 3 × 2.8 = 33.6 m³

Step 2: Calculate surface area – S = 2(LW) + 2(LH) + 2(WH) = 2(4×3) + 2(4×2.8) + 2(3×2.8) = 24 + 22.4 + 16.8 = 63.2 m²

Step 3: Calculate absorption – A = S × α = 63.2 × 0.25 = 15.8 sabin m²

Step 4: Apply Sabine formula – RT60 = (0.161 × 33.6) ÷ 15.8 = 5.41 ÷ 15.8 ≈ 0.34 seconds

This RT60 of 0.34 seconds is appropriate for a vocal booth: short enough to avoid excessive reverberation that would cloud the recording, yet long enough to retain a natural vocal quality. If the engineer wants slightly longer reverberation (say 0.5 seconds) for a warmer sound, they would reduce foam coverage or introduce less-absorptive materials in some areas. The calculator makes iterative design exploration straightforward.

Absorption Coefficients for Common Materials

The following table illustrates how absorption coefficients vary by material and frequency. Note that absorption is frequency-dependent: most materials are less absorptive at low frequencies and more absorptive at high frequencies.

This table demonstrates that hard surfaces like concrete reflect sound (low absorption), while porous materials like fiberglass and thick fabrics absorb sound (high absorption). Notice how acoustic foam becomes increasingly absorptive at higher frequencies. When designing a room, material selection must account for this frequency dependence; simply covering all surfaces with foam that works well at 1000 Hz may leave low-frequency boomininess uncontrolled.

Target RT60 Values for Different Room Types

Different functional spaces have different RT60 targets based on their use:

Speech-critical spaces (classrooms, lecture halls, offices): 0.3–0.8 seconds. Short reverberation ensures that speech remains clear and intelligible. Too much reverberation causes overlapping syllables that are difficult to understand.

Recording studios (music, voice): 0.3–0.6 seconds. A controlled, relatively short RT60 allows the engineer to capture clean, direct recordings without excessive ambiance. More reverberation can be added electronically in post-production.

Live music performance spaces (concert halls): 1.5–2.5 seconds. Orchestras and classical ensembles need moderate reverberation to fill the hall with sound and provide acoustic richness. Too much reverberation muddies performance; too little sounds clinical and dead.

Churches and cathedrals (choral and organ music): 2.5–5+ seconds. The large volume and hard reflective surfaces create extended reverberation that complements liturgical music and choral works. Intelligibility of spoken word is often sacrificed for acoustic grandeur.

Small rooms (bedrooms, home studios): 0.3–0.8 seconds. Furnishings (beds, curtains, carpets) naturally provide absorption. Minimal treatment is usually needed.

The calculator allows users to explore how material choices move calculated RT60 closer to these targets, enabling data-driven acoustic design.

Sabine Formula Limitations and Eyring's Correction

While the Sabine formula is widely used and remarkably accurate for moderate absorption levels, it has limitations. The formula assumes that sound bounces randomly throughout the room (diffuse field assumption) and that absorption is evenly distributed. In highly absorptive rooms (α > 0.3), the Sabine formula tends to overestimate RT60. For such spaces, Eyring's formula provides a correction:

This calculator uses the standard Sabine formula, which is appropriate for most architectural and studio design scenarios. For highly damped rooms, Eyring's formula or measured data is more accurate.

Frequency Dependence and RT60 Curves

Real materials have different absorption coefficients at different frequencies, meaning RT60 also varies with frequency. A room might have an RT60 of 0.5 seconds at 1000 Hz but 0.8 seconds at 125 Hz. This frequency-dependent behavior is called a "RT60 curve." Designers typically specify RT60 at octave bands (125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz) and aim for a smooth, balanced curve. The calculator allows selection of reference frequency, enabling exploration of frequency-dependent behavior.

Using the Calculator

Enter the room's length, width, and height in meters. Select the predominant material type or choose custom and enter an average absorption coefficient (0–1). Select the reference frequency. The calculator applies the Sabine formula to compute RT60 in seconds. Use this result to assess whether the room meets acoustic targets for its intended function. If RT60 is too long, increase absorption (add soft furnishings or acoustic panels). If RT60 is too short, consider reducing absorption or, for speech clarity, this is often desirable. The calculator facilitates rapid iteration and educational exploration of acoustic principles.

Practical Considerations and Limitations

The Sabine formula provides a single-number estimate useful for design but does not account for room shape, surface curvature, or acoustic defects like flutter echo or focusing. A room with parallel hard walls may exhibit flutter echo (rapid repetition of sound) even if RT60 is within target. Curved surfaces can focus sound to create hot spots. In practice, acoustic design combines RT60 analysis with considerations of diffusion, bass trapping, and placement. Additionally, occupancy affects absorption—people absorb sound, and a lecture hall full of students has different acoustics than an empty one. This calculator treats the empty room condition; adjusting for occupancy requires more detailed analysis.

Field measurements using a sound level meter and calibrated noise source are the ultimate verification. The Sabine formula is a design tool, not a substitute for acoustic testing and adjustment. For critical applications (recording studios, concert halls), consulting an acoustician is advisable to optimize the final result.