Free Absorption Calculation Spreadsheet — Sabine + Eyring RT60 (Excel)

Calculating RT60 by hand is straightforward once you understand the structure, but spreadsheet errors are common — wrong surface areas, incorrect absorption coefficient frequencies, mixing Sabine and Eyring in the same cell, forgetting air absorption in large rooms. This article walks through a purpose-built RT60 calculation spreadsheet layout that avoids these errors and handles both the Sabine and Eyring equations across all six octave bands.

The spreadsheet has five tabs: Room Input, Material Library, Calculation, Compliance Check, and Notes. Each tab has a specific function, and they feed each other through named cell references — no hard-coded values.

The Physics Behind the Spreadsheet

Both equations calculate RT60 from room volume and total absorption area, differing only in how they handle the absorption term.

Sabine (ISO 3382-2:2008 §A.1):

RT60 = 0.161 × V / A

Where V is room volume in m³ and A is total absorption area in m² (sabins), calculated as:

A = Σ(Si × αi) + 4mV

Eyring (ISO 3382-2:2008 §A.2):

RT60 = 0.161 × V / (-S × ln(1 - ᾱ) + 4mV)

Where S is total surface area in m² and ᾱ = A/S is the average absorption coefficient.

The critical difference: at ᾱ = 0.10, Sabine gives RT60 = 0.161V/0.10S and Eyring gives RT60 = 0.161V/0.105S — a 5% difference, negligible for design purposes. At ᾱ = 0.40 (a well-treated room), Sabine gives 0.161V/0.40S and Eyring gives 0.161V/0.511S — a 28% difference that matters. The spreadsheet calculates both and flags when they diverge by more than 10%, alerting you to use Eyring.

Tab 1 — Room Input

The Room Input tab collects all geometric parameters. The layout:

Below the geometry, there is a surface assignment table. Each row represents one surface or zone, with columns for surface name, area (m²), and material selection (dropdown linked to the Material Library tab):

The surface area column uses validation to flag if the entered areas do not approximately sum to the total surface area (within ±5%). This catches the most common spreadsheet error: forgetting a surface or double-counting.

Tab 2 — Material Library

The Material Library tab contains absorption coefficient data for 80 common construction materials across the six standard octave bands. It is structured as a named table so the Room Input dropdown can reference it:

Note: absorption values for people are expressed in m² per person (metric sabins), not as coefficients. The spreadsheet handles this with a conditional formula: if the material type is "person," multiply by count instead of area.

You can add rows to the Material Library for project-specific materials (e.g., a specific manufacturer's panel with tested ISO 354 data). The Room Input dropdown refreshes automatically when you add rows.

Tab 3 — Calculation

The Calculation tab is the core of the spreadsheet. It computes absorption area A at each octave band, calculates RT60 using both Sabine and Eyring, and presents the results in a comparative table.

Column structure for each octave band:

Row by row for each surface:

• Area (m²) — pulled from Room Input
• α (absorption coefficient) — pulled from Material Library via VLOOKUP
• Absorption area contribution Si×αi (m² sabins)

Results table:

The "Use Eyring?" row applies a conditional formula: if ᾱ > 0.20, flag yes. At 500 Hz and above in this example, the average coefficient exceeds 0.20, so Eyring gives the more accurate result.

The spreadsheet automatically selects the appropriate value (Eyring where ᾱ > 0.20, Sabine where ᾱ ≤ 0.20) and feeds this "best estimate" row into the Compliance Check tab.

Tab 4 — Compliance Check

The Compliance Check tab compares the calculated RT60 against the limits for the selected room type and standard. A dropdown at the top selects the applicable standard:

• ANSI S12.60 (classroom)
• BB93 (UK school)
• ISO 3382-1 (performance spaces)
• WELL v2 Feature 74
• Custom (enter your own limits)

For BB93, the 500–2000 Hz average is evaluated instead. For ISO 3382-1 concert hall targets, mid-frequency RT60 (500–1000 Hz average, occupied) is compared against the recommended ranges for the room type.

A conditional formatting rule turns the Status cell green for PASS and red for FAIL, and a summary cell at the top reads "COMPLIANT" or "NON-COMPLIANT" for quick reference.

Worked Example: 200 m³ Open-Plan Office

Room parameters:

• 10 m × 8 m × 2.5 m = 200 m³
• Total surface area = 250 m²

Sabine RT60 at 500 Hz: 0.161 × 200 / 13.0 = 2.48 seconds

This is nearly five times the WELL v2 Feature 74 limit of 0.5 seconds for open-plan offices. A typical target for this space would be 0.5–0.6 seconds, requiring a total A of approximately:

A required = 0.161 × 200 / 0.55 = 58.5 m²

Additional absorption needed: 58.5 − 13.0 = 45.5 m²

Treatment options:

Option A — Acoustic ceiling tiles (NRC 0.90): 80 m² × 0.90 = 72 m² additional absorption. More than sufficient. Post-treatment A = 72 + 13 − 4 (removing plasterboard) = 81 m². RT60 = 0.161 × 200 / 81 = 0.40 s (slightly over-treated).

Option B — Suspended baffles at 50% coverage (40 m²): 40 m² × 0.95 (two-face) = 76 m² effective. Similarly over-treats, but baffles work well in rooms with structural soffits that cannot receive a ceiling tile system.

Option C — Ceiling tiles (50 m², NRC 0.90) + rear wall panel (20 m², NRC 0.90): A = 45 + 18 = 63 m² additional. Post-treatment A = 13 + 63 = 76 m². RT60 = 0.43 s. Good balance of ceiling and wall treatment.

Tab 5 — Notes and Air Absorption

The Notes tab documents all assumptions and the air absorption correction. For rooms above 500 m³, the air absorption term 4mV becomes significant at high frequencies. The air absorption coefficient m (per metre) depends on temperature and relative humidity:

For a 2000 m³ room at 20°C / 50% RH, the air absorption at 4000 Hz is 4 × 0.0151 × 2000 = 120.8 m² — comparable to the surface absorption and essential to include.

For rooms under 500 m³ (the majority of calculation scenarios), air absorption is negligible and can be omitted without meaningful error.

Common Spreadsheet Errors to Avoid

The most frequent mistakes in manual RT60 spreadsheets:

1. Using NRC instead of octave-band coefficients. NRC is an average of the 250–2000 Hz bands, rounded to the nearest 0.05. It suppresses frequency-dependent behavior. Always use the six-band values.

1. Forgetting to subtract the existing surface contribution when adding treatment. If you add an acoustic ceiling tile over an existing plasterboard ceiling, the plasterboard absorption should be removed from the total and replaced by the tile absorption, not added on top.

1. Using Sabine when ᾱ > 0.20. The spreadsheet flags this automatically. If you are using a manual calculation, check ᾱ after computing the initial A.

1. Using field-measured absorption data in place of laboratory data. Field measurements include room geometry effects and should not be used as input to a Sabine/Eyring calculation, which assumes laboratory conditions.

1. Not separating occupied and unoccupied calculations. For concert hall design, the unoccupied RT60 must be close to the occupied value to avoid the characteristic "over-bright unoccupied hall" problem. The spreadsheet supports a toggle between occupied and unoccupied seat absorption values.

Using AcousPlan Instead of the Spreadsheet

The Quick RT60 Calculator performs the Sabine and Eyring calculation online with the same material library, automatically selects the appropriate equation, and generates a compliance check against the standard you specify. For more complex multi-room projects or iterative treatment design, the full room simulator allows you to adjust surface materials and see the RT60 update in real time.

Related Resources

• Quick RT60 Calculator — browser-based Sabine + Eyring calculation
• What is RT60? — explanation of reverberation time and why it matters
• Sabine Equation — When It Fails — in-depth analysis of Sabine's limitations in non-diffuse fields
• Room Acoustics Fundamentals — foundational concepts before running the calculation
• NRC Absorption Calculator — how to interpret and use NRC data