RT60 Acoustics & Sabine Formula Guide

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 primary metric for describing how "live" or "dead" a room sounds.

• Short RT60 (< 0.5s) — "Dead" room, very controlled sound. Ideal for recording studios and critical listening spaces.
• Medium RT60 (0.5-1.0s) — Balanced acoustics. Good for speech clarity in offices, classrooms, and conference rooms.
• Long RT60 (> 1.0s) — "Live" room with noticeable reverberation. Common in gymnasiums, lobbies, and worship spaces.

The Sabine Formula

Developed by Wallace Clement Sabine in 1898, this formula remains the foundation of architectural acoustics:

The constant 0.049 applies when using imperial units (cubic feet). For metric (cubic meters), use 0.161.

Calculating Sabins

Absorption (Sabins) is calculated by multiplying each surface area by its NRC value:

Total room absorption is the sum of Sabins from all surfaces (floor, ceiling, walls) plus any furniture, people, or other absorptive elements.

Example Calculation

Consider a 30' × 40' × 10' conference room:

Volume = 30 × 40 × 10 = 12,000 CF

RT60 = 0.049 × 12,000 / 1,570 = 0.37 seconds

This room is well below the 0.5s target for conference rooms, providing excellent speech clarity.

Target RT60 by Room Type

Different spaces have different acoustical requirements based on their function:

Understanding NRC

NRC (Noise Reduction Coefficient) is a single-number rating representing a material's average absorption across four frequencies: 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz.

• NRC 0.00 — Perfectly reflective (no absorption)
• NRC 0.50 — Absorbs half the sound energy that hits it
• NRC 1.00 — Absorbs all sound energy (in theory)
• NRC > 1.00 — Possible due to edge effects and diffraction (common with baffles)

ASTM C423 Testing

Manufacturers determine NRC through standardized testing per ASTM C423. Results depend heavily on mounting method:

• Type A (direct mount) — Material adhered directly to substrate. Lower NRC.
• Type E-400 (16" air gap) — Simulates plenum mounting. Higher NRC, especially at low frequencies.

The NRC values in our calculator use Type A mounting for direct-mount products and Type E-400 for ceiling tiles in typical grid installations.

Floor Material NRC Values

Ceiling Material NRC Values

The following values are from manufacturer-published test data (ASTM C423):

Acoustical Ceiling Tiles (ACT)

Fiberglass Panels & Specialty Products

Wall Material NRC Values

Using the Calculator

The RT60 Calculator guides you through the process:

Step 1: Enter Room Dimensions

For rectangular rooms, enter Length × Width × Height. For complex shapes (L-shaped, T-shaped), click "Use Complex Shape" and enter floor area and perimeter directly. Use the Areas tool in the utility drawer to calculate complex shapes.

Step 2: Assign Surface Treatments

For each surface (Floor, Ceiling, Walls), add treatments with their areas:

• The "Total" shows the calculated surface area
• The "Assigned" shows how much you've allocated to treatments
• Green = fully assigned, Red = over-assigned

You can add multiple treatments per surface. For example, a ceiling might have 800 SF of ACT and 400 SF of drywall soffits.

Step 3: Select Room Type

Choose a room type preset to set the target RT60. The calculator shows whether your current configuration meets the target.

Step 4: Review Recommendations

If current RT60 exceeds the target, the calculator shows how many Sabins of additional absorption are needed and translates that to square feet of treatment at various NRC ratings.

Treatment Selection Tips

Ceiling First

The ceiling is usually the largest unobstructed surface and the easiest to treat. Start here. A good ACT ceiling (NRC 0.70+) often provides enough absorption on its own for typical commercial spaces.

Consider the Floor

Carpet with pad (NRC 0.55) provides significant absorption. If the floor is hard surface (concrete, tile, LVT), you'll need more ceiling or wall treatment to compensate.

Walls for Fine-Tuning

Wall panels are expensive per Sabin compared to ceiling treatment. Use them strategically:

• Conference rooms: Panels on the wall opposite the display/camera
• Open offices: Panels on columns and end walls
• Gymnasiums: Upper wall areas where they won't be damaged

Don't Over-Treat

A room that's too dead feels uncomfortable and fatiguing. If your RT60 is already at or below target, stop adding absorption. Some reflection is natural and helps occupants orient themselves spatially.

Baffles and Clouds

When ceiling plenum space is unavailable (exposed structure) or you need maximum absorption, hanging baffles or acoustic clouds are effective. Because both sides are exposed to sound, they can achieve NRC > 1.00.

Need Help?

For complex spaces, multi-room projects, or critical listening environments, consider consulting an acoustical engineer. The calculator provides estimates based on the Sabine formula, but real-world results depend on room geometry, furnishings, and frequency-specific requirements that simplified calculations don't capture.

Questions about the calculator? Send us a message.