Calculate sound absorption coefficient from incident and reflected power, or look up values from a 10-material database. Includes Sabine/Eyring reverberation time and RT60 target comparisons.
The sound absorption coefficient (α) measures how much sound energy a material absorbs versus reflects, on a scale from 0 (perfect reflection, like polished marble) to 1 (perfect absorption, like an open window). This deceptively simple number is the cornerstone of architectural acoustics, determining the reverberation time, speech intelligibility, and overall acoustic character of every enclosed space — from concert halls designed for 2-second reverb to recording studios aiming for near-zero reflections.
Absorption is frequency-dependent: porous materials (carpet, fiberglass, foam) absorb high frequencies effectively but are poor at low frequencies, while panel absorbers (thin plywood, drywall) resonate at low frequencies and reflect highs. This is why professional acoustic treatment uses a mix of absorbers, diffusers, and bass traps to create a balanced frequency response. The Noise Reduction Coefficient (NRC) provides a single-number rating by averaging α at 250, 500, 1000, and 2000 Hz, though this simplification can mask critical frequency-dependent behavior.
This calculator computes α from incident/reflected power measurements, provides a 10-material database with absorption values at six standard frequencies, and calculates reverberation time using both the Sabine formula (accurate for low absorption) and the Eyring formula (more accurate for highly absorptive rooms). RT60 results are compared against target ranges for different room types.
Architectural acousticians, studio designers, HVAC noise engineers, and audio enthusiasts all need to calculate absorption and reverberation time. This calculator provides the complete workflow: measure or look up α, compute total room absorption, and predict RT60 with both Sabine and Eyring formulas — then compare against standard targets. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain.
Absorption coefficient: α = (P_incident − P_reflected) / P_incident = 1 − P_reflected/P_incident. Total absorption: A = Σ(αᵢ × Sᵢ) in sabins (m²). Sabine RT60: T = 0.161·V / A. Eyring RT60: T = 0.161·V / (−S·ln(1 − α̅)). NRC = (α₂₅₀ + α₅₀₀ + α₁₀₀₀ + α₂₀₀₀) / 4.
Result: α = 0.70, A = 84.0 sabins, RT60 = 0.63 s
α = 1 − 0.003/0.01 = 0.70 (70% absorbed). Total absorption = 0.70 × 120 = 84 m². Sabine RT60 = 0.161 × 330 / 84 = 0.63 seconds — suitable for a classroom or conference room.
| Mechanism | How It Works | Best Frequencies | Examples | |---|---|---|---| | Porous absorption | Sound enters pores; viscous friction converts energy to heat | Mid-high (500+ Hz) | Fiberglass, foam, carpet | | Membrane (panel) | Thin panel vibrates at resonance, dissipating energy | Low (50–500 Hz) | Drywall, plywood | | Helmholtz resonator | Air in a neck resonates, absorbing energy at a tuned frequency | Narrow band | Perforated panels, bottles |
Wallace Sabine developed his formula in 1898 at Harvard, establishing the field of architectural acoustics. His formula T = 0.161V/A assumes difuse sound field and low average absorption. Carl Eyring's 1930 modification −S·ln(1−α̅) corrects for higher absorption levels. The Millington-Sette formula further refines the calculation for rooms with very non-uniform absorption distribution.
1. Define target RT60 based on room purpose (speech, music, recording, etc.) 2. Calculate existing absorption from construction materials and furnishings 3. Determine additional absorption needed: A_added = 0.161·V/T_target − A_existing 4. Select treatment materials matching the needed frequency range 5. Verify with Eyring formula and acoustic modeling software 6. Measure after installation; adjust placement as needed
α = 0 means the surface reflects all sound (perfectly reflective, like ideal concrete). α = 1 means it absorbs all sound (perfectly absorptive, like an open window). No real material achieves exactly 0 or 1, but fiberglass panels approach α = 0.99 at mid-high frequencies.
NRC (Noise Reduction Coefficient) averages absorption at 250, 500, 1000, and 2000 Hz to give a single rating. NRC 0.00 = full reflection, NRC 1.00 = full absorption. Architects use NRC for quick material comparisons, but it ignores low-frequency behavior.
Sabine's formula (T = 0.161V/A) assumes low, uniform absorption and works well for live rooms. Eyring's formula uses −S·ln(1−α̅) in the denominator and is more accurate when α > 0.2 or when absorption is not uniformly distributed. For dead rooms (α > 0.5), Sabine overestimates RT60.
Home theaters target RT60 of 0.3–0.6 seconds. Shorter reverb improves dialog clarity and surround-sound imaging. Too dry (< 0.2 s) sounds unnatural and lifeless. Strategic placement of absorption and diffusion panels is key.
In measurement, yes! This apparent paradox occurs because the test specimen can absorb sound from a larger area than its physical footprint due to edge diffraction effects, especially at low frequencies. This artifact is common in impedance tube and reverb chamber measurements.
For porous absorbers, thickness determines the lowest frequency effectively absorbed. As a rule of thumb, a porous absorber works well above the frequency where its thickness is about ¼ wavelength. A 2-inch panel absorbs well above ~1700 Hz; 4-inch extends this to ~850 Hz.