The acoustic absorption coefficient α represents the fraction of incident acoustic energy absorbed by a surface or by a room’s effective boundaries. It typically ranges from 0 (fully reflective) to 1 (fully absorptive).
- Reflection method: α = 1 − |R|², where |R| is the magnitude of the pressure reflection coefficient.
- Level-based reflection estimate: |R| = 10^((Lr − Li)/20), using incident level Li and reflected level Lr in dB, then α = 1 − |R|².
- Surface impedance method: R = (Zs − Z0)/(Zs + Z0), with Z0 = ρc (air characteristic impedance). Then α = 1 − |R|².
- Sabine method: Equivalent absorption area A = K·V/T60 (sabins), where K = 0.161 in metric units and K = 0.049 in imperial units. Then α = A/S.
- Select a method that matches your measurement data (reflection, impedance, or Sabine).
- Choose your units system. This matters most for the Sabine method.
- Optionally enter a frequency to label frequency-dependent results.
- Enter the required values for the chosen method and click Calculate.
- Review the result α, the interpretation, and intermediate values. Use Download CSV or Download PDF for reporting.
| Method | Inputs | Output α | Notes |
|---|---|---|---|
| Reflection | |R| = 0.70 | 0.51 | Moderate absorption surface. |
| Impedance | ρ = 1.21, c = 343, Re{Zs}=450, Im{Zs}=-120 | 0.42 | Depends on Zs at chosen frequency. |
| Sabine | V = 120 m³, T60 = 0.9 s, S = 260 m² | 0.083 | Room average, not a single panel. |
1) Meaning of the absorption coefficient
The absorption coefficient α is the fraction of incident acoustic energy absorbed instead of reflected. Values near 0 indicate reflective boundaries, while values near 1 indicate strong damping. Designers use α to predict reverberation, speech intelligibility, and subjective comfort.
2) Frequency dependence and reporting
Absorption is usually frequency dependent. Many finishes absorb more at mid and high bands than at low bands. Use common octave-band centers (125, 250, 500, 1000, 2000, 4000 Hz) to label calculations. Exported results stay traceable to the measured or modeled band.
3) Choosing a method that matches your data
Pick the reflection method when you know the pressure reflection magnitude |R| or can estimate it from incident and reflected levels. Pick the impedance method when you have complex surface impedance at the boundary. Pick the Sabine method when you have room geometry and a measured T60.
4) Reflection method with quick checks
The reflection approach uses α = 1 − |R|². If you enter sound levels, the calculator estimates |R| = 10^((Lr−Li)/20) before applying the same formula. As a sanity check, |R| = 0.80 gives α = 0.36, while |R| = 0.50 gives α = 0.75.
5) Impedance method for resistive and reactive behavior
Boundary impedance captures both losses and storage. The calculator forms the characteristic impedance Z0 = ρc and computes R = (Zs−Z0)/(Zs+Z0), then α = 1 − |R|². Defaults ρ ≈ 1.21 kg/m³ and c ≈ 343 m/s suit typical indoor air, but adjust for temperature-critical work.
6) Sabine method for room-average absorption
With reverberation time, the calculator estimates equivalent absorption area A = K·V/T60 where K = 0.161 (metric) or 0.049 (imperial). It then computes α = A/S. This is an average over boundaries and furnishings, not a single material panel rating.
7) Interpreting typical outcomes
Hard surfaces such as glass, tile, or sealed concrete often behave as low absorption at mid frequencies, while thick fabrics, carpet, and upholstered seating can be moderate. Deep porous absorbers and tuned treatments can be high. Compare results across frequencies and keep the same test assumptions for fair comparisons.
8) Practical workflow using exports
Run scenarios by changing frequency, method, or boundary parameters, then export CSV or PDF for documentation. For rooms, iterate toward a target T60 by adjusting surface areas and materials. For product testing, keep geometry consistent so you can compare samples reliably.
1) Can α be greater than 1 or less than 0?
Measured or modeled inputs can yield nonphysical values because of noise, leakage, or assumption mismatch. Enable clamping to keep α within [0,1] for quick comparisons, then investigate the measurement chain for formal reporting.
2) Should I use one frequency or several?
Use several frequencies whenever possible. Most absorbers are not flat with frequency, so a single number can mislead. Run octave-band centers and export a small table to see the trend.
3) What is the difference between NRC and α?
α is frequency specific. NRC is a single-number rating derived from selected mid-band absorption values. NRC is useful for quick comparison, but design decisions should use band results.
4) When is the Sabine method appropriate?
Sabine works best for room-average estimates when the sound field is reasonably diffuse and you have reliable T60, volume, and surface area. It is less reliable in tiny rooms, very absorptive spaces, or strongly directional layouts.
5) Why does the impedance method use complex Zs?
Real boundaries both dissipate and store energy. The real part models resistive loss, and the imaginary part models reactance. Together they set the reflection magnitude, which determines α.
6) What affects α most in the reflection method?
The magnitude |R| dominates because α depends on |R|². Near |R| = 1, small measurement errors can change α noticeably. Keep Li and Lr measurement geometry consistent.
7) What do the CSV and PDF exports contain?
Exports include your selected method, units, frequency label, all inputs, intermediate values, and the final α. Use them for reports, audit trails, and spreadsheet comparisons without retyping.