Sound Absorption Coefficient Calculator
Calculate the sound absorption coefficient of a material, the total absorption of a room, the RT60 reverberation time using Sabine's formula, or the treatment area needed to hit a target. Switch between four calculation modes, enter your values, and get a detailed breakdown with show-your-work steps.
Formula
Worked example
A 120 m3 room has: a 60 m2 concrete floor (alpha=0.03, giving 1.80 sabins), 80 m2 plaster walls (alpha=0.05, giving 4.00 sabins), and 20 m2 acoustic panels (alpha=0.50, giving 10.00 sabins). Total A = 15.80 sabins. RT60 = 0.161 x 120 / 15.80 = 1.22 s, a moderately live room.
What is the sound absorption coefficient?
The sound absorption coefficient (alpha, written as the Greek letter alpha) is a number between 0 and 1 that describes what fraction of incident sound energy a material absorbs rather than reflects. A value of 0 means the surface is a perfect mirror for sound: all energy bounces back. A value of 1 means the surface is a perfect absorber: no energy is reflected. In practice, most building materials fall between 0.01 (painted concrete) and 0.95 (thick mineral wool). Because absorption varies with frequency, published values are usually given at six standard octave-band centre frequencies: 125, 250, 500, 1000, 2000, and 4000 Hz. The Noise Reduction Coefficient (NRC) is the arithmetic mean of the 250, 500, 1000, and 2000 Hz values, rounded to the nearest 0.05.
How to use each calculation mode
This calculator has four modes. In "Alpha from intensities," enter the incident and absorbed intensities in watts per square metre; the ratio gives alpha directly. In "Alpha from SPL," enter the incident and reflected sound pressure levels in decibels; the calculator converts the dB difference to an energy ratio using the formula alpha = 1 - 10^((Lr-Li)/10). In "RT60 reverberation time (Sabine)," enter the room volume and up to three surface groups, each defined by its area (m2) and alpha value. The Sabine equation RT60 = 0.161 x V / A then predicts how long it takes for sound to decay 60 dB after the source stops. In "Required treatment area," enter a target total absorption and the alpha of the material you plan to use; the calculator tells you how many square metres to install.
Sabine's formula and its limits
The Sabine formula (RT60 = 0.161 x V / A in metric, or 0.049 x V / A in imperial feet) is the classic tool for predicting reverberation time. It assumes the sound field inside the room is perfectly diffuse, meaning energy is uniformly distributed in all directions at every point. This assumption holds well for rectangular rooms without extreme proportions and for rooms where the average absorption coefficient is below about 0.3. At higher absorption levels, the Eyring-Norris formula RT60 = 0.161 x V / (-S x ln(1 - alpha_avg)) gives more accurate results because it accounts for the nonlinear relationship between absorption and decay rate. For rooms with mixed dimensions or strong geometry effects, room-mode analysis or acoustic simulation software is needed for precision design.
Frequency dependence and the NRC rating
Sound absorption is strongly frequency-dependent. Thin, hard materials tend to reflect bass and absorb very little at low frequencies. Porous materials (mineral wool, foam, heavy fabric) absorb well at mid and high frequencies but need to be thicker, or mounted with an air gap behind them, to be effective at bass frequencies. A rule of thumb: maximum absorption starts at a frequency whose wavelength is about four times the material thickness (plus any air gap). So 50 mm of mineral wool reaches peak absorption around 1700 Hz; the same material with a 100 mm air gap is effective down to around 850 Hz. The NRC value summarises mid-frequency performance in a single number and is useful for comparing products, but always check the full frequency table when designing for a specific acoustic target.
Typical sound absorption coefficients at 500 Hz
| Material | Alpha (α) at 500 Hz | Category |
|---|---|---|
| Concrete, unpainted | 0.02 | Hard surface |
| Brick, unpainted | 0.03 | Hard surface |
| Plaster on lath, smooth | 0.05 | Hard surface |
| Hardwood floor | 0.05 | Hard surface |
| Vinyl tile on concrete | 0.05 | Hard surface |
| Plate glass | 0.04 | Hard surface |
| Gypsum board (drywall) | 0.07 | Semi-reflective |
| Carpet on concrete | 0.20 | Soft finish |
| Heavy carpet on pad | 0.35 | Soft finish |
| Drapes, light | 0.11 | Soft finish |
| Drapes, heavy velvet | 0.35 | Soft finish |
| Upholstered chairs (unoccupied) | 0.44 | Furnishing |
| Acoustic ceiling tile, standard | 0.70 | Treatment |
| Mineral wool, 50 mm | 0.65 | Treatment |
| Mineral wool, 100 mm | 0.90 | Treatment |
| Polyurethane foam, flexible | 0.85 | Treatment |
| Fiberglass panel, 25 mm | 0.55 | Treatment |
| Fiberglass panel, 50 mm | 0.80 | Treatment |
| Bass trap, corner-mounted | 0.75 | Treatment |
Mid-frequency (500 Hz) alpha values for common building and treatment materials. Values vary by manufacturer and installation method.
Frequently asked questions
What does an absorption coefficient of 0.75 mean?
It means the material absorbs 75% of the incident sound energy that strikes it at that frequency, and reflects the remaining 25%. A coefficient of 1.0 would mean all energy is absorbed; 0.0 would mean all energy is reflected.
Can alpha be greater than 1?
In theory, no - an alpha above 1 would imply more energy is absorbed than arrived. In practice, measurement results from reverberant chambers occasionally produce values slightly above 1 (for example, 1.05) due to edge-diffraction effects and imperfect test conditions. These values are usually reported as-is and should be treated as approximately 1 when used in calculations.
What is the difference between absorption coefficient and NRC?
The absorption coefficient (alpha) is a frequency-specific value - you need one number for each frequency band of interest (typically 125, 250, 500, 1000, 2000, and 4000 Hz). The Noise Reduction Coefficient (NRC) is the arithmetic average of the alpha values at 250, 500, 1000, and 2000 Hz, rounded to the nearest 0.05. NRC is a convenient single-number summary of mid-frequency performance, but it hides frequency-specific behaviour. For bass-heavy environments, always check the full octave-band data.
How accurate is Sabine's formula for calculating RT60?
Sabine's formula is accurate for rooms with a diffuse sound field and a room-average absorption coefficient below about 0.3. In highly absorptive rooms (average alpha above 0.3) it overestimates the true RT60 because it does not account for the nonlinear saturation of absorption. The Eyring formula handles this more accurately. For small, non-rectangular rooms with strong modal behaviour (below about 300 Hz), neither formula is precise and full acoustic modelling is needed.
What RT60 is optimal for different uses?
Recommended RT60 values depend on the room size and purpose. Speech-focused spaces (classrooms, boardrooms, courtrooms) benefit from 0.4 to 0.8 s to maximise clarity. Recording studios typically target 0.2 to 0.5 s. Home theaters aim for 0.3 to 0.5 s. Multi-purpose halls work well at 0.8 to 1.5 s. Orchestral concert halls are designed for 1.8 to 2.2 s, and large cathedrals may have RT60 values of 4 to 8 s.
What is the formula for finding the required treatment area?
The total absorption added is equal to the treatment area multiplied by its alpha: A_added = S x alpha. So the required area is S = A_needed / alpha, where A_needed is the additional sabins required on top of what the room already has. This calculator computes A_needed as (target total absorption) minus (existing absorption), then divides by the material's alpha to give the area to install.
What unit is a sabin?
A sabin (named after Wallace Clement Sabine, who pioneered room-acoustics science around 1900) is the unit of sound absorption. One metric sabin is the equivalent of one square metre of perfectly absorbing surface. It is a derived quantity: you get the sabins for any surface by multiplying its area in square metres by its absorption coefficient at the frequency of interest. One imperial sabin equals one square foot of perfect absorption.