Absorption Coefficient

Same wave, two surfaces: hard tile (alpha ~0.02) fires the echo back, thick foam (alpha ~0.95) turns it to heat - alpha is the 0-to-1 soak-up score.

What it is

A 0-to-1 score for how much sound energy a material soaks up instead of reflecting it back.

Key facts

• Scale: 0.00 = perfect reflector (eats nothing), 1.00 = perfect absorber (eats everything)
• Symbol is alpha (Greek a); formula: alpha = sound energy absorbed / sound energy hitting the surface
• Glass/painted concrete/tile sit near 0.01-0.05; heavy drape ~0.5-0.7; thick mineral wool or deep foam reach 0.80-0.99
• alpha is frequency-dependent: rated at 125, 250, 500, 1000, 2000, 4000 Hz - bass is the hard one to absorb
• Rule of thumb: a panel only absorbs well above the frequency whose quarter-wavelength equals its thickness
• NRC (Noise Reduction Coefficient) = average of alpha at 250, 500, 1000, 2000 Hz, rounded to 0.05
• Sabine reverb formula: RT60 = 0.161 x V / A, where V = room volume (cubic metres)
• A = total absorption in sabins (m2) = sum of (each surface area x its alpha)
• Sound speed in air = 343 m/s at 20 C; wavelength = 343 / frequency
• Doubling total absorption A roughly halves the reverb time RT60; lab alpha can read above 1.0 (edge diffraction), treat as ~1.0

How it works

1. Sound wave hits a surface carrying acoustic energy
2. Hard, dense, smooth surfaces reflect most of it back into the room (low alpha)
3. Soft, porous, fibrous materials convert sound into tiny heat via friction in the pores
4. Trapped air moving through fibres loses energy, so less bounces back (high alpha)
5. Thicker and deeper material catches lower frequencies (longer wavelengths)
6. Add up area x alpha for every surface to get the room's total absorption A

Real examples

• Glass window: alpha approx 0.03 at 500 Hz - reflective, makes rooms ring
• Heavy stage curtain hung in folds: alpha approx 0.5-0.7 mid-band
• 100 mm rockwool panel: alpha approx 0.95+ across mids and highs
• Painted brick gym wall: alpha approx 0.02 - the classic boomy, echoey nightmare
• Audience in upholstered seats: each person adds roughly 0.4 m2 of absorption

How it helps in live sound

• Boomy/echoey room? Add high-alpha gear: heavy drapes, mineral-wool panels, not thin foam
• Target the back wall first - it sends slap-back echo straight at the mic and stage
• Bass boom needs THICK absorbers (100 mm+) or corner bass traps; thin panels won't touch it
• A full audience is your best absorber - the empty soundcheck always sounds brighter and worse
• Estimate gear needed: pick desired RT60, solve A = 0.161 x V / RT60, then split into panels
• Hang drapes in folds (gathered ~50%) not flat - folds raise the effective alpha a lot