It is a number telling you how much of a sound bounces back when it hits a surface instead of passing through or being absorbed.
Same incoming sound, two surfaces: a hard wall reflects ~97% (R approx 0.97, echoey) while a drape reflects ~20% (R approx 0.2) and turns the rest to heat.
What it is
A number from 0 to 1 telling you what fraction of sound energy a surface bounces back instead of absorbing.
Key facts
Reflection coefficient R = reflected sound energy / incident (incoming) sound energy.
R ranges 0 to 1: R=0 means total absorption, R=1 means total reflection (perfect mirror).
Energy balance: R + A + T = 1 (A = absorption coefficient, T = transmission coefficient).
Absorption coefficient alpha = 1 minus R for a non-transmitting wall; tables list alpha, not R.
Brick/concrete/glass: alpha approx 0.01-0.05, so R approx 0.95-0.99 (reflects nearly all).
Heavy curtain/thick foam: alpha approx 0.6-0.9, so R approx 0.1-0.4 (reflects little). Open window: alpha=1, R=0.
Speed of sound approx 343 m/s at 20 C; ~30 ms delay (~10 m extra path) = audible slap-back echo.
Doubling power = +3 dB; doubling distance in free field = -6 dB (inverse square law).
Wavelength = speed / frequency: 100 Hz = 3.43 m, 1 kHz = 0.343 m, 10 kHz = 0.034 m.
Sabine reverb: RT60 = 0.161 x V / A (V = room volume m3, A = total absorption = sum of alpha x area).
How it works
Sound wave hits a surface carrying incident energy.
Part passes through (transmission T), part turns to heat (absorption A).
The leftover energy bounces back as the reflected wave; R = that fraction = 1 minus alpha.
Hard, dense, smooth surfaces store little and reflect most (high R).
Soft, porous, fluffy surfaces convert sound to heat and reflect little (low R).
Many reflections stacking up build reverb, slap-back and flutter echo.
Real examples
Bare gym with concrete + glass (R approx 0.97): wash of echo, vocals turn to mush.
Carpeted lounge with sofas and curtains (R approx 0.2): dead, intimate, easy to mix.
Two parallel tiled walls in a corridor: high-R both sides = audible flutter echo zing.
Stage back wall 10 m behind a singer: reflection returns approx 60 ms late = obvious slap-back.
Hanging heavy drapes (R drops from approx 0.95 to approx 0.3) instantly tames a boomy hall.
How it helps in live sound
Treat high-R parallel surfaces (glass, tile, render) with drapes/banners to kill flutter and ringing.
Aim mains so first hard reflection point gets carpet, bodies or absorption; bodies cut R a lot.
Angle speakers down/in so energy hits absorptive crowd, not bare reflective back walls.
Use RT60 = 0.161 V / A to estimate how much absorption (low-R material) you must add.
Pull mics away from hard reflective boundaries to dodge comb filtering from the bounce.
Outdoors R is near 0 (no walls): no reverb help, bring more PA and watch low-end loss with distance.
Everyday analogy
Like throwing a tennis ball: a brick wall fires almost all of it straight back (R near 1), a thick curtain swallows it and barely returns a dribble (R near 0).
Watch out
Myth: a hard surface absorbs the sound. Truth: hard, dense surfaces have HIGH reflection (R near 1) and absorb almost nothing; soft porous stuff is what actually absorbs.
Fun fact
An open window is the perfect sound absorber (alpha = 1.0, R = 0) because the sound just escapes and never comes back, which is why acousticians measure absorption in sabins, literally square-metres-of-open-window equivalent.
Key takeaways
R = fraction of sound energy bounced back; 0 = all absorbed, 1 = all reflected.
R + A + T = 1; for a solid wall R = 1 minus alpha (the absorption coefficient).
Hard/dense = high R (echoey, lively); soft/porous = low R (dead, controlled).
High R on parallel walls breeds flutter echo and slap-back; treat with absorption.
Speed of sound 343 m/s sets your delay times: approx 30 ms = audible slap-back.