Diffusion

Flat wall throws one harsh slap-back; a bumpy diffuser scatters it into many weak directions, same energy, no harsh focus.

What it is

Scattering sound off a bumpy surface into many weak directions instead of one harsh straight-back echo.

Key facts

• Speed of sound in air = 343 m/s at 20 degrees C (rises ~0.6 m/s per +1 degree C).
• Diffusion SPREADS reflected energy; absorption REMOVES it. A diffuser keeps energy in the room but de-focuses it.
• Wavelength lambda (m) = c (343 m/s) / frequency f (Hz). 100 Hz = 3.43 m; 1 kHz = 0.34 m; 10 kHz = 0.034 m.
• A surface only scatters wavelengths near its bump size; low frequencies (long lambda) bend around small bumps and pass straight back.
• Scattering coefficient (s): 0 = pure mirror reflection, 1 = energy spread fully in all directions (ISO 17497-1).
• Diffusion coefficient (d): how UNIFORM the scattering is across angles, 0-1 (AES-4id / ISO 17497-2). Different metric to s.
• Schroeder QRD diffuser well depths follow a quadratic-residue sequence n^2 mod p (p = a prime); deepest well approx = lambda/2 of lowest target frequency.
• +6 dB SPL = doubling of pressure (x2 voltage); +3 dB = doubling of power (x2 watts); -3 dB = half power.
• Slap-back/flutter echo becomes an audible discrete echo once delay exceeds ~30-50 ms (Haas: reflections under ~5-35 ms fuse with direct sound).
• Effective diffuser band roughly 250 Hz to 5 kHz; below ~250 Hz use bass traps (absorption), best placed at first-reflection points and rear wall.

How it works

1. Sound wave hits a hard FLAT wall and bounces back as one strong mirror-image reflection (a slap-back echo).
2. Swap to a BUMPY/uneven surface whose bump depth is near the sound's wavelength.
3. Each part of the surface re-radiates the wave at a different angle and a slightly different time.
4. The one strong reflection is broken into many weak ones spread across many directions.
5. Total energy stays in the room, so it stays lively, but no single harsh echo or harsh focus remains.
6. Result: smoother decay, no flutter, clearer music and speech without a dead room.

Real examples

• A QRD (Schroeder) diffuser panel of wells at different depths behind a drum kit or on the rear wall.
• Skyline / 2D diffuser: a grid of pillars at random heights spreading sound both horizontally and vertically.
• A brick or stone feature wall, bookshelf, or coffered ceiling acting as a cheap natural diffuser.
• Curved / polycylindrical panels that scatter mids and highs across the room.
• Empty hard squash-court or bare hallway = pure flat reflection = nasty flutter echo (the opposite of diffusion).

How it helps in live sound

• Hang diffusion on the rear wall of a hard room (gym/hall) to kill flutter echo without making the room dead and lifeless.
• Treat first-reflection points: absorption at side walls for clarity, diffusion at the rear to keep ambience.
• Diffusion only scatters mids/highs (~250 Hz-5 kHz); for boomy low end use bass traps (absorption), not diffusers.
• Use diffusion to tame vocal harshness and slap-back off a back wall behind the audience or band.
• Bookshelves, rigging, bunched drapes and rough walls already diffuse; don't over-treat a room that already scatters.
• Slap test: clap once. A fast metallic 'zinging' flutter = parallel hard walls needing diffusion or absorption.