Scattering

Flat wall fires back one hard echo; a bumpy diffuser splits the same energy into many soft, even reflections.

What it is

Sound hitting a bumpy, textured surface gets broken up and spread evenly in many directions instead of bouncing back as one sharp echo.

Key facts

• Speed of sound in air = 343 m/s at 20 degrees C (about 1235 km/h); rises ~0.6 m/s per +1 degree C
• Scattering needs surface bumps roughly the same size as the wavelength: depth/spacing in the ballpark of the wavelength to be diffused
• Wavelength formula: wavelength (m) = speed of sound (343 m/s) / frequency (Hz)
• 100 Hz = 3.43 m wavelength; 1 kHz = 0.343 m (34 cm); 10 kHz = 0.034 m (3.4 cm) - bigger bumps scatter lower notes
• A surface acts as a flat MIRROR (specular reflection) when its bumps are much smaller than the wavelength: angle in = angle out
• Scattering Coefficient (s): fraction of reflected energy sent OFF the specular angle, scale 0 (pure mirror) to 1 (fully scattered) - ISO 17497-1
• Diffusion Coefficient (d): how UNIFORM the scattered energy spread is, scale 0 to 1 - ISO 17497-2; NOT the same as scattering coefficient
• Schroeder diffuser well depths set the low cutoff: max well depth d gives lowest diffused freq f = c / (2 x d) roughly
• QRD (Quadratic Residue Diffuser) wells follow n squared mod p, where p is a prime (e.g. 7, 11, 13, 17)
• Scattering REDIRECTS energy, it does NOT absorb it - sound level barely drops, unlike foam which converts it to heat

How it works

1. A wavefront hits a rough surface whose bumps are around one wavelength in size.
2. Different parts of the wave hit ledges at different depths, so each reflects with a slightly different timing (phase).
3. Those out-of-step mini-reflections head off at many angles instead of one.
4. Energy that would have been one strong echo is spread thin across the room.
5. Low frequencies (long wavelengths) need deep, wide diffusers; high frequencies scatter off small bumps.
6. Result: reflections are softened and even, but the energy stays in the room (no deadening).

Real examples

• A bookshelf crammed with random-depth books is a natural broadband diffuser on a rear wall.
• Schroeder/QRD diffuser panels behind a stage to kill flutter echo without deadening the room.
• A brick wall scatters highs but mirrors bass, because mortar bumps are tiny vs a 3 m bass wavelength.
• Rock-up venue with a flat back wall: a slap-back echo you fix with diffusion, not just foam.
• Stage-set scenery, trussing and gear that randomly break up reflections at a gig.

How it helps in live sound

• Hard slap-back off a flat rear/back wall? Hang or stand diffusers there, not just absorption - keeps the room alive.
• Put ABSORPTION at first reflection points (side walls/ceiling) for intelligibility; use DIFFUSION on the rear wall to tame echo without going dead.
• Diffusion only works above its rated cutoff (often ~300-500 Hz) - it does NOTHING for boomy bass, treat low end with bass traps.
• A book/record shelf or scenery flat is a free, gig-friendly diffuser for a boxy room.
• Diffusers don't lower SPL much - if a room is too loud/reverberant overall, you still need real absorption.
• Aim for even decay, not a dead room - over-foaming makes vocals sound lifeless; scatter to keep natural ambience.