A diffuser scatters sound in many directions instead of absorbing it. It keeps a room lively and spacious while breaking up strong, distinct echoes.
Diffusion basics
Beginner
What diffusers do
- They spread reflected sound out in time and direction, so no single reflection stands out.
- They remove less energy than absorbers, so the room doesn’t sound dead.
- They help prevent flutter echo, the zingy ringing between two parallel hard walls.
Common types
| Type | Looks like | Scatters |
|---|---|---|
| Quadratic residue (QRD) | Rows of wells of different depths | Mainly in one plane |
| Skyline (primitive root 2D) | A grid of blocks of different heights | In all directions |
Don’t sit too close
A diffuser needs distance for the scattered sound to blend. A common recommendation, attributed to diffuser designers Cox and D’Antonio, is to sit at least three times the longest wavelength it diffuses. That’s about 2 m for a diffuser working down to 500 Hz, or 1 m for one working down to 1 kHz.
Where they go
Diffusers are often used on the rear wall behind the seats and on parts of the ceiling. Dolby recommends a mix of sound-absorbing and sound-diffusing treatment in rooms with overhead speakers.
Diffuser behaviour is well established, but the distance rule comes from a secondary source and practice varies.
Diffusion in more depth
Advanced
Well depth and frequency
For a QRD, each well depth is set by the sequence n² mod N, where N is a prime number, and the design wavelength. The deepest well sets the lowest frequency it scatters well. For N = 7, a design frequency of 500 Hz needs a deepest well of about 196 mm. The well width sets the upper limit, at about half a wavelength.
Side walls: absorb, diffuse or leave?
Floyd Toole’s research found that reflections arriving from the sides are generally beneficial in small rooms. He says you’re free to redirect them with diffusers or absorb them, as long as whatever you use works evenly across the range. Removing only the treble with thin panels changes their tone and is worse than leaving them. There’s no single right answer, and preference varies.
Sources
- Diffusion (acoustics) (overview)
- Ballestero et al. (ICA 2019): Quadratic residue diffuser design
- Subwoofer Builder: QRD diffuser design (citing Cox and D’Antonio)
- Dolby Atmos Home Theater Installation Guidelines
- EBU Tech 3276: Listening conditions for the assessment of sound programme material
- Toole (2006): Loudspeakers and rooms for sound reproduction, a scientific review, JAES