Maximum effect diffusers

Maximum Sequence Diffusers (MLS)

Maximal sequence diffusers , often also called MLS diffusers , use mathematical principles of number theory to distribute sound waves evenly in space. The basis is a special sequence of the numbers $+1$ and $-1$ , known as the maximal sequence .

The mathematical secret: White spectrum

What makes these consequences so valuable for acoustics is the property of their Fourier transform: they possess a white spectrum .

Audio technology: A corresponding signal sounds like white noise.
Architectural acoustics: A surface built according to this pattern scatters incident sound almost perfectly evenly in all directions.

Structure and construction

A classic MLS diffuser consists of a sequence of depressions and elevations (two different depths).

Separation plates: Thin, solid partitions should be installed between strips of different depths. These prevent pressure equalization movements and improve dispersion in the case of oblique sound incidence.
The optimal depth: The maximum scattering effect is achieved at the frequency whose quarter wavelength ( $λ/4$ ) exactly corresponds to the depth of the depressions.
Fringe width: In order for the scattering to work precisely, the width of a single fringe should be a maximum of half a wavelength ( $λ/2$ ).

Advantages and disadvantages

Although MLS diffusers theoretically provide perfect dispersion, they have a limitation in practice:

Bandwidth: MLS diffusers are relatively narrowband. Their effective range usually only extends over about one octave . For this reason, they are often supplemented or replaced today by wider-band designs (such as QRD diffusers).

From line to surface: 2D diffusers

While a strip diffuser scatters sound only in one plane perpendicular to the strips, two-dimensional diffusers allow for hemispherical scattering .

This is achieved by mathematically multiplying two independent maximum sequences. The result is a checkerboard-like pattern of square fields with two different heights, which effectively distributes sound both horizontally and vertically.