MLS Diffuser Design Guide

Education · 11 min read · ResonAia Editorial

Explore MLS diffusers. Binary pattern acoustic treatment with simpler construction than QRD. Learn the math, design considerations, and when to choose MLS.

What is an MLS Diffuser?

A Maximum Length Sequence (MLS) diffuser uses a binary pattern (wells at only two depths) to scatter sound. Unlike QRD diffusers with their multiple well depths, MLS designs have just "on" and "off" states, making them simpler to build while maintaining effective acoustic performance.

The MLS pattern comes from pseudorandom binary sequences used in signal processing and communications. These sequences have special correlation properties that translate into broad, even scattering when used as diffuser patterns.

The Mathematics of MLS

Generating the Sequence

MLS sequences are generated using Linear Feedback Shift Registers (LFSRs). For a register of length n, the sequence length is 2^n - 1:

n=3: 7 elements
n=4: 15 elements
n=5: 31 elements
n=6: 63 elements

The feedback taps determine which register outputs combine (XOR) to produce the input. Different tap configurations produce different sequences, all with the same length.

Example: 7-Element MLS (n=3)

Using taps at positions 3 and 2 (notation: [3,2]):

Starting state [1,0,0]:

[1,0,0] → output 0
[1,1,0] → output 0
[1,1,1] → output 1
[0,1,1] → output 1
[1,0,1] → output 1
[0,1,0] → output 0
[0,0,1] → output 1

Sequence: 0, 0, 1, 1, 1, 0, 1 (then repeats)

Translation to Well Depths

0 in sequence → shallow depth (often zero or minimal)
1 in sequence → full design depth

Full depth is typically λ/4 at the design frequency, where λ is wavelength.

Design Parameters

Sequence Length

Choose based on panel width and desired well width:

Sequence	Elements	Typical Panel Width
n=4	15	12-18" (300-450mm)
n=5	31	24-36" (600-900mm)
n=6	63	48-72" (1200-1800mm)

Longer sequences provide better low-frequency performance but require larger panels.

Well Depth

For design frequency f₀:

Full depth = c / (4 × f₀)

Where c = speed of sound (~343 m/s)

For 1000 Hz: depth = 343 / (4 × 1000) = 0.086 m = 86 mm

Well Width

Similar considerations to QRD:

25-50mm for extended high-frequency response
50-75mm for easier construction
All wells same width in standard MLS design

MLS vs QRD: When to Choose MLS

Advantages of MLS

Simpler Construction Only two well depths means you can build MLS diffusers without CNC. Cut blocks to two heights and arrange in pattern.

Thinner Profile Maximum depth is λ/4 versus λ/2 for QRD at the same frequency. An MLS targeting 500 Hz is half the depth of an equivalent QRD.

Good Scattering Performance At and above design frequency, MLS provides effective, relatively uniform scattering.

Disadvantages of MLS

Narrower Effective Bandwidth QRD works well over approximately 3 octaves. MLS effective range is typically 1.5-2 octaves.

Less Uniform Scattering at Low Frequencies Below design frequency, performance degrades faster than QRD.

Binary Limitation Having only two levels limits some acoustic optimization possibilities.

Decision Guide

Choose MLS when:

You're building by hand (no CNC)
Space is limited (need thinner panels)
Primary concern is mid-high frequencies (500+ Hz)
Budget is constrained

Choose QRD when:

CNC is available
Maximum bandwidth is important
You need strong low-frequency scattering
Panel depth isn't constrained

Construction Methods

Block Assembly (No CNC)

Cut blocks: Two heights: zero (backing only) and full depth
Prepare backing: Plywood or MDF panel
Apply blocks: Follow MLS pattern, glue to backing
Finish: Sand, paint, or wrap

This method produces professional results without CNC equipment.

CNC Routing

Generate pattern: Use our MLS designer preview to calculate the sequence and well depths
Generate G-code: Bring the calculated dimensions into your CAM software for a two-depth pocketing operation (direct export from the designer is coming soon via early access)
Machine: Standard pocketing CAM strategy
Finish: Sand and apply finish

Faster than block assembly but requires CNC access.

Panel Construction

For larger installations, MLS patterns can be created at the panel level by alternating full panels of absorption and reflection. This creates architectural-scale diffusion for large spaces.

2D MLS Patterns

Like QRD, MLS can be extended to two dimensions for hemispherical scattering:

Method 1: Product array Multiply 1D sequences to create 2D grid. Simple but creates some pattern regularity.

Method 2: Folded sequence Arrange 1D sequence in 2D using prime-length rows. Better randomness properties.

2D MLS creates skyline-like visual patterns that scatter sound both horizontally and vertically.

Common Applications

Control Room Rear Wall

Standard application. MLS scatters rear reflections while maintaining room energy. Thinner profile is often advantageous in smaller studios.

Ceiling Treatment

Where depth is constrained (suspended ceilings), MLS provides diffusion in less space than QRD alternatives.

Live Room Boundary Treatment

Alternating MLS panels create variable acoustic environments without making the room dead.

Broadcast Studios

Voice frequency focus (300-3000 Hz) aligns well with MLS effective range.

Verification

After installation, verify with measurement:

Impulse response should show reduced discrete reflections
Frequency response should be smoother at diffuser frequencies
Room should feel more spacious without increased reverb

If specific reflection problems persist, consider:

Additional panel coverage
Repositioning existing panels
Supplementing with targeted absorption

Design Resources

Our MLS diffuser designer preview:

Generates sequences for any valid length
Calculates dimensions for your frequency target
Visualizes 1D and 2D patterns

Join early access for build-file and G-code export for CNC or block-assembly methods.

MLS, Diffuser Design, Binary Sequence, Acoustics

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