April 30, 2011

Diffuser panels: why you need them and how to make your own

An acoustic diffuser is generally more difficult to design and build, hence more expensive. They are also a little more difficult to integrate into a room. In the average room they are a rarity, but are often seen in high end rooms and recording studios. Do you really need one in your room?

Diffusion vs Absorption

An acoustic absorber simply absorbs sound waves and such a panel can be placed anywhere it is needed to reduce the overall room reverberation and address the most damaging reflections. In many domestic rooms, a point is quickly reached where the room begins to sound dead. Diffusers can be used to address problemmatic reflections without absorbing their energy. Instead, an incident sound wave is dispersed equally in all directions. This has two effects; the first is that a strong first reflection is reduced and the second is that the energy is redirected so that it remains in the room but the sound arrives later and at a lower level, having travelled a longer path. A further benefit is that a small room actually sounds bigger when treated with diffusion, while a room treated only with absorption tends to sound more controlled until it becomes dead.

Placement considerations

Absorbers can be placed anywhere in a room, but diffusers are subject to placement rules.

1. Minimum listening distance

Diffusers become ineffective at close range. The minimum distance depends on the specific panel. Often 1m or more will be required, but where a listening position is close to a wall behind, a low profile diffuser may allow a distance of 0.5m.

2. Diffusers have more impact when the path of relatively early reflections

If sound waves have reflected off multiple boundaries before reaching the panel, their impact will be reduced.

3. Diffusers have a limited effective bandwidth

Based on their geometry, an upper and lower limit is established. One typical example would be a 150mm deep diffuser with an effective bandwidth of 1 - 7 kHz.  One octave below it becomes progressively less effective and below about 500 Hz, sound waves will behave as if reaching a flat wall.

4. Don't overdo it!

There is such a thing as too much diffusion, and it's likely to create imaging confusion!

Types of diffusers

Most diffusers are either 1D or 2D. A common example is the QRD (Quadratic Residue Diffuser). It is popular because it is one of the simplest. A 1D QRD is the very simplest, and works in one direction.

In this example, an RPG unit is shown. Diffusion occurs horizontally. If rotated 90 degrees, it would work vertically.

A 2D QRD works in both directions.

Quite similar is the Skyline diffuser, which is essentially the same thing without the grid.

Confusion over diffusion!

There are some panels that are confused with diffusers, when in fact they could be best described as scatterers. Panels that feature curves or small pyramids are not true diffusers. Curved panels simply create more specular reflections. They are like a curved mirror, while a true diffuser is more like a matte painted wall.

The unfortunate aspect of this is that panels that aim to look more artistic will tend to compromise their performance. Repeating panels and designs based on equations yield the best results. 

Mixed panels

Some products include absorption as well as diffusion. Real Traps diffusors are filled with rigid fibreglass so that they transition to absorption below their effective diffusion range. This means they add a small amount of bass trapping, so if enough is used, the bass performance will also improve. Other products also aim to absorb over a wider bandwidth.

Do you want to design or build your own?

An acoustic diffuser is an ideal DIY project. They are easy to design. Design your own >

More about Diffusers >


  1. It's good to know how to make the diffuser panel ourselves as the price of buying from some where else.Thanks so much for your detailed explanation.

  2. So from my understanding we can build these from simple products purchased at our local hardware store. Are there any hidden difficulties when it comes to the construction of our homemade diffuser?

  3. Be prepared for it to be tedious. Consider a fast setting glue and consider how you are going to hold them in place. Think about weight and how you will fix the finished panel in place.

  4. Paul, do you know if there are any available on the market like the RPG skyline but not made of the cheap looking polystrene??


  5. I've talked about this with a local manufacturer and hope to bring some to the market.

  6. For some good tested DIY diffuser designs, see


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