Should You Use Diffusion or Absorption at First Reflection Points?

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You’ve carefully considered your room, chosen your gear, and have set up your speakers to the correct millimeter but still find your mixes lack clarity and precision – this is where acoustic treatment is necessary to create a professional mixing or recording space.

By treating the acoustics in your home studio, the overall quality of sound in your room will increase considerably.

Direct Sound and Reverberation

Sound in a room consists of two components, the direct sound, and its reverberation. Reverberation is made up of reflections that occur when sound waves meet an obstruction and bounce back to the listener at a slight delay. If left untreated, these reflections can cause serious problems for you and your mixes such as comb filtering and flutter or slap echo – this is the main issue we’re looking to address in acoustic treatment.

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A recording studio requires some reflections, so the playback reproduces the spatial information from the room to give ambiance to your recording. A mixing studio should sound neutral and be mostly dead to represent this recording as accurately as possible. A common way to achieve this is by creating a reflection-free zone around your listening position leaving only the direct sound with no coloration.

Reflections cause a mix to be colored by the room through reverberation, directional cues, and standing waves affecting how you’ll perceive the frequency content and localization of the mix.

The natural acoustics of your room could affect your decisions when mixing a track and will often translate poorly in different listening environments i.e. your car. My lecturer once explained this as ‘you can take a mix out of a room, but you can’t take a room out of a mix’.

Of course, it is possible to avoid acoustic treatment in a production studio. Headphones can be used to ignore room acoustics entirely; this is a good option when an appropriate room isn’t available however requires good quality headphones and even so may still be difficult to determine how the mix would sound to your audience using speakers.

First Reflection Points

Treating a room involves placing acoustic panels at the first reflection points, these are the boundaries sound collides with directly after leaving your speakers. Commonly the walls on either side, behind and in front of your speakers, the ceiling, and the floor or desk dependent on which sound reaches first. First reflection points can be the biggest factor affecting a room’s sound quality.

Sound propagates spherically in all directions however it may help to envision the sound waves as rays of light. In the classic mirror experiment, whilst sitting in your listening position, ask a friend to move a mirror along a wall – the points which you can see a speaker in the mirror can be marked as the first reflection points. This is the easiest method without using specialized software or lots of complicated math.

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Once the first reflection points have been determined, we arrive at our main question:

Would you use Absorption or Diffusion panels at first reflection points?

The question is debated amongst professionals and can quickly become an overwhelming part of treating your room. Both absorbers and diffusers can decrease echo, localization, and coloration however each takes on a different role to do so.

When sound collides with a surface it can be reflected with little change to the wave characteristics, altered and transmitted as scattered reflections with little intensity or absorbed into the material. This is determined by the acoustic treatment within the room.

What is Absorption?

Absorption panels work to take in sound energy and transform the energy into heat to remove reflections from the room. It’s important to focus on the rate and level of absorption in your room as 100% absorption in a room is generally not ideal.

Materials such as polyurethane foam and glass wool make effective acoustic panels due to their high absorption coefficients – the ratio of absorbed sound to transmitted sound. Absorbed sound doesn’t cause reflections therefore the listener will only hear the direct sound from the speaker. 

The thickness of the panel is also an important factor to create an effective absorber. Generally, the thicker the panel, the higher the absorption coefficient however as space is often an issue in home studios, you should aim to use panels with at least 4-inch depth.

Absorbers are advantageous over diffusers in that less depth is required to treat low frequencies as the speed of sound in porous materials is lower than air. They are most effective at noise control and will help to decrease reverberation and sound level to create a dead room.

Sound can also form high-pressure waves in the corners of a room or where 2 boundaries meet, absorption panels can be placed to soften corners and stop the build-up of low frequencies. 

Some argue absorption alone can be enough to cover your acoustic treatment entirely, particularly in a mixing room where the aim is to create a reflection-free zone at the listening position to reproduce the mix accurately. This is somewhat true and ideal in smaller spaces however using diffusers as a complementary function to absorbers will improve the sound quality.

What is Diffusion?

Unlike absorption panels, sound is reflected back into the room by diffusion panels however is dispersed by the variation of surface depths on the panel. The reflections are now split into many pathways causing each to have a much lower intensity and removing directional cues, so your brain ignores them. However, sound energy is conserved.

Diffusion can be a way to treat a wall producing echo and reduce room coloration without creating a completely dead room. Instead, ambiance and coverage are improved giving a wider stereo image and making the room seem bigger. They can also help stop spill in a recording.

 A good diffuser will disperse reflections evenly in every direction.

Online tutorials often suggest alternative, homemade solutions to diffusers, most commonly using bookcases. Whilst bookcases can provide a nice sounding room, they’re mostly going to absorb sound. The depth variations in diffusers have to be carefully calculated to ensure even propagation and books generally won’t provide this accuracy – This could cause directional cues to be produced by uneven reflections.

Setting up a studio is ultimately an experiment so feel free to test these homemade solutions as you might find some good, cheap solutions especially if you have limited access to professional acoustic panels. One downside to diffusers is that they’re often more expensive than absorbers due to needing more technical manufacturing.

Diffusers are most effective when they’re further away from microphones and speakers hence why they’re commonly found on the back walls of studios. In smaller rooms, artifacts caused by diffusers are more audible and could be picked up in a recording or mistaken as part of the mix during editing.

Diffusion on the sidewalls and ceiling above the listening space will disperse lateral reflections. Lateral reflections play a role in describing the envelope of the sound to the listener. Whilst absorbers will remove this energy entirely, diffusers will maintain the sound’s information.

There are a few different kinds of diffusers to consider, Quadratic Residue Diffusers (QRD) are more tolerant in smaller rooms however curved diffusers can create inner corners where the diffusers meet forcing focused areas of sound. Partially absorbing diffusers or resonant absorbers are a good compromise when space is limited.

The math for diffusion is much more complicated than absorption so it can be more difficult to get diffusion placement right the first time – It may be helpful to use software such as FuzzMeasure or RoomEQ Wizard to test how diffusers affect your room.

As mentioned, depth plays a huge role in the frequencies an acoustic panel is able to treat. Low frequencies have longer wavelengths than high frequencies so require thicker acoustic paneling. Diffusers are usually too shallow to have any effect on low frequencies and are better suited to treating mid and high frequencies.

People often talk about the importance of diffusers from a ‘nice extra’ to ‘critical’ and both are correct in their own rooms. A good rule of thumb is to use less diffusion in smaller rooms.

Using Absorption or Diffusion in your Studio

It is ultimately important to use both absorbers and diffusers within your room if you have access to both. Between the two, your mids and highs will be covered by your diffusers without removing the sound entirely whilst absorbers will keep annoying bass frequencies, making your mixes sound muddy, away.

Gun to your head, you have to pick one?

In a mixing and editing studio, absorbers are cheaper and will solve any acoustic problems your room is facing. In a recording room, they would kill too much of the ambiance and leave your recordings dry so diffusion might be a better option here. 

Acoustic treatment can make a huge difference in recording and mixing studios alike however researching the treatment needed in your studio can become quite confusing because every room is so different. Ultimately no how-to guide or YouTube video will better inform you of the treatment needed in your studio more than testing it for yourself.  

Always trust your ears.

To read more about this topic and to learn about the advantages and disadvantages of studio monitors and acoustic treatment, see our article Do I Need a Pair of Studio Monitors?