I wrote a short piece trying to explain diffusers. I hope it will be of use to the members. I am a retired acoustician with masters degrees on both acoustics and electronics.
How does sound diffusers work and how to use them
“Good sound” isn’t just the quality of the music or the sound mixing of a track. “Good sound” most often is the quality of your listening room. Soundproofing a room will not create a good sound, nor damping the reflections will. Often what is missing is a well-placed acoustic diffuser.
During my long life I have experienced that the concept of sound diffusion is often harder for a layperson to understand than absorption. However, we must strive to create an experience that rich and full instead of flat and sterile. The enjoyment of music and the clarity of voice can only happen in a live room that has a wide and smooth reverberation. That can only be achieved with using diffusers along with absorbers.
What Is a diffuser?
To understand what a diffuser does, you need to first understand a few principles of the physics of sound. Sound waves behave like light. When you shine a light at a light reflective surface, the light reflects at you. Sound waves act like light. However unlike light, every hard, flat surface can serve as a mirror for sound.
If we continue in the light analogy, an absorber is like a black wall, very little light is reflected from it. Whereas a diffuser is like a surface where small bits of broken glass of different colours scattered on it. They don’t block the light; they reflect them at so many angles that the reflected colour is white.
Sound absorption panels are made of soft materials with lots of air pockets. Those prevent sound waves from bouncing back at you. Whereas a sound diffuser allows for the sound to reflect—but it breaks up the reflection to many angles so that you do not hear a distinct echo. By not deadening the sound they create a pleasant sounding room.
Diffuser types
Diffraction in acoustics involves a lot of nuances. What type of diffuser you need depends on your needs and the room. The industry has given names to various diffusers: 1D, Quadratic (QRD), Skyline, etc. What does any of that mean?
The so called 1D diffusers are panels whose names seems to be derived from the number of planes on which sound is diffused. They are half-cylinders and diffuse sound waves left to right if they are positioned vertically and up and down if they are positioned horizontally. They are not very effective. Your money and space is better utilised on a QRD or Skyline diffuser.
Quadratic (QRD) diffusers are designed to diffuse sounds at different frequencies. They will often use several vertical slots that are calibrated for a specific frequency range. This will allow diffusion of sounds at a wide range of frequencies in both ends of the spectrum.
Skyline diffusers works at multiple angles. It scatters sound both horizontally and vertically. They are the best choice if you have the space. They are called a skyline diffusers because from the side they look like a city skyline. They are formed with (often) wooden columns of different lengths glued together. To build a skyline diffuser, it’s important to use a special diffuser calculator to aid you in calculating the lengths of wood required.
Where should I place the diffusers?
To understand where to place a sound diffuser for maximum efficacy, you must know what you want out of your listening experience. Are you setting up a home theatre? Are you mixing music and create high-quality masters? Are you an audiophile that wants to achieve the best sound possible in their room? Your purpose is the major factor to decide the placement of diffusers. Another factor is the speaker placement and where your “sweet spot” is located. However, for a good sounding room the decision is not the choice between sound diffusion and absorption. Both are important for a pristine listening experience.
You must place diffusers at the primary reflective points. That is where the sound from your speakers will first hit where the sound will reflect -- a wall or surface. When you have neither diffusion nor absorption, those reflections will produce reverberations and echoes, which will diminish the listening experience. In a home theatre most of the sound will come from the front, so having diffusion along the rear wall is a must. However, using 1D diffusers along the side walls to correspond with side speakers is also wise.
The limits of diffusers
Low frequency sub-bass sound waves are difficult to deal with if diffusers are the only thing you’re working with. Bass frequencies have very long wavelengths and most materials just aren’t capable to deal with them. To manage bass in a room special absorbers must be used. They are often called bass-traps and should be placed in the corners of your listening room. They work in conjunction with your diffuser and absorber set. That way you can control both the high-end and low-end sound of the room.
Employ an expert*
Recognising that sound diffusers may improve the quality of sound in your room is only the first step. There are lots of aspects involved in room acoustics and even experienced do-it-yourself'ers may need help. An expert will help you not only show you but will also help you to understanding how to best use a sound diffuser in your room, whether to use a diffuser/absorber combo, how to place skyline diffusers, etc.
*This is not an ad for my services. I’m retired. However, I’m happy to help ASR members within the limits of a forum communications.
Note: I’ve edited the diffuser types section to clarify what 1D diffusers are.
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