So you start reading the topic of room acoustics to properly treat your room with an objective of having an even and flat response. You then encounter two technical terms: “Absorption” and “Diffusion”. So you search for more information but it seems not clear. Finally you asked this question: “Which is more important; Absorption or Diffusion in my home studio acoustic treatment?” If you already know absorption and diffusion very well, go to the “Final Recommendations and Best practices” section below.
The answer to this question requires some deep thought and it depends on the room size, your intended application and existing room acoustics. Let’s discuss what is diffusion and absorption so that you will know when to use them.
What is Diffusion?
Let’s start by understanding these two terms separately. Let us start with diffusion. The easiest way to understand a sound diffusion process is to “randomly” scatter sound waves throughout the rooms. In an untreated room, you have a reflective wall that is perfectly perpendicular to your floors. According to the laws of Physics, the law of incident is equal to the law of reflection. When an incident sound wave at 0 degrees (horizontal to the floor) will strike at the wall surface; it will bounce in the same direction (still at 0 degrees but in opposite direction). This can cause standing waves particularly if you have parallel walls and floors in your room. The opposing effect will result to cancellation of sound waves at some point in the room making the response uneven. On the other hand if the incident wave is 45 degrees, the angle of reflection is also 45 degrees. See figure below:
Angle of incidence is equal to angle of reflection
As you might suspect, having a highly reflective flat surface can introduce unnatural echoes and cancellation of sound in other areas of the room. By using a diffuser (the material that enhance diffusion); you can randomly reflect the sound waves throughout the room thus making the reverberations and room acoustic response natural. This is because the angle of incidence is not equal to the angle of reflection. Below is an example of the acoustic diffuser:
Barrel diffuser
Image credits: Sound Seal
As you have observed, the wall is the not anymore perfectly flat and there are curvatures that you see. These are called barrel type diffuser. There are so many types of diffuser and discussing each one of them is beyond the scope of this post but the concept is still the same. In the above screenshot, since the wall is not flat but curved, incident sound waves will be reflected in different directions not equal to the angle of incidence.
When you analyzed very carefully, diffusion does NOT remove sound from the room but it will simply reflect them randomly in different directions creating a more naturally reverberating environment.