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.
What is Absorption?
Absorption is a process of removing sound energy from the room. While a diffuser reflects sound waves to different directions, an acoustic absorbent absorbs incident sound waves and reflects only a percentage of the incident sound waves. Depending on the degree of absorption, a highly absorbent material (such as rigid fiber glass) will only reflect very little but absorbs a lot of sound (turning the sound energy to heat energy when they hit the absorbent material). There are 3 factors affecting the quality of absorption:
a.) Absorption coefficient (frequency dependent)
b.) Thickness and porosity of the material
c.) Air gap
Imagine the cross section of the particular absorbent material below with air gap and placed in the highly reflective wall:
How sound absorption works
As you have noticed; the red arrows are the sound waves. At first, a strong unabsorbed sound wave originating from the source (nearfield monitor for example) will pass through the absorbent material of thickness “T”. When they pass the absorbent material, the sound will be absorbed. How good it will absorb depends on the material absorption coefficient which also depends on the frequency of the sound. A fraction of the sound will still pass through the material because of its porous characteristics. The reduced sound energy will then travel through the air gap and reflected by the wall (this is your untreated wall, concrete and painted wall for example). The reflected sound will then pass again the absorbent material, gets absorb again and then finally only a tiny percentage of the sound will finally bounce back to the source. If you are monitoring at low volumes during your mixing/mastering session, then nothing will be reflected back to the source and this is the advantage of monitoring at low volumes (aside from preserving your ear).
If the absorbent material is thick (2 inches for example), it will have an ability to strongly absorb both bass, mid and high frequencies. However if it’s thin, it will only effectively absorbs middle to high frequencies. If the material is not porous, it will not allow sound to pass through but instead it will be reflected back to the source which is not desirable.
Final Recommendations and Best practices
Knowing how acoustic diffuser and absorber works, it is not difficult to find out when they should be used in your home studio:
1.) In studio control rooms (where the mixing and mastering are performed), you should absorb reflected and standing waves as much as possible because this will clutter throughout the room and affects the frequency characteristics being monitored. Therefore, it makes sense to apply MORE absorbent material in the control rooms to reduce this reflected sound energy as much as possible. In fact in small control rooms, diffuser is not entirely needed because there is no effective reverberation and you need to treat your room entirely with acoustic absorbent material. Since absorption works by removing sound energy (reflected sound waves), what you are hearing from your nearfield monitors are EXACT direct sound waves so you will be able to make accurate judgement.
2.) In recording and live rooms, musicians play together. A good example is the environment to record drums, guitar amp/cabinets, etc. Natural reverberation is critical in this recording environment as it will need to be captured by the microphones very well. Therefore you should treat the room with acoustic diffuser to randomly reflect the sound waves to different directions creating a natural room response.
3.) However when recording vocals, the objective is to record it as dry as possible (without effects or coloration from the recording environment). It is therefore crucial to treat the vocal recording environment with highly sound absorbent material. In this way, the sound wave that reaches the microphone is coming directly from the vocalist mouth and not the reflections coming from the room.
4.) In bigger control rooms, the room introduces reverberation due to the size of the room. Aside from implementing absorption to minimize unwanted reflections, you should also consider treating the room with diffusion materials to create a more natural reverberating environment. But in most control room environment (particularly in home studio which is often small), treating it with absorbent material is the ultimate priority.
Content last updated on August 7, 2012