Absorption vs. Diffusion in Home Studio Acoustic Room Treatment

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

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