WARNING - Diffusion Can Make Home Recording Worse
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Home recording studios are notoriously small in acoustic terms. These small rooms tend to have big problems with the low frequencies and not enough space for proper diffusion.
In this article I will explore why we need to be careful when adding diffusion to our home recording studios.
1) What is a small room?
Before we can jump into the discussion of diffusion we first need to define what a small room is in acoustic terms. Acoustic terms means how sound experiences the room not us. Sound needs much more space to "feel comfortable" in a room and sound pleasant to our ears. This is why professional studio tend to be very large in comparison to home studios.
A small studio defined by F. Alton Everest and Ken C. Pohlmann in "The Master Handbook of Acoustics" states that a small room is 1,000 cubic feet, a medium room is 3,400 cubic feet and a large studio would be 8,000 cubic feet (28.3m3, 96.3m3 and 226.5m3)
A small home recording studio in this example would have an 8 ft ceiling height (2.43m) a width of 10 ft (3m) and a length of 12.32 ft (3.7m). This size is very typical of a home recording studio.
2) What Will The Room Be Used For?
In professional studios there is always a live room and a control room. It is very difficult if not impossible to blend the two perfectly. However, I am a fan of recording in one room and more and more music is made this way. So, the first question you should ask yourself is how you want to use your room. If it is a live room than diffusion could be useful. If it is a mixing room then you probably want to address the low modal frequency problems with absorption before adding diffusion.
If you are recording and mixing in your room than you have to decide if you value a live room sound for recording or a flatter response for mixing and mastering. I personally love an accurate stereo image and a tight sounding drum kit and acoustic instruments. For this reason, I would lean towards making my room ideal for mixing and the recordings would also sound tight and even.
However, if you like a huge drum sound or a big piano sound you may want to give the instrument lots of space. A small room will sound small, but diffusion can make the room sound bigger than it really is.
3) The Problems with Diffusion in Small Rooms
A) Distance From the Diffuser
The main issue with diffusion and specifically quadratic phase grating diffusers is that they need space to fully form a diffuse field. The other problem is that to diffuse at low frequencies the diffusers also need to be very deep, which takes up floor space into your already small room.
Now how much space do you need. Great question and I have heard a few different answers. Some online forums such as the Arqen step diffuser website say that you should use the 3x rule. This means that your listening or recording spot should be three times the distance of the lowest wavelength diffused.
If your diffuser goes down to 300 Hz then the wavelength for 300 Hz is 1,130 (speed of sound) / 300 = 3.77ft. Three times 3.77ft = 11.3ft from your diffuser. In a room that has a 12 foot length that would be impossible.
Well, why do you have to sit so far away? Another great question. Quadratic diffusers and prime root diffusers all scatter energy evenly back in the room, but it takes distance for the scattering to fully diffuse. Before that ideal distance the diffusion will sound out of phase and you will get a comb filtering effect. All that means is what you hear will not be what the sound really is coming from your speakers or instrument. You wouldn't want to listen near in a space that has comb filtering. You may want to record there if you think it sounds good, but it won't sound accurate to the instrument being recording just different.
What about polycylindrical diffusers? These diffusers can work differently from phase grating diffusers like a QRD or Prime Root diffuser. A polycylindrical diffuser will simply scatter energy back in an indirect form so there should not be phase issues. These could be a better option in smaller rooms, however they take up space and would need to be filled with insulation to increase low end absorption.
B) Low Frequency Modal Issues
Smaller rooms suffer from serious low frequency modal issues. This means that the low end from 20Hz - 100 Hz will not sound accurate when listening to your speakers or when recording low frequency instruments like bass, drums, and even voice.
To remedy this issue you need to absorb as much low frequency energy as possible. In a small room that is not soundproof a lot of the low frequency energy may travel through the walls, which is a good thing for your room acoustic, but could be bad for your neighbors or roommates. If you have soundproofing then the low frequencies will be trapped and bounce around your room, worsening the low end problems.
All this is to say that in small room you probably should cover all your walls and ceiling with absorption that is at least 5" thick. A common theme in home studio design is to put diffusion on the back wall. However, the back wall is where some of the worst low frequency issues occur. So if you diffuse down to 300 Hz on the back wall then all the low end reflects right back to your listening spot. Instead I recommend using absorption on the back wall to at least absorb down to 125 Hz. Bass traps in the corners and the upper corners where your wall and ceiling meet will also help absorb the low end in your small room.
To support my argument here is a quote from Philip Newell, one of the most respected and great thinkers on studio design: "In small rooms absorption is the only solution, for whilst diffusion may in some circumstances maintain the stereo imaging over a large 'sweet area,' it nonetheless returns considerable energy to the listening position, and this will colour the sound (Newell, 390).
C) LEDE Myth
You may have heard of a control room design called the Live End Dead End or LEDE for short. This design says that a control room should have a fully absorptive front end and an entirely diffuse back wall. The problem with this philosophy is that for the back wall to work as intended you need to diffuse well into the low frequency range. This requires very deep quadratic diffusers to properly diffuse down to 20-100Hz and even then it is difficult. Small room simply cannot give up enough space for proper low end diffusion.
Conclusion
Small rooms are challenging in many ways and acoustics may be the top issue. My recommendation for home recording studios in the 1,000 - 2,500 cubic foot range is to focus on absorption. Diffusion could be fun to experiment with on the back wall, but they are hard to build and expensive and you may waste money or get sub par acoustics in the end. However, I am not one to discourage experimenting, because at the end of the day it is your studio.
A good point to remember too is that a flat listening monitoring experience is not what you may want. If you do not plan to mix or master your work, diffusion may add life and a joy to the listening experience. In fact listening rooms often use diffusion for this purpose.
However, if you want an accurate response when listening back then diffusion may not be the way to go.
Works Cited:
“DIY Sound Diffusers FAQ.” Arqen.com, 14 Dec. 2013, arqen.com/sound-diffusers/faq/#:~:text=This%20corresponds%20to%20a%20safe. Accessed 6 Mar. 2024.
Everest, Frederick A., and Ken C. Pohlmann. “Acoustics of Small Recording Studios.” Master Handbook of Acoustics, McGraw-Hill, New York, 2015.
Philip Richard Newell. Recording Studio Design. New York ; London, Routledge, Taylor & Francis Group, 2017.
