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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.
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Works Cited:
“DIY Sound Diffusers FAQ.” Arqen.com, 14 Dec. 2013, arqen.com/sounddiffusers/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, McGrawHill, New York, 2015.
Philip Richard Newell. Recording Studio Design. New York ; London, Routledge, Taylor & Francis Group, 2017.
Books I Recommend (Affiliate Links)
Home Recording Studio: Build It Like The Pros by Rod Gervais: https://amzn.to/48ONVF6
Home Recording Studio Design by Philip Newell https://amzn.to/4b7zyxd
Master Handbook Of Acoustics by F. Alton Everest and Ken Pohlmann: https://amzn.to/3Olwio7
0:00 Intro
1:03 What is a small room?
2:42 What will you use the room for?
4:36 Problems with small rooms
4:43 Distance from the diffuser
8:50 Small rooms have low end problems
10:52 LEDE issues
14:08 Conclusion