19 A Reflection: Rear Wall Diffusion
Updated: Apr 22, 2021
The decision to build a diffuser into the studio came about from an aesthetic appeal, and a curiosity in their acoustic characteristics, having never really worked in a studio with diffusion before. After doing some research, I found that some of the benefits of diffusion suited my room and its usage. Essentially, diffusers can make a room 'sound' bigger than it actually is, which, in the case of my small basement, would be much appreciated! Aside from this, it can also help 'treat' a room's acoustics without resorting to too much absorption, which can result in a dead-sounding room. Because I want to occasionally record in my studio as well as mix, keeping a slightly livelier acoustic environment would be preferable. So how do acoustic diffusers accomplish these results?
How Diffusion Works
If you've heard of diffusion before, you may think of a device that simply changes the reflective direction of a sound wave, and, although this is true, it is also combined with changes in time and intensity of the reflected sound waves. As diffusers work with reflecting rather than absorbing sound energy from a room, they help keep the room from sounding flat and lifeless from over-absorption, whilst reducing bad reflections, such as flutter echo and comb-filtering. But how do they make a room sound bigger?
If we imagine a sound wave reflecting off of a standard, flat wall, all returning signals to our ears will come from the same direction, at the same time, with the same intensity. Because of this, we can 'localize' where the reflection is coming from, determining the distance of the wall. Our ears are pretty good at being able to localize sound, and this is how you can go into any room and hear how big or small the space actually is. However, if we place diffusers in a room, they will actively displace the three fundamental parameters for localization, hindering the ear's ability to determine the reflection point, and essentially removing the auditory position of the wall, making the room sound larger than it actually is.
Although the reflections that are created by a diffuser are random, they still need to be produced evenly to create a smooth scattering. Meaning the size and spacing of the diffuser wells and fins need to be correctly calculated for the size of the space and the target frequency range. Proper calculations are required to ensure comb filtering isn't occurring when the reflected sound interferes with the direct sound. As tempting as it might be to throw up random blocks of wood or shelves full of books behind your listening position, you may end up doing more harm than good!
Fortunately, as I'm not a mathematician, there are a few handy tools available online to aid in these calculations. First off, the studio designer John H Brandt has excellent resources on his website, such as the excel file: Control Room Mode Calculator. As well as calculating the room modes for different room dimensions, it also provides some information on ideal diffuser specifications for the particular sized room. Below is a screenshot of one of the tabs in the calculator, displaying the theoretical modes for the empty basement, along with other useful information.
As we can see at the bottom, the diffuser lower and upper-frequency range of my room has been calculated, as well as the suggested well width and diffuser depth.
I could then plug these numbers into a program called QRDude to finish off the design of the diffuser. I picked the number of wells to use (basically how wide I wanted the diffuser to be, but they have to be a prime number) and tweaked the numbers to match the diffuser's characteristics to the ideal frequency range. We can also see the minimum distance to the seating position, which I checked wouldn't be too close, otherwise, you can begin to hear some unwanted artifacts of diffusion. This is obviously useful knowledge, not just for the listening position, but for when I'm doing any recording in the room.
One choice that did need to be made was whether to build a 1D (aka Quadratic) or 2D (aka Skyline) diffuser. 1D diffusers, as you might have guessed, diffuse sound over one dimension, most often the horizontal plane, as this is where our ears live, whereas 2D diffusers work on both horizontal and vertical planes. 2D diffusers inherently absorb more sound than their 1D counterpart and they tend to attenuate sound further still as they scatter the sound waves in multiple directions. This is useful when you need to place the diffuser near the listening position, such as mounted on the ceiling above. However, they are technically not as efficient as 1D diffusers, which are also more effective at lower frequency diffusion. Finally, as shallow as it sounds, the look of the studio does come into play, you want a room that you are happy with visually if you are working in there day after day. In the end, I decided on a 17 well quadratic diffuser. As I would be sitting more than a couple of meters in front of the rear wall I opted for the greater diffusion efficiency on the horizontal plane. At this stage in the planning process, I was also contemplating a skyline diffuser for above the listening position, although ultimately decided against it because of the already low ceiling.
Now it was time to build the thing, as I wasn't entirely sure of the best way to build a quadratic diffuser, I bought a build plan from Acoustic Fields and then replaced the measurements with my own which were specific for my room. I decided to build my diffuser out of birch plywood, not just because it's hardwood, but it would also fit the visual aesthetics of the rest of my studio including my desk and sidecar.
To begin with, I had to measure out all the routing lines for the top, bottom, and back pieces...
...then the routing could begin. Unfortunately, my 6mm router bit was slightly blunt, which I didn't realise at the time, causing my lines to come out just under 6mm, meaning I had to do two passes on each run for the fins to slot in perfectly.
The finished routing...
Next, I checked that everything slotted together, I could then sand down and oil the parts for the finished look.
Once I was happy it all fitted together, I could then glue and screw the frame in place, glue in the fins, cut out the block mounts, and glue them in place ready for the vertical panels to sit on top of.
Once the diffuser was fully assembled and the glue had set, I just needed to carry it down into the basement and slot it straight into the empty frame. As everything was measured to the nearest millimeter it was a nice snug fit that didn't need any screwing or gluing to hold in place.
And that's how the diffuser was built! Next time we'll be looking at tidying everything up with a bit of fabrication.
Frank Leonard Walker
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