Part 3: Room Treatment
By Fred Gilpin
Creating a uniform energy decay across the entire frequency spectrum and controlling reflection is the main goal in treating your mix room. The use of both absorption and diffusion will create a natural sounding room. If you don’t have the ability to measure the energy decay in your room, a simple rule of thumb is to treat 25 to 30 per cent of the walls and ceiling with absorption, 25 to 30 per cent with diffusion, and leave the rest bare. Randomizing the panel locations on each surface adds to the overall diffusion in the room.
Mid and HF absorption can be done with soft porous material like semi-rigid fibreglass or open cell foam. You want a density of at least 3 PCF (pounds per cubic foot). A 4-in. thick panel of this density will have quite uniform absorption down to 250 Hz and is usable down to 125 Hz. Higher densities (like 6 PCF) only offer a minimal increase in absorption (mostly in the 125 Hz octave), but do offer more resistance to abuse. Spacing the panels off the wall increases the low frequency absorption. For example, a 2-in. thick panel with 2 in. of air space behind it will provide virtually the same absorption as a 4-in. panel mounted to the wall.
The depth of diffusers determines the lowest frequency they are effective to and the width of the cells determines the highest frequency. I recommend at least 6 in. of depth and a cell width of 3/4 to 1.5 in. to provide good broadband diffusion. Diffusers must be made of solid material to reflect and scatter the waves. There are manufacturers selling sculpted open or closed cell foam products they are calling diffusers; they are not.
For a lower budget scattering solution (pseudo diffusion) you can use simple curved surfaces. Cardboard concrete forming tubes (Sonotubes) can be cut in half and affixed to a thin piece of plywood to create scattering panels. Using random sizes of tubes will create more scattering (i.e. 6-, 8-, and 12-in. tubes). Stuff insulation inside the half rounds so they don’t resonate and to add some low-mid absorption to the panels.
The first treatment step is to identify the locations causing the primary reflections from your loudspeakers. Sit at the mix position and have someone walk around the room holding a mirror against the walls. Any place you can see the loudspeaker in the mirror is a surface that will cause a reflection back to the mix position and you want to place an absorber at that location. Make sure to find the reflection off the ceiling and floor between the front wall and the mix position, as well. There’s not much you can do about the floor. Placing a 4-in. thick, 4 x 4-ft. absorber behind each loudspeaker will absorb the low mid waves that wrap around the speaker and bounce off the front wall.
Corners of the room will have a buildup of low frequency energy, so this is the first place to control that energy. Stand two unopened bags of pink fibreglass insulation one on top of the other in each corner of your room. You can hide the bags behind a piece of cloth. You’ll be surprised by how much this simple treatment will tighten up the bottom end in your room.
Room modes are caused by low frequency reflections. The sound leaves the loudspeaker and travels to the opposite wall and reflects back. When the wave reflects off the wall, there is a polarity reversal (i.e. reflected wave is now 180 degrees out of phase with the next wave leaving the loudspeaker) and when these waves meet in the centre of the room, they will cancel each other, causing a very deep dip at that point. This happens between the front and rear walls, the side walls, and the floor and ceiling.
To find these modes, sit in the centre of the room, turn on an oscillator, and slowly sweep the frequency between 30 and 150 Hz. You will hear peaks and dips. When you find one that sounds like you turned the oscillator off, that is a room mode. Walk around the perimeter of the room and find the spot where that frequency is the loudest. There will also be a corresponding spot on the opposite side of the room. You now know which direction that mode is going in your room. That is where you place your tuned low frequency absorber for that room mode.
Remember to check the floor for the up/down mode. A “cloud” above the mix location is effective in controlling an up/down mode. The cloud should be as wide and long as half the height of your room and can be made from a piece of 1/4- to 3/8-in. plywood with 4 in. of absorption on the face. Hang the cloud 6 to 12 in. below the ceiling to increase the low frequency effectiveness.
Tuning low frequency absorbers is beyond the scope of this article. F. Alton Everest, in his Master Handbook of Acoustics (pg. 183), shows a straightforward way to measure the tuning of low frequency absorbers. Don’t bother with the formulas on the internet; they are all over the map and most won’t even get you close. The room modes will only be a few hertz wide, so you need to measure while you tune the absorbers.
Fred Gilpin is an acoustic consultant, designer, and the owner of FGA Electroacoustics. He has 35 years of experience and expertise designing studios and providing acoustical solutions at facilities around the world. Fred is based out of Abbotsford, BC and can be reached at firstname.lastname@example.org.