Studio Owners Part 3 : Room Treatment
RECORDING
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 fredgilpin @ me . com .
By Fred Gilpin
Design Guidelines For DIY
Studio Owners Part 3 : Room Treatment
This is part three of a three-part series . Parts one and two appeared in the Nov ./ Dec . 2015 and Jan ./ Feb . 2016 issues of Canadian Musician .
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 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 .
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