Soundproofing a control room involves many different variables and many different options to deal with those acoustic anomalies. It is too complex of a subject to deal with in one sitting. This is the first of a three part series on how to soundproof your control room. We will cover room size, room cut off frequency, parallel walls, and low frequency resonances and how to deal with these issues.
Lets begin with the room size. Room size determines what kind, how large, and where the resonances we will face are located. These are low frequency resonances which cause signal exaggeration or loss at our microphone or listening/monitoring position. They are in all of our rooms in some amount or another. We can’t get away from resonances, so we need a room size ratio that spreads them out inside the room. We need to spread them out, so they don’t get together and party at our acoustical expense. If they are spread out within the room, we can use smaller sized acoustical treatments to deal with them.
Finding the least offensive height to width to length ratio is critical from the beginning. A good room size and volume ratio in the beginning will keep our costs down in the future as we move forward through the complete project. Some recommended height,length, and width room ratios are 1:1.14:1.39 for a small control room of around 1,600 cu. ft. Do not go any smaller than this ratio. A large control room would be 1:1.50:2.10 which would have a total volume of 3,150 cubic feet.
Cut Off Frequency
Our next issue is the room cutoff frequency. Every room size has a frequency that stands for where modal frequencies are close enough together that they all merge together and give us a baseline to work with. Frequencies above this resonant frequency will not be as much as an issue as frequencies below this number. At frequencies above the cut off frequency, the room will treat them fairly uniform and predictable. Below the cut off frequency, resonances will predominate. The room’s resonant frequency can be computed using a formula which involves reverberation time and room volume. The higher the cut off frequency the better. Higher cut off frequencies go with larger rooms.
Parallel Surfaces – Bad
Parallel surfaces with our walls is another concern. If we can’t splay or angle our walls with at least an eight degree splay, 10 degrees is preferable, we must make the existing parallel surfaces uneven and irregular. To accomplish this, we can use the industry accepted live end/dead end room treatment protocol. First, we make the area in front of our monitoring position directly between our monitors absorptive. We use absorption techniques on the ceiling and also on the side walls to minimize reflections from the ceiling, front, and side walls. We want to hear the direct sound from our monitors which includes, hopefully, just the sound on the recording. Any sound that has reflections in it is room sound. This is our dead/end part of the room set up equation. We have treated the front wall, side walls, and ceiling with absorption. Our rear wall will be diffusion to minimize any delayed time signature that could be thrown at us from the rear wall at our monitoring position. The rear part of the control room will be our live/end of our control room.
Low frequency issues are our next issue. We will need two types of low frequency absorbers. We will need a frequency specific absorber and a broadband absorber. Diaphragmatic absorbers can handle the frequencies below 80 Hz. and poly cylindrical absorbers can handle the frequencies above 80 Hz. and also it can use sound re direction on those frequencies above 125 Hz. due to its curved shape. Both diaphragmatic and poly cylindrical absorbers can be built by anyone with basic carpentry skills. When building diaphragmatic absorbers careful attention must be paid to the absorbers depth for the depth of the unit determines the lowest frequency at which the unit will absorb. Poly cylindrical absorbers require more skill level to build because of their curved face surface.
Make sure you calculate the correct room size from the beginning. Follow the ratios given. If you have to build another wall in your room to make it conform to the correct ratios of width, height, length, and room volume, do so. It will be worth it in the end. Find the room’s resonance frequency and make sure it is high enough to not cause issues. Anything above 200 cycles is a good start. No parallel surfaces for our sound energy to rattle back and forth between. Bass energy must be controlled through the use of absorbers both frequency specific and broadband. More next time on speaker placement and specific room treatments.