The acoustics of a room need to be designed to meet the purpose of the room. In the recording room the acoustics may be designed "enhance" the sound of the recording and in control room the acoustics need to very controlled and predictable so the engineer, producer, and musicians can trust what they are hearing in the monitors. In a church or a theater the sound needs to project into the room.A few tips about control room design.
A control room needs to have balanced acoustics, meaning a proper balanced absorption the frequencies and reflections are well controlled and diffused. If all the walls are covered with foam the room will sound boomy, and the mixes will have too much high frequencies and little bass out in the real world. The other extreme would be a room with too much bass absorption, causing the mixes to sound muddy outside of the studio. A control room should have only a little less reverb time than the average living room. Do not over do it. A completely dead room is not the real world and it will make it very difficult to get a good mix.
Symmetry; layout the room so that the left and right sides of the room are symmetrical. This helps with your stereo balance.
Use short racks (28" or 70cm) to minimize reflections in the mix position.
A recording room for voice (spoken) also needs to be designed to have flat acoustic properties with very short reverb times. For music recording longer reverb times, with a slight increase in bass reverb time is considered by many people to be desirable. The feeling is longer bass reverb time helps to give music a little more power. Too much though and the recordings will become boomy.Low Frequency Absorbers
A resonant panel or diaphragmatic absorber is used to absorb low frequencies. They work by vibrating at these low frequencies and turning the sound energy into heat. The drywall in wall construction acts as a resonant panel and absorbs a considerable amount of bass. If you a ever been in a empty masonry or concrete room you might have noticed the amount of bass build up when you try to talk in the room. Compare that to an empty room in the average house with sheetrock walls there will be some reverb but the bass build up like in the concrete room. When you calculate the acoustic absorption of the room remember to account for the bass absorption of the drywall.
The simple way to build a resonant absorber is to mount panel on a 2 by 4 frame with fiberglass batt (R11) or 703 inside of it. If you use 703 leave at least a 1/4" space between the panel and the fiberglass or else the panel won't be able to resonant properly.
Approximate Plywood Panel absorption peaks on a 2 by 4 frame 3.5 " deep
1/8" = 150 Hz
1/4" = 110 Hz
3/8" = 87 HZ
You can also build them into the corners of the room which will give a variable depth increasing the bandwidth of the absorption.
Membrane absorbers work like a resonant panel but with a thin membrane instead of plywood panel.
I had a recording room with plaster over brick walls that had a nasty problem at 250 Hz, well it just happens that R19 insulation with the paper facing the room has a peak absorption @ 250 Hz. I built two 8' high by 4' wide frames in two corners. I put in R19 with the paper facing the room. They fixed the bass problem right up. I finished them by adding some 1/4" wood molding on top of the frame and stretched some fabric across the frames to finish them. The molding was spacer to keep the fabric from touching the paper.
RPG's Corner Bass Trap is an example of a commercial membrane absorber. See the links page for a link to RPG.
Sections on Helmholtz resonators and slat absorbers coming soon.
Placement of absorption materials
Reducing First Order Reflections
In the mix position you will want to setup what is known as a Reflection Free Zone (RFZ). In the RFZ you will want to eliminate any early reflections of sound. By reducing the early reflections you increase what is known as the initial time delay (ITD) gap. This gap is the time between the arrival of the direct sound and the first reflections in the room. A 20 ms ITD gap is a good minimum target. The purpose of increasing the the ITD is to allow you to hear the time domain localization cues from the recording room. The controls room's ITD gap needs to be longer then the recording room's ITD. This allow allows you to hear the reflections in the recording room better.
Any surface within 11 feet of the mix position could cause a first order reflection, but treating every surface within 11 feet of the mix position is not possible or necessarily desirable. The International Telecommunications Union (ITU) and the European Broadcasting Union (EBU) recommends levels of reflections earlier than 15 ms relative to the direct sound should be treated to reduce them to at least 10 dB below the direct sound for all frequencies in the range 1kHz to 8kHz. In the area of the room between the monitors and the mix position any surface that has a 21.5 foot complete path between the monitor and the mix position can also cause a reflection.
One way to find the areas that need to be treated, will require two people and a mirror (a plastic one is preferred since you may have to tape it to a pole to reach some areas) To help find the placement location have one person move a mirror along the side walls of the control room with the other person siting at the mix position. Any point the monitor can be seen from the mix position in the mirror mark those points on the wall. Repeat this process for the ceiling and back wall. Any of these points that are less then 11 ft from the mix position will cause early reflections and is where you will want to locate absorption materials. In the front half of the room measure from the monitors to the marked reflection point to the mix position if it is less than 21.5 feet be sure to treat those areas as well. You may want to treat an area larger than what you have marked, since you wonÕt always be in the exact mix position.
The reason for the two measurements is, if you measured 22 feet from the speaker to the back wall and back to the mix position your reflection time is still under 19 ms because the monitor is a few in front of you the sound has traveled 3-4 ms before it reached your ears so the reflection will reach you in 15-16 ms. In the area in front of the mix position you could have a reflection from something more than 11 feet away reach you in less than the 19ms since the spot is is less than 11 feet from the monitors.