My first step was to learn more about acoustics, studio construction and so on. My first instinct was to buy a bunch of studio foam and cover the walls with it. As I learned more about acoustics I learned that was probably not the right approach for what I had in mind. The following resources were helpful:
Master Handbook of Acoustics, by Alton Everest. A fairly thick book, quite readable. Although it is accessible to a beginner, one gets the feeling that there's more depth to the book than you think one first reading. I've gone back over it several times, trying to internalize the formulas and tradeoffs he discusses.
recording.org One of the message board is devoted to studio construction, acoustics and so on. Reading the archives is a real education, and questions get answered quickly.
Studio Tips Forum (formerly a Yahoo acoustics group) Another message board, similar to recording.org. You see many of the same faces, but also some different people, different philosophies, etc.
Auralex Web Site Information about this company's foam products and more.
RPG Web Site RPG specialized in diffusion products. There are product pages and other technical information on this site. There's one very interesting and concise paper by RPG systems that explains the issues and some solutions for small studios.
Real Traps web site. Real Traps is a small company owned by Ethan Winer, who moderates recording.org. Besides info on his products, the site has a good primer on acoustics, and a fair amount of practical information on acoustics and acoustic treatment.,
Ethan Winer's page on bass traps has more information, including plans for building your own bass traps.
John Sayers' site and discussion forum. The site itself is interesting, with lots of nice photos of studios John has designed (great for ideas) and the forum is, again, similar to the yahoo group or recording.org, with some overlap of people, and some that are unique.
homerecording.com Yet another site with information and a discussion board.
ETF Software ETF is a very interesting program that turns a PC and a measurement mic (I'm using a $39 Behringer) into an acoustics analysis system. You can download the software and try it for free.
Cara CAD Software An inexpensive modeling program that can predict your room's acoustics from mathematical models, once you enter the room's dimensions and layout using a simple CAD program.
Very brief summary of what I think I've learned:
This is really dangerous, since there are many complicated books written on the subject and to be an expert probably takes a professional a lifetime. From what I can tell, even the experts don't always agree on fairly basic things. So this is really just a 10,000 ft level description of what I've gleaned, and may be very misleading, if not entirely wrong. This stuff is way too complicated to be summarized briefly, even if I fully understood it. You'd be better off reading the RPG paper referenced above, or Ethan Winer's pages, as an introduction. But in the interest of setting the stage for the rest of the process, and outlining the way I'm approaching my room, here it goes anyway...
For acoustic music, I want a fairly live room, not as dead as some multitrack studios. At the same time, my room is on the small side, not really large enough, nor shaped optimally to have the best acoustics. For small rooms, the best bet is to have a significant amount of absorption to eliminate as much of the room as possible. Also, in my case, the single room will serve as both recording space and a control room, which ideally require different acoustics. For mixing, one wants a fairly dead room, while the recording room can be more lively. I will strive for a compromise and see what I can do.
Room modes: every room has "modes", resonant frequencies that are a function of the length between the various hard surfaces. Modes are relatively easy to calculate. For a room with 2 parallel walls X feet apart, there is a fundamental mode at 1128/X, the frequency where the walls are one wavelength apart. There are also modes at all multiples (harmonics) of this frequency. Ideally modes are spaced somewhat evenly and relatively close together, so that no one frequency stands out. Problems arise in small rooms where there are sparse modes through the lower frequencies, and also in rooms whose shape produces the same mode from multiple dimensions. In the worst case, a square room, 10 X 10 X 10, has the same modes in all three dimensions, which will reinforce those frequencies.
One way to reduce the impact of modes is to introduced broadband absorption, to absorb the resonant frequencies. One can also build traps tuned to specific frequencies if needed.
Reverb time: The reverberation characteristics of rooms are measured as the time it takes the reverberation of an impulse, like a clap, to be reduced by 60 DB. This time is called the RT60 of the room. This time can be calculated based on the room size, shape, and materials on the walls, and it can also be measured. The RT60 is frequency dependent and may vary at different frequencies. I will be using the ETF software to measure the RT60 of my room. Fairly dry studios have an RT60 of 300 milliseconds or less. Acoustic music is usually aimed at 600 milliseconds. For my purposes, somewhere in between this range seems appropriate.
Reflections: Even with a short reverb time, strong reflections can make the room difficult to mix in. A reflection that causes a short delayed version of a sound to be mixed with the direct sound creates "comb filtering", which is the effect exploited by phase shifter and chorus guitar stomp boxes, and that can certainly impact the way music sounds. For the mixing area, I need a "reflection free zone" where I hear the monitors, not immediate reflections off walls and other surfaces. Parallel surfaces may also cause reflections that bounce between walls and ceiling and floor, causing problems, so I may want to do somethign to help break up the parallel walls.
There are three basic types of surfaces I can work with to address both room modes, reverberation time, reflection, etc.
 | Reflective: hard surfaces that reflect sound. This is what the room mostly has to start with... |
| Absorptive: surfaces that absorb sound, like acoustic foam. It's important to absorb sound at all frequencies. Simple acoustic foam, or carpets may absorb highs but not the lows, leaving the room sounding bass heavy and muffled. To absorb lower frequencies, one can use "bass traps" or various structures designed to resonate and absorb sound at specific frequencies. |
| Diffusive: surfaces that diffuse sound, essentially bouncing it around. This is similar to reflection, but scatters the sound in many directions rather than bouncing in a single direction, mirror-like, from a reflective surface. Diffusion is claimed to make a room sound bigger, and more alive, which is intriguing for acoustic music. However, there are seems to be issues with using diffusion in small rooms, contradictory as that sounds. Commercial diffuser products are expensive, but the information for building your own seems to be readily available (The patents from RPG - a company that makes diffusors - are fascinating to read, and tell you exactly how their products are made if you want to go that much work yourself), so this may be an interesting area in which to experiment. |