Homeward Bound—The Move to the Small StudioPart One: Isolation Issues and Speaker Placement 6/01/2004 8:00 AM Eastern
Most people don't understand that converting a bedroom into an accurate recording/mixing room is more difficult than building a facility from the ground up. The process may not be fast or cheap, although sometimes we get lucky. As the audiophile community discovered long ago, you can't just put a pair of speakers into a room and expect it to sound good.
There are a number of issues that engineers need to pay attention to if they want a professional, accurate home working environment. Home studios are plagued with the same problems as large facilities — and some small spaces are even gnarlier. If you've been reading articles in Mix or have visited my Website (www.bobhodas.com), some of what I say here might sound familiar, but it is all worthwhile because the laws of physics haven't changed much in the past 20 years.
The most expensive issue is isolation, but we also need to look seriously at room orientation. Next is symmetry. First-order reflections definitely need to be addressed and, of course, that monster problem of how to control the bass. (I'll address these two topics in Part 2.) For this article, let's assume that your room has parallel walls — your basic shoe box.
A room's dimensions are the first restriction when building a home studio. This is the first issue that all studio designers face when working from the ground up, but in most homes, you're stuck with what you've got. Designers calculate room modes for rooms with parallel walls, so be aware that the charts shown here are based on assumptions that won't always apply to every room.
In these measurements, the assumption is also made that a room is infinitely contained; i.e., a concrete bunker. The less contained a room is, the more low frequencies will pass through the walls. Of course, you will be subject to the first peaks and dips of the wavelength, but some of these frequencies (starting with the lowest) are going to pass right through walls and windows and just keep on going. Wavelengths that escape are not going to bounce back into the listening area out-of-phase to haunt you in the form of cancellations. It all depends on the stiffness and density of your walls. Some studio designers even work this concept to control the room response by letting some bass escape. It doesn't work too well if you have neighbors who are on the receiving end of all the bass you just let out of your house without a license. Or, maybe you need isolation from your neighbors — or airplanes. Depending on how they are built, walls may vibrate and absorb some frequencies. Studio designers may also use this technique, although the safest bet is to build your walls as stiff as possible so that they don't flex. You can always add strategically placed membrane absorbers once the room is scoped out.
The mode charts also assume that the speaker is against or in the front wall. This works for some home studios, but obviously not many. The speaker's distance from the walls is going to determine reflections that will mix in with the direct signal either in or out of phase, causing dips or peaks in the response. The seating placement determined by the mode chart isn't necessarily the right spot to put that chair. And don't forget that you must have parallel walls and floor/ceiling: No cut-off corners, no L shapes, no angles, no wet bar along the sidewall, no open walkway into the next room. A big rack of gear right behind the mix position doesn't help much, either.
If your room has parallel walls, your speakers are on the front wall and your studio is in the basement or just very well-built, then there is merit to the charts here. And as I stated above, even if you don't fit this criteria, checking these factors is not a bad place to start. JBL offers a free, easy-to-use and accurate Excel-based calculator on its Website at www.harman.com/wp/index.jsp?articleId=131.
Most home studio owners will be stuck with the existing room dimensions, but if you have the freedom to move a wall, consider creating a room size that fits into the Golden Ratio: a ratio of length-to-width-to-height that minimizes the effects of modes. It spaces them so that they do not pile up, causing large bumps or dips in the frequency response. There are a number of well-accepted ratios depending on the room's volume. You can play around with the mode calculator mentioned above to find a good ratio that works within your current room size restrictions or use a program that helps determine an ideal room size. I use two programs: AcousticX from Canadian designers Pilchner Schoustal (www.pilchner-schoustal.com/old/acoustic-x/index.html) and RPG's Room Sizer, located at www.rpginc.com/products/roomsizer/index.htm. Once your room size is scoped out, it's time to see if you can make noise in it.
Isolation is key: Unless you live alone, you generally can't make as much noise as you want at all hours of the day. This also applies if you have neighboring apartments. Addressing isolation in an existing house structure is a difficult issue. Most homes are not built with much sound isolation in mind, and the hollow walls and stud spacing are not ideal. Unless you are prepared to build a room within a room, you will never be totally isolated from the rest of the house or your neighbors. Many of my clients know their neighbors and made arrangements to listen in the neighbor's house while they had the music cranked in their studio. This will give you a good idea of how much work you have ahead of you.
On the cheap, you can do several things: Replace your interior door with a solid-core exterior door. Make sure you address the air spaces around the doors with some type of insulation. Remember that any little air space will leak sound like a sieve. This also applies to your HVAC ducts, but most home studio budgets can't accommodate that. If budget allows, purchase medium-priced, mid-quality doors that are used in home theaters, such as the ones available at www.owenscorning.com/around/sound/products/door.asp.
Basic isolation can be added to the walls and ceiling by simply adding another layer of mass. If you're going to go to this amount of trouble, instead of using ⅝-inch Sheetrock, try MDF or Wonderboard. These materials have much more mass than Sheetrock. Be sure that when you install an extra layer, it is glued and screwed and that the new seams do not overlay the existing seams. A new material on the market, called QuietRock and QuietWood, has a significantly increased amount of isolation. For more information, visit www.quietsolution.com/construction-building.html. I recommend it, and you may find that you will simplify the construction process with this material. Any of these wallboards can be made more effective by floating them on a resilient channel. This creates an insulating air space and isolates the new wall movement from the existing wall — very effective, and, of course, more labor and money. There are various forms of this channel, so shop around. Visit www.silentsource.com/rsic.html, www.kineticsnoise.com or www.auralex.com/sound_isolation_rc8/rc_discussion.asp to get started.
I hope I don't have to tell you that two or three layers of glass are better than one. Glass is expensive, but if your neighbors complain, you're history. Find a good window dealer that carries insulated windows.
Unless you plan on floating a floor (which is expensive), you'll have to live with structure-born transmission. It is tough to control, but an alternative is to treat the ceiling in the room below you with QuietRock, but that will still only give you minimal low-frequency isolation because the bass will travel throughout the house via supports.
The old wives' tale that states, “having speakers in your face eliminates room problems” is a crock: There is no escaping the laws of physics. Before you can think about acoustic treatments, you need to optimize the speaker positions in your room. This is the single most important thing that you can do to improve your system response. It is very important that you become extremely familiar with your speakers. Take a good look at the manufacturer's frequency response charts. Remember that these are anechoic measurements, and as soon as you put your speaker in a room, the bass response will start to change significantly. Bass response will build up even more when you place the speaker against the wall or in a corner. But the response charts are useful for knowing the speaker's limitations. For example, Yahama NS-10s can roll off dramatically after 100 Hz, so you don't have to be too concerned about deep bass problems when positioning them. Using a wall or corner may even be to your benefit. You will also want to pay close attention to the recommended position for proper phase alignment. For some speakers, it's directly aligned with the tweeter; for others, it's a point between the woofer and tweeter. It depends on the design, so check the manufacturer's literature. When you position your speakers and listening position, make sure this alignment point intersects your ear position.
Again, I am assuming that your room has four walls with equal spacing; i.e., the common rectangle — no L shapes. Step one is determining which wall your speakers should be on. If your room is square, then this doesn't matter. If your room is rectangular, then there is no rule of thumb as to whether you should put your speakers on the long wall or the short wall — it all depends on the dimensions. This is not an easy thing to figure out without an analyzer. It will take some time and effort, but the payoff is tremendous. The speaker distance from the front and sidewalls will also dictate the listening position. This is one of those “chicken-or-the-egg” situations: All of these factors are interactive, so plan on moving things around a lot.
To figure out which wall to use, place the speakers on two intersecting walls: one on the short wall and one on the long wall. Place the speakers at listening height in an approximate left or right speaker position as if you had stereo speakers. (Remember the chicken/egg problem.) Run a mono send from your CD player to one speaker at a time and seriously listen to the bass. Keep yourself centered between the sidewalls, and move forward and back six inches at a time. Your distance from the speaker will play a part in the frequency response. You should also move the speaker forward and back and side-to-side six inches at a time in this process. You should be able to get a feel for which speaker has a flatter bass response. Remember, more bass is not necessarily better! Listen for smooth and connected bass from mid down to low.
Now you've done the hard part! While finding the proper wall, you also found the proper speaker and listening position. Take measurements of the winning speaker distances from each wall and set the speakers up symmetrically. Figure 1 and figure 2 are examples of just how different the walls can be and what you should listen for. Remember that there is no rule of thumb here; sometimes the speakers will perform best on the long wall and other times they'll be better on the short wall. Sometimes they want to be right against a wall. However, I've never seen a room where putting the speakers in corners worked.
If you don't want to go through the process mentioned above, you could hire a guy like me to do the math, analyze your room and tell you where your speakers should be placed. Or you could buy Room Optimizer by RPG Inc. (www.rpginc.com/products/roomoptimizer/index.htm) and try to figure it out yourself. While these calculations work quite well in theory, they are neither perfect nor fool-proof. The program is based on perfectly symmetrical rooms with walls that do not flex or pass signal. I normally compare the direct measurement results with my calculations, and have found that the RPG program is often very close; I usually only make minor changes to positions to fine-tune the system. But there are times when, due to construction, for example, the program doesn't get the job done and an analyzer and your ears are needed.
I can't stress the idea of symmetry enough. If your speakers are not placed symmetrically in the room, then they will have different frequency responses. This means that your music will sound different in the left and right speakers, your center image will be off-center and your depth of field will suck. So make sure that the left and right speakers are equidistant from the sidewalls. The same applies to the speakers in regard to the front wall.
Why is the above true? Below 200 Hz, your speakers are fairly omnidirectional. The signals that bounce off the walls and ceiling are going to mix in with the direct speaker signal. This delayed bounce will cause comb filtering. The time delay, and thus, frequency of interaction are dependent on the speaker distance from the walls. If the left and right speakers are different distances from the walls, then the cancellations will occur at different frequencies. You wouldn't put one speaker on the floor and the other on a stand, would you? (This is also true for first-order reflections above 400 Hz, which will be addressed in Part 2.) Figure 3 demonstrates what happens to the bass when speakers are placed asymmetrically in a room. One speaker looks great (red curve), but the other (blue curve) is in bad shape. If you try to treat the above bass problem acoustically, you will run into trouble. A treatment that works for one speaker will not work for the other, and perhaps make it even worse.
Pay attention to the placement of your equipment in the room. Most people don't consider this factor, but having a bunch of gear on the right side of the room and nothing on the left is going to change the way the speakers behave. Try to design a layout that is balanced side-to-side as much as possible. Remember, it's all about symmetry! Next month, I'll tackle room reflections and acoustic treatment, including the tough task of bass control.
Bob Hodas is one of the top acoustical consultants in the business, known for his 30 years of experience in detailed testing and measurement techniques, plus amazing ears to boot. He's first-call for studios all over the world, tuning rooms and tweaking speakers, and he's published dozens of articles on the subject. Lately, however, he's been busier than ever tuning project studios: As more producers and engineers make the transition to home rooms, they realize how difficult it is for mixes created at home to translate to the commercial studio. For those considering building a studio in their existing space, Hodas shares his tips in this two-part design series. In part one, he discusses the unique isolation challenges in existing rooms and details the finer points of speaker placement. Next month, he breaks down room treatment and EQ.