How to Choose the Right Room

By Michael Cooper

Imagine looking at a colored piece of paper through yellow-tinted glasses. The paper appears to be a light green. But it is really light green? The yellow lenses are changing your perception. Maybe the paper is really dark green. Or possibly blue. Or even turquoise.

I So it is with room acoustics. No matter what room you're recording or mixing in, the size, shape, specific dimensions, and textures of walls, floor, and ceiling will change the tonal coloration of the sound you hear in that room. Poor room acoustics are one of the biggest obstacles to creating great mixes and, to a lesser degree, to recording great tracks. A bad sounding room is like a big filter, making it impossible for you to hear the difference between the aural equivalents of dark green, light green, blue, and turquoise. If you can't hear it correctly in the first place, how can you possibly know when you've got it right?

We've All Been Here

How many times have you added some bottom end at, say, 100Hz to a kick drum or bass track to correct a thin-sounding mix, only to have it sound too boomy on your friend's stereo? Maybe your control room naturally attenuates 100Hz, causing you to think that your mix needs a boost at that frequency when it's really okay after all. Your friend's listening room may not affect 100Hz, so you hear the unnecessary EQ boost on his or her system and it sounds boomy. Or, worse still, maybe your friend's listening room naturally boosts 100Hz by the same amount that your control room cuts it. Now you'll hear twice the boost at 100Hz! Had you left the kick and bass alone, your mix would have sounded only half as boomy in the other room.

You can't change your friend's listening environment, let alone all of the other living rooms, bedrooms, bars, elevators, automobiles, and health clubs your music might by heard in. But you can avoid compounding any imbalances in those environments by making sure that your mix room is as flat as possible from the lowest bass to the highest highs, with no peaks or dips in the frequency spectrum caused by the room itself.

There's a lot more to acoustics than simply evening out the frequency response, but the end goal is the same in each case; to create a neutral, accurate listening environment that will allow you to produce mixes that sound reasonably good (if not outstanding) on any system in any room anywhere. (Okay, an outhouse in Montana is probably hopeless.)

Every Room Counts

Acoustics are almost as important in the studio/tracking room as in the control room. Most of us are hip to the fact that the specific microphone we choose to record an instrument or vocal with will inexorably change the way that track sounds. But think about it: The room in which the instrument is recorded will affect the sound the microphone receives. A boomy room makes for boomy tracks. A bass-deficient room makes for thin-sounding tracks.

The importance of project studio acoustics has increased dramatically over the last few years as high-quality recording and mixing tools have come down in price. Now that the average Joe can afford to record and mix CD- quality (or better) sound at home, the main thing that's standing in his or her way when it comes to getting an awesome mix is a confusing-sounding room that hides the truth of what's going onto tape or disk.

Rescue 911

Happily, you don't need to record in a high-end, commercial studio or have a sky-high budget to get great acoustics. For a moderate cost, you can turn an ordinary bedroom into an extraordinary mixdown room. Living the starving musician lifestyle? You can even improve your room's acoustics somewhat using items you probably already own.

Over the next few months, we'll explore practical ways to transform your current studio or music room from a sonic battleground into a sonic playground. We'll strip away the voodoo jargon and put aside the acoustician's logarithmic equations. We'll take a real-world look at what materials are available on the market, and discuss their pros and cons. Where something does the job cheaper and better, you'll hear about it here.

Our focus will be on methods that require the smallest investment of your time, so you can be playing music in your new sonic shelter as soon as possible. We'll concentrate on techniques and products that require minimal alteration to existing walls, floors, and ceilings so that your home isn't turned into a real estate agent's nightmare should you decide to sell a few years down the line. If you rent, a little spackle and paint should keep your landlord from freaking out when you move out.

Because most control rooms still follow the stereo paradigm, I don't plan to address surround sound monitoring in this series. Surround sound presents many additional variables, and acoustical control for such listening environments is a developing science. Besides, we don't yet know how important surround sound will become as a consumer format for music.

Our Battle Plan

In this first installment, we'll discuss what problems to listen for in an "acoustically challenged" room. After all, ya gotta know what the problem is before you can correct it. We'll also show you how to determine which are the best rooms in your house, from an acoustical standpoint, to track and mix in.

Part Two of our series will cover solutions for soundproofing. We want to be able to keep the sound from our control room monitors from bleeding into microphones in other rooms. We want to avoid barking dog overdubs on our tender love ballads. We don't want the police to drop by to give their opinion of a raging guitar solo. This we can do.

Part Three is where we get serious about control rooms. We'll discuss where to place your speakers, mixer, and outboard gear from an acoustical standpoint. We'll tame your room's flutter echoes and balance the reverb time, so your mixes sound clear and the stereo imaging rock solid. You'll learn how to turn your control room into a classic Live-End Dead-End (LEDE) listening environment - the professional paradigm for small control rooms - with wall panels, tube traps, acoustical foam, and diffusers. We'll discuss computer software to help tune your room. When we're finished with your control room makeover, your current monitors and amplifier will sound as if they cost twice as much. And, just so starving musicians don't feel left out, we'll reveal how you can turn bookshelves, picture frames, clothes racks, closets, area rugs, sofas, even mouse pads into acoustical aids.

In Part Four, we'll learn how to identify and tame standing waves in the control room. Remember that 100Hz scenario we discussed earlier? A standing wave was the probable culprit. We'll discuss the pros and cons of using electronic equalization to fine-tune your control room, and give a step-by-step method for doing so. Finally, we'll delve into the special considerations involved in treating tracking rooms and isolation booths. You'll be making better mic placement choices in your remodeled tracking room, and reaching for the ol' EQ knob on your mixer less often, all because you'll be able to hear what's really going on. You'll be on your way to cutting master-quality records.

That's the plan. We may shift things around from one installment to another, though, based on space considerations, so be sure to check out the entire series so you don't miss anything. But enough about the future - let's dive into a bad room!

Mommy, It Hurts When I Listen

In order to fix an acoustical problem, you must first learn how to recognize it. Let's take a brief stroll through hell together, and discover some of the things that can be wrong with a room.

Walk around your control room and tracking room(s) while you clap your hands. Does the sound slap back with a short series of sharp, discrete echoes, instead of decaying with a smooth reverb tail? These beastly echoes are known as flutter echoes. In a worst case, they can resonate at a distinct pitch. If you're not sure what flutter echoes sound like, find a narrow hallway with smooth bare walls and clap your hands. They're unmistakable.

Flutter echoes are caused by high-frequency sound bouncing back and forth between parallel surfaces - either from wall to wall, or from ceiling to floor. When flutter echoes occur close to your mix position (where you sit when you're mixing), two things happen: First, the stereo imaging becomes "ghostly", so that it's hard to pinpoint exactly where individual tracks are panned. Second, the comb filtering (a closely spaced series of peaks and dips in the frequency spectrum) makes the audio from your monitors sound subtly phasey. It's like the sound you hear when you talk down a garden hose, but much less pronounced. Comb filtering can make the high end of your mix harsh or uneven. You may be tempted to EQ the mix to make it sound smoother or warmer, when in fact the mix may be fine - it's your acoustics lying to you again.

Assuming you already have a control room or music room set up, listen to some major label CD's that have a variety of instruments and vocals that cover the full audio spectrum from low bass to sparkling highs. Do the mixes sound overly bright, or bassy and cloudy? If you're listening to full-range reference monitors and hear these problems, the reverb decay times of your room are probably imbalanced. That is, the bass, mids, and highs are dying away at different rates with respect to one another.

Even if you're listening to nearfield monitors, what you're hearing is a combination of the direct sound hitting your ears in a straight path from the speakers and the reflected sound that bounces off of the walls, floor, and ceiling before it reaches your ears. The reflected sound consists first of extremely short echoes (called early reflections), which quickly blend together into a continuous wash of reverb. If the frequency components that make up the reverb decay at different rates, the monitors' tonal balance will perceptually change.

The change in tonality can be dramatic. If you find yourself constantly boosting the highs in your mixes, only to hear them sounding brassy and tinny when played on other systems, then your mix room probably has a longer reverb decay time for the bass frequencies than for the highs and mids. Covering the entire surface of all of your walls with carpet of acoustic foam can cause this condition. "Room tone" is a term that is often used to describe, in part, the collective effect of the various reverb decay times in a room.

The overall reverb decay time of a room is also very important. If your overall reverb decay time is too short, you'll tend to add too much processing to your mixes. If the decay time is too long, you'll tend to produce overly dry mixes.

Do some bass notes always seem to boom in your mixes, while others disappear? The culprit is standing waves. The exact dimensions of your control room are a recipe for sound pressure to build up and/or weaken at specific bass frequencies. Depending on where you're seated when you mix, you'll hear these specific bass frequencies either much louder or much quieter than they're actually being printed to the track. For instance, if your room causes a standing wave at 98Hz, the open G string on a bass guitar will either sound boomy or radically lower in level, depending on where you're sitting. If your room has uncorrected standing waves and you're unaware of what frequencies are being affected, you'll be EQing the bottom end of your mixes like a dog chasing its tail - never really getting anywhere, and never knowing why. You may boost a weak bass frequency from the mix position, only to hear it become incredibly boomy from another position in the room. And God only knows what that mix will sound like in another room.

All of the above problems can be tamed - if not always completely, at least to the point where they become more or less minor influences while tracking and mixing. In fact, some rooms, because of their dimensions, inherently produce more standing waves than others. If you can set up in a room that has fewer problems to begin with, you'll end up spending less time and money correcting those problems. So let's discuss some simple ways to determine which are best rooms in you house for music production.

Go To Your Room!

In many instances, practical matters may force you into choosing a specific room for music production. For example, if you have a basement with a reasonably high ceiling, that's the space you'll want to use for tracking drums, because the earth surrounding the outside walls is the best soundproofing money can buy! When it comes to choosing between two or three equally attractive rooms for music production, however, analyzing three factors can lead you to making the wisest decision on which one to use. These factors are size, shape, and dimensions.

The smaller your room, the more likely its frequency response will be uneven in the critical bass region below 300Hz. For this reason, try to choose the largest room available to you for music production. The BBC concluded years ago that a room under 1,500 cubic feet in volume (for home warriors, that's roughly a 15-by-12-foot room with eight-foot ceilings) is not practical for critical work, as it will present so many problems as to make an acoustical overhaul too costly and potentially unsuccessful.

But a variety of acoustical materials and techniques have emerged in the past few years that allow smaller rooms to become amazingly accurate, and at a significantly lower cost than was possible before. Size does matter, but don't take 1,500 cubic feet as an absolute cutoff in choosing your room(s). As we'll see shortly, the shape and dimensions of a room are equally important. To calculate the volume of a room, multiply the width times the length times the height in feet. Be sure to include all alcoves and open closets.

If all the rooms available to you are small, try to pick the largest one for your control room. If you record bands of any size, you may need the largest room to track in. However, the control room is where you make all of your final decisions for both mic placement and mixing. If you can't hear the truth, you're just guessing at what to do. Therefore, ergonomic considerations aside, the control room should be the room with the best acoustics. You can always use close miking to minimize room tone in your tracking room.

The shaped of the room is also very important. Choose a room that is more or less rectangular, not a square or (worse) a cube. Standing waves become a big problem when two or more occur at the same frequency. If any two of the three dimensions (width, length, and height) of your room are either equal or whole number multiples of one another, they will produce multiple standing waves at the same frequencies. You should avoid this at all costs!

For control rooms, the symmetry of the room is also very important. If the room has an alcove off to one side but not the other, imaging may be compromised. This is because the timing and number of room reflections will be different on the two sides of the room, causing stronger or more numerous ghost images to pull tracks slightly toward one side or another in the stereo field.

Room Dimensions Matter

By measuring a room's dimensions and performing some simple multiplication and division, you can quite accurately predict the frequencies at which standing waves will be produced in that room. Knowing ahead of time what the standing wave profile of a room is, you can avoid setting up in a room that would require a lot of acoustical treatment to sound good.

Another name for a standing wave is "room mode." Standing waves that resonate between two opposing surfaces, such as walls, are called "axial modes." These are the boogers of studio acoustics. They are the strongest, and therefore the most audible, of room modes. Tangential modes (in which the sound bounces off four surfaces) and oblique modes (involving all six surfaces - four walls, floor, and ceiling) are much weaker, as well as formidably difficult to calculate. For these reasons, axial modes are the only ones we'll concern ourselves with.

The formula for calculating the axial modes of a room is: f=1,130/2L -where f is the frequency of the mode in Hertz and L is a room dimension measured in feet. The approximate speed of sound at sea level is 1,130 feet per second, at normal room temperature and humidity.

For example, a room that is ten feet long will produce an axial mode/standing wave at 56.5Hz (1,130/(2x10)=56.5). This is the fundamental axial mode for the length of this room; that is, it is the lowest standing wave that the length of the room will produce. Axial modes also occur at integer multiples of the fundamental, in this case at 2x56.5=113Hz, at 3x56.5=169.5Hz, at 4x56.5=226Hz, and so on.

By plugging the length, width, and height of a room into the above formula, we can assemble the results in a table (see below). The numbers show a room's axial modes. Some rooms may have two or more dimensions each for length, width, or height because of closets or alcoves that extend the room proportions along part of a wall. Be sure to include these measurements when tabulating axial modes, adding columns of data as needed. It's only necessary to calculate the modes up to 300Hz. Above this frequency, standing waves become less problematic (for reasons we'll discuss momentarily).

Table 1. The axial modes (in Hertz) for a room measuring 16.5' L x 10.5' W x 7.7' H (1,334 cubic feet), derived from the formula f=1,130/2L. Modes that occur within 5Hz of another mode are shown in parentheses. f1 through f8 represent the fundamental mode and its harmonics for each room dimension. Only the critical modes under 300 Hz are listed.

  Height Width Length
f1 (73.4) 53.8 34.2
f2 146.8 (107.6) (68.5)
f3 (220.2) 161.4 (102.6)
f4 293.6 (215.2) 137
f5 (269) 171  
f6 205.2    
f7 239.4    
f8 (273.6)    

Once you've made a table of the axial modes for a given room, you can draw a graph that provides a graphical representation of the modes. This is very helpful, because it lets you see the spacing between the modes. In general, modes only become a major problem when they are spaced too far apart or pile up on top of one another (within 5Hz).

A large room produces a greater number of axial modes compared to a small room. In the ideal room, these modes would be numerous, equidistantly spaced, and close enough together (but not too close) that the sound pressure levels across the entire bass frequency range would be more or less equal in strength - a room with a perfectly "flat" frequency response! No particular frequency would be boomy or weak. However, such a room does not exist, even in the very best of studios. The gap between large commercial studios and smaller project studios is not as huge as one might thing. Sooner or later, we all have to deal with acoustics!

As a room shrinks in size, its axial modes generally get spread farther and farther apart (because there are fewer of them), causing isolated frequencies to be boosted or weakened. If the axial modes produced by the three different room dimensions include common frequencies, the pile-up at those frequencies causes dramatically exaggerated boosts and dips in bass response.

No matter what the size of a room is, axial modes become increasingly numerous per octave as you go up in frequency. For example, a room may produce three axial modes over a 50Hz span. From 50 to 100Hz, this would cause three boosts/dips over the span of an octave. But three axial modes occurring between 1,000 and 1,050Hz (also a 50Hz span) exist within a span of less than 1/10 octave. So you can see that, as the frequency rises, axial modes begin to fill in octaves more densely and produce a more uniform distribution of sound pressure across the frequency spectrum (i.e., a more uniform frequency response). For this reason, we only need to concern ourselves with the axial modes below 300Hz. They're the ones that stand out like a sore thumb and challenge us during mixdown.

Studies have been made to determine the most favorable ratios of room length, width, and height to effectuate uniform distribution of axial modes. Unless you're prepared to build new interior wall, floors, and ceilings, however, you'll likely be struck with the dimensions of the building you're in. In any case, I highly recommend that you use the "table and graph" approach shown above to make an acoustical record of all the rooms you're considering to use for music production. Compare the graphs for each room, and find the one that has the least number of common modes ≠ that is, modes that occur within 5 Hz of each other. This is the room you should use for your control room (as long as ergonomic factors also support that decision). The second best room should be your tracking room.

Be sure to keep your graphs and tables handy for future reference: They will prove to be highly useful when we discuss acoustical remedies for standing waves in Part 4 of our series. In the meantime, keeping the axial room modes of your control room in mind will help you to avoid making unnecessary (and unflattering) EQ decisions during mixdown.

I'll close with a helpful tip for untreated rooms: If a particular bass frequency sounds boomy or weak, move a foot or two towards or away from your speakers; that is, out of the peak or trough of the suspected standing wave. If the level of the problem frequency shifts dramatically, don't boost or cut it with EQ! You're hearing your room, not the mix.