Illustration: Mike Cruz
More recording and mixing is being done in very small rooms than ever before. Small-room recordists include not only amateurs and weekend engineers in their home studios but also serious professionals doing critical music projects for major labels. Much of the conventional wisdom about room acoustics, measurement, and treatment fails to take into account the unique behavior of small rooms. This article explains some important but often misunderstood or overlooked acoustic principles relating to low frequencies in small rooms.
Many people believe that room acoustics is a complicated subject that can be understood only by those who have a Ph.D. in math or physics. However, the behavior of sound waves in small rooms is actually pretty simple, at least for the purpose of addressing problems that are common to recording studios and control rooms.
All acoustic anomalies are caused by reflections off the walls, floor, and ceiling. I make a distinction, however, between problems caused by reflections at low frequencies (below about 300 Hz) and those caused by midrange and high-frequency reflections. Above 300 Hz, reflections are perceived mainly as echoes, ambience, and reverb. Below 300 Hz, skewed frequency response is a much bigger problem. In all cases, waves bounce around the room much like a cue ball on a pool table; unlike a cue ball's, though, their behavior changes in three dimensions.
PEAKS AND NULLS
Contrary to popular belief, all small rooms have peaks (boosts in amplitude) and nulls (dips in amplitude) at all low frequencies, not just the mode frequencies determined by the room's dimensions (see the sidebar “Acoustical Physics”). That situation has a profound effect on how you should approach low-frequency acoustic problems. For any given low frequency, every room has places where a peak exists and others where a deep null exists. Likewise, for any given location in the room, there will be frequencies with a peak in the response and others with a null. Specifically, a null occurs at a distance from most boundaries (walls, floor, and ceiling) equal to one-fourth the wavelength of the frequency. (For more information, see Wes Lachot's article, “Bass Waves in the Control Room,” at www.ethanwiner.com/waves_wl.html.) Other nulls occur at odd multiples of that distance: three-fourths of the wavelength, five-fourths, and so on. Similarly, peaks occur at two-fourths of the wavelength, four-fourths, six-fourths, and so on.
Some boundaries have stronger peaks and nulls than others, due to multiple reflected waves coming from different directions and combining in the air. In fact, all acoustical problems in all rooms are caused by reflections. Because the peaks and nulls in small rooms occur at regularly spaced frequency intervals, the net result can be considered a type of comb filter. That is exactly how flanger and phaser effects work, except in this case the filtering happens acoustically in the air as the waves collide, reinforcing or canceling each other. The general term for this phenomenon is acoustic interference.
The effect is much more pronounced in small rooms than larger ones because the walls are closer together and the reflections are therefore stronger. The strength of the reflections also depends on the density of the walls, with rigid walls reflecting more frequencies and lower frequencies. Indeed, the worst environment for a home studio is a basement, because cement walls are more rigid than standard Sheetrock walls. One of the great ironies of acoustics is that thick, rigid walls that improve isolation between rooms cause more acoustic problems within the rooms than they solve. With standard walls made of one layer of Sheetrock, the lowest frequencies pass through to some extent and are partly absorbed when the wall vibrates in sympathy. By reflecting more frequencies and lower frequencies, walls made of cement or multiple layers of Sheetrock increase the damage caused by acoustic interference.
FIG. 1: This frequency response was plotted at 1 Hz intervals in a control room measuring 16 x 10 feet and 7.5 feet high. As horrible as it looks, this is typical for small, untreated rooms.
As I explained, peaks and nulls occur at predictable quarter-wavelength distances from every room boundary. The nulls are often strongest at the rear wall because the loudspeaker's wave front (the initial change in air pressure caused by a sound) that travels the length of the room is strongest in that direction. But other reflections occur at other boundaries, and they combine in and out of phase to bolster or reduce the quarter-wavelength nulls. To create a deep null, the opposing wave fronts must be nearly identical in level. It takes very little contribution from an errant reflection arriving from somewhere else to disturb the precise balance needed to create a deep null.
If you want to find out just how serious the acoustic problems are in your own control room, you'll need a sine-wave generator and a tape measure. But you can't just place a mic or your ear at a distance from a wall and expect to find a deep null. You also have to move up and down, and left and right, to find where the null is least influenced by a competing reflection off the floor, ceiling, or another wall.
In most rooms, the peaks caused by acoustic interference are usually less than 6 dB, but the nulls are typically 30 dB deep or more (see Fig. 1). In fact, most people first become aware of bass problems in their rooms when they notice a lack of bass at the mix position compared with other locations. A slight peak is not nearly as noticeable or damaging as a deep null. Further, many nulls have a very narrow bandwidth.
FIG. 2: Acoustician Doug Ferrara prepares to measure the response of a typical project studio''s control room.
WHAT YOU KNOW IS WRONG
Standard real-time room analysis using pink noise to measure the frequency response in ⅓-octave bands completely misses the peaks and nulls that exist in all rooms. When pink noise is analyzed in bands, the levels of all frequencies within each band are averaged together. Even measuring at 1/12-octaves is far too coarse to see the true room response. I have observed peaks and adjacent nulls at less than 1/12-octave spacing in many small rooms. Unless you measure the exact frequencies of the peaks and nulls that occur where you place the microphone, however, a room might appear perfectly flat when it actually has many large, hidden aberrations.
One excellent way to measure the true low-frequency response of a room without having to use special equipment or analysis software is to play low-frequency sine waves and measure the result at various locations (see Fig. 2). Such a test is useful because all music ultimately consists of sine waves that sustain for some length of time. When a bass player holds a long note in a slow ballad, that note contains primarily two sine waves: the fundamental pitch and its second harmonic. Likewise, kick drums produce mostly sine waves, though the waves often fade quickly and are accompanied by the click sound of the beater against the drumhead. Cymbals, maracas, and every other musical instrument create sound composed entirely of sine waves, even if those waves sustain only briefly.
Even if you measure the room using a bandwidth of 1 Hz (whether with pink noise, impulses, or sine waves) you must consider another issue: the physical size of acoustic nulls is often extremely tiny, so the notion of a sweet spot for mixing is meaningless unless you're willing to clamp your head in a vise. In one test, I located the physical center of a deep null at 100 Hz in my own control room. I then moved the microphone four inches to the side — equivalent to turning your head a little — and the level rose by 15 dB. When the mic was 18 inches from the null center, the level was 20 dB higher. If you measure a room's response using sine waves 1 Hz apart, you'll still need to measure every frequency at dozens of locations within a cubic foot or two of the mix position to get the true picture (see Fig. 3).
One saving factor is that our ears are spaced several inches apart. When one ear is in the center of a very deep null for a given frequency, the other ear is likely to be out of that null, though it might be in the center of another. The severity, narrow bandwidth (high Q), and small physical size of acoustic nulls are the main reasons equalization can never correct low-frequency problems in small rooms. Whatever you do to flatten the response in one location will surely make things much worse somewhere nearby.
FIG. 3: Any DAW program can measure a room''s response with no special test gear. This Sonar project plays 260 sine waves in 1 Hz increments from 40 to 300 Hz. Each tone on the first track sustains for one second and is played through the speakers. A microphone at the mix position records to the second track, which then reveals the room''s response at each frequency.
The old-school method of acoustically treating low frequencies is to calculate the room's modes based on its dimensions and then design custom bass traps that target those specific frequencies. That approach is inadequate because it addresses only the modal frequencies, ignoring the peaks and nulls caused by acoustic interference that occur at all other low frequencies. Further, even if you consider only the first five axial modes for each dimension, that would still require building bass traps for as many as 15 different frequencies. A much better solution would be to use broadband absorption, because that flattens the response throughout the entire low-frequency range.
In some severe cases — say, with a room that's 8 × 8 × 8 feet — it could be useful to complement broadband absorption with traps that target the enormous resonance that exists when all three dimensions are the same. In this case, the resonance is at 70 Hz, but similar resonances exist at all multiples of 70 Hz, so broadband trapping is still needed because the related frequencies must also be treated. (Besides, any room that's only 8 × 8 × 8 feet probably doesn't have space for a sufficient number of traps that target such a low frequency.)
MIX AND MATCH
Another important point, which again defies conventional wisdom, is dispelling the myth that you can learn to make great mixes in an untreated room. The biggest problem most people have when mixing is getting the right bass levels. Often a mix that sounds correct in your control room will sound boomy when played elsewhere. Most small rooms have a deep null at the mix position in the vicinity of 80 Hz. (The exact frequency depends largely on how far you sit from the rear wall.) As a result, you tend to mix with too much bass to compensate for what you're hearing. When a mix can be made to sound good both inside and outside your control room, it's said to be portable.
Studio designers often advise playing a commercial CD of the same type of music as you're mixing, with the goal of matching the bass levels to obtain a portable mix. The problem with that approach is that matching bass levels with a commercial CD works only if both songs are in exactly the same key. Let's say your song is in the key of E, and your room has a response similar to the one shown in Fig. 1. Whether the bass is playing a low E or the octave above, either the fundamental frequency or the all-important second harmonic will align with the deep null at 82 Hz, making the bass seem very thin even though it really isn't. But if the reference song is in the key of A, either the low A or the octave above will align with the enormous peak in the response. With those two particular keys, at least, trying to make a well-balanced mix by matching bass levels is doomed to fail.
I visit a lot of audio newsgroups and Web forums, and participants often ask if they should buy a subwoofer to improve their ability to mix. Although a subwoofer can help compensate for inadequate loudspeakers, it will not solve the problem of an irregular response caused by acoustic interference. Often a subwoofer just compounds and hides the problem. In truth, even if your monitor speakers cost as much as your house, the response you hear will still vary by 30 dB or more in a typical small, untreated room.
In this article, I explained some of the common acoustic problems facing owners of personal studios. I also showed why conventional thinking doesn't always apply to the small rooms often used for recording. If you'd like to learn more, I created a short video clip that illustrates some of the concepts discussed here (www.realtraps.com/videos.htm).
Understanding small-room acoustics is not especially complicated. All you need to know is that virtually all problems are caused by waves reflecting off of a room's boundaries. Armed with that knowledge, you are on your way to solving your studio's acoustic shortcomings.
Ethan Wineris based in Connecticut, where he and partner Doug Ferrara design bass traps and other acoustic treatment for RealTraps. Visit them atwww.realtraps.com.
FIG. A: A node occurs when direct and reflected waves that are out of phase with each other collide in the air.
Nodes, modes, and standing waves are three key properties that all rooms have, and they are closely related to each other. A node is a place in a room where a null or dip in the frequency response occurs. A node is caused when two waves meet in the air and combine out of phase (see Fig. A).
In a typical case, waves emitted from a loudspeaker reach a wall and are reflected back into the room. At some distance from the wall the original wave will have a positive pressure, while the reflected wave is negative — or vice versa. If the reflected wave is exactly equal in level and is exactly 180 degrees out of phase with the original, the waves would cancel completely at that particular location. At other levels and phase relationships, the waves will cancel to a lesser degree. (When they're in phase, they increase in level instead of canceling.) Total cancellation never occurs in practice, because no wall is 100 percent reflective at any frequency.
A mode is a natural resonance that occurs in a room, and the frequency of the resonance depends on the room's dimensions. A typical rectangular room has three fundamental mode frequencies: one for the length, another for the width, and another for the height. Sound travels at a speed of approximately 1,130 feet per second, so the resonant frequency between two opposite walls can be determined by the following formula, in which “Feet” is the distance from one wall to the other:
1,130 Frequency = _____ Feet × 2
Twice the distance is used because a wave travels from one side of the room and back to complete one cycle. Each dimension actually has a series of modes, because higher frequencies can also occupy the same distance. That is, wall spacing that exactly fits one cycle of 70 Hz also accommodates two cycles of 140 Hz, three cycles of 210 Hz, and so forth.
The most common type of mode is the axial mode, which occurs between two opposing surfaces such as the floor and ceiling. There are also tangential and oblique modes, which are weaker and thus have less impact on the room's response. Tangential modes complete one or more cycles after bouncing off four room surfaces, literally like a cue ball going around a pool table in a diamond shape. Oblique modes are weaker still and bounce off all six surfaces to complete one or more cycles.
A standing wave is a wave that's not moving — it literally stands still. Standing waves occur at node locations in the room, and they result when two equal yet opposite waves arrive from different directions and collide. A few inches away, just outside the node, the waves are traveling toward each other. There's no motion, though, at the one precise location where the wave fronts meet. (This is much like the isometric exercise of pushing your hands together.) Some people wrongly consider modes and standing waves to be the same thing, because standing waves can occur at modal frequencies. But they are not at all the same, because one is a wave and the other, a mode, is merely a propensity to vibrate. Moreover, opposing waves can create nulls at nearly any frequency in any room, not just those frequencies that correspond to the room's dimensions.
I want to share with you some conventional wisdom that is valid, in spite of the fact that few people understand why. Many recordists who don't have proper acoustic treatment have learned to play mixes in their cars in order to get a better sense of the bass levels. Of course, most car stereos are a poor second to a good pair of monitor speakers. Yet that method works surprisingly well, discounting the nuisance of having to keep burning CD-Rs to play in your car. Many people think that a car is a good place to assess mixes because we spend so much time listening there, but they also listen through their studio monitors.
As you know by now, acoustic reflections cause a series of peaks and dips throughout the entire low-frequency range. Those problems are much worse than an overall lack of bass or an overall increase in bass, for which you could more easily compensate. For whatever ills most car stereos have, they do not usually suffer as much from acoustic comb filtering, because much of the low-frequency energy passes right through the car's lightweight walls to the outside. By passing through the walls instead of being reflected, the low-end response is more uniform than in many rooms. You can easily prove this to yourself: play some bass-heavy music fairly loudly in your car, then roll up all the windows and get out. All you will hear outside the car is the bass that escapes through the walls and windows instead of being reflected.
Note that some car stereos, and many boom boxes too, have a permanent loudness-type compensating boost at the upper-bass range. That boost is intended to fool inexperienced listeners into thinking the system has more bass than it really does. If your car or boom box has such a boost, you'll have trouble hearing bass accurately. But many car stereos, including the stock stereo in my aging '93 Camry, have a surprisingly flat and extended low end.