Good acoustics are often the dividing line between professional and personal studios. After all, the gear that’s used to record and mix much of what we hear on the radio these days is quite similar to what can be found in a well-appointed personal studio. Largely, it’s those big, beautiful, acoustically accurate rooms for recording and mixing that separate the big guys from the little.

But by giving a little attention to the acoustics in your own studio, you can improve the quality of your mixes so that they compete with the best the majors have to offer.

BASIC CONSIDERATIONS

Before getting into acoustics, make sure that there are no weak links in your monitoring system. Monitoring systems for critical listening must have a fairly flat frequency response from about 60 Hz (or lower) to 16 kHz (or higher). The power amp should also have as flat a frequency response and as low a distortion spec as possible. Fortunately, most of the studio-grade near-field monitors and power amps on the market today meet these specifications. Therefore, selecting the "right" system is often just a matter of personal taste.

The monitoring system must be set up symmetrically within the room. The distance between the speakers should be the same as the distance from each speaker to your ears, thus forming an equilateral triangle with your head. For near-field monitoring, your speakers should be about two to four feet apart, depending on their size and dispersion and what is most feasible ergonomically. Also, the center of this equilateral triangle should be equidistant from the room’s side walls.

Unfortunately, falling short of sonic accuracy is common, even when high-quality gear is placed symmetrically in the room. The overall sound is often boomy and muddy, the bass is too loud or too soft, the high-end is dull or harsh sounding, and the imaging is blurry and undefined. Room acoustics can play a significant role in creating, and reducing, these problems.

In pro studios, room acoustics are considered a top priority. Typically, owners spend lots of money on professional consultation, premium construction, and first-rate sonic treatments, sparing no expense to achieve problem-free, acoustically "neutral" monitoring environments. However, overcoming acoustical problems is not outside the financial realm of the personal studio owner. You should expect to spend at least the same amount of money for acoustical treatment as you did for your monitors.

The acoustical problems that occur most commonly in small monitoring rooms are room resonances (standing waves), speaker/boundary interference, early reflections, and poorly diffused late reflections. These problems can be overcome in three easy steps.

Step 1: Controlling Resonance and Reflections

The first step deals with low frequencies–from 20 Hz to 500 Hz. This frequency range affects the smoothness of the bass and low mids: if the room’s acoustics are balanced, the bass and low mids will be full and warm; if the room has significant frequency boosts in this range, the sound will be boomy or muddy; and if the room has significant frequency dips in in this range, the sound will be thin and hollow. The goal in Step 1 is to flatten out the room’s low-frequency response so as to avoid erroneously mixing music to compensate for the boosts or dips caused by the acoustic environment.

Resonance and standing waves. The way low frequencies behave in a room is dictated largely by the room’s dimensions. Certain frequencies, due to the lengths of their respective sound waves, are reinforced as they move between the room’s boundaries (walls, floor, and ceiling), creating resonant boosts in volume at those frequencies. These resonances are commonly referred to as standing waves.

You can estimate the most prominent resonant frequencies of a room by using the following equation:

f₁ = 1,130/2L = 565/L

In this formula, f₁ represents the resonant frequency, and 1,130 represents the speed of sound in air under "normal" conditions, which are defined as one atmosphere of pressure at sea level at 21 degrees Celsius. L represents the length of the room in feet. For example, if the room is ten feet long, there will be a natural resonant volume boost in the room at 56.5 Hz. In addition, natural boosts in volume will occur at multiples of this frequency: f₂ = 113 Hz, f₃ = 169.5 Hz, f₄ = 226 Hz, and so on.

These resonances become more closely spaced and their volumes diminish as you move up the frequency spectrum. Therefore, in small rooms, resonances are typically not as problematic above 200 Hz.

Speaker/boundary interference. Because low frequencies are omnidirectional by nature, they reflect from all nearby room boundaries. These reflections adversely affect low-frequency response, making the bass sound as though it’s coming from different directions.

These slightly delayed reflections of the original signal cause comb-filtering peaks and dips within the range of frequencies above the modal resonance range (typically 200 Hz in a small room) to an upper limit of approximately 500 Hz. The increasingly directional nature of sound above 400 Hz makes speaker/boundary interference less of a problem for mid and high frequencies.

Standing waves and speaker/boundary interference can cause frequency-response deviations as high as 15 dB. This amount of level variation could keep you guessing about proper levels for all the bass frequencies you mix.

Identifying the problems. The best way to tell if you have a problem with standing waves or speaker/boundary interference is through a combination of listening and measurement. First, listen to a finely engineered CD through your monitoring system at a decent mix level. The CD you select for this exercise should have a tight bass sound and minimal reverb. Some of my favorites are Tchad Blake’s mixes on Crowded House’s Woodface album and Sheryl Crow’s two latest albums, Sheryl Crow and the Globe Sessions. Tchad Blake uses interesting imaging and minimal reverb, which makes his mixes great for critical listening exercises. (But then, that’s the style of music I mix; you may prefer something else).

As you listen to these CDs through your system, notice whether the bass sounds tight, smooth, and consistent in volume. If the mix sounds full and warm, then your room naturally promotes good bass response. Larger "small rooms," rooms with lots of windows, and rooms with lightweight walls tend to balance bass frequencies nicely.

However, if the room rumbles and booms with the music, or if the bass either sounds mushy or alternates between high and low levels, then you may have a problem with resonance or speaker/boundary interference. At this point, it’s a good idea to take a precise measurement of your room’s resonant characteristics.

Simple measures. Measuring for room resonance and speaker/boundary interference requires a high-resolution frequency analyzer, rather than the usual octave or 1/3-octave real-time analyzer. Octave and 1/3-octave analyzers average out too much information to be useful for this task. Fortunately, some new software programs allow you to perform high-resolution acoustical analysis affordably from a computer (see the sidebar "Acoustical Programs for the PC").

For example, Figure 4 (in the print version of this article, from February, 1999) depicts a high-resolution, low-frequency response graph made with AcoustiSoft’s ETF 4.0 software. Measurements such as these can help you better identify the problem areas in your studio. Given the high resolution of the measurement, narrow notches in the response aren’t that bad, but you should pay attention to the general frequency-response trends. Notice, for instance, that the average signal level is around -16 dB, with a boost of 8 dB centered around 300 Hz, a 12 dB boost around 125 Hz, and one sharp 16 dB boost around 55 Hz.

Based on this measurement, you can guess that our monitoring system would sound muddy in this room because of the 300 Hz boost, too bassy because of the 125 Hz boost, and too boomy because of the peak at 55 Hz. Given that the specifications for the loudspeakers used in this test are flat throughout their low end, it can be assumed that these low-frequency response deviations result from room influences.

Fixing the problems. A good way to smooth out resonance and boundary-reflection problems is by optimizing the location of the speakers and the listener in the room. Resonance and boundary reflection are less pronounced in certain areas of a given room. In fact, changing the location of the loudspeakers and listening position often results in a drastic change in sound from the previous location.

Optimizing speaker and listener locations used to be a process of trial and error. Now, however, there are several PC-based programs that can model the acoustics of your room and help you find a location where room resonance and boundary reflections are minimized.

I used a speaker/listener optimization program on the same monitoring system shown in Figure 4. Then, after changing the location of the listening station, I remeasured the low-frequency response. The optimization program worked well: the average signal level is still about -16 dB, but the significant boosts shown in Figure 4 have been smoothed out. The only exception is that the boost at 55 Hz remains.

The frequency response still appears to have a lot of variation because of the high resolution of the measurement. If measured using a lower resolution of 1/6-octave or 1/3-octave, however, the frequency response would appear as a nearly flat line.

After optimizing the placement of the speakers and listener, you can do several other things to further reduce any problematic boosts in the low frequencies. Applying normal acoustical foam works well to dampen high- and mid-frequency energy but doesn’t adequately absorb low frequencies. You can absorb low frequencies by using a bass trap, which is any acoustical device that absorbs low-frequency energy in a room.

Often, the best place to put bass traps is in the corners of rooms, because that’s where low-frequency energy collects. As the low-frequency energy is absorbed, the various peaks are reduced, resulting in a smoother bass sound overall. The average listening room benefits from having about 1 percent of its total volume dedicated to bass trapping.

Many companies manufacture broadband bass traps, but one in particular, Acoustic Sciences Corporation (ASC), also offers affordable acoustical consulting for treatment of low frequencies using a test that they developed called the M.A.T.T. (Musical Articulation Test Tones) test.

Step 2: Reducing Early Reflections

The second step deals with frequencies of 500 Hz and up. This range has a critical effect on the accuracy of the monitoring system’s imaging and its mid- and high-frequency tonality. The biggest detriment to mid- and high-frequency accuracy is the presence of early reflections.

Early reflections. When listening to your monitors, you hear a combination of the direct sound from the speakers followed by the reflections of the direct sound from the room’s boundaries (walls, ceiling, and other hard surfaces). Reflections that hit the ear within 20 milliseconds of when the direct sound is produced are heard as part of the direct sound and are called early reflections. Because sound waves travel at a rate of about one foot per millisecond, most of the first reflections that make their way to the listening position in a small room qualify as early reflections.

Early reflections often add audible comb-filter distortion to the direct signal, tainting the frequency response with a variety of boosts and dips. Early reflections also tend to blur the stereo imaging between the speakers, making it difficult to accurately hear the exact position of sounds within the stereo field.

Identifying early-reflection problems. The best way to determine whether you have a problem with early reflections is to listen for and measure them. For this exercise, play a well-mixed CD that has clear and precise imaging, such as one of those mentioned in Step 1.

As you listen, notice whether the locations of the instruments are clearly identifiable in the stereo spread or whether they blend between the speakers. You should be able to hear the various instruments coming from specific points in the stereo field. Problematic early reflections, however, will degrade the aural clues that help us identify stereo imaging, and the resulting mix will sound blended and fuzzy.

Measuring for early reflections requires a high-resolution analyzer that can generate an impulse response, or an energy-time curve, of your environment. Or you can use one of several acoustics analyzer programs mentioned in the sidebar "Acoustical Programs for the PC" to generate an energy-time curve. This kind of measurement will give you a clear idea about any problems you might have with early-reflection levels.

The direct sound from the speakers is at 10 milliseconds, with the early reflections occurring between 10 to 30 milliseconds–a period of 20 milliseconds. In a balanced acoustical environment, the reflections between 10 and 30 milliseconds would be 15 to 20 dB below the level of the direct sound. In other words, early reflections should be virtually inaudible.

The early reflections are about 15 to 18 dB below the level of the direct sound for nearly 15 milliseconds. Although not perfect, this particular room would have good imaging and would be free of significant comb-filtering in the mid- and high-frequencies. Prior to this test, the room had already been treated with acoustical foam at strategic points to reduce early reflections. If measurements or listening tests confirm that your room has problematic early reflections, you should consider a similar treatment with acoustical foam.

Fixing early reflection problems. The goal in fixing early reflections is to reduce them to an inaudible level, which is typically about 15 to 20 dB below the level of the direct sound. This is where sound absorption materials, such as acoustical foam, work wonderfully. Companies such as ASC, RPG, and Acoustical Solutions market a variety of sound absorption products. Generally, it is best to use products that have sound-absorption coefficients greater than 1 and that absorb frequencies down to 400 or 500 Hz.

You don’t need to cover every inch of your walls with this stuff: using too much absorption can make the room sound too dry. Rather, determine the best places to put sound absorbers to reduce early reflections. You can easily do this by using the "mirror trick."

To perform the mirror trick, sit in the mix position facing the speakers. Then, have someone move a picture-size mirror flush along the walls, ceiling, and other surfaces to your sides and front. (You are allowed to turn your head, of course.) Any spot on the walls or ceiling where you can see the face of the speaker in the mirror should be covered with sound-absorption material. If you are unable to see the front of the speaker in the mirror, it is best to leave the surface alone. Once you get the concept, you can perform this operation without a mirror. It’s also a good idea to cover the wall space behind and between the speakers with sound absorption to reduce any diffraction reflections from the speakers.

There will be a noticeable improvement in your system’s imaging and frequency response once you have reduced early reflections in your room. You will immediately hear sounds in your mixes that were previously masked.

Step 3: Diffusing Later Reflections

The third step, which also covers frequencies of 500 Hz and up, deals with late reflections. Unlike early reflections, late reflections arrive outside of the ear’s integration time and do not necessarily affect the accuracy of the monitoring system. In fact, late reflections are desirable for creating acoustical "spaciousness" in the room. Without them, the room would sound like a dry, anechoic chamber.

The problem is that small rooms have such a low density of reflections that later reflections typically sound sparse and choppy in their decay. You can improve this situation by increasing the diffusion in the room.

The concept of diffusing late reflections in small rooms proceeds from the fact that mid- and high-frequency sounds typically reflect from a flat surface at a single angle only. However, when mid- and high-frequency sounds strike a diffusing surface on a wall (such as a quadratic residue diffuser), they reflect back into the room at many different angles. This results in a more complex spread of sound, which is known as diffusion. Spreading the reflections out in space and time also reduces their volume levels.

The best way to identify diffusion problems is by listening for them. Sit in the mix position, and have another person clap loudly in front of each speaker. This simulates the sound of transients coming from the speakers. Does the room have a noticeable echo at the mix position? Do the reflections sound well blended, or do they sound harsh and fluttery? If you notice echoes, your room would benefit from added diffusion.

Fixing diffusion problems. Improving diffusion is generally done by placing diffusive surfaces along the back wall of the room. For example, when sound strikes a cylindrical (as opposed to flat) object, it reflects into the room laterally over a 120-degree arc. This creates a uniform spreading of the reflection back into the room. Diffusers can be as simple as bookshelves or cylindrical objects, or as complex as primitive root and quadratic residue diffusers. However, some diffusers are much more effective than others.

The most effective diffuser is the quadratic residue diffuser. First conceived and proposed by acoustics researcher Manfred Schroeder, it was commercially introduced into the audio world by Dr. Peter D’Antonio of RPG Diffusor Systems, Inc. A quadratic residue diffuser is essentially a box that comprises a series of parallel "wells" of varying depths. The depth and width of the wells are calculated to give an effective diffusion of a specific range of frequencies. In addition, these units reflect sound laterally over a 180-degree angle.

The primitive root diffuser is also highly effective. Its well configuration is based on a different mathematical sequence than that of the quadratic residue diffuser. Both these types of diffusers are commercially available through RPG Diffusor Systems.

Placing a diffuser at each point where sound first reflects from the back wall will improve diffusion and will result in a more natural and "spacious" decay. Plan on covering about 60 percent of the rear wall with diffusers if you want to achieve a highly noticeable diffusion effect in the room.

Diffusion is like icing on the cake for room acoustics: it gives the room a pleasant, spacious ambience that often makes the room much easier to work in. Once you’ve completed this last step, your room’s configuration will most likely resemble the one shown in Figure 9.

Upon Reflection

An accurate monitoring system in a balanced acoustical environment allows you to clearly hear the imaging, tonality, and other nuances in your mixes. Smoothing out room resonance and boundary reflections, reducing early reflections, and diffusing late reflections are the best methods of improving a studio’s listening environment. As you go through each of these steps, take advantage of the various tools mentioned in this article, and don’t be shy about contacting people for advice.

In the end, good acoustics will not only make your music and your mixes more fun to work with and listen to, they will also increase your efficiency and make the task of mixing easier. Ultimately, you will turn out more reliable and professional-sounding mixes–and more of them.

Geoffrey Goacher is the founder of Acoustical Research Associates, which specializes in research and communications on audio and acoustics for critical-listening environments.

Acoustical Programs for the PC

High-quality acoustical measurement systems have traditionally been too expensive for the average audio enthusiast. These systems have therefore remained in the domain of acoustical consultants and audio designers, who have bigger budgets. Recently, however, significant advances have been made in the availability of affordable measurement systems for home-studio owners.

AcoustiSoft’s ETF Room Acoustics Analyzer, Liberty Instruments’ LAUD, and JBL’s SMAART are all affordable, Windows-based software programs that will turn your computer into an acoustics analyzer. Each of these companies can give you further instruction on taking and interpreting measurements of your room if you need help.

Several new programs are also available for optimizing location of monitors and the mix position. KB Acoustics’ Visual Ears, Pilchner-Schoustal’s Acoustics-X, and RPG’s Room Optimizer are three PC-based programs that will model the resonances and boundary reflections of your room and help you find an optimal location for loudspeakers and listeners. These programs usually work very well, provided your room fits the program’s criteria.