The EM Poll

The Well-Tempered Studio

Feb 1, 1999 12:00 AM, By Geoffrey Goacher

           

Improve the sound of your personal studio in three easy steps.

FIG. 6: The direct sound (green) and any early reflections (red) are heard as one sound.

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 graphed in Figure 7 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.

FIG. 7: An energy-time curve. The direct sound is shown at 10 milliseconds. The reflections in this graph, from 10 to 30 milliseconds, are the early reflections (a total period of 20 milliseconds).

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 (see Fig. 8). 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.

FIG. 8: Diffusers and cylinders scatter reflections in many different directions.

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

FIG. 9: The use of corner bass traps, strategically placed absorbers, and back-wall diffusers can significantly improve room acoustics and monitoring capabilities.

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.