The Taming of the Room

Jun 1, 2007 12:00 PM, By Robert Hanson

STUDIO DESIGNERS SHARE FIXES FOR COMMON ACOUSTIC PROBLEMS

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FINDING THE FIX

Even after identifying the chief problem areas within a studio, there is still the task of determining which materials will solve certain problems and how much of a given acoustic surface treatment is needed. And to further complicate things, each material has its own inherent strengths, weaknesses and special mounting needs.

“For absorption at midrange and high frequencies, there are myriad possible materials that can be used,” explains Hallowell. “We have used a number of products, including compressed Fiberglas, commonly available as Owens-Corning 703 or 705 insulation board, which is inexpensive, easy to work, but sheds fibers in the air. We've used acoustical cotton, which is similar to Owens-Corning 703 but produces fewer fibers. There is acoustical foam such as Sonex, sintered or foamed aluminum, Fiberglas duct liner, carpet, drapery and many other products. When using products that can release particles into the air, like Fiberglas, it is a good idea to cover the Fiberglas with an acoustically transparent material like panel fabric. For any product you have in the room with you, you should also be aware of how flammable it is.

“The above types of absorptive treatment can vary dramatically as to how much they absorb and at what frequencies,” he continues. “Most of that information is available online at the manufacturer's Website. Once you know how much is absorbed and at what frequency, you should calculate a rough idea of the final reverb time for the room using manual calculation or a software calculator. Each product you select can also be adjusted in terms of the frequencies absorbed by using thicker versions or leaving a gap of various dimensions behind the absorber to increase low-frequency absorption.”

“These products absorb energy by means of friction, which means that they are only effective when placed in the area of a wave's maximum particle velocity and not its maximum sound pressure,” adds Swist. “Any frequency's energy that is not transferred though a boundary will have a particle velocity of zero at that boundary and a maximum particle velocity at a distance of one-quarter of its wavelength from that boundary. So in practice, if someone mounts a 1-inch-thick Fiberglas panel directly on a wall, the effective bandwidth of the absorbed frequencies will only be 4 kHz and higher. So one can ascertain that the use of this type of direct wall mounting would severely color the reverberant characteristics of a room by absorbing only high frequencies and allowing the lower bands to remain uncontrolled.”

Swist explains that this problem is solved by mounting the panels away from the wall with frames or other mounting systems. “One can then calculate the effective absorption bandwidth of the panel by using the one-quarter wavelength measurement and incorporating that into the overall reverberant field design goal. Obviously, there is a limit to the distance one can mount these panels to obtain lower-frequency performance, and therefore other means are needed to control the low-frequency energy.”

Dealing with a buildup of low-frequency energy is never an easy task. The problem becomes all the more difficult to treat when dealing with the often-cramped dimensions of a spare bedroom or den that is being converted into a studio. “Low-frequency absorption treatment is a bit more specific,” explains Hallowell, who adds that there are many off-the-shelf products designed to absorb low frequencies. “Some are targeted for specific frequency ranges, such as RPG Modex, and some are more broadband, such as RPG Modex Broadband.” Hallowell stresses that whatever absorption products are installed, one should calculate the predicted quantities and types of absorption needed before installing. You can adjust the quantities and placement of the products, as needed, as you complete the installation.

“You should also consider where and how much diffusion you need in the room,” he continues. “There are a number of off-the-shelf diffusion products available, depending on your needs. A good option are quadratic residue diffusers, such as RPG Omniffusors and Diffractals, or the Auralex SpaceArray. You could also try mixed absorption/diffusion products such as RPG BAD [Binary Amplitude Diffsorbor] panels or Kinetics' TAD [Tuned Absorber/Diffuser Panel]. You commonly see an area of diffusion at the rear of the room behind the engineer in stereo mixing rooms to get a smooth return from the back wall.”

GUESS AND CHECK?

Estimating the type and quantity of acoustic material necessary to treat a room is only one part of the equation. To make the most of any potential improvements, users must develop an understanding of where and how to apply surface treatments, as well as how to check their work and make adjustments.

“You don't want to overuse the Fiberglas or foam because they don't absorb low frequencies,” says Yanchar. “So the more of that that you have in a room, the more you're boosting the low frequencies as a default. You have to balance that, which usually requires you to cover 50 to 60 percent of the room with materials like that. As far as placement, the corners are the most efficient place for countering both high and low frequencies. And then as far as reflections on the side and ceiling, the common way of optimizing the position of those materials is to have someone hold up a mirror and place surface treatment wherever you see monitors reflected in the mirror.”

“The only quantitative way I know of checking your work is to do a real-time analysis of the room,” says Hallowell. “Whatever type of analysis you do, it can be relatively expensive and requires a certain amount of knowledge about how to test and what to do with the results. Recent years have seen more and more lower-cost test units, such as the NTI AL1 Acoustilyzer and the Sencore SP495, but there is still a cost and a learning curve. The ears of an experienced engineer are also a valuable qualitative testing tool.” Hallowell also recommends taking a recording done in your room to other familiar rooms to see how it translates. “If your mix has less bass or high end than you thought when you mixed it in your room, you may need some renovation.”

BEST-CASE SCENARIO

While owning a purpose-built, professional project studio may be out of the reach of most working professionals, treating an existing room can yield some surprisingly good acoustic results. “Some rooms were designed well in the first place, whether by luck or good planning, so the best-case scenario might be only minor tweaking to get your desired sound,” Hallowell says. “If the room has good mode characteristics and sufficient noise isolation, then it may only need minor tweaking of the surface treatments or re-arrangement of the near-field monitor placement.”

“You can actually do a fairly decent job and get pretty good results,” adds Yanchar. “Basically, it all falls apart at some low frequency because of the space available. We've done some rooms in some very tight spaces that actually perform as well as or close to the biggest rooms we've done.”

“The best-case scenario is staggeringly good,” concludes Pelonis. “I have been custom-designing prefab, modular acoustical systems for decades. I have several projects every year that fit this description. If there is one thing I'd like to impress upon your readers it is the value of talking to a professional designer to help them with the treatment specifications, whether it's custom-designed or off-the-shelf. I think that many designers are open to working with situations that, for whatever reason, don't have the option of a complete build-out.” n


Robert Hanson is a former editor at Mix and Remix.

Step by Step: D.I.Y. Acoustics

Although it's impossible to account for the myriad variables that come into play when attempting to treat the acoustics of a project studio or production environment, designer Larry Swist has broken the process down into four basic steps that can serve as a rough guide for anyone who is interested in improving the sound of their workspace.

  1. Start by eliminating as much noise intrusion as possible. Place CPUs and hard drives in a separate room or closet, or purchase or fabricate some sort of soundproof enclosure. This is especially important for users who record live tracks within the control room.

  2. Create a wide-bandwidth absorptive surface in the front of the control room, right behind the monitor system. Special emphasis should be placed on creating a cavity in the corners that can be packed with high-density Fiberglas to obtain some bass-trapping effect. This usually has a profound effect on accuracy and imaging quality.

  3. Create a back wall in the control room with as many random surfaces and varying depths as possible. This will help create a more diffuse reflective condition. Many QRD-type diffusers are available on the market, but it's possible to approximate this effect with bookcases, CD racks and shelving. This approach does not conform to a specific number theory, and thus it does not have a predictable effect as with real QRD diffusers. And as with the front of the control room, it is extremely beneficial to try to provide some form of bass trapping in the back corners, as well. This can be done with a number of approaches, including framing and panels.

  4. If you have the luxury of enough ceiling height, it is beneficial to install an absorbent cloud system that hangs from the ceiling (at least over the console and mix position). These can be made from 2-inch, fabric-wrapped, high-density panels suspended on hooks and wire. Be sure your ceiling can take the added weight and that you mount these panels securely.
    Robert Hanson






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