The article titled "Your Studio's Control Room" tackled the subject of acoustics in enclosed spaces, applied to the control room of a recording studio. With the goal of achieving a reliable listening environment in which to make precise decisions during recording and mixing, the article introduced some concepts such as a room's natural modes, how to calculate them and how they affect a room's response. It also discussed various considerations when placing monitors in the control room and, finally, stressed the need to back up all theoretical calculations with empirical measurements, and how to do so. These measurements let you evaluate how the room actually behaves and thus design acoustic treatment suited to the room's problems.
This article picks up from there, examining the different acoustic materials available to solve these problems, as well as how to use and place them. Frictional absorption, bass traps, diffusers... these elements can help us achieve a good sound field in the control room if they are used correctly.
To show where and how to use these materials, we present a well-known design for recording control rooms, and in the same spirit as the previous article, many of the solutions presented can be implemented without a large financial investment.
Types of materials
In general, there are two ways to get rid of unwanted reflections in a room.
The first is absorption, which uses materials that reduce the energy of reflections, making them less harmful.
The second method is diffusion, which consists of "breaking up" the waves so they reflect in different directions, thus avoiding sound focusing.
Each solves specific problems, and they are generally used in combination to achieve an even sound field.
Absorption
Absorption reduces the energy of reflections by turning it into other forms of energy, usually heat or motion.
Every time a sound wave strikes a surface, part of the wave is reflected and part is absorbed. The ratio between the amount of energy absorbed and the incident energy is called the material's Absorption Coefficient, usually represented by the letter alpha (a).
Because of the huge differences in size among audible wavelengths—from about 17 mm to about 17 meters—materials don't usually absorb all frequencies equally. That's why the absorption coefficient is expressed as a function of frequency, in octave and/or third-octave bands.
Absorption coefficients of some materials
The NRC (Noise Reduction Coefficient) is an average of the coefficients—rounded in steps of 0.05—at 250, 500, 1k and 2k Hz, and it is used to indicate an acoustic element's absorption with a single number.
Although it can be useful for getting a quick idea of the degree of absorption you can expect from a material, it doesn't really define the material's behavior (materials with the same NRC can behave in very different ways, as you can see in Figure 1), so it is better to study the various absorption coefficients to determine the right material.
Materials with the same NRC and different behaviors
With the same NRC, material A would be suitable for treating high frequencies, and material B for low frequencies.
Porous materials
High and mid frequencies are treated with this type of element. Also called frictional absorbers, they work as follows: the waves penetrate the pores, and the friction of the air particles against the material's internal walls reduces their motion, turning acoustic energy into heat.
Rugs, carpeting, curtains, tapestries, clothing... all of these are porous materials that absorb high frequencies, and their presence shouldn't be ignored. A clear example occurs in live situations, where it's common to correct the highs during the show to compensate for the absorption effect of the audience's clothing.
Professional acoustic materials are usually made from foams with a rigid skeleton. The pores in the foam cause frictional absorption, and the rigid skeleton increases this effect: the less the material's particles move, the greater the relative motion of the air against them, and therefore the greater the absorption.
However, these materials have little or no effect on the lowest frequencies. For long wavelengths the small pores are virtually "invisible," and the waves reflect as if they were striking a smooth wall.
It wouldn't be the first time someone had a bass problem and tried to fix it by covering the walls with carpet... the result is that the problem gets worse, since the new high-frequency absorption exposes the excess low end (which will have hardly been affected).
Thickness and distance
The thickness of the material and the distance between it and the wall it's mounted on affect its absorption capacity.
Recalling a concept from the previous article, there are pressure maxima at the room's walls. This means that at those same points there is a minimum in the velocity of the air particles, since at the moment they reach their maximum excursion from their starting point, their velocity is zero before they start the return trip.
Therefore, if the material is placed a quarter of a wavelength (λ/4) away from the wall, it will be located at the point where particle velocity is greatest, increasing friction and with it absorption.
Figure two shows a graphic representation of a wave's pressure and velocity, where you can see the point of maximum velocity. Frequencies whose quarter wavelength matches (or is similar to) that distance will see their absorption by the material increase, since the material will be located at the point where the particles move the most.
Likewise, it's easy to deduce that the thicker the material, the greater the number of frequencies whose λ/4 falls "inside" the material, thus widening the absorption bandwidth toward the mid frequencies (Figure 3).
In short, the bandwidth and absorption capacity of porous materials increase if you leave an air gap between the material and the wall, and also the thicker the material is.
Bass traps
This is the name given to devices that deal with low frequencies. These waves are large and carry a lot of energy, so they are the hardest to combat. Depending on the absorption method, we can talk about active traps and passive traps. The former work by getting certain elements (usually rigid, flexible panels) to resonate when the sound wave strikes them, turning acoustic energy into motion.
The latter are usually made of very dense foams that work because of the trap's own size and its placement (usually in corners), preventing the waves from building up there and thus damping their energy.
The bandwidth of active traps is usually narrower than that of passive ones, although it can be widened by placing absorbent material inside them.
In general, active traps work better for treating specific resonances, since their absorption is greater and focused on certain frequencies. That's why they must be designed and placed carefully, because a mistake can make the trap not work or, worse, work at the wrong frequencies.
Passive traps, on the other hand, usually offer less absorption but act more evenly, so they are ideal for evening out the room's low end in general. However, if large amounts of absorption are needed, the size of a passive trap can skyrocket.
Diffusion
As for the second method of combating unwanted reflections, diffusion, its aim is to prevent sound focusing by scattering sound rays in multiple directions.
To do this, diffusers usually have geometric shapes of various sizes and arrangements, so the wave reflects differently off each one, resulting in a more even sound field. The more variation in sizes there is in the diffuser, the wider the frequency range over which it is effective.
Figure 5 shows the Skyline diffuser from the manufacturer RPG, and Figure 6 shows Auralex's Space Array. In both cases you can see the variation in sizes and depths of the geometric shapes that make up these diffusers.
Similarly, a bookshelf with books of different sizes or various decorative items placed on a wall can work as makeshift diffusers.
LEDE
Now that we know the most common materials in acoustic treatment—frictional high/mid-frequency absorbers, bass traps and diffusers—let's look at the "instructions for use."
One of the best-known designs for recording control rooms is called LEDE (Live End - Dead End), in which the wall behind the listening position has diffusers to improve dispersion (Live End), and the opposite wall, from which a large part of the monitors' early reflections arrive, is heavily damped (Dead End) to prevent phase cancellations and improve the stereo image.
The side walls usually get treatment to control the remaining early reflections and prevent flutter echo, and the ceiling usually has a mix of absorption and diffusion. Bass traps are added if needed.
Although there are naturally more complex and modern designs, this technique has been widely used because it is easy to implement and effective. With this design in mind, let's see how to place the acoustic materials.
Placement
The idea is to cover the wall behind the monitors with material that absorbs high and mid frequencies. Acoustic foam is ideal, but if you don't have enough money, a thick, heavy curtain or a tapestry can help control the early reflections coming off this wall. As mentioned earlier, leaving some distance between the material and the wall will increase performance and widen the bandwidth toward the mid frequencies.
If you don't have enough material to cover the whole wall, or if you find that this is excessive treatment, start by covering the areas around the monitors and the space between them.
As for the side walls, the goal is to reduce the early reflections coming from them, since they can cause comb filtering at the listening position.
To find the points where these reflections occur, you can use a very simple technique based on geometric acoustics: while you stay seated in the listening position, someone slides a mirror along the wall. The points from which you can see one of the monitors reflected (especially the tweeter, since the highest frequencies are the most directional) are first-reflection points, and that's where you should place absorbent material.
With this same technique you can locate the points on the ceiling where material should go, and if you have one, a diffuser placed right above the listening position will improve the stereo image and the sense of space.
As for the back wall, you can place diffusers opposite the monitors, with some absorbent material between them, or vice versa.
Finally, bass traps should be placed in the corners formed by the walls and the ceiling, starting with the front ones (closest to the monitors) and then moving to the rear ones.
If you need to place more than one trap in each corner, start at the ceiling and work your way down toward the floor, and if there isn't room to place them vertically, they can be placed horizontally (in the corners formed by the walls and the ceiling).
Flutter echo
This phenomenon is caused by mid- and high-frequency waves reflecting back and forth between two parallel walls, and it can impair sound clarity and speech intelligibility. You've probably heard this kind of echo when clapping your hands in an empty room: it's that ringing, metallic sound.
Naturally, the critical point where this phenomenon must be controlled is the listening position. To do so, absorbent material is placed on both side walls at the height where your head usually is in the listening position.
Other considerations
When placing absorbent material on parallel walls, the pattern between them should be opposite; that is, the spots with material on one wall should face the untreated spots on the other, and vice versa. The only exception is flutter echo treatment, where it's best for the material to match on both sides.
Be careful not to "overtreat" the room. Working in a room that's too dead in the high and mid frequencies isn't pleasant, and likewise excessive bass absorption can be disastrous. In control rooms, no more than 45–70% of the total wall and ceiling surface should be covered with absorbent material.
There are many resources on the Internet for building acoustic treatments, especially bass traps. Be careful with them. Just because they're online doesn't mean they're reliable, and most of these traps are active designs, which, if poorly designed, can have negative consequences for your room. If you go the "do it yourself" route, it's advisable to follow one of the books on acoustic treatment available in specialized bookstores.
If, on the other hand, you decide to buy the acoustic material, many manufacturers now offer complete room-treatment kits that are cheaper than buying each material separately. The seventh figure shows one way to place the materials included in Auralex's Deluxe Plus Roominator.
Whatever treatment you use, always check the result with measurements in the room to make sure the new materials have the desired effect.
Conclusions
Through the ideas presented in these two chapters devoted to control room acoustics, we have introduced two subjects as vast as they are fascinating: room acoustics and acoustic treatment.
As mentioned in the previous article, hiring a professional with the right knowledge and tools is always the best option for tackling a subject as complex as the acoustics of a recording studio.
However, it's also true that there are a number of basic measures that can greatly improve the sound field in a control room and that don't have to involve a large financial investment.
If you've followed these tips, you've probably noticed improvements in your studio control room's acoustics. Feel free to visit the Home Studio section of the ISP forum and send us your comments.
NOTE: the previous article mentioned a free spectrum analyzer—Inspector Free—from the company Elemental Audio. This company has been acquired by Roger Nichols, and the new address for the free plugin is www.rogernicholsdigital.com, Downloads section.
