A seemingly elusive topic full of misinformation and opinions from every side. Let's start off with what acoustic treatment is, and why having "flat" monitors isn't good enough.
My monitors are flat, why do I need treatment?
When monitors are placed into a room, the room ends up having a far greater effect in how you perceive the monitors to sound. If your mix position sits in a null (a dip in volume) at 100hz, you may just think your monitors of choice don't have a good bass response, when in reality the bass is being cancelled out through phase cancellation (when two signals combine and cancel each other). A room will also have modes, which are buildups of certain frequencies, or nulls, which are a reduction of certain frequencies. Furniture such as a desk, and untreated walls, will also introduce early reflections and comb filtering. Early reflections are when sound bounces from one reflection point, straight to your ears. Comb filtering is when sound blends with other sound waves, and similar to phase cancellation, cancels frequencies out.
In other words, putting your monitors in an untreated space could be likened to putting a graphic equalizer on your monitors and randomly pushing the faders up and down in a random fashion. You wouldn't do that to your monitors and expect an accurate representation of your music, would you?
Acoustic Treatment.
Acoustic treatment is the application of sound absorptive and reflective materials to achieve a desired listening environment. This can be achieved by using a combination of absorption, diffusion, and deflective/reflective surfaces. For this article we will only be looking at absorption.
Absorption is a material that does as the name implies, absorbs sound. Some materials are much better suited than others for this particular task. For instance, a 2" thick piece of pyramid foam will stop an echo in a room, but does nothing for lower frequency signals. The reason being the higher the frequency, the shorter the wavelength and easier to stop. For comparison, a 1khz sound wave is 13.5 inches in length, while a 100hz sound wave is 11.25 feet!! Bass takes mass to stop simply because the waves we want to target are very long and powerful. A room treated in "acoustic" foam is deceptive, as the shorter wavelengths are easily tamed making the room sound treated, yet leaves the low end wildly untamed. Mineral wool insulation is the preferred treatment choice of most pro studios, and is highly effective of stopping frequencies of all lengths when constructed correctly. The thickness of mineral wool traps can go anywhere from around 2" to several feet in depth as a means of heavy bas strapping in a studio construction. The panels in the above photo are what is known as broadband absorption, as they cover a wide range of frequencies in what they will absorb, and are the best bang for the buck purchase for a budding studio.
Proper Placement.
The best method would be to measure your room with a dedicated measurement microphone, and using a program such as Room EQ Wizard to tell you what is exactly happening in your space. Such a microphone can be picked up relatively inexpensively at around $100.
A relatively surefire way of treating a room though, is treating all the corners which is where bass builds up the most, and all first reflection spots. These include the side walls, ceiling above the desk, and back wall. While a combination of diffusion and absorption can be used, diffusion does need a minimum distance from the listener to be best effective, and therefore may not be an option to many in smaller rooms. The good thing about broadband absorption and bass trapping is that it is difficult to place too much in your room. If the highs become too damped, wood slats can be attached to the front of the panels to cause diffraction, and bring back some high end energy.
What should I use?
GIK and RealTraps both make professional room treatment options for those unable to DIY. Utilizing even just a few 2x4ft broadband panels will greatly increase the clarity of your monitors, and tighten up the sound significantly. Attacking the first reflection spots should be your first concern, along with a panel straddled across each corner to help control bass.
Having an accurate listening environment is paramount in getting the most out of your studios monitors. An accurate listening environment means less guess work in your mixes, and greater translation across playback systems.
For those that can DIY, effective treatment can be made for much less than purchasing, ad in most cases is more affordable than store bought absorption.
Next week's article will be on how to construct proper broadband absorption traps.