What's the Deal with Acoustic Treatment?

What Does Treatment Do?

Back in the first section of this site, we talked about how sound is a pressure wave, moving through a medium. Well, it turns out that at the speed of sound, pressure waves misbehave in all sorts of troubling ways. Acoustic treatment is the effort to fix problems that are created by the nature of the room itself.

Common Acoustic Problems

Room Modes

A room mode affects the volume of specific frequencies by reinforcement and cancellation. This occurs when the dimensions of a room coincide with specific frequencies and the wave isn’t given enough space to decay naturally; in essence, it bounces between the walls so that the bounce lines up with the initial wave (and subsequent bounces), reinforcing itself over and over at one point in the wavelength, and canceling out at the other end of the wavelength.

We call the static locations in the room where a frequency has reinforced itself an antinode and a canceled zone a node. As we learned earlier, when waves are out of sync with each other, they are called “out of phase.” The easiest way to calculate trouble frequencies in a particular room is to use a mode calculator like this one. Calculators can only do so much in predicting modes, so testing a room after it’s constructed is always needed to determine exactly how it behaves.

Due to the physics involved, provable via simple algebra, the larger the room, the less the engineer has to worry about modal waves. If the room is large enough for all audible frequencies to decay without any modal interaction (as is the case in a large arena), the sound is similar to being outside. In small rooms, like a basement bedroom (the mainstay of home studios), modal waves are a huge issue.

Even though the most obvious and troubling waves are caused by waves bouncing between parallel surfaces (axial), modal interaction occurs between four (tangential) and six surfaces (oblique) as well, and all across the spectrum. The math quickly becomes complex enough that hiring a professional acoustician is usually the right way to approach any serious studio design.

The way we normally treat most modal waves is through absorption and diffusion, as explained below. Mic selection and placement is also a great way to minimize the issue with minimal effort.

Besides modal waves, the simple reflections off surfaces adjacent to speakers and the studio back wall can cause havoc and uncertainty with what the engineer is actually hearing. These issues are solved in similar ways as modal problems.

Flutter Echo

Flutter echo is the high pitch you hear after clapping in a room. It occurs when a high-frequency wave bounces repeatedly between hard surfaces—basically a high-frequency modal wave. Usually, it’s one of the easier things to treat, since high frequencies are readily absorbed or scattered. Thin studio foam and packing blankets do practically nothing for low-frequency problems, but they can easily solve issues like flutter echo.

Another common solution is to simply do away with parallel surfaces.

Comb Filtering

Comb filtering occurs when a signal takes multiple paths towards the listener, combining at different places in the wavelength. For example, there is a direct path from studio monitors to the engineer, and another path out to the wall, then to the same engineer; that second path takes slightly longer to reach the engineer, so it doesn’t reinforce the original wave perfectly. In fact, the amount and frequencies that are canceled out, when plotted on a frequency response graph, resemble a comb.

The same type of reflections can also destroy the stereo image that should be created when listening to right and left speakers; for example, the sound from the right speaker can bounce off the left wall, and appear (in part) to come from that direction. This issue only exists with mid to high frequencies, so it’s fairly easy to treat.

Outside Noise

There are many types of noise that can interfere with recording. The easiest to deal with is usually electrical hums and interference. The first thing you’ll notice in a pro audio rack is a power conditioner (the most common is made by Furman). This usually eliminates the 60Hz hum of a standard electrical grid, and helps filter out any other interference that may come through “dirty power.” Unshielded cables can also attract outside interference, similar to an antenna; it can get to the point where you record a radio station with your audio, or the audience can hear it coming through the speakers during a live show.

Every piece of equipment generates its own level of electrical noise. This can range from barely audible to distracting and unusable, especially when equipment fails. The level of noise in a given signal is called the “noise floor.” The lower the noise floor, the more the engineer may boost the signal without it affecting the final mix substantially. If the noise in a recording is too present, and it can’t be recorded any other way, there are algorithms that have various success at cleaning it up. The most popular program for audio rescue is iZotope RX 6.

Isolation from outside noise is usually the most expensive audio issue to treat. The sound that travels through a structure follows “flanking paths,” such as studs, pipes and ductwork. Since low frequencies require materials that are usually a combination of large and dense, they get expensive; getting a studio to sound completely isolated is usually prohibitively expensive. Getting a studio up to a professional standard of isolation usually requires walls within walls and a “floating” floor, all decoupled from adjacent surfaces. The noisier the neighboring area, the more difficult it is to isolate; it’s still possibly to get acceptable noise levels next to a freeway or airport, but it’s usually more expensive.

Fortunately, many instruments can be decently recorded with frequencies below 70Hz sharply cut, which takes care of the lowest rumbles (including most HVAC noises). That being said, pro studios still construct specialized air conditioning and heating vents to maximize isolation and minimize noise.

Common Acoustic Solutions

Some solutions are better than others, in both effectiveness and budget-friendliness. The first thing to consider is the space you have to work with. If it’s a bedroom in a house, the options are significantly different than if you’re building a commercial studio from the ground up. Pro studios often have floating floors, walls within walls, and no parallel surfaces. If you have a single, tiny square room in a basement apartment you’re renting, those things simply aren’t an option. There’s only so much you can do to improve a space that wasn’t built from the ground up to be professional; the most important step in terms of cost-effectiveness, time-savings, and sanity, is to accept the reality of your space.

If at any point in reading this and the related studio design section you decide to get a good pair of headphones and build a tiny iso booth, it may be the best decision you make. This goes for schools. Often a small isolation room next to a lab of computers, each with their own headphones, is the best solution for a music production class. That being said, there are many tools available to improve any space.

Absorbers are usually the first thing people think of when they hear “studio treatment.” They usually come in the form of studio foam, fiberglass/mineral wool panels, and velour curtains/packing blankets. Each of these materials is significantly different, so care should be used when selecting one for a given space.

A good place to begin comparing materials is an absorption coefficient chart like Bob Gold’s Absorption Coefficients; the number given represents a percentage of absorption at the given frequency (0.55 = 55% absorption). A quick read through will tell you that the more dense and thick a material is, the lower the frequency that will be absorbed. For this reason, 1″ studio foam and packing blankets don’t do much beyond preventing flutter echo. Velour curtains help a little more, but for real, low-frequency help, fiberglass and mineral wool is usually the way to go (thick foam can help as well, depending on the specific use). Also, different materials and placement help for different specific frequencies, so it’s good to know exactly what frequencies are causing issues and where they are located in the room.

The most common acoustic panel is made of rigid fiberglass, then covered with breathable (acoustically-transparent) fabric. To increase the low-frequency coverage, an air gap is maintained behind each panel, where space allows. When an absorber is specifically designed to treat low frequencies, it is called a bass trap; these are most commonly placed where two or three walls meet, since that’s where low frequencies build up the most. Other common places for panels will be discussed in the next section.

When more isolation is desired between artists in the same room, studios often use movable panels called gobos. Below are a couple of 4×7′ homemade, rolling gobos filled with 6 inches of mineral wool. They are sometimes filled with sand and built with a soft and a hard side, for different applications.

Homamade rolling gobos for absorptive acoustic treatment

A room full of absorbers is going to have an unnaturally low amount of reverberant energy. Anyone who has been in an anechoic chamber (a room with practically no reflections) can attest to the need for some reverberation. Enter diffusion. A diffusor is anything that scatters sound.

Every hard surface in a room is a diffusor, from non-parallel walls to bookshelves. In general, planned diffusion is the best, especially when it’s form and placement is backed up by measurements. Curved surfaces are common diffusors; they get rid of parallel surfaces and scatter sound at an infinite amount of angles. They are easily constructed with pliable wood and some insulation behind.

Another now-common diffusor is the quadratic residue diffusor (QRD). These come in two different forms; one is comprised of a series of long wells made of wood (1D), the other resembles a city skyline /inverted skyline (2D). QRD diffusors are based on elementary number theory and are very effective at diffusing over several octaves. A good, free program to calculate well depth/city skyline height is QRDude. Making your own QRD diffusors can be challenging. RPG, GIK Acoustics, RealTraps and other companies have pre-made panels available. A 2D QRD diffusor can look like this:

After absorption and diffusion have done all they can, resonators take on specific trouble frequencies that are hanging around. This is where exact measurements, simple math and exact carpentry usually meet. Resonators absorb a very narrow set of frequencies based on their construction; being off by a fraction of an inch can significantly alter their effectiveness.

One type of resonator is basically a rigid, flat, wooden box. The resonating frequency of the box should match the trouble frequency in the room, so it will absorb some of the acoustic energy. Inside the box is insulation, but not touching the resonating surface—this would dampen the vibration and lessen the resonator’s effectiveness.

Another type is the Helmholtz trap/resonator. There are many types, but the slot resonator is one of the most common. They’re constructed of slats of wood with slat width and spacing determining the affected frequency. Insulation is placed behind the slats so it touches them, and an airspace is left behind. They can be placed directly on walls or bridging corners. Often, slot widths are varied to affect more frequencies. Slot resonator calculators are readily available online.

As stated previously, isolation is usually the most difficult acoustic issue to treat. Here are a few of the ways pro studios accomplish their goals. Many of these methods are prohibitively expensive and an impractical for the home studio.

Isolated Slabs and Floating Floors: Instead of pouring a single concrete slab for the entire building, a separate slab is poured for each room to minimize sound transmission through the floor into adjacent rooms. The space between slabs is filled with expansion material, and the edges all have a haunch (essentially their own foundation). Slabs can also be floated with jacks or springs specially designed for that purpose. Sand-filled decks are sometimes used to aid in cable routing, and may marginally help isolation (especially as it pertains to keeping audio wiring separate from high-voltage lines).

Floating wooden decks don’t have enough mass to properly isolate a floor. Every solid assembly (like a wall or floor) has a resonant frequency that travels right through it; for a properly-built concrete slab, it sits at about 10Hz, which is where a studio floor needs to be to achieve maximum benefit. Resonant frequencies above 20Hz may help if chosen to help a specific issue, but will more likely either hurt or have no net benefit if applied without proper measurements.

Walls: It usually takes more than a simple stud and sheetrock wall to get an acceptable level of sound isolation. Figure 4.10 on page 63 of Home Recording Studio: Build it Like the Pros, by Rod Gervais shows that by varying the placement of materials, and not the amount, you can get better isolation. He shows a difference of 40dB of attenuation versus 63 dB between two placements. This demonstrates the importance of not just decoupling and having a lot of mass to isolate sound, but properly placing mass as well. His 63dB level of attenuation is achieved with two walls, with space between, insulation in both, and a double layer of drywall on the outside of each (no drywall on the inside!).

One thing that may be overlooked when constructing walls is the small gap that is often along the bottom of each wall. If the room isn’t airtight, then it isn’t sound tight either.

There are many other ways to help isolate walls, including existing ones. Some of these include resilient channels or RISC clips to decouple sheetrock from studs, staggered studs or double frame assemblies, light steel framing instead of wood (approx. the same benefit as wood with a resilient channel), and using concrete blocks.

Ceilings: The ceiling and floor are the same as walls; sound travels up as easy as down or to the side. However, gravity insists that vertical surfaces can carry much greater loads than horizontal ones, so a structural engineer should approve any increase in the mass of a ceiling (above or below it). The best time to consider isolation between floors is before construction. It’s easy to build a room within a room if the basement is 9 or 10 feet high; if the supporting structure is reinforced, it’s a simple task to add 1″ of gypsum concrete to the upstairs floor; a detached garage also avoids the issue altogether.

A structural engineer could approve several layers of added drywall to the underside of floor joists (between joists), then added fiberglass insulation. Resilient channels work as well on ceilings as walls to decouple existing ceilings from joists. Ceilings may also be suspended or independently framed.

Damping: Some products provide a non-rigid joint between two rigid surfaces. This converts some of the acoustic energy to heat within the mass instead of transmitting it into the room. A popular one is Green Glue, which adds some efficiency to multiple sheets of drywall without much additional mass.

Glass: Studios normally have windows, so that everyone in a session can see each other (ideally). standard window glass (float) and plexiglass are the least-effective materials for studio windows. Tempered glass is a little better. The best is laminate glass, similar to a car windshield; this type of glass has a plastic layer in the center that acts as a dampener. Having two sheets of glass with a few inches of air between and keeping the seal completely tight are standard practice. The mass of the glass should match the mass of the drywall.

Other: Doors follow basically the same rules as glass and walls—an airtight seal and enough mass is essential. Lighting and electrical are also sources of concern as far as both flanking pathways and electrical interference goes. The most common culprit for electrical hum are electronic light dimmers—don’t go there, if possible.

Resources for this Section

There are myriad reasons why this book is the first recommendation when people are looking for studio construction advice. In the author’s words, “I’m not going to bury you with math, nor confuse you with the complex analysis that an acoustic engineer performs . . . I’m going to show you the practical side of this industry . . . I am also going to let you know where not to waste your time and money” (p.47).

Room EQ Wizard is a free tool for analyzing your room. RTA mic not included.