The acoustical behavior of your room is quite literally the foundation for its audio performance. Poor room acoustics can prevent you from ever having a great sounding room, regardless of how good the sound system is. On the other hand, great acoustics simply make everything sound better, whether or not a PA is involved.
Thin panels only absorb high frequencies, so overuse of these can dull the sound in a room.
Let’s look at some of the considerations for getting a room to naturally sound its best.
First, we need to understand that there are several categories of acoustical concerns. One of these key areas is the room geometry and construction methods. There’s an old joke among acoustical consultants that sometimes the best acoustic treatment for a poorly designed room is a bulldozer. That’s because the way a room is built, both in terms of construction materials and methods, and in the shape and size of the room, determine a lot about its behavior.
Problematic issues like sound transmission (unwanted sound leaving or entering the room) and standing waves (resonances, essentially) cannot be fixed with acoustic panels; they must be prevented during construction. The concept of ‘soundproofing’ a room by installing a bunch of panels as a solution, is a myth.
Another category is mechanical noise concerns, such as the level of background noise generated by an HVAC system, or the rumble from nearby equipment. We won’t dive into this area in this article, but keep in mind that this is another piece of the puzzle that is very difficult to address once construction is complete, but it is relatively easy to plan for ahead of time.
The next category, and the one we’ll spend the most time on in this piece, pertains to interior acoustics. This is the area concerned with things like the quality and decay time of reverberation, and beneficial versus distracting reflections. It’s the one area of acoustics that can be pretty easily shaped after construction, and it can make a big difference in the overall sound quality of a room, whether it be a worship space or elsewhere in the facility.
The most obvious and common interior acoustic treatment product is the absorptive panel. These are usually fabric-wrapped wooden frames with absorptive material inside, such as Fiberglas or Rockwool. The most important thing you need to know about these is that the thickness of the panel determines how low in frequency it will be effective.
Thin panels only absorb high frequencies, so overuse of these can dull the sound in a room. Using thicker panels (for example, 4 inches thick) will do a better job of curtailing lower midrange frequencies that may be contributing to “muddiness” in the room (see Table 1).
It is also possible to use acoustic panels where the front surface is reflective for high frequencies (often by using Fiberglas with foil on one side and facing the foil side outwards). This has the benefit of absorbing midrange frequencies while not overly absorbing the high end.
The absorption coefficient “a” indicates the amount of energy absorbed at each octave band; 1 is the theoretical maximum, meaning all energy is absorbed at that frequency, although test conditions often result in slightly higher readings. Notice how much better the 4” panel does at absorbing energy in the 125 Hz and 250 Hz octaves).
Another very useful absorption product is the acoustical banner (sometimes referred to by the trademarked name, Lapendary banners). These are sheets of insulation wrapped in either fabric or vinyl, hung from the ceiling either sideways (like a banner) or catenary style (picture a hammock). Rooms that require a lot of treatment often benefit from these, since the ceiling can be a useful place to pack in a lot of product.
Determining the amount of needed absorption is somewhat straightforward, but not entirely within the scope of this article. Nonetheless, I will give you a head start, while leaving you with a bit of homework to research the rest. The following formula can roughly predict the reverberation time of a space, if you know the total volume, absorption coefficients of the products you intend to use, and the total surface area (in square feet) that you will employ with those products:
RT = 0.049 V / (S * a)
where “V” is the room volume in cubic feet; “S” is the total surface area of absorption products in square feet, and “a” is the absorption coefficient of the octave band you want to predict.
The trick is to get the really important octave bands (mainly those centered at 125, 250, 500, 1,000, 2,000, and 4,000 Hz) to have similar reverb times, and remember that other room treatments (like carpet or acoustical ceiling tiles) will also play a part.
A good average for many medium-sized rooms might be in the 1.5 second territory, although that certainly varies, depending on room size and program content. Shorter reverberation times are often better for speech intelligibility and in small rooms, while somewhat longer times are more natural-sounding in larger rooms and are also usually better for unamplified music (think orchestra, choir, and organ).
While absorption products may be the first thing people often think about when considering acoustic treatment, and they are usually necessary on a decent percentage of room surfaces, we don’t want to automatically use them to treat all acoustic issues.
Sometimes we have specific reflections (such as a slap from a back wall, or a flutter echo between two parallel walls) that we need to remedy, but we don’t want to overly absorb the room and make it too “dead.” That’s where diffusion comes into play. With diffusers, you get the benefit of breaking up problematic reflections, while otherwise retaining the acoustic character of the room. Diffusers are usually considerably more expensive than absorption products, but they are a very important part of having a balanced-sounding room.
Finally, sometimes we actually want to add points of reflection to a room. Acoustic clouds, for example, purposely reflect energy coming from the stage back down into the audience, thus helping acoustic energy from the stage stay focused to the seats.
Clouds can also help enhance the feeling of corporate worship, because the congregants can hear each other better. You may have also noticed “shells” behind an orchestra or choir, where the reflected energy helps the performers hear each other better and helps direct their energy toward the audience.
As you can see, absorption is only a piece of the puzzle, albeit a very important one.
At the very least, you should consider the different absorption capabilities of different panel thicknesses and remember that it is possible to overdo absorption.
In addition, plan for diffusion, particularly along the back wall (to break up a nasty echo from the PA) and to mitigate flutter echo between parallel surfaces.
Finally, an acoustical consultant can work wonders with your room, and (in my opinion) should be part of any construction or renovation project.
Although this introductory treatment of interior acoustics may get you pointed in the right direction, and make you aware of some of the considerations, there’s no substitute for a seasoned professional.