Room Resonance
What is Resonance?
Let's first define resonance in general. From Wikipedia: "Resonance...occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency...of the system, defined as a frequency that generates a maximum amplitude response in the system. "
In mechanics, this frequency depends on the mass, stiffness, and damping of the system. While all real systems include some type of damping, the concept is often illustrated using a simple mass-and-spring model.
At a certain frequency of stimulation, the undamped system will oscillate to higher and higher levels, growing quite literally out of control. The animation above shows the simple harmonic motion of an undamped system with an impulsive input. It continues moving back and forth with a uniform amplitude, never getting any higher or lower. Easy to picture, but not realistic.
When real systems (with some damping) are stimulated at resonant frequencies, they tend to "hold on" to the energy for much longer. Instead of spiraling out of control, they just continue to move...sometimes, for a very long time!
Resonance in Real Rooms
In this discussion, the system is the room geometry and construction, with the input stimulation coming in the form of sound waves, usually musical playback from your speakers. The byproduct is ringing, booming, droning...there are many names, but it causes a perceived increase in level at that frequency and makes sonic events last longer than they should. This is generally considered an undesirable aspect of musical playback or recording.
Each set of parallel surfaces in the room has some capacity for resonance at a certain frequency, with the main variable being the distance between the surfaces. A secondary variable is the stiffness of the surface, which can range from poured concrete to weakly supported 1/2" drywall. Calculators can predict the resonant frequencies of your room, but only measurements tell the real tale.
In rooms, we call these resonant frequencies "modes". As you see below, there exists a fundamental mode at the lowest frequency the parallel surfaces will support. Then there exist higher-order modes that are integer multiples of the fundamental. This is similar to a string on a violin, which produces a fundamental tone as well as harmonics that add to the richness and timbre of the emanated sound.
Again, this is easy to picture, but usually does not quite reflect reality. Actual rooms do not exhibit perfectly stiff, perfectly flat walls, with perfectly uniform distance between them. Flexure of the wall surface, a very slight tilt to a wall, and obstructions such as cabinets, bookshelves, and or pilasters will affect the strength and frequency of resonance. Finally, the presence of aggressive sound control devices, whatever their mechanisms, can alter the effective mode frequency in a real room.
What does not change is that for the frequencies which resonate between surfaces, the maximum pressure will be found at the boundaries, and in the case of higher order modes, at the mid point or 1/3rd and 2/3rd points. This is very useful information.
What Can We Do About This?
Optimizing the distances between surfaces and increasing the damping (at the right locations!) are the two primary approaches. We'll explain each of these in some greater detail.
Rooms built using preferred dimensional ratios usually have more even distributions of resonant frequencies, meaning the side-side resonance does not overlap with the front-back resonance. Generally, try to avoid building rooms that have matching width, length, or height, and also avoid integer multiples. For a more advanced approach, rooms built with angled walls or ceilings can exhibit less severe modes, but they may be harder to predict.
Here's the good news: no matter what the size or shape of the room, resonant frequencies can be controlled through the use of bass traps, which work as acoustic dampers. A portion of the sound wave's energy is converted to heat each time it impacts the trap (somewhere between 20 and 200 times per second) so it dies down much quicker than without damping. Does it matter where they are placed if they are to provide this damping? Yes, it does. For most types of bass traps (including TubeTraps), they should be positioned in the highest sound pressure zones.
Best Types of Bass Traps to Use
It may come as no surprise that we think TubeTraps are the best device for damping bass in your room. The primary reasons are twofold:
- TubeTraps operate by a principle of converting high pressure sound energy into high velocity sound energy. They do this by establishing a pressure differential between the interior and exterior of the unit, causing the air particles to equalize, or seek lower pressure (see entropy), moving through a flow-resistive material at relatively high speed. In this movement, the kinetic energy is easily converted to heat, and the wave's energy is reduced.
- TubeTraps are not tuned devices, meaning they work equally well across the entire bass range*, regardless of the specific resonant frequency in the room. This is in contrast to diaphragmatic or helmholtz-style pressure absorbers, which work over only a narrow bandwidth, leaving you hoping you predicted the mode frequency correctly - and that the addition of the treatment did not change it!
Hopefully with this you've learned a thing or two about room resonance and how to control it. Now, get out there and enjoy some music with your new knowledge!
*The low frequency extension of a TubeTrap improves as the diameter increases, but all models extend deeper into the bass range than most alternative treatments. Additionally, TubeTraps provide strong absorption all the way up past middle C, or 262 Hz.