In this article, we’ll examine what is a bass trap (bass absorber) and why we recommend putting bass traps in certain places in the room. We’ll explore the options and the benefits of each. The basics are simpler than most people think. First, some background.
No discussion of bass traps and bass trapping would be complete without an explanation of modal activity. Whatever the modal activity might be, efficiency is the name of the game in small spaces which means you’ll always want to start trapping where the buildup of bass is greatest. Low frequencies will build along every hard boundary surface (walls), but they will intensify in corners where two hard boundary surfaces meet. That means starting in the corners whenever possible, but it doesn’t necessarily end there. There are also great bass trapping benefits to be had by trapping the perimeter walls. Let’s look at how bass trapping works and where to do it most effectively.
How it works
It’s not magic, just physics!
One of the questions we get the most here at GIK is, “how do bass traps work?” Remember your high school physics? If not, here’s a simple crash course on energy conservation.
Energy is energy no matter where it’s derived from. The First Law of Thermodynamics states that energy cannot be created or destroyed. Energy can only be changed to another state. In other words, it can change from solid to liquid to gas and back again, but the total amount of energy available in a system remains constant. These changes have observable effects on the way we experience the world. When you remove heat from water, it turns to ice. If we give some heat back to the ice, it returns to a liquid state. Give it even more heat energy, and it turns to a gas. The amount of energy has not changed, only how it is perceived.
Think of room acoustics as that same concept in reverse. We’re taking the motion of a gas (air) at a certain temperature and causing it to slow down via friction. That friction gives off heat to conserve the total amount of energy. When some energy is turned into heat, the intensity and strength of the waves is reduced. Rub your hands together quickly. Feel the heat? The same principle is at work. The density of our hands is just much higher than the density of air. Still, the friction is resisting our attempts to move our hands and turning some of that motion energy into heat.
Whenever you turn on a source emitter (like your speakers), you’re essentially filling the room with energy (motion) that bounces off of every surface available. Bass traps work by providing resistance, generally in the form of an insulation material with the right properties, specifically proper gas flow resistivity. When the energy in the room encounters the insulation material, friction causes the kinetic (motion) energy to be changed to thermal (heat) energy, which results in a loss of amplitude. That reduction in amplitude brings peaks down and valleys up by reducing the strength of one or more interfering waves. We experience that as a reduction in volume as peaks are decreased and an increase in volume when valleys are brought up.
Where to place them
Start in the corners.
Why? Is there something magical about corners? Yes, sort of. Low frequency pressure builds up wherever there is a boundary. Each room dimension has 2 boundaries which are generally parallel to each other. It’s actually intuitive if you think about it. If you have pressure building along each wall and traveling to the outside ends, it stands to reason that it will pool in the corners since you’re now where you’re at 2 boundaries. When you place a panel at 45 degrees across a corner, you’re getting the most ‘bang for the buck”; you’re producing the greatest attenuation using the least amount of space. This is because you’re not only creating an air gap behind the sound absorber, but you’re also at the ends of 2 or more of the room’s dimensions. Efficient use of space is critical in small spaces where bass trap placement options can be limited.
People ask a lot whether they should fill the corner with something like our Tri-Trap Corner Bass Trap© or straddle the corner with a panel (GIK 244 Bass Traps or Monster Bass Traps). While the question has to take into account other factors, all things being equal adding mass back into the corner significantly increases the conversion of kinetic energy to thermal energy. Why? That’s a little harder to explain, but Jeff Hedback of Hedback Designs put it very well: “An appropriate analogy might be that the flat corner panel is like driving through downtown Atlanta during rush hour while the Tri-Trap© is like the same drive but adding 10 miles of construction zones, lane restrictions and traffic cones. It wears you out!”
The bottom line is, fill the corners if you can. If you can’t for whatever reason, don’t let that stop you from straddling the corners with panels 4″ thick or thicker. As a matter of fact our Monster Bass Trap is actually designed to provide a greater boost at 80Hz than the Tri-Trap©.
Flat walls are good candidates for bass absorption too.
We’ve done a lot of talking about corners and modal problems. That said, not all bass problems in a room are modal in nature. While corner bass trap placement is very efficient and will help with overall decay time control, not all of the frequency related problems can be solved purely by treating corners.
Some of the deepest valleys and highest peaks come off of an untreated back wall. Usually that’s going to be the longest dimension of the room, so thicker panels, like our Monster Bass Traps are the right way to go. Center the Monsters on the back wall to help reduce the bass reflections off of the rear wall to reduce the intensity of those reflections and thus, the intensity of any cancellations caused by these reflections. If you have them on stands or have the space in general, give them 4-6″ of airspace. The increase in absorption numbers is almost the same as adding that same amount of insulation; you’re making a 6″ panel perform closer to a 10″ panel.
Our Monster Bass Traps are already stellar performers well into the deep bass so don’t worry if you can’t get the gap. You’re still getting excellent results.
Another on wall situation where bass control is appropriate is behind the monitors. In small rooms, monitors end up being very close to boundaries. This can cause serious frequency response issues due to SBIR. To learn more about SBIR, see our article written by Bryan Pape.
Most rooms benefit tremendously from broadband treatment directly above the mix position as well. Something like our 244 Bass Trap is perfect. The reason is pretty simple: many of the rooms we work with have ceiling heights under 9′, which is often the smallest dimension in the room. That means that the primary mode, where there’s the most energy, will be all the way up in the 60Hz-70Hz range. That creates big problems in critical places in the spectrum as there will still be plenty of energy available to cause problems into the 2nd and 3rd octave. The 244 Bass Trap is designed to get well down into that range even when flat mounted on a surface.
Very small rooms benefit a great deal by using 4″ absorbers (like our 244 Bass Traps) on the side walls as well. It’s almost impossible to not sit in a bad modal position of the width dimension in a small space. Having something like a pair of 244 Bass Traps directly to your sides can do a lot to minimize these problems.
Every small enclosed space is going to need low frequency control in order to be enjoyable and useful. Otherwise modal activity, SBIR, and random wave interference, both constructive and destructive will dominate the sonic landscape. There are several ways to do this, and each has particular advantages in terms of pure absorption, aesthetics, space-use and even budget; all of these things need to be taken into account when making bass trapping decisions. You maybe constrained in one of these areas, and that’s okay. Here’s something we say a lot here at GIK: don’t let something you can’t do stop you from doing the things that you can.