An inconvenient truth

In attempting to improve the bass in a room, we are met with an inconvenient truth – effective bass traps need to be large. One can have an effective bass trap or a small bass trap, but not both at the same time. To give some perspective on what it means to treat the bass range seriously, we’ll start with a fairly extreme example.

Figure 1.0 - Bogic Petrovic’s design with diffusers covering bass traps over all walls and the ceiling.

Petrovic’s design encloses the entire room within a single giant bass trap, as the plan reveals:

Figure 1.1 - Floor plan showing Petrovic’s conceptual layout where bass traps are shown hatched. The thick dotted line denotes timber slats.

This is an ideal solution to the bass problem, even though it isn’t realistic for a home system.

The next best thing looks more like this:

Figure 1.2 – Bass trap layout for a dedicated room

This room has vertical bass traps in the corners, each of them extending from floor to ceiling and straddling the corner. The pink shading around the boundary depicts a bulkhead trap. In a dedicated room, this layout should be attempted where possible. Doors and windows should be located away from the corners.

Back to reality

For many, this kind of “ideal world” thinking is merely a fantasy. Now we’ll start from the other end of the spectrum where most enthusiasts begin. Often the first thing people try are small foam traps. Some of them are only 300mm wide. The simple truth is that these are not real bass traps. I measured one such trap here:

Red Spade Audio blog: Small foam bass trap

The smallest bass traps worth using are 600mm wide. The smaller traps are only effective in the midrange.

Figure 1.3 – foam bass traps

Serious bass traps

Small foam bass traps can offer some improvement, but they really should be considered as auxiliary bass traps. The heavy lifting is best achieved with vertical corner bass traps that are more like 800mm wide and run from floor to ceiling.

Aim for one in each corner of the room. If your space isn’t a simple rectangle, but open out into other spaces, then attempt to place traps in the other corners.

In many rooms the corners of the listening area are not the acoustic corners. In this case, the traps should be placed in the corners as seen by bass sound waves, as shown below:

Figure 1.4 - Possible bass trap locations where the listening room is located within a larger acoustic space.

All the locations shown have good potential. In reality, which will be used is more a matter of pragmatics than acoustic performance. All of the traps could end up being located outside the actual designated listening room area.

Is there such a thing as too many bass traps?

Yes – just ask your wife!

From a performance point of view, you can never have too many bass traps. As you add more bass absorption, the bass response becomes smoother and the decay performance improves. The only downside is that you can end up a room that sounds dead. This is a mid and high frequency problem and it’s easily avoided by choosing traps with a membrane. In some rooms, especially large open plan reflective spaces, the extra absorption is desirable. In some situations, having bass traps with mid and high frequency absorption can mean reducing the amount of midrange treatment required. Small rooms may be reasonably well served with just a few bass traps.

What can I expect?

Bass traps improve the decay performance in a room and the subjective impact is that bass sounds tighter and much better controlled. The bass sounds much “faster.” The improvement when a room has been seriously treated can be quite dramatic. It is the kind of difference that can even be heard from another room without paying attention. My first impression on adding serious traps to my listening room was like moving the system outdoors. It was a startling improvement. Not many have experienced a room with serious bass damping and this is a big part of the reason why bass traps are not found in many home systems.

Realistic expectations

Aim for at least two large corner traps in a typical listening room. This is the minimum requirement for a noticeable improvement. Four is certainly better. As a rule, one should use as many as the room can accommodate.

Bass bliss is not automatically assured. Placement and integration are also critical to getting good bass. In many cases, especially where a subwoofer is used, EQ and measurements are also required.

Part Two of this guide will cover treatment for the mid and high frequencies. 

About the Author

Paul Spencer is a StereoNET Technical Contributor. Paul is a long time StereoNET member, and owner of Red Spade Audio, specialising in Room Analysis and Custom Audio Design.
For more information visit http://www.redspade.com.au/audio/