The importance of room damping

Room damping or more specifically bass damping, is an essential part of accurate bass reproduction. All charts shown here are based on a REW simulation of a single subwoofer in the left corner in a small room 4.65m wide x 3.9m deep x 2.85m high. You quickly and easily generate similar results. They will differ based on the chosen listening position. Here we are focusing on the room itself, without any acoustic treatment.

What is bass damping?

Bass damping is simply absorption in the bass range. I refer to it as bass damping as we are dealing with room resonances that need to be damped. You can achieve it with room construction or added treatment. Here we will focus on the former. Plasterboard/drywall light framed walls work well as bass traps. Sound waves cause the boards to flex and in the process, energy is absorbed. The effectiveness is based on the depth of the air gap behind, the presence of insulation in the cavity and the mass and stiffness of the membrane.

Here you can see the impact of damping on the frequency response.

 

The top navy blue line is a room with minimal damping. That means a very low absorption coefficient of 0.1. You can see this is a poorly performing room with the worst peaks and dips. The tell tale sign is that they are all narrow Q (sharp and narrow). The best room has an enclosure with very high absorption coefficients (0.9). Each chart increases the absorption coefficient in increments of 0.1. A coefficient of 0.1 will absorb 10% of a sound wave. The black line represents the kind of damping you might see in an anechoic chamber. Obviously it isn't practical as a listening room due to likely cost and pragmatic issues. This level of damping would only be achieved with a great deal of added acoustic treatment.

Here are three selected rooms.

The navy blue line again shows the worst case scenario. The magenta line shows a room that is quite good and may represent many lightly constructed rooms with plasterboard/drywall and a timber floor. The green line shows a room that is very highly damped. It would probably require added bass traps, or special construction. 

Understanding waterfall plots

 

In this waterfall, you can see just how bad this room is. Not only are the peaks very sharp, but they also decay at a very slow rate. After 300 ms they have only decayed by 10 dB. While this is a problem in the bass range, it is even worse above 100 Hz where the ear is more sensitive to time domain effects. The result would be a cavernous sound. Fortunately few rooms would be this bad.

In this waterfall, you can see much better performance. Here one of the limits of the simulation becomes clear. In a real room, the bass region below 80 Hz would be worse, but above it would probably be better. Still, it isn't too difficult to see how this is a big improvement.

 

With very high damping in this room you can see that the response is much flatter and any peaks and dips are much broader. The decay is very even and rapid. If a room were built to perform this well (not likely), very little extra effort would be required for exceptional performance. Unfortunately this level of damping requires a very well designed room and extensive added bass traps.

Bass traps vs room modification

It is important to combine acoustic treatment with a room that also provides damping within its envelope. The room damping tends to be restricted in bandwidth and is not likely to do all that is required. If it is left out, the additional space required for larger bass traps may not be practical. The best and most sensible approach is to combine the two different types of damping. With this approach in mind, the bass traps used should be of the porous broadband type. Membrane-based traps are not likely to have the required bandwidth.

Bass trap couch

Can a couch also work effectively as a bass trap? That depends partially on where it is placed. If in the middle of a room, it isn't likely to be very effective. However, if you are in a small room where you are forced to place the couch very close to the rear wall, then it can work quite well as a bass trap. A couch is a fairly large area of a foam near the floor to wall junction. You can make it more effective by putting foam between the couch and the wall, and foam that fills in the space between the couch and the floor. The difference is shown here:

Comparing the two it seems that peaks and dips are changed. Not all the changes are an improvement. The dip just above 100 Hz related to boundary interference is made effectively worse, but this is mainly because it remains while the level comes up on either side. Minor dips are added around 150 Hz, but the peak and dip higher up are both smoothed out. 

Overall I'd call it a slight improvement and some improvement in the decay time is also seen. This is a good result, especially considering that no extra space is taken up in the room. A single foam mattress was placed in between the couch and wall. Most of the improvement is in the lower midrange, above 80 Hz.

Do small foam traps actually work?

The short answer is NO! Small foam "bass traps" achieve very little. Foam traps must be oversized in order to be effective, because foam isn't the ideal material. If you put four foam single bed mattresses in four vertical corners, you would get an acceptable result, but the 1x1ft foam wedges sold as bass traps are well short of being enough. Shown below is what I call a "minitrap" which is about 0.3 x 0.3 x 2m in size, placed in a corner.

 

Here you can see the response is virtually identical. When bass trapping is adequate, we should at least see the Q of the peaks smoothed out. 

The waterfall shows a more complete picture. Again it confirms no real improvement. A great deal more trapping is required. Grey is the empty room, green is with the minitrap.

Bass measurement update

 

A recent in-room measurement showing how effective the bass traps and EQ combination is. You can see a minor peak around 44 Hz. I have a major mode there, and a little more attention to getting the EQ right would bring that down. The consistency of the decay and the damping of modal ringing is the thing to note. For those not accustomed to looking at such plots, this is a very good measurement.

Thinking differently about room treatment

Introduction

The conventional approach to room treatment is often adopted without considering or being aware of the options I'm going to present here. The main key to getting it right is to consider the speakers and the room together as a system. Many of the ideas presented here are based on the work of Dr Earl Geddes, however, the opinions expressed are my own.

The conventional approach

In a typical dedicated or semi dedicated room, the conventional approach to room treatment usually involves the following:

• bass traps (typically removable)
• absorption panels for first reflections

The result is usually a fairly dead sounding room. The cost is quite high and the result often removes ambience from the room and takes some of the life out of the music in the name of improving imaging.

While this approach can work quite well, an alternative is presented here which many will find more satisfying.

Considering speakers and the room together

Speakers and the room together form a system. The relationship between the two is critical. Conventional speakers are often designed for their on axis response, while neglecting the power response. As a result, the off axis output which is radiated to the room has a different response. The ambient sound is therefore coloured relative to the direct sound. For this reason, room treatment is often used to reduce the damage.

An alternative approach is to first start with speakers that are designed to interact with the room in a way which does not require damage control. Some options include:

• controlled directivity speakers with waveguides
• open baffle speakers with behaviour approaching constant directivity
• omnidirectional speakers which radiate evenly in all directions

Some notable examples include:

• AI Audio loudspeakers or their Gedlee kit versions
• Linkwitz Orion or the Nao
• Linkwitz Pluto or B&O Beolab

What are we trying to achieve?

If we start with any of these speakers, we no longer need to be concerned as much about reducing reflected sound in the same way. The sound radiated by the room now matches the character of the direct sound. So what are we trying to achieve now?

I suggest the following guidelines:

• high level of bass damping
• modest amounts of diffusion
• little if any absorbing panels
  
• careful speaker placement

The bass challenge

Firstly, the room itself should provide a high level of damping in the bass range. Idealy this should include the entire envelope acting as a large bass trap. Installing multiple layers of plasterboard (drywall) joined with a flexible adhesive such as Liquid Nails will achieve this goal. If the existing structure is very solid and undamped (masonry/concrete), then false walls and ceiling is a good place to start.

When starting with a room with significant damping, the problem of room modes is much easier to deal with. In addition to starting with a well damped room, it's also advisable to place multiple bass sources in locations determined by measurement. Three bass sources will typically provide a good balance of price and performance.

Read more about the multi sub approach >

Broad band treatment

Most domestic rooms can be considered acoustically small. They require different treatment to large spaces such as a commercial theatre of concert venue. Unlike these venues, it's preferable in a home environment to retain as much reverberant energy as possible. As a result, we should start with diffusion. It's important to avoid placing diffusers too close to the listening positions. Nearfield placement results in poor performance. Ideal locations include the wall behind the speakers, ceiling and side walls.

Experimentation should be used with absorbing panels. When forced to sit with a wall close behind, it may be worthwhile placing absorbing panels on the wall. Idealy such panels should be as thick as possible and relatively dense to be effective over a broad range of frequencies. Panels that are too thin will only work at high frequencies.

Speaker placement

In nearly all cases, speakers should be given room to breathe. Firstly, the tweeter should be as close as possible to seated ear level. They should have at least one metre clearance behind and to side walls, but preferably more. In the case of omni speakers, Linkwitz recommends that they be placed wider and closer to the listener than open baffle speakers.

Read more about omni placement at Linkwitz lab >

Geddes loudspeakers have unique recommendations for toe in. Geddes recommends 45 degree toe in relative to the side walls. This means the axis of each speaker will cross in front of the listening position. As a result, the listening position will be off axis. This is discussed in an online discussion at DIY audio. View thread >

This recommendation may or may not apply to other speakers.

Conclusion

Ideally the decision to purchase speakers should be made while also considering room issues. If your room isn't a dedicated room and treament isn't an option, then it becomes even more important to choose speakers with a well behaved polar response. In this case especially, omni, open baffle and controlled directivity designs should be seriously considered.

More information is avaible in the white papers and scientific papers on the Harman International website >