The Difference with Bass and Midrange Treatment

In every room there is a division called the Shroeder frequency, which separates two regions that behave differently. Below this point, which is typically around 200 Hz, we are dealing with room resonances or room modes. In the bass range, we simply add as much absorption as we can to damp the ringing that results from room modes. We place bass traps in a room, their locations based on places where bass tends to gather – the corners. At higher frequencies, we focus our attention instead on room reflections.

Figure 1.0 – the Shroeder frequency (SF) divides two distinct acoustic regions

The measurement above shows frequency response as measured in the listening position. You can see the modal region where peaks and dips are spaced further apart. The key point here is not so much to determine the location of this frequency, but to simply understand that it identifies the need for different approaches above and below.

The difference between small and large rooms

Much of the literature and examples of acoustic treatment are related to large spaces such as auditoriums and commercial cinemas. It’s critical to understand that these spaces require a very different approach. The dimensions are often so large that room modes are not a significant factor as they occur below the low frequency cut-off of the system. The much larger dimensions also mean that reverb time is much higher. The surface to volume ratio is also much lower. All this means much more absorption is required. Often entire walls may be treated with absorption. In commercial cinemas, absorption panels are hidden behind heavy curtains.

In a home cinema, it would be a mistake to cover all the walls with similar curtains. Firstly, because panels are required behind the curtain to work in the midrange. Secondly, because this would make the room sound too dead. As a general rule, the smaller the room, the sooner it sounds dead as treatment is applied. Many make the mistake of copying the treatment scheme of commercial cinemas, failing to understand how such a treatment scheme suits one application but not a domestic system.

Key factors

Listening rooms vary a great deal and these differences lead to different treatment schemes.

Some of the factors affecting the treatment required for your room include:

* Room dimensions – includes the entire space seen by sound waves
* Room shape
* Scattering objects in the space
* Absorption present in the space
* Absorption properties of surfaces (curtains and carpet vs specular surfaces)
* Objects in the space

The first mistake that people make

A typical mistake is to simply add a rug to a timber floor along with curtains. These will often serve to tame a bright room enough to remove an objectionable impression, but in reality this isn’t a good treatment scheme. The problem is that rugs and curtains tend to only absorb high frequencies and the result is an uneven reverb response. This can be seen in the reverb time that is typical of rooms that have not been properly treated.

When I perform acoustic room analysis as a consultant, I often see a reverb time like the blue line in an untreated room.

Figure 1.1 - Reverb time of a treated (black) and untreated (blue) room

The plot above shows the reverb time plot of two different rooms. The room shown in blue is a medium sized untreated room. The measurement is typical of most such rooms, with a lot of midrange reverberation, but more absorption at high frequencies. Above 5k you can see considerable roll-off. The average is around 0.6 s. The black line shows a room that has been treated. As well as having a lower reverb time that is below 0.3 s, you can see that the reverb is more consistent, with high frequencies no more attenuated. The room shown above in blue could be considered an average listening room. The treated room is smaller, which also contributes to the lower reverb time.

What is the subjective impact? An untreated room has a fair amount of coloration and imparts its own character to the music. Some rooms sound pleasing without treatment more often by pure luck than by design. However, in the vast majority of cases, adding treatment will improve a room. The process is all about gaining control and creating the sound that you prefer. A treated room will often sound more “dry,” however it’s always a matter of choice.

First determine your goals

First we consider what needs to be changed about a room. Ideally, it’s best to start with a reverb time plot. These can be generated with the free program REW – Room EQ wizard and a measurement microphone.

It’s also a good idea to have a clear idea of what you would like to achieve. Do you want to create the impression of a larger room? Do you want to create a very dry sounding home theatre? Or do you want to retain some “room personality” but create a more pleasant sound? Do you need  to fix a bright sounding room? Do you want to create a big sound? Do you want to sit in the sweet spot and enjoy very sharp imaging? Or do you most want to create a larger than live experience that is more like a live concert event? These goals all affect the approach that one should take. One size doesn’t fit all.

Types of acoustic panels

The most common product for dealing with mid and high frequencies is an absorption panel. They may consist of foam in various profiles, sizes, shapes and colours.

Figure 1.2 – profiled raw foam

Other panels use rockwool, polyester or rigid fiberglass wrapped in fabric. Overly thin panels should be avoided. In general, a good midrange absorber should be around 50mm in thickness. The choice of material and fabric are related mostly to cost and aesthetics.

Raw foam panels are generally the cheapest but they aren’t especially attractive. Colour can be applied by a flocking process, which gives the foam a more attractive velvet-like appearance whilst applying colour

Figure 1.3 – flocked foam panels

* In a large or bright room, a treatment scheme might focus mostly on these panels
* In a smaller room, less of these by proportion should be used

Midrange absorbers

In some cases, selective absorption is desired. In a large room with an unbalanced reverb time caused by heavy curtains and thick carpet, it may be desirable to absorb midrange and avoid too much high frequency absorption. This is also true in small rooms that more quickly start to sound dead due to too much high frequency absorption.

One example is the Pyrotek Cosmo panel. It has a decorative metal facing that covers a semi rigid polyester absorber at the core.

Figure 1.4 – Cosmo midrange absorber (Picasso)

The panel absorbs midrange, but high frequencies are absorbed to a lesser degree. This serves to balance the impact of carpet, rugs and curtains, which tend to absorb only the highs.

The Cosmo panel comes with various designs, each with a different absorption profile.

Figure 1.5 – Absorption co-efficients of different Cosmo panels

Diffusers

As a rule, it’s best to focus primarily on absorption. Absorption panels are easier to use and generally more cost effective. However, diffusers also serve an important function. When used correctly, they tend to replace earlier reflections with later arriving ones which have been further attenuated. The sound energy is retained in the room and reflections that have been diffused generally sound more natural than those that are simply reflected off a flat surface.

The most common type of diffuser is a Quadratic Residue Diffusers (QRD). These come in two types – 1D and 2D.

Figure 1.5 – 2D QRD (skyline) diffuser

Figure 1.6 – 1D QRD diffuser

A little extra care is needed in the placement of diffusers as a certain distance is needed between the panel and listening position for them to work. This distance varies depending on the panel in question. If the listening position is close to a wall, special care is needed if diffusers are to be placed on this wall.

In most treatment schemes, diffusers should be considered as the icing on the cake.

Tips and suggestions

Preferences and goals

Many choices made in treating a room are related to the specific goals and listener preferences. There are many viable solutions to any system and room and the sound of a system can be radically changed with treatment. Many make the mistake of thinking that it’s all about creating a dead and dry sounding room that looks and sounds like a recording studio. In reality, using acoustic treatment well is about getting the result that you prefer. It is simply another aspect of your sound system that allows you to create the kind of sound that you want.

Don’t overlook the bass

The most common mistake that people make in treating a room is to ignore the bass. Virtually all rooms require bass traps to get the best bass performance. This aspect is covered in Part 1.

Take time to adjust

It’s important to realize that the initial impression of a new treatment scheme may be less positive due to a different prior reference. Sometimes it can take some time to adjust to a different sound. After an adjustment period, you may find that your preferences have shifted and that what you previously considered as “correct” now sounds wrong.

This has been a very brief introduction to acoustic treatment in domestic listening rooms. As with many aspects of audio reproduction there are choices that are always right, some that are always wrong and a considerable grey area that involves personal preference. When treating a room, one should always attempt to use as many bass traps as possible. One should always avoid making a room sound dead or leaving a bright room untamed. A rug is usually a good idea over a reflective floor, but curtains should be chosen based on decoration preferences rather than their acoustic properties. It’s a good idea to aim for a fairly flat reverb time plot that is usually at least a little lower than an untreated room.

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/