RealTraps - Rear Wall Treatment Strategies

Tips for avoiding damaging reflections

..MANAGING THE REAR WALL..

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By Jim Lindenschmidt

By now, most people interested in improving the sound of their listening rooms are familiar with the two beginning maxims of room treatments: add bass traps to as many corners of the room as possible, and create a Reflection-Free Zone (RFZ) at the listening position. In general these are the best places to begin, and often these two steps alone will get you most of the way toward a great-sounding room.

However, there are other strategies beyond these two basic maxims. Probably the third most important treatment strategy in most rooms, after the above two steps, is the rear wall behind the listener. To begin, we'll go over the most common problems with the rear wall that should be addressed. Most rear walls, particularly in rectangular rooms, exhibit two acoustics issues that must be dealt with to achieve maximum sound quality.

"Most rectangular rooms, and many other types of rooms, exhibit a strong bass buildup on the rear wall."

PROBLEM #1: BASS BUILD-UP

The first problem is perhaps less obvious or intuitive. Most rooms exhibit a strong bass buildup at the rear wall, which is the usual location for the producer's couch at the rear of a recording studio control room. It's common for mix engineers to check a mix by sitting (or lying!) on the producer's couch while listening - the enhanced bass there helps the engineer experience the sound of bass-heavy listening environments such as dance clubs.

In addition to the peak at the rear of the room, there's also a deep null at the center of the room front-to-back. This null is related to the peak at the rear wall. In more technical terms, a room has mode frequencies whose wavelengths correspond to the room's dimensions. To illustrate, we'll use the room in the figure below which is 18 feet long left to right:

Room Modes

The frequency associated with an 18 foot wavelength is approximately 62 Hz. However, room modes extend from one end of the room to the other and back, so the first-order (lowest) mode is actually at 31 Hz because the length of the room encompasses only half the wave. The wavelength of the lowest mode at 31 Hz is therefore twice the room length. This results in a 6 dB peak at 31 Hz at both the front and rear walls, with a corresponding deep null in the center of the room front-to-back. The second-order mode at 62 Hz is one octave above the first mode, and it yields a peak at the center of the room. This series of peaks and nulls repeats at subsequent octave frequencies. Top

As a demonstration, you can try the following experiment in your listening room:
  • First, play some familiar music with complex bass so that it excites a variety of frequencies and not just one note or one key. Better, you can play bass-heavy pink noise through your system.
  • While the audio is playing, stand near the back wall facing the speakers. In most rooms you'll hear a strong bass build-up.
  • With the music still playing, begin to walk slowly toward the front wall. As you move forward, the bass should gradually level out. Then when you get to the center of the room you should hear noticeably less fullness.
  • In many cases, the 38 percent point of the room - or where you are 38 percent of the way into the room from the rear wall - exhibits the flattest bass response. This so-called "38 percent rule" was first described by Wes Lachot, and is explained in more detail in our Room Setup article.

The key here is that adding bass traps in the rear of the room reduces the extent of the peaks, and the corresponding null in the center of the room is also improved. The net result is thus a flatter response over the entire room. Top

"Note that bookshelves or random uneven surfaces are not diffusion."

 

RealTraps Diffuser
The RealTraps Diffuser is also an excellent bass trap.

 

"In general, you will get best results from spacing the trap away from the wall a few inches."

PROBLEM #2: HIGH-FREQUENCY REFLECTIONS

In addition to bass problems at the rear of the room, reflections from the rear wall also occur at mid and high frequencies. Luckily, high frequencies are, in general, easier to deal with than low frequencies. There are basically two choices: we can either absorb or diffuse these frequencies.

ABSORPTION OR DIFFUSION?

Both absorption and diffusion are effective at a rear wall, with different costs and a somewhat different sound. Absorption removes sound energy from the room by converting it to a tiny amount of heat, whereas diffusion scatters the sound waves evenly in various directions.

Note that bookshelves or random uneven surfaces are not diffusion. Also note that it's difficult to build a diffuser precisely enough for it to be effective. For instance, a good QRD diffuser contain wells of varying but precisely calculated depths. These wells work together to produce the diffusion. For more detail about what diffusion is and how it works, see our video All About Diffusion. This video also lets you hear the sound of absorption and diffusion, and how they are different from a flat reflective surface.

In general, our take on diffusion is that it is "icing on the cake" in a great room, but it's not necessary for good sound. And because Diffusers are our most expensive panels (they have a lot of parts and are very labor-intensive to build), they aren't always the most cost-effective solution. That said, if you have a budget for diffusion, and bass trapping and early reflection control are in place, there's no more effective rear wall treatment than RealTraps Diffusers, and specifically our Diffuser Modules.

Note also that Diffuser Modules are exceptional when placed around the entire perimeter of the room (except at reflection points). This strategy will transform just about any room into a world-class listening environment, as shown in our video Hearing is Believing. Top

 

 

DiffuserModule
RealTraps Diffuser Modules provide additional bass trapping in the important wall-floor corner, and require no permanent mounting.
PRACTICAL TIPS

Of the RealTraps product line, the following options are highly effective rear wall treatments, listed in order of effectiveness and cost.

Diffuser Modules

First and most effective are our Diffuser Modules, as shown at left. These units provide exceptional bass trapping at low frequencies, and also give all the benefits of diffusion at higher frequencies. If you want the best possible solution, this is the way to go.

HF Mondo Modules

Next are HF Mondo Modules. These are the same size and shape as Diffuser Modules, but rather than diffusion the upper units are broadband absorbers that are equally effective at bass and treble frequencies. Both modules are thick enough to provide outstanding bass trapping, and they effectively deal with problems at high frequencies as outlined above. Both modules are freestanding and require no permanent mounting.

HF MondoTraps

From here we move away from the modular kits to traps that are mounted on the rear wall or used with stands. In general, you'll get the best results by spacing traps away from the wall a few inches, as shown in our post base mounting tutorial. HF style MondoTraps give superior performance for a permanently mounted trap, offering plenty of high frequency absorption as well as bass trapping.

HF MiniTraps

Lastly are our most popular and arguably most cost-effective rear wall treatments - HF MiniTraps. These too should be mounted with post bases to space them away from the wall for best results. HF MiniTraps are a great choice when a trap with a slim profile is desired for space reasons. They offer good bass trapping down to below 100 Hz, as well as plenty of high frequency absorption.

LISTEN! BEHIND YOU!

Rear wall treatments are an important step toward creating a superior listening environment, second only to overall bass trapping and early reflection management. Please contact us if you'd like some specific advice on the best rear wall treatments for your room and your budget! Top

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