ACOUSTIC PRIMER

OVERVIEW

In our Acoustic Primer, we briefly go over basic room acoustics theory, common issues encountered in typical recording and listening environments, and how Cylinder Traps^TM can help to overcome these issues. We won't get into complex math-based equations, but instead will keep it simple, providing various treatment options utilizing Cylinder Traps based on specific goals.

THE ROOM IS EVERYTHING

Well, almost. Without a source of sound, the room will not react and impose its own fingerprint on the sound before it reaches your ears.

A quick and dirty way to illustrate how a room imposes its signature on reproduced (or produced) sound is to stand in any room and clap your hands. Now go outside devoid of walls and clap them out there. You should hear quite a bit of difference. Although perhaps not quite as drastic, you should be able to hear a substantial difference when comparing a carpeted family room to a smaller tiled bathroom, as another example.

Like the sound coming from your hands, other sources of sound cannot escape the effects imposed by the room either, including home theater systems and live instruments.

If you have ever gotten used to a playback system over a long period of time in a particular room, then moved the system to a dissimilar room, you may have noticed quite a difference in the sound with familiar music. Likewise, if you are a musician and have ever spent months practicing in the same room with an acoustic instrument, then moved to a new rehearsal space, you most likely noticed a change in the sound of the room, possibly quite drastic, which is not uncommon.

Microphones, even tightly-patterned cardioid variants, are not impervious to the effects of the environment they are in, even at relatively close distances within an inch or two! When the mic is moved further away from the source, the room plays an even bigger role in the final recording.

Typically in less than perfect recording rooms, a compromise is often made between increasing naturalness or realism in an instrument by pulling the mic away from it, to drawing the line when there is just too much interplay of bad room sound. In rooms that have desireable qualities, there is less or no compromise, and pulling the mic further away from the source will often enhance the recording by allowing the room's quality signature to come through in the recording. Alternatively, depending on treatment, the room may add relatively very little to the recording, giving the engineer more options in post.

Home theater systems are just as susceptible to the ill effects of an untreated room due to the room's inescapable role it plays on the resultant sound that eventually reaches the listeners' ears.

In the case of most home theater setups, the loudspeakers are relatively far away from the listener, allowing the room to play a large role in the final sound. In a recording studio mixing environment, near field monitoring attempts to overcome the room's effect on the sound by placing the loudspeakers closer to the engineer, giving the room less of a chance (or less time) to mangle the audio before it reaches his or her ears.

A relatively flat frequency response is a virtue in a mixing environment, and nearly any room will adversely effect this response curve the further the source is away from the listener. The severity of peaks and dips in the response vary by room and are caused by the room's attributes (dimensions, types of surfaces, etc.). Heavy bass trapping can reduce these effects by smoothing out the response curve, allowing the true accuracy of the source to be heard.

Cylinder Traps utilize their broadband absorption characteristics to provide as much effective bass trapping as any other broadband absorber on the market of equal volume. The same holds true for absorption in the remaining audible frequency range.

Peaks and dips in the response curve well below 150 Hertz is the most challenging acoustic problem facing most untreated small to medium-sized rooms, and Cylinder Traps are up to the task as long as proper size selection and positioning is adhered to. We often concentrate on this problem area first since it is the most challenging to overcome and is typically the largest contributor of poor room sound.

A COMMON ISSUE AND A POPULAR SOLUTION

During the design and construction of a living or family room in a typical home, sound quality was likely the least of concerns. Therefore, it is not unreasonable to believe the sound in such an environment could be greatly improved.

Time has shown quite a difference in the design of a typical room in a modern home compared to that of a home built in the eighties, for example. At least in Western US homes, gone are the acoustic "popcorn" ceilings and heavy carpet in favor of harder surfaces, which have a direct and profound effect on the sound of these rooms. In smaller-sized rooms, the abundance of these hard surfaces can wreak havoc in a recording or listening environment, where proper acoustic treatment can show great improvements.

The clapping of hands is a good initial measure of a room's reverberant quality (or lack of), as well as quickly displaying possibly large amounts of slap-back echo and other issues. However, this technique is not a good measure of where small to medium-sized rooms often suffer the most; namely drastic peaks and nulls in the lower frequency spectrum around and well below about 150 Hertz. Consequently, this is the most difficult issue to overcome, and typically requires a substantial amount of low-frequency absorption in the form of sheer volume.

THE ROLE OF ABSORPTION

As mentioned above, for most small to medium-sized rooms, the prime target area of acoustic problems that need to be addressed typically center around low-frequency-related issues where only large amounts of acoustic treatment will suffice. Specifically, effective low-end absorption and control is our first and foremost concern in these types of rooms. Other types of acoustic problems are minor in comparison, and can be taken care of with lighter "artillary." The problems associated with severe peaks and dips in the lower frequency ranges mentioned may include:

• 1. Flabby, tubby, and/or uneven bass response.
• 2. Low-frequency "overhang".
• 3. Weak bass response.
• 4. Radically varying bass response in the room depending on location.
• 5. Inability to translate mixes on varying playback systems.

Effective control of these low frequencies require a good number of absorbers that are effective well below 150 Hz. Absorbers that fit this criteria are typically quite large. In the case of Cylinder Traps, the diameter of the trap needs to be at least twenty inches [20"]. A ring of 9" Cylinder Traps, for example (or any 9" tube shaped broadband absorber/diffusor) around an instrument or throughout a room is just not going to cut it, as these traps are only effective down to about 150 Hz. This, however, does not mean that the smaller diameter Cylinder Traps do not serve a purpose or have their place in a room's overall treatment.

INGREDIENTS FOR EFFECTIVE LOW FREQUENCY CONTROL

By "effective low frequency control," we mean effective smoothing of frequency response in the critical lower bass regions around and far below 150 Hertz. In order to attain effective low frequency control, generally speaking, the top three main ingredients of concern are:

1. An acoustic absorptive material with the right properties, thickness, and area. For example, four to six inch-thick 24" X 48" Owens Corning 703 is often a popular candidate.

2. Proper "coupling" of the absborptive material to air space and/or room boundaries. For example, the absorptive material mentioned above straddled across a room corner. The position of the absorptive material, the air space behind the absorber, as well the perpendicular walls that make the corner, are critical. Some broadband absorbers contain an "integrated" air space. Others rely on positioning alone to determine how the air space, if any, effects the performance of the absorber.

3. Enough of the strategically placed materials listed above in order to provide an acceptible level of low frequency control. In most cases, eight or more of the absorbers listed may be needed depending on their sizes. In small to medium-sized rooms, this type of low frequency control is typically just one problem area encountered that needs to be addressed.

Note: In some designs and positioning, the amount of air space varies and may not even be part of the equation, as well as the proximity of the room boundary. In the case of Cylinder Traps, this air space is integrated within the trap itself, although Cylinder Traps will become more efficient in low frequency absorption when placed near a room boundary, just like other broadband absorbers. That said, they do not lose as much of this efficiency when moved away from a boundary compared to most other designs. This is because most of the critical air space stays in-tact when the trap is moved.

THE ROLE OF DIFFUSION

Diffusion is an important ingredient in retaining the "naturalness" and "space" in sounds the way they are often perceived by humans in the real world. Rehearsing in a small, dead room with no diffusion is often very uninspiring, unnatural, and extremely fatiquing for a musician, the result of which will be directly translated into the recording. A good balance of random diffusion can help to overcome these issues by giving more of a sense of liveliness, space, and brightness in a room with little or no reverb time otherwise caused by natural, numerous reflections.

Diffusion is probably the least understood attribute in the role of sound reproduction and acoustic treatment. Many solutions are usually expensive and the results are often unpredictable. Poly diffusion is more predictable, but also less flexible in its range of use compared to other types of diffusion. Poly diffusion is typically more useful when it is further away from the source and/or listening positions. When the source radii of the poly diffusors are varied in the room, this creates a higher level of randomness in diffusive output, translating to higher flexibility and naturalness. A room of Cylinder Traps of various sizes serves this purpose and goes a step further by allowing quick and easy adjustment in the amount of diffusion.

Because Cylinder Traps allow quick and easy adjustable diffusion, any mystery in the role diffusion provides in the recording or listening experience can quickly be realized through experimentation. We highly recommend using your own ears and judgment when varying the diffusion in the room. The possibilities are endless when you consider all of the options available to you.

THE ROLE OF REFLECTION

Simply stated, reflections within a room provide dimensionality around the source sound, along with other cues that make up the "quality" attributes of the room. The density and varying lengths of multiple reflections are a prime contributor to the room's inherent reverb sound.

Cylinder Traps can be used to remove a very large portion of a room's reverb decay time, if desired. Some would describe the result, when implemented thoroughly, as a "roomless" sound, which can be quite powerful in certain scenarios.

In addition to decreasing reverb time, making the most out of a room's reverb qualities, if present, are relatively easy utilizing Cylinder Traps due to their portability. Rooms can easily be partitioned into live-end/dead-end spaces with minimal effort, each space having variable diffusion attributes based on the traps' positioning. It is interesting to note that through creative placement, the source sound can easily travel from one space to the other, taking advantage of each space's acoustic qualities.

An interesting experiment is to place a ladder in a room lined with Cylinder Trap columns that are stacked two-high (72" high columns). Then, as a single instrument is playing in the room, climb up on the ladder so that your ears are approximately mid-way between the ceiling and the tops of the Cylinder Trap columns. You might be surprised at how much different the sound is up there, as if it is a different room all together.

Alternatively, you can perform the hand clap test while climbing up the ladder. Don't forget to clap your hands in the opposite zone from where your ears are as well.

THE SHAPE OF THE ABSORBER AND ITS ACOUSTIC EFFECT

When talking specifically about "static" broadband absorption, the shape of the absorber will not effect its absorption capability in theory. However, if the shape dictates the method of air coupling to a room boundary, the shape could indirectly effect performance, for better or worse. For example, the air "coupling" to a room corner in the case of Cylinder Traps is much different than a panel trap straddled to a corner.

Tube shaped traps typically offer acoustic benefits panel traps do not, however. Namely, integrated poly diffusion, which cannot be achieved by a flat panel. Many acousticians agree that a good balance of absorption, diffusion, and reflection are key in obtaining desireable results across many applications. Cylinder Traps help to provide all three attributes due to their cylindrical shape, and go an important step further by allowing these attributes to be varied very quickly and easily.

The reason why Cylinder Traps help to provide [sustain] reflection is because they can be placed further apart from each other compared to flat panels when attempting to tame reflections from walls. This is due in part to the concentration of diffusion around the trap. Even if the diffusive side is facing the wall, there is still diffusion happening in the space between the traps, which can help to break up distinct reflections from walls in close proximity to the traps.