Originally Published from www.audiophilia.com July 1998 by Jon T. Gale
While I have always taken pride in my open mind when it comes to things audio, a healthy dose of skepticism has served me well – I’m a “show me ” kind of guy. When a dealer answered my query as to the efficacy of Acoustic Sciences Corporation’s TubeTraps with, “Take a pair home and try ’em out”, I did just that. After carefully placing the traps in my listening room (OK, I plopped them down in the corners) I could not believe my ears! No other component had ever had such a profoundly positive effect on the sound of my system. And this was in a small room, well treated by conventional standards.
Before getting to the discussion of the TubeTrap itself, it’s worth spending a moment discussing the sort of problems they are intended to address.
There are two distinct forces at work when sound pressure is introduced into an enclosure such as a listening room. Mid and high frequencies, their wavelengths being far shorter than any room dimension, do not excite room modes. These frequencies travel in a ray-like manner, “beaming” about the room like shafts of light, or like a billiard ball striking and rebounding from the cushion of a pool table. The classic clapping of hands while walking about the listening room will excite these frequencies, manifested by an audible ringing accompanying the sound of the clap. Low frequencies, on the other hand, whose wavelengths are far greater than any room dimension, behave quite differently. The propagation of these frequencies results in room modes, or massive peaks and dips in sound pressure caused by reinforcement and cancellation. These peaks manifest themselves as high-pressure hot spots in the listening room, resulting in very audible frequency response anomalies.
A very real, yet rarely discussed, problem related to low-frequency room modes, is that of masking. Excessively long low-frequency decay severely masks transients and, more importantly, low-level midrange detail. The massive energy of what I’ll term bass echo can literally swamp delicate low-level information. Another fascinating property of the bass resonance is its ability to shift the tone of a bass note as it decays, directly affecting what is often referred to as pace and timing. The shifting effect of a bass resonance can be explained in terms of room resonance. If a bass note with a frequency near, but not equal to, a resonant frequency of the listening room is launched into that room, it will still manage to excite the room’s resonant frequency. The result is that the original note is heard to decay not at it’s true frequency, but at the resonant frequency of the room. This results in a type of smearing which is detrimental to the intelligibility of the original signal.
The TubeTrap is a fabric covered porous cylinder (comprised of 1″ thick compressed fiberglass) that, when sealed, acts as a pressure zone device. Sound pressure building up around the outside of the hollow tube creates a pressure differential between the pressure inside and outside the tube. In an effort to eliminate this differential, the sound wave responsible for the pressure zone attempts to pressurize the hollow core of the tube. In order to do so, it must travel through the tube’s resistive fiberglass which, in turn, dissipates the wave’s energy as heat. Simple in design, relatively simple in execution and simply indispensable for improving the acoustics of your listening room.
Founded in 1984 by Art Noxon, ASC supplies acoustical products to studios, auditoriums, churches, and a multitude of other venues, from their headquarters in Eugene Oregon (Mr. Noxon has also generously devoted precious research time to developing high articulation speech rooms for the hearing impaired). Noxon’s research led him to the sealed cylinder as a device for absorbing excess low frequency energy. While high and midrange frequencies can be absorbed by small devices, relatively large devices like the TubeTrap are necessary to effectively tame frequencies below 100Hz. ASC manufacturers traps with diameters of 9, 11, 16, and 20 inches, successively larger tubes having the ability to absorb successively lower frequencies.
In my experience, it is quite common for audiophiles to treat their rooms unevenly, resulting in short decay times in the middle and high frequencies, and much longer decay times in the bass. Such uneven treatment is almost as bad as no treatment at all! As with any acoustical treatment, including ASC’s TubeTrap, a balanced approach is best.
Wisely, ASC provides a free service to potential customers the result of which is a computer-generated drawing of the subject listening room along with a recommended set of traps and their placement. (Fig. 1 contains the ASC-provided drawing of my room). The provided drawing is most useful, and can be used as a guide in purchasing additional traps. Clearly, ASC knows their product, and are constantly at work to refine both its appearance, its technology, and its usefulness. As these products aren’t exactly cheap, I urge anyone interested in investigating them to avail themselves of ASC’s free services.
In my opinion, one should take a gradual approach to the full use of traps. Much will be learned about both the sound of the listening room and the best use of the trap itself, by piecing together a full complement incrementally.
The first pair of traps should be inserted in the front corners of the listening room behind the loudspeakers. As half of the trap’s diameter is covered with a reflective paper, (useful for dialing in the high frequencies) start with the reflective side out. In practice, I have found that only when using traps as side wall absorbers should their reflective side be placed towards the wall. As the fabric covering itself tends to soak up very high frequencies, it is easy to suck the high-frequency life out of the room. Care must be taken to use the trap only for its intended purpose: pressure absorption.
The next pair of traps should be placed in the rear corners behind the listening position. It is common to place larger traps in the rear corners and smaller traps in the front corners. However, if your listening position is at or forward of the room’s midpoint, as is mine, place the larger set of traps in the front corners.
The next, and usually final, step is to put one trap at the midpoint of the front wall. This trap, often referred to as the imaging trap, helps tame cross-correlated reflections and takes care of the room resonance which occurs at the midpoint of any of a room’s walls.
If finances permit, traps can also be used to treat a room’s four side-wall reflections. Although there is some benefit to bass damping at these positions, low-frequency energy is more concentrated in a room’s corners and should be dealt with first. Using a more conventional (and less expensive) damping panel at the side walls is advisable. Such products can also be purchased from ASC.
Placing smaller traps, or ASC’s half rounds, at third, quarter and half-way points along a room’s walls completes what would be considered a full acoustical treatment. Keep in mind that the goal is not to treat every point of reflection in your room. Traps should be used only at those locations resulting in resonant peaks in the room, as dictated by the room’s dimensions. As a rule of thumb, traps should be placed at even fractions of the room’s dimensions.
Improved midrange articulation will likely be the first thing you will notice in a room treated with traps. This is due to the elimination of bass resonances that previously drowned out midrange detail. Subtle vocal “pops”, clear, intricate violin sections, stunningly defined kick drums that just load the room and then disappear – all this and more will be yours once low-frequency resonances have been adequately dealt with. Low-level information, once obscured, will be readily audible. Soundstaging also benefits substantially from this additional level of detail, for it is in these details that are buried the aural cues of the recorded space. The breathing sound of a large hall, the cut-through-the-hall energy of treble transients, along with increased delineation of percussive instruments, are all easily perceived after treatment. Dynamic range is also greatly enhanced. Previously masked micro-dynamics emerge, as transients are clearly delineated from their decay. A “trapped” room also allows one to listen at a much lower volume level while gaining in dynamic impact. In an untreated room, one usually finds themselves raising the volume in an attempt to attain more clarity.
Yet another benefit of a trapped room is the newfound sense of ease afforded the listener. With the room effectively removed from the equation, the sense of listening into the recorded space is greatly enhanced, the listener no longer required to strain to hear the music against the room’s own sonic signature. It’s worth keeping in mind that ASC’s TubeTraps aren’t bass absorbers per se, they are resonant absorbers. They won’t remove the bass energy from your room, rather they will tame the resonances caused by this energy. Bass peaks will still be strong at the room’s resonant frequency, but the absorption of this resonance will reduce their severity.
While some may balk at the cost of the TubeTraps, their positive effect on the sound of your audio system will likely rival that resulting from the upgrade of a major component. I trapped my room midway into a major system upgrade and found the improvement fully the equal of any amp, cable, or source upgrade I’ve made.
The ASC TubeTrap is unquestionably one of the most effective acoustical treatments available to the audiophile. With careful set-up, and used in conjunction with other tools, the audiophile now has the means to address the most important component in the playback chain – the listening room itself.