Your Room: The Final Link
IAR's New Reference Listening Room
There's a special joy and excitement in pursuing the audiophile hobby. And this excitement reaches a climax when, on that rare occasion, you make a sonic breakthrough with your audio system. We've all had that experience at least once. Say you're installing a new phono playback component or speaker system. You spend hours and hours experimenting with aligning them to get everything optimized. Then finally you sit back and listen to music on your new updated system. Usually the sonic improvement is rewarding but nevertheless merely incremental. However, if you pursue the holy grail of your hobby diligently, then with luck perhaps once a year you achieve an improvement so striking that your system is transformed, literally born again. When you hear this, you can't believe it. And then, in an excited orgy unmindful of time or hunger, you pull out several dozen records from your collection, and play familiar snippets of each. The sound is unbelievably better. You literally rediscover the music that's on these treasured records, and later in the rest of your library. Finally, after hours that seem like minutes, at once exhausted and enervated, you can regain contact with the rest of the world. Your audio system has once again become that magic machine that transports you to other worlds. And you have achieved this magic, by your selection of components and your work in setting them up.
I've just been through such a magic experience myself. IAR's new laboratory in Vista has several small listening rooms, typical of small apartment size rooms with low ceilings. But the main listening room for the reference system has magnificent dimensions of 25 by 30 feet, with a 14 foot cathedral ceiling. These dimensions allow the full development and propagation of very low bass waves, the wide stereo staging needed to recreate the full breadth of a symphony orchestra experience in the concert hall, and the long reverb tail that can enhance perception of the original hall ambience encoded in a good recording (a la Madsen and the subsequent delay devices that appeared on the market. As in any listening room, the layout of the stereo system and the acoustic treatment of the surfaces needed to be optimized. And there are many acoustic parameters to juggle, as there are many desiderata to optimize. You have to optimize overall reverb time at all frequencies; control standing waves for all dimensions and at all frequencies; and control reflections from the surfaces near the speakers, near the listener, from the side walls, and between all pairs of opposite surfaces, with respect to both frequency and time delay. These reflections affect the system's amplitude and phase response, its transient attack and decay envelope at various frequencies, the time smearing that can malform the original musical information plus obscure subsequent musical information, stereo spread and localization in all three dimensions, perception of recorded ambience, and finally the tactile coherence of the stereo image, including not only the instruments but the recorded space surrounding those instruments. You can see that all this is a tall work order, with many variables that make for many hours of experimentation for each new room you tackle. And the larger the room, the more variables there are to juggle (by at least the square of the increase in room dimensions).
The end results are worth it. After not hours but weeks of experimental work, this reference listening room and its system has now given IAR quite simply the finest stereo imaging in the world, using commercially available components. Don't take my word for it. A number of independent experts, who have heard some of the best audio systems in the world (including those of other leading reviewers), say they've never heard anything like it. Furthermore, thanks in part to the control of time smearing reflections and excess reverb, the system has given us some of the best transparency and clean purity to be heard anywhere.
The weeks of experimental research in room acoustics have taught me a lot about the subject. Most of what I've learned demonstrates that today's prevalent theories and guidelines on room acoustics and treatment are wrong and even backwards. They led me up many blind alleys, for example prescribing the use of absorbent materials on the walls around the speakers (the LEDE technique). The sonic results I obtained by following accepted guidelines ranged from mediocre to satisfactory, but none gave me the special magic I thought should be possible. So several times, after trying vainly to optimize all the above parameters following prescribed tactics, I tore everything down, stripped the giant room bare, and started again from scratch. Finally, I had to develop my own new tactics and guidelines, proceeding by ear and by the technical results gleaned up to that point. Incidentally, it's truly amazing what human hearing can detect and then analyze, as minute changes in the reverb and reflection patterns of the room cause the subtlest variations in the music and in the stereo imaging. The ear/brain gradually learns what to listen for, how to interpret it, and then what adjustments to make where in the room's acoustic treatment. The new tactics and guidelines I learned form a gold mine of material for a series of articles we'll run in IAR, on how you can acoustically treat your room for better stereo imaging and clearer sound. And the series starts here, with our review below of the ASC Tube Traps.
When all the experimental research into system arrangement and room treatment finally jelled, and IAR's new reference listening room gave me that spectacular breakthrough in stereo imaging, I went into one of those exhilarating audiophile orgies. For days, I kept pulling recordings with good imaging out of the library, and being blown away by the spectacular imaging discoveries on each new disc. I was rediscovering the music in the lab's collection, with symphony orchestras and choruses palpably spread before me in 3D space. With over 6000 records plus hundreds of pre-recorded tapes and now CDs in IAR's library, I still have a lot of rediscovering musical joy ahead.
Of course, this spectacular stereo imaging breakthrough in IAR's new reference listening room serves not only my musical enjoyment, but also yours. This room and system setup can do justice to the finest stereo components being evaluated and compared for review. It puts to the test not only their stereo imaging capabilities, but also their transparency, resolution, clean purity, and even phase behavior -- since all these contribute to the preservation of subtle musical information that is required to yield the best imaging. Also, this superior imaging, together with the low mud factor obtained by controlling room reverb, allows us to hear better into the inner textures of music (especially on complex material), which allows us to better evaluate how well components under review are preserving musical textures, subtle detail, intertransient silence, etc.
ASC TUBE TRAPS
Even the manufacturer doesn't know how good these are. He modestly recommends that you merely use one in each corner of your listening room, to control the room's low bass standing wave modes. But they are so powerful and flexible that you should use them every 3 feet or so, along every surface, to control reflections as well, if your budget and wife's sense of decor will allow.
Your listening room is the final link in your audio system, the final link in the recording/reproduction chain. Your room's acoustics, with its many variables, have a profound effect on the sound of the music you'll hear, in many ways. Yet the treatment of listening room acoustics is still an untamed wilderness, the most primitive area of today's music reproduction systems. ASC Tube Traps are the most powerful tool yet developed, to help you come to grips with this vital problem and successfully tame this wilderness.
What are Tube Traps? Essentially, cylindrical sound absorbers. Each cylindrical module is about 3 feet long, and there are two models, 9 inches and 11 inches in diameter. The 3 foot long cylinders may be stacked end to end, to form columns that are 6 feet or 9 feet long. These columns should then be stood vertically, in the corners of your room and around its perimeter, every 3 feet or so. The columns (or individual 3 foot long modules) should also be placed horizontally on your ceiling, hung by convenient eye screws. There are also half cylinder models for midwall placement, and other variations we'll get into below.
Tube Traps vs. Flat Panels
What's the functional difference between ASC Tube Traps and the hordes of flat panel or foam acoustic absorbers on the market? The most important is frequency range. Flat panels of sculptured foam or fiberglass can only absorb acoustic energy from the midrange upward in frequency. They are useless in the bass and warmth regions, and even in the lower midrange. That's like having speaker systems with no woofers. In contrast, manufacturer's measurements of the ASC Tube Traps show uniform acoustic absorption down into the warmth region for the 9 inch diameter model, and down through the upper bass for the 11 inch model.
This contrast has some crucial sonic consequences for you. If you employ flat acoustic absorbers, you'll have to put up enough along the walls to take care of bright reflections from hard surfaces, audible echoes, and lowering excess reverb decay time in the upper frequencies. But this can cause your room to be too dead in the midranges and trebles, which gives a lifeless sound with poor imaging and ambience. And it can cause a skewed tonal balance, with your system sounding too warm, since those absorbers soak up only higher frequencies while leaving the warmth and bass at full level. Worst of all, these flat absorbers do nothing to lower and control excess reverb decay time in the bass, warmth, and even lower midrange regions. This leaves your system sounding muddy and boomy.
The ASC Tube Traps, on the other hand, don't have these drawbacks. Because of their cylindrical shape, they can be set to absorb more unwanted upper frequency energy from the room as a whole, while taking up less wall surface area than flat absorbers would. This allows you to leave bare wall areas all around the room, which, as we'll discuss later, is crucial to obtaining good imaging and ambience, while avoiding a too dead sound. Because they have flat frequency response of absorption down through the warmth region and into the bass, they leave your room's tonal balance sounding pretty neutral, instead of too warm, dull, and bass heavy. Most important, only ASC Tube Traps address, and conquer, the dreaded mud factor.
The Mud Factor
What is the mud factor? It's hard for you to hear its pernicious presence, but you sure can appreciate the sonic improvement when it's gone. The mud factor is caused by excessive and excessively long lasting room reverberation in the warmth region, with the adjacent upper bass and lower midrange contributing somewhat. If your room has distinct echoes, excess reflections, or excessively long reverb decay in the upper frequencies, you can easily hear their sonic effects. At upper frequencies, the human ear/brain is very sensitive to timing and directionality, so it can easily detect the objectionable time smearing caused by upper frequency reflections and excess reverb, as well as the distracting directions they come from. But at lower frequencies, the ear/brain is less sensitive to timing and directionality. So it is not actively irritated by lower frequency reflections and excess reverb. Nevertheless, this delayed lower frequency information, ricocheting around your room, is a sea of lingering acoustic mud that does make it much more difficult for you to hear succeeding new musical information coming out of your speakers. As a result of this mud factor, your system will sound muddy and untransparent, throughout the frequency range, but you won't be able to figure out why, because you don't really hear anything wrong with your listening room's acoustics.
Virtually every listening room has this mud factor. Stuffed furniture, carpeting, drapes, etc. are just like the flat absorbent wall treatments, in that they absorb only upper frequencies, so they don't help the mud factor. When your room has this mud factor, you may find yourself constantly turning the volume level up, hoping in vain to hear the music more clearly. But of course, as you turn up the volume level of musical information from the speakers, the sea of mud also rises, so you gain nothing. Finally you run into power amp clipping and emotional frustration. ASC Tube Traps soak up this sea of mud. Suddenly your system sounds much more transparent, at all frequencies, because now your can hear the new musical information coming out of your speakers. And, with your system sounding much more transparent, you'll find that you can enjoy clearly hearing all the music at much lower volume levels. Also, transients will sound louder and more dynamic, since their full amplitude will appear against a background of silence, instead of being half buried in a sea of unperceived acoustic mud.
Note that this finding from our research contradicts conventional wisdom about absorbent acoustic treatment, which has always taught that you need to turn up your volume control and use more amplifier power when you put more absorbent material in your room. Using the ASC Tube Traps to absorb the mud factor not only improves the transparency of the final component link of your playback system, your listening room. It also allows you to use less amplifier power and a more transparent amplifier, while avoiding the blurring distortion of running your power amp into clipping, and giving you better loudness and dynamics.
More and Better Bass
Similarly, using the 11 inch diameter Tube Traps to absorb bass gives you not only better quality bass but also, surprisingly and again contrary to conventional wisdom, more impactive and louder bass. ASC Tube Traps can be used to address and solve a number of bass problems, and thus accomplish a number of types of improvements in bass quality and quantity. The most obvious problem is bass standing waves, the acoustic resonances of your listening room due to its dimensions between opposing pairs of surfaces. This resonant bass ringing of your room sounds just like the ringing bass boom of some speakers and amplifiers. Since IAR Journal 3 we've shown how even a single overshoot in an amplifier's bass transient response can produce boomy, undefined, heavy, and muddy bass. Ringing, such as seen in the bass transient response of some speakers (especially vented bass systems) sounds even worse. Most listening rooms have prolonged ringing at their bass resonant frequencies (and harmonics), so they are the worst offenders of all, completely negating the work you put into the rest of your system to get deep, accurate bass. Again, note that other acoustic absorbent treatments, from flat panels to stuffed furniture, only absorb upper frequencies, and do nothing to solve lower frequency room resonances (indeed, they make the resonances sound worse, by thrusting the lower frequencies into greater prominence as they absorb the upper frequencies only). Some sound studios use very large moving diaphragm panel absorbers (equivalent to a flexible wall or floor) to quell the room's acoustic bass resonances, but these panels are clumsy and expensive, and they have resonances of their own, so they solve one problem but contribute others. The 11 inch ASC Tube Traps, placed at the pressure antinodes of the room's several fundamental resonances plus their low order harmonics, are a compact, convenient, relatively attractive and inexpensive alternative. They are also nonresonant, so they don't add colorations. And they are effective down into and through the upper bass (down to about 50 hz). ASC also makes, to custom order, even larger diameter Tube Traps, which absorb unwanted resonant energy down to l0 hz (a 15 inch diameter model is forthcoming as standard).
The other bass problems are less well known. It is no longer popular to place your speakers in room corners, nor even right against the wall, since this excites the room's resonances worse and also does not give the best stereo imaging. But nowadays, with your speakers placed away from the walls, the distance from the speaker to the nearby wall surfaces corrupts the bass (and warmth region) in various ways. Regardless of their mid and high frequency forward dispersion patterns, virtually all loudspeakers radiate omnidirectionally in the bass and warmth regions (dipoles radiate all frequencies to the rear, but virtuously do not radiate any energy to their sides). Thus, virtually all speakers radiate full energy at the nearby wall, floor, and ceiling surfaces.
For simplicity here, let's consider just the energy directed toward the wall at the rear of the speaker. That acoustic energy will reflect from the wall back toward the listener. So you'll hear a double version of the music, the original which came directly from the speaker and the time delayed reflection from the rear wall. The result is time smeared music (but see below on LEDE). Furthermore, as this reflected acoustic energy passes the speaker on the way to the listener, it combines with whatever new acoustic information is then coming out of the speaker on its direct path to the listener. At the frequency where the distance to the rear wall equals one wavelength, and st all multiples of that frequency, the acoustic energy will be reinforced, giving not only tonal coloration from too much amplitude at those frequencies, but also a boomy overhang (similar to resonant ringing) to the bass quality at those frequencies. Then, at half that fundamental frequency, farther down into the bass, the distance will be half a wavelength, and there will be almost complete acoustic cancellation of your bass by the wall reflection at that frequency, and all multiples of it. If you use ASC Tube Traps to absorb all that low frequency energy when it arrives at the rear wall, it won't reflect back toward the speaker and listener. So you won't get those phony boomy peaks at the full wavelength frequencies. And you won't get those cancellations at the half wavelength frequencies. You'll get a flatter tonal balance with less coloration, and better quality transient information in the bass and warmth regions. Note that the lowest frequency aberration caused by the rear wall reflection was a cancellation, not a peak, and that it was at a bass frequency. Getting rid of this lowest frequency aberration, by using absorbent Tube Traps, will therefore increase the amount and impact of bass energy from your system. Once again, this finding contradicts the conventional wisdom that absorbent room treatment soaks up and diminishes the loudness or power you'll hear from your system st the absorbed frequencies.
The obvious bass application of bass absorbers may be to reduce the room's standing wave resonances, a phenomenon that takes place between pairs of the room's Surfaces. In this role they improve the quality of bass transients and reduce the mud factor by eliminating ringing overhang, while reducing excess bass amplitude (and reverb decay time) at the room's resonant frequencies. But in this more subtle application, the absorber treats a phenomenon that takes place between a single room surface and the speaker. And here it serves not only to again improve the quality of bass transients and lower tonal colorations, but also to increase the amplitude of bass, that bass formerly lost to cancellation interaction between the speaker and a single nearby surface.
Obviously, you should use bass absorbers on not just the rear wall, but also the side walls and ceiling near the speakers. These other single surfaces also cause bass cancellation at other frequencies, so you'll hear your system's bass increased to its correct amplitude at these several frequencies which were being cancelled.
This single surface cancellation effect, correctable by ASC Tube Traps, applies not only to speakers. It also applies to live instruments. Bass viols and 'cellos have some of their natural warmth and bass cancelled by interfering reflections from the nearby floor, with tonal colorations and time smearing caused by additive reflections at other frequencies. So ASC Tube Traps, lying horizontally on the floor, could be used to make live instruments sound more natural. And indeed, the world premiere concert using ASC Tube Traps has already taken place -- and everyone marveled at the more powerful bass they heard from live instruments.
Single surface interference problems also occur between you the listener and the surfaces near you, for the same reasons and in the same ways. So for better bass you should also treat the surfaces around your listening position.
So far, we've spoken of this single surface interference only in terms of complete cancellation and reinforcement (which effectively double the amplitude). But in between these extremes, and at all the frequencies lying between those frequencies where these extreme phenomena occur, there is still unwanted interference from single surface reflections, and there is still some corruption of what should be the correct amplitude level at all these other frequencies, either a gain or loss. You can think of this interference, between direct and reflected acoustic energy, as two sine waves combining, not at their maxima or minima (giving complete cancellation or reinforcement), but rather at some other points during their cycles. Furthermore, as Henning Moller has pointed out, the resultant of these .two sine waves combining, at all those frequencies where their maxima and minima are not synchronized, is a new wave whose phase is all screwed up relative to what it should be, and relative to either the direct or reflected signal. Here you are paying attention to your system's absolute phase polarity, having your subwoofer in the correct polarity, and even phase aligning your subwoofer (which we found to be audibly important in testing the Spica subwoofer) --but then the reflections from nearby surfaces totally screw up the phase of your bass. Absorbing the bass energy at the surfaces with ASC Tube Traps cures all these problems too, and Henning has found that the improvement in bass phase response is audible. This corroborates the work done recently by KEF, in improving the bass phase response of speakers, which also found that correct bass phase response is audibly important (see our report in Hotline 37).
Flexible Upper Frequency Absorption
As you see, your room has a myriad of acoustic problems in the bass and warmth regions, which are uniquely solved by the ability of the ASC Tube Traps to absorb these frequencies. Now, what about the midrange and treble regions, which are also absorbable by the various flat panel materials. Here the ASC Tube Traps again win out, this time due to their flexibility. After you place all the Tube Trap columns in your room to optimally deal with the various problems above requiring bass and warmth absorption, you can then individually adjust each Tube Trap as to the amount of upper frequency absorption, and as to the direction of upper frequency absorption. One half of the cylinder's circumference has a sheet of material that reflects midrange frequencies (some trebles are still absorbed by the decorative outer material), while the other half absorbs all frequencies. You adjust how much upper frequency energy is to be absorbed, and from which direction, simply by rotating the entire cylinder in its installed location. This user adjustment turns out to be a crucial tool for modifying the upper frequency reflection patterns and time delays reaching your ears, as well as the overall room reverberation decay time at these upper frequencies. The precise reflection patterns from different portions of your room's surfaces, their direction, and their time delay are critical parameters affecting the stereo imaging of your system Once again, the Tube Traps' manufacturer doesn't realize how powerful his product is with respect to this flexible upper frequency adjustment.
There's an interesting story to document this finding of our research in evaluating this product. While optimizing the stereo imaging of our reference system, we sought to determine how sensitive and powerful the positioning and rotation of the ASC Tube Traps were in achieving the best imaging. We had Tube Trap columns set up every 3 feet or so around the perimeter of our reference listening room, each column reaching up close to the ceiling. Note that, because of the huge room dimensions of 25 by 30 feet with a 14 foot cathedral ceiling, a given movement of a Tube Trap column or rotation of it would have proportionately far less effect upon the room and its stereo image than that same displacement or rotational movement in a smaller worn. Therefore, the following findings are conservatively large for smaller listening rooms. We found that, if we moved any Tube Trap column by a displacement of merely one quarter inch in its location, we could hear the difference in the quality of the stereo imaging. And, if we rotated some columns, to change the amount and direction of upper frequency absorption, by as little as 1.5 degrees, We could hear the difference in the quality of the stereo imaging. That's how potent and flexible these ASC Tube Traps are in handling upper frequency reflections!
No doubt at this juncture some tin ears of the ABX double blind switch box persuasion are shaking their heads, muttering that here's another golden eared tweak just imagining that he's hearing some phenomenon that sensitively. Sorry, Charlie. We got proof. We couldn't believe that these ASC Tube Traps could so powerfully and sensitively affect stereo imaging in such a huge room that a mere 1.5 degree rotation would audibly affect stereo imaging. So we set up an experiment to prove it to ourselves. We set up a 6 foot long (2 unit) column on its side, at the junction of ceiling and wall in back of the listener, at a distance of about 10 feet from the listener. In rotating this column for optimum imaging, we found that the front stage was markedly degraded if the Tube Traps were rotated one eighth inch at their circumference from the sonic optimum, which translates into 1.5 degrees with the 9 inch diameter column. Repeated tries at rotational optimization by ear always came up with the same alignment. These two units were centered in back of the listener.
Then, as a further challenge, we added a third unit off to one side. The sonic effect of this third unit was drastically decreased compared to the first two. First because the first two units were already aligned in place, so the third unit could at most have contributed a further third of the total effect upon imaging from this ceiling/back wall junction. Second because this third unit was farther away from the listener, so its sonic effect would be less perceptible at the listening seat (and it wasn't close enough to the side wall to start causing any effects there). Now, with this third unit installed, came the challenge of aligning its rotation by ear. Could an optimum rotation point be heard? Would it seem to be sensitive to this mere one eighth inch rotation? And, most crucially, would this be provable, by the chosen alignment point physically matching the alignment point already chosen and fixed in place on the first two units already aligned? Yes, yes, yes. The outer fabric seam along the length of each Tube Trap indicates the center of the upper frequency absorbent side. After the third unit was aligned by ear to its seeming optimum for best front stage imaging, its alignment was visually checked against the first two units, by inspecting how closely the seams lined up (these seams were out of sight during the optimization by ear, since they pointed upward toward the ceiling). Did the seam of the third unit line up with the seam of the first two units? Yes it did. Within one sixteenth of one inch (about 44 minutes of arc)!
Upper Frequency Diffusion
You've seen many, many room problems in the lower and upper frequencies that are addressed by the ASC Tube Traps' absorbent capabilities. But the Tube Traps do more than absorb sound. Their round cross section is also perfect for diffusing sound. The half of the cylinder which reflects upper frequency energy will diffuse the sound it reflects, as opposed to flat surfaces such as your room's walls.
What is diffusion, and what do flat walls do that is different? Simply speaking, flat walls reflect an incoming coherent acoustic wavefront straight back out, as a still coherent acoustic wavefront. This helps the ear/brain to identify the flat wall as a source of secondary radiated sound, to hear the colorations introduced both by the time delayed path from that wall and by the nature of that wall's material, and also to hear the time delayed energy itself as a coherent packet that smears the original music. You don't want the ear/brain to hear where your room's wall is, because that distracts from the correct imaging of the original stage and hall as it was recorded. You don't want the colorations of the wall's reflected energy corrupting your neutral playback system And you don't want delayed energy arriving in tightly bunched coherent packets that the ear/brain can pick out as information, which contradicts the original direct arrival information from the speakers and so confuses the perception of music. But if, instead of being reflected as a coherent unit, that wavefront could be scattered in all directions as it is reflected, these sonic problems would be minimized. If the sound is scattered in all directions as it is reflected, very little of it will come directly from that reflecting wall to you. Most of it will be scattered to other parts of the room, and this acoustic energy will eventually reach you at scattered times, from scattered directions, and with a wide variety of colorations acquired from its subsequent reflections. This is known as a diffuse sound field. You are surrounded by this ambient, reverberant field of diffuse acoustic energy, but no single part of it sticks out as identifiable in any parameter of time, direction, or coloration. Thus it enriches the ear/brain's perception of space and ambience on the original recording, without being distracting, confusing, or otherwise calling attention to itself. Additionally, a diffuse sound field set up in a good room has a uniformly decaying reverberation tail at all frequencies, with no gaps or severe discontinuities.
Thus, you can set up ASC Tube Traps every 3 feet or so around the perimeter of your room (and at other surface junctions, e.g. the wall/ceiling), as needed to control the room's bass and the warmth region's mud factor. Then you can micro-locate and micro-rotate each tube to deal with reflected energy in upper frequencies. By rotation, you control the amount and direction of upper frequency absorption. And, where upper frequencies aren't absorbed, they're diffused.
Which models of the Tube Traps should you buy? The 9 inch diameter full round model is the original, and the only one available until now. We've spent months with this model, learning all the tricks it can do, and there are still aspects to explore. The 11 inch diameter model is brand new, and should be identical except that its absorptive capacity is greater (about 6 sabines across the frequency spectrum), and it extends down to lower frequencies (50 hz vs. 120 hz, according to the manufacturer's measurements). Both factors make the 11 inch model a better absorptive value than the 9 inch model, in spite of its higher cost. This new 11 inch model is also available in a 2 foot long instead of 3 foot long cylinder, which is more economical, and also allows stacking with a 3 foot long section (either 9 inch or 11 inch diameter), to give a 5 foot long instead of 6 foot long column, which might strike some as being more decor conscious. A forthcoming 15 inch diameter model will extend absorptive coverage below 50 hz, down into the low bass.
ASC has announced a third model for the near future, which is a half round section of a 9 inch diameter, 3 foot long tube. This half round cylinder model is for the room's flat surfaces (not corners or wall edge junctions), but that's just where you need the full cylinder's rotation feature, which we found so sensitive for achieving superb imaging from our room, since we could adjust the absorption and diffusion of single surface reflections. This half round model, soon coming off the assembly line, is to be used with its flat side flush against a wall surface. The half round side facing the room is fully absorbent at lower frequencies, extending down about as far (150 hz) as the 9 inch full round model, but with less absorbent capability (5 sabines less). At upper frequencies, the central third of the half round side has a reflective strip running the length of the tube, which reflects and diffuses most upper frequencies back into the room (again, the outer decorative cloth still absorbs some treble energy). Meanwhile, the outer third on each side of this central strip is fully absorbent at all upper frequencies. This makes the half round column effective at suppressing boundary layer support of upper frequencies along the bare wall surfaces, and also keeps the half round columns, spaced at say 3 foot intervals along your walls, from talking to each other with reflected energy. This half round 9 inch Tube Trap model costs just half the price of the full round 9 inch model, so it is a cost effective way of obtaining the perimeter coverage of your room we suggest. Nonetheless, its lack of adjustability for amount and direction of upper frequency absorption should limit its usefulness in tuning your room for best stereo imaging, and we found this feature very important in evaluating the full round 9 inch model. When this forthcoming half round model starts coming off the assembly line, we'll check out this aspect for you and issue a review update.
Where to Install Tube Traps
Where should you install the ASC Tube Traps? For the present, let's consider doing your room using the original 9 inch full round Tube Traps with which we've had the most experience, plus the new 11 inch full round model for added bass extension.
Let's start with the corners. A standard rectangular room has eight corners where three surfaces meet, sometimes called tri-corners. These are the four corners at floor level, plus the four corners at ceiling level. These eight tri-corners have a unique property. They are the only locations in your room where every standing wave of your room, at every frequency and in every direction or dimension, has a pressure maximum (and velocity minimum). Consequently, placing acoustic pressure absorbers at these unique locations accomplishes more to quell your room's many standing waves (resonant modes) than would placing such pressure absorbers at any other locations. ASC Tube Traps are designed to be pressure absorbers, so the most effective place to start using them is in your room's eight tri-corners. This is also the location that the manufacturer recommends - indeed as the only place to employ his cylindrical Tube Traps. But, as we saw above, Tube Traps are valuable for tackling many more problems than merely a room's standing waves, so we recommend the corners as merely the starting point.
Note, incidentally, that conventional acoustic absorbent materials, such as panels and foam, are acoustic velocity absorbers rather than acoustic pressure absorbers. Therefore the common practice of placing them flush against room surfaces such as walls (or in corners) is incorrect. So doing limits their effectiveness to merely the treble frequencies, because at most frequencies there is minimum acoustic velocity (and maximum pressure peaks) within a couple of inches from every surface and corner. And, with their effectiveness thus limited by flush wall placement, one tends to use more and more of these absorbent panels in a vain attempt to solve the room's problems, until one suddenly finds that the room has become too dead (especially at upper frequencies) with all these absorbent panels, while the room's resonant mode and mud factor problems have still not been solved. So the room winds up sounding tonally dull and reverberantly dead, while still sounding muddy. The correct way to use absorbent panels and foam would be to suspend them in the middle of the room, away from surfaces (or perpendicular to them). Such placement intrudes into the middle of your room's space, and might not sit well with those of you who value your room's space and decor.
In contrast, since the ASC Tube Traps are designed to be primarily pressure rather than velocity absorbers, they perform at their best when placed unobtrusively flush against the walls and in the corners. And because they are performing at their best in such locations, fewer are needed and the wall surfaces do not need to be blanketed with absorbent treatment. Therefore one can conquer the room's resonant mode and mud factor problems, while still having neutral (not dull) tonal balance and a richly reverberant (not dead) room that does not require much amplifier power.
The tri-corners of your room are the obvious place to use the brand new 11 inch diameter model Tube Trap, to obtain the best control of the room's bass resonant modes down to the lowest frequency. The most economical approach would be to merely use four of the 2 foot long 11 inch diameter sections, one on the floor of each corner. You could then stack a 3 foot long 9 inch diameter section on top of each, giving you a 5 foot column in each corner. The whole hog approach would be to run 11 inch diameter sections from floor to ceiling in all four corners, using an appropriate mixture of 2 and 3 foot long sections to achieve your ceiling height. An intermediate approach would be to place eight 11 inch diameter sections, either 2 or 3 foot long versions, one in each of your room's eight tri-corners (it's easy to suspend Tube Trap sections from the ceiling with the 5/16 inch screw thread built in each end). Again, 9 inch diameter sections can be added in the four corners, between the 11 inch sections at the floor and ceiling corner junctions.
Your choice as to which approach you take in the corners will be dictated by your budget, various parameters of your room, and also by your taste in bass. The more 11 inch sections you put in your room's eight tri-corners or four corners from floor to ceiling, the tighter and drier the quality of the bass will probably become, due to improvements not only in the room's various standing wave patterns, but also in the bass impact and phase accuracy, which had been compromised by reflection interference (as discussed above). But there might come a point in adding corner 11 inch sections where some listeners (who prefer boomy bass) feel that the bass quality has become too dry for their taste. Also, in some rooms the bass resonant modes are more pronounced, in amplitude excess and/or long lasting ringing boom, than in other rooms. These variables are controlled by factors such as whether the room's three dimensions are nearly identical or multiples, vs. ideally staggered; whether the walls, floor and ceiling are rigid or flexible (flexible surfaces absorb some bass energy, but then add their own ringing colorations); whether there are adjoining hallways or cavities with their own boomy resonant modes; etc.
After you have installed some combination of Tube Traps in your room's corners, you can control the room's resonant modes further by adding Tube Trap columns at the one half, one third, or one quarter way points along every one of the four walls and along the ceiling (where the columns are hung horizontally from screw eyes). This placement effectively works further on the harmonics of the fundamental bass resonances for all three room dimensions. If your room is large, even these harmonics are low enough in frequency so that they should be attacked with 11 inch diameter rather than 9 inch diameter units. When deciding between the two diameters, keep in mind that 150 hz is the point at which the 9 inch unit has lost significant absorptive capability, while the 11 inch unit is still at maximum absorptive capability. And remember that, at 150 hz, the half wavelength pressure peaks are merely 3.7 feet apart.
If adjoining hallways or cavities are suspected of having resonant modes, they too should be treated with their own sets of Tube Traps, installed at their corners or at the ends of at least their longest dimension.
Installation of the Tube Traps in the corners and at these half, third, or quarter way points will solve all the room's standing wave problems. It should also suffice to control the room's reverberation decay time in the upper bass and warmth regions, thus eliminating the dreaded mud factor. Therefore, in addition to hearing better quality bass, you should also notice that your system sounds much more transparent and dynamic. The Tube Traps have now completed their mission of controlling unwanted acoustic energy that resonates in the room's volume, between pairs of opposite surfaces.
Perimeter Reflection Control
Installation of these Tube Traps outside the corners has also begun to establish the perimeter system for control of reflections from single surfaces, thus providing the many other sonic benefits discussed above. Once you have conquered the room's resonant modes and the mud factor, additional Tube Traps placed around the perimeter of your room serve mainly to control unwanted single reflections from the wall and ceiling surfaces. These additional units can probably be 9 inch diameter rather than larger cylinders, since upper bass should have been well controlled by all the 11 inch diameter units you have already installed. These additional perimeter columns should be placed (and rotated) so as to absorb and diffuse unwanted reflected packets of acoustic energy, adjust overall room brightness and upper frequency reverberant decay times, control tonal colorations and time smears from reflections, and optimize the reflected and reverberant energy from all directions for best stereo imaging. That's a tall order, much more complex than the simple absorption of room resonances and reverberant decay in the bass and warmth regions. But the ASC Tube Traps are flexible enough to meet this intricate challenge, if you are patient enough to explore and exploit their flexibility.
In our experimental research so far, we found it best to place Tube Trap columns, running virtually the full height from floor to ceiling, at about 2 foot intervals around the perimeter of the U defined by the wall in back of the speakers and the side walls, as far out from the back wall as the speakers are placed (which ideally is 5 feet or more, so that the first reflection from the back wall reaches the listener at least 10 milliseconds after the direct speaker sound). Then the remainder of the side walls, and the wall in back of the listener (also ideally 5 feet or more away from the listener), are covered by full height Tube Trap columns spaced at 3 to 4 foot intervals. Depending on the shape and height of the ceiling, it might also be wise to have horizontal Tube Trap columns, running across the full width of the ceiling (i.e. perpendicular to the direct line between speakers and listener), spaced at 3 to 4 foot intervals. The approximate nature, of this interval spacing for the additional perimeter columns, allows you to keep the exact half, third, or quarter way columns you installed above, and then fill in the perimeter of the room with additional columns (if needed) to obtain the approximate 3 to 4 foot interval spacing. We have also had very significant sonic results from placing yet another horizontal column of Tube Traps across the ceiling, at the junction where the ceiling meets the wall in back of the listener. It might therefore be helpful to install horizontal Tube Trap columns along the length of all such junctions of room surfaces (along the four ceiling to wall junctions, and the four floor to wall junctions, in between the vertical columns).
The wall and ceiling surfaces between Tube Trap columns should be left bare, not covered with flat absorbent materials -- at least until after you have fully explored the Tube Traps' own upper frequency absorptive capabilities by rotating them.
It is important that all parts of every room surface participate in radiating reflections to the listener, and in sustaining a directionally diffuse and evenly decaying reverberant field. In fact, the most significant finding of our experimental research with Tube Traps is that the widely touted Live End Dead End (LEDE) room treatment concept is wrong and even backwards. The LEDE concept teaches that the entire 3 dimensional U end of the room occupied by the speakers should be treated to be totally dead, so as to avoid all reflections from the surfaces at that end of the room. The theory is that early reflections must be killed, since they color the sound, cause time smearing of the music, and degrade the stereo imaging by giving false directional cues within 10 milliseconds of the original direct speaker sound (as does the secondary radiation from cabinet diffraction that we've written about). But such early reflections (those less than 10 milliseconds after the direct sound) can also be eliminated by placing the speakers 5 feet or more away from every reflecting surface. The listener should also be placed 5 feet or more away from every reflecting surface, so that he does not hear any reflected sound sooner than 10 milliseconds after the direct sound from the speakers. Note that this implies that a good listening room should be at least 20 feet long, allowing a reasonable 10 feet between listener and speaker plane. The LEDE concept was originated for small monitoring rooms in recording studios, where it does have relevance because early reflections cannot be eliminated by proper spacing. But in a proper size listening room, the LEDE concept actually degrades stereo imaging and makes music sound lifeless.
Imagine two flashlights or car headlights shining in the inky darkness. They are two isolated pinpoints of light. They do not illuminate anything. Illumination requires reflection. You cannot see anything by these two lonely lights, unless that something reflects light (recall that a wet blacktop road at night is invisible even in the glare of your car's headlights). Likewise, a pair of speakers in the dead unreflective end of a LEDE treated room are like two pinpoints emanating sound. They do not illuminate a stereo stage nor create one. The sound field between and around these two lonely speakers has at best a phantom, ghostlike quality. When on the other hand the entire speaker end of your room is acoustically illuminated by the speakers, the entire room end can become a stereo stage with incredible tactile reality, bringing music to life everywhere - and giving you superb imaging with a continuous curtain of sound between and beyond the speakers (no hot spots of lonely light) and with depth to the stage and tactile air around the instruments. How do you get the speakers to illuminate their entire end of your room? By reflections.
First reflections arriving less than 10 milliseconds after the direct sound from the speaker are bad, as are reflections that arrive later but are coherent time packets or uniform directional packets. But secondary, tertiary, etc. reflections, arriving at scattered times and from diffuse directions are beneficial. Indeed, they are psychoacoustically crucial to helping the ear/brain to reconstruct a 3D stereo stage all around those two lonely pinpoints of light, your speakers. Some of this has already been proven by the work of Madsen, who showed that delayed sound is important, and Damaske, who showed that the delayed sound should be incoherently scattered in time/phase, and should be non-identifiable as to the direction whence it came (diffuse sound coming at the listener from the side of his head is the best in this regard). How can you get the three desiderata all together: time delay, scattered time, and diffuse non-identifiable directionality? You could go to the expense of making your listening room totally dead (like an anechoic chamber) and then add an electronic reverberant delay unit, plus an extra amplifier and speakers to the side of the listener's chair. Or you could save a whole lot of money by not buying the extra electronics, the extra speakers, and all the extra absorbent material needed to make your room totally dead. Instead, put your listening room to work. Get rid of just the unwanted single surface reflections (as well as of course the paired surface resonant modes dealt with above), and then control the remaining surface reflections so you can put them to good use. They will generate your delay, your scattered time, and your diffused direction.
Our research findings also suggest that, if anything, it is the listener's end of the room that should be dead, not live as the LEDE concept suggests. Once the speaker end of the room and the side surfaces had been properly set up with controlled and diffuse reflections, then reflections coming from the wall in back of the listener actually detract from hearing into the stage up front, from hearing into the concert hall acoustics as they were captured on the recording. These reflections coming at the listener from his rear serve only to define the listening room's dimensions and tell the ear/brain that there's a wall in back. This information is confusing and misleading, being at odds with the hall's acoustics encoded in the recording, and so should be eliminated. But controlled reflections coming at the listener from his front and his sides are very important in enhancing his perception of the stereo imaging encoded in the recording, so these should be preserved and optimized.
After all Tube Traps have been set up and optimized around the perimeter of your room, including the wall in back of the listener, we'd recommend that you then try adding flat absorbent material to this wall in back of the listener. Any thick material might be satisfactory, including heavy fabric, carpeting, acoustic foam, batting material, or fiberglass type insulation. Speaking of carpeting, there is one room surface that you cannot place speakers or listener 5 feet or more away from, namely the floor. Therefore, in order to kill early reflection paths between speakers and listener, the floor should be covered with heavy carpeting between the speakers and the listener, and fully surrounding the listener to his rear as well (see mini article below). The floor area from the speaker plane back to the wall in back of the speakers should be left bare, however, since reflectivity is needed in the stereo stage area. Heavy carpeting will kill the unwanted early reflections in the upper midrange and trebles; to kill them also in the bass, warmth, and lower midrange regions would require sprinkling Tube Traps horizontally about on the floor, which would pose an unreasonable tripping hazard for most folks.
How should you adjust the rotation of the Tube Traps? The ear/brain is most sensitive to many acoustic phenomena from the midrange upwards. When we hear a room as too live or dead in its reverb decay time, we're hearing primarily these upper frequencies. When we hear distinct room echoes in its reverb decay pattern, again we're hearing primarily these upper frequencies. Likewise for hearing colorations of reflections, time smearing of the music itself by reflections, distinct time packets of reflected energy, directionality of these packets that detracts from the stage's stereo imaging, etc. Rotating the ASC Tube Trap changes the amount of absorption, and the direction of absorption, for all frequencies above 400 hz. That covers all these upper frequencies for which the ear/brain is most sensitive to all these phenomena. Therefore, merely rotating the cylindrical model Tube Traps enables you to control all these phenomena.
To optimize all these phenomena for your own unique listening room, and for the speakers you are using, you should learn how to listen for and optimize all these phenomena. It's an iterative process, with the Tube Traps themselves being the teacher that progressively educates your hearing even while you use them as a tool to manipulate these phenomena. As you home in on the best optimization, you'll be able to isolate more clearly the sound of the various phenomena being adjusted, so you can make better progress instead of stumbling around blindly. Here then are some starting guidelines, which should get you close enough to optimization so that you can begin making solid progress immediately.
Tube Trap columns should be positioned against the back wall precisely behind the center of the speakers. The fabric seam running the length of the column (which indicates the side of maximum upper frequency absorption) should face the speaker's rear. About 2 feet to either side of these columns, sets of columns against the back wall should be placed, with the seams at about 45 degrees, so that the seams toe inward toward the speakers. All remaining columns in that U end of the room, up to the plane of the speakers, should have their seams pointing directly at the wall, with corner columns having their seams pointing directly into the corner apex.
At the listener end of the room, all columns against the rear wall in back of the listener should start pointing straight out, with possible optimization tending to make them toe in toward the back of the listener's head. We found that the optimum toe in was a bit less than this, with the corner columns in back of the listener having their seams pointing toward the opposite corners of the room.
Along the side walls, up to the U end defined by the speaker plane, start all columns with their seams pointing at 90 degrees to the side wall, i.e. pointing directly at the speaker end wall of the room. Fine tuning will then take the seam slightly toward the side wall, or slightly toward the room, from this forward facing alignment.
When you fine tune the Tube Traps' rotational alignments, change only one column at a time, and listen to the differences in stereo staging, etc. At first, try moving the seam 1 inch to either side of our recommended starting points. Pick your favorite of these three rotational alignments, and work your way around the room for all columns. Then repeat the process, this time rotating all columns one half inch to either side of your previously picked favorite alignment. In the end, you should be able to hear the improvement and optimization of stereo imaging, etc. by rotating every column merely one eighth of an inch.
After you've finished rotating all columns to their optimum (see further tips below), you can experiment with lateral placement along the wall (this usually affects reflections less than rotation, which is why you should do it after rotation, even though that's physically the more inconvenient tactic). Try moving one column at a time, an inch at a time, and listen from your listening seat to what changes occur in the imaging and ambience from that section of the stage or side walls. After your experience with rotation, your hearing sensibilities should be finely tuned to such changes, and to achieving optimization. Ultimately, you are free to space the columns as far apart as a foot intervals, or as close as 2 foot intervals (even closer in the wall area directly in back of the speakers). Be sure to keep each column at the same rotational alignment as you move it laterally, and after listening to the new lateral placement, try more micro-rotation optimization, since the angles with respect to the speakers, listener, and room will change with change in lateral position.
Because you might wind up spacing the room perimeter columns as closely as 2 feet or as far as 4 feet apart, and because only listening will tell you whether you prefer the tighter bass from the 11 inch units or the lesser control of the 9 inch, there's no way to predict how many Tube Trap sections of which type you'll wind up wanting to use. Therefore, it would be wise for ASC dealers to adopt a plan where they can let you buy more than you'd possibly need on approval, and you can pare down your purchase from this number by listening. Even better, some ASC dealers could become expert installers, suggesting spacing and placement for your perimeter and ceiling columns. Of course, you should still do a lot of further experimentation with alignment and placement yourself. No dealer would have enough time for this, and it's uniquely your room, your system, and your ears that count.
After you've completed all rotational alignment and lateral positioning to your satisfaction, then is the time to assess whether you think your room still sounds too bright, or has some reflective hot spots where bare wall areas are between columns. If so, then you can apply limited mounts of flat absorbent material to soak up some upper frequency energy. Recall that flat absorbent materials, being velocity absorbers, work best if they are placed away from wall surfaces (see also discussion below).
Theory of Absorbing Reflections
Is there some theory to back up our suggested starting points for rotational alignment of the Tube Trap columns? Yes, and we can learn more about room treatment and the psychoacoustics of optimum stereo imaging by thinking about it. Let's begin with the wall in back of the listener. Pointing the seams straight into the room maximizes the Tube Traps' absorption of upper frequencies, coming directly from the speaker plane. Then, toeing in the seams of the outer columns toward the back of the listener's head maximizes the killing of all first reflections from the rear wall that would have been directed at the listener. This short circuits the first sound that the listener would have heard (from his rear direction) after he hears the direct sound from the speakers: the coherent packet following the path from speaker to rear wall to listener. Thus, the first sound coming at the listener from his rear will all be secondary, tertiary, etc. reflections. These are later in time and weaker in level (effectively moving the listener even farther away from the rear wall than the minimum recommended 5 feet). Furthermore, they are not a coherent packet in time and they are diffuse in direction (effectively minimizing their distractive effect). By spacing Tube Trap columns at closer intervals than the recommended 3 to 4 feet along this wall in back of the listener, you can achieve even greater suppression of this unwanted first reflection from the rear, at all frequencies. Or, by adding flat absorptive material to this wall, you can achieve this greater suppression at merely the upper frequencies (which are the most distractive to the ear/brain).
This first reflection from the rear, coming as a time coherent packet from a specific unwanted direction so soon after the direct sound from the speakers, is very destructive sonically -- especially to stereo imaging and your perception of the recording hall space instead of your listening room space. And so it's very important to get rid of it. That's why we suggest a dead end to the rear of the listener, which again is just the reverse of the LEDE concept (although LEDE does correctly teach that the rear field reflection should at least be diffuse).
The destructiveness of this rear first reflection does not just apply to speakers in a small listening room; it also applies to live music in a huge concert hall. You can hear this for yourself with the following experiment. From a good seat in the middle of a concert hall, you hear the giant hall space in front of you, just as you can hear portrayed from a well miked recording played through a system that images very well. When sitting in the center of the concert hall, you also hear the hall space to the sides and rear of you, but that's beside the point of the demonstration here. Now, as you move backward in the concert hall, the volume of the huge space in front of you is literally growing. So close your eyes and pay attention to the perceived size of the concert hall just in the frontal direction. You should indeed hear the subtle sonic cues telling you that the hall space in front of you is getting larger: an increase in reverberant ambience and certain decay times. And now, for the crux of the experiment, carry this progression to the extreme. While the orchestra is playing, start at some distance from the center rear wall of the hall, or better yet diagonally from a rear corner if no seats intrude; start ideally at a 15 to 20 foot distance (10 feet will do) from the rear wall, or from both rear and side walls in the case of a corner. Close your eyes and slowly walk backward toward the rear wall, or diagonally into the rear corner. As you near the corner or rear wall, you now have literally the whole concert hall volume in front of you, the maximum hall space and depth for that 3D stereo image being portrayed in front of you. That's the fact of the matter, so that's how your ear/brain should hear it, right? Wrong! As you get close to the rear wall or corner, the whole huge concert hall in front of you magically disappears! And you find yourself sonically in a small apartment size listening room, with an orchestra playing in this teeny room through pint size speakers. What on earth has happened? The first reflection from the rear wall (or double the effect if you've backed into a corner) has become much stronger relative to the direct sound from the orchestra and relative to all the long tailed reverberant sound diffused throughout the hall. This first reflection energy arrives as a time coherent packet from specific directions. And it arrives very soon after the direct sound from the orchestra. All this conspires to tell your ear/brain that you are standing in a small room, with the walls merely a few feet away, and an orchestra playing in this small room. That huge concert hall space with its incredible depth, which your eyes tell you is in fact there, has literally disappeared for your sense of hearing. Suppose you had the most perfect imaginable stereo imaging fragilely recreated by a mere pair of speakers and a playback system, playing the most perfectly talked stereo recording -- so perfect that it could virtually, as they say, transport you to the concert hall. It hasn't a prayer of sustaining that desired illusion if it has to contend with that pernicious first reflection from your listening room's rear wall. Because here you are, literally transported to the concert hall, and even a live orchestra in an actual huge concert hall, generating and radiating the acoustic information that is by definition perfect stereo imaging and the sound of being in the concert hall, is defeated by first reflections from a nearby surface. Incidentally, this listening experiment with live music in a concert hall obviously has implications for how you should select the best sounding seat in a hall -- but that's another story (with further complicating factors), another IAR article.
Our recommended rotational alignment for the Tube Trap columns along your room's side walls (and the ceiling) is related to the same theoretical basis. The first reflection from the speakers to the side walls to the listener tells the ear/brain that the listening room's side walls are there, and you want them to disappear in favor of the concert hall's side walls as encoded in the recording. Aligning the Tube Trap seam to face forward toward the speaker end of the listening room intercepts what would become the first reflection energy as it arrives from the speakers, at all frequencies. Meanwhile, the other half of the Tube Trap, which reflects and diffuses upper frequencies instead of absorbing them, is facing the listener end of the listening room This provides the benefit of sustaining (instead of absorbing) and diffusing the many secondary, tertiary, etc. reflections that constitute the room's reverberant field - especially the part of the field in the listener's end of the room, which is the part that counts most, since that's where the listener is located, and ultimately the only sound that counts is what reaches the listening position.
When you get to the fine tuning stage for the rotation of the side wall (and ceiling) Tube Trap columns, you'll find that turning the seam of a column toward the room makes the recorded stage and hall ambience deader from that section of the side wall covered by that column, since too much delayed reverberant, diffuse energy is being absorbed at upper frequencies. This reverberant energy is crucial to perceiving the recorded stereo 3D image and ambience, thanks to its significant delay (via the Madsen effect) and thanks to its diffuse directionality and temporal incoherence (via the Damaske effect). On the other hand, turning the seam toward the adjacent side wall increases the perceived recorded ambience, but also increases the coherent first reflection packet from that section of side wall (or ceiling). Your goal should be to maximize the recorded stage ambience and width as heard from your listening chair. But don't rotate the column seam so far toward the side wall that you begin hearing where your room's side wall is (in the section of wall covered by that column). As soon as you do begin to hear this section of side wall (courtesy of the first reflection), this acoustic information conflicts in the ear/brain with the desired perception of the recorded stage and hall width, so the latter will be degraded. If you wish, you can also experiment with slightly offsetting the seams of the Tube Trap sections constituting each column. You might try turning the lower one toward the room a bit, to better kill the first reflection at ear level, and the upper one toward the side wall a bit, to better reflect and diffuse reverberant energy with a longer and more indirect delay path to the listener.
Finally, let's consider the U at the speaker end of the room. The wall section immediately in back of each speaker should be made dead across the entire frequency range. The primary reason is again the first reflection. The first reflection coming off the wall immediately in back of each speaker has the shortest delay, since its path length is the shortest. Even with the speakers placed the recommended 5 feet or more away from this wall, the delay is too short, so the reflection still time smears the music (longer delays, such as those from reflections elsewhere in the U shaped stage area of the room, enhance ambience and imaging via the Haas and Madsen effects). This first reflection from directly in back of the speaker also directly combines with the speaker's frontal output, so it causes the worst interference patterns, thereby coloring the tonal balance with honks, boominess, etc. And furthermore, reflected energy from directly in back of the speaker comes at the listener from exactly the same direction as the original radiated sound from the speaker, so it contributes nothing further to the width of the image, or to making the speakers' locations disappear within the stereo curtain of sound.
Most box speaker systems radiate their upper frequencies primarily forward, not toward this wall area at their rear; But their lower frequencies, from the lower midrange down (depending on driver diameters and box dimensions), radiate omnidirectionally. So closely spaced Tube Traps are vital against the wall immediately in back of each speaker. Flat absorbent materials are ineffective in handling these lower frequencies. Turning the Tube Trap seam toward the speaker seems sufficient to absorb the slight upper frequency energy radiated rearward by box speakers. Recall that the column directly in back of the speaker has its seam pointing straight out, while the columns 2 feet to either side at the rear have their seams toed in to approximately face the back of the speaker. Be sure to experiment with the exact seam toe in of these latter columns. You don't want maximum upper frequency attenuation from these two columns at the rear to the sides of each speaker, since some secondary, tertiary, etc. upper frequency reflections are needed in this area, in order to establish depth and ambience throughout the whole center stage. Align the seams so you hear stage ambience and depth being optimized, throughout the center half of the stage width.
If your speaker is a panel dipole or has rear facing tweeters, then more upper frequency absorption might be appropriate, perhaps in the form of flat absorbent material on the wall immediately in back of the speakers.
The remaining wall (and ceiling) surface area of the U end of the room, up to the speaker plane, has longer delay paths and provides indirect multiple reflections for upper frequencies. It also provides reverberant energy coming at the listener from all locations on the stereo stage other than the speaker locations (the Bose effect, which is beneficial when properly tamed). So you want the upper frequencies to be reflected and diffused, not absorbed, throughout this remaining surface area. This is accomplished by having all column seams facing into the adjacent wall surface or corner.
Tube Trap Operating Principles
How do the ASC Tube Traps operate? The AES paper given by designer Art Noxon reveals the principle of the device, which is patented. The structure of each cylindrical Tube Trap section consists of a tube of rugged wire fencing called hardware cloth, with end caps of a solid material such as Masonite. Just inside the supporting tube of hardware cloth is another tube, of a soft absorbent material such as Fiberglass. And inside of this absorbent tube is nothing but air -- the Tube Trap is hollow inside. Deceptively simple, yes? Like most clever inventions are, once you see how they work. The theory of operation is equally simple and clever. The Tube Traps are intended to function best in areas of acoustic pressure peaks and velocity nulls, which are against the room's surfaces and in its corners. The tube of absorbent material forms an acoustic resistance. This creates a pressure difference or gradient between the peak pressure zone outside the tube, and the air trapped inside the hollow tube (remember that the end caps are sealed). This pressure gradient in turn creates a velocity airflow from outside the tube to inside (or vice versa in the case of a negative pressure, i.e. rarefaction, peak). As the airflow passes through the same acoustic resistance that created the pressure gradient in the first place, its kinetic energy is absorbed, being turned into heat. This is the same way that all soft, porous materials absorb acoustic energy: making air velocity work while going through a resistance, thereby dissipating itself as heat. But, as noted, all the other flat sound absorbers on the market, from sculptured foam to Fiberglass panels, have to be placed in zones of peak velocity to be effective, which means in the middle of your room, away from walls and corners. The key difference in the ASC Tube Trap is that it creates a pressure gradient from pressure peaks, and then creates its own air velocity from this pressure gradient. And it accomplishes this by the simple expedient of wrapping the conventional flat sheet of absorbent material into a tube shape, leaving the tube's inside empty, and sealing the ends of the tube. Note also that the ASC Tube Trap operates differently from old fashioned bass absorbers, such as Helmholtz resonators and diaphragmatic resonators, both of which are physically very large, very expensive, and add resonant hangover colorations of their own.
The selection of the absorbent material is critical to the proper operation of the Tube Trap. The material has to be thick and dense enough so that it has a high enough acoustic resistance to set up a significant pressure gradient -- and so that a significant amount of work is expended by the air velocity in fighting its way through it, in order that enough energy is dissipated and hence absorbed. But it also has to be thin and porous enough so that a high enough air velocity is allowed to develop, in order that the absorbent material has significant effectiveness in dealing with the only parameter it can, velocity.
The functioning of the Tube Trap is also described by its inventor in terms of an electrical equivalent circuit. The hollow interior trapped air constitutes a capacitive compliance, and the acoustically resistive absorbent material restricting the outside air's access to this interior air volume constitutes a resistor in series with the capacitance. This series resistor and capacitor constitutes an RC network, with a low frequency cutoff determined by this RC time constant. The inventor states that this RC cutoff is dependent only on the tube's diameter; this is why the 11 inch diameter tube is effective to lower frequencies than the 9 inch diameter tube.
One half of the cylinder's periphery has a thin sheet wrapped around the outside of the tube. This thin sheet of say rubber or compliant plastic, described as a limp mass, reflects upper frequencies (it is designed to start reflecting above 400 hz), but is transparent to lower frequencies. This feature gives the cylindrical model Tube Traps the ability to reflect or absorb these upper frequencies, depending on how you align their rotation. On the half cylinder Tube Trap, a narrow strip of this thin material runs down the center of the half round.
The whole assemblage is enclosed in a decorative cloth, which also absorbs some treble energy from all sides. The seam of the cloth is placed opposite the reflective side of the cylindrical models, so it represents the center of the side that absorbs upper frequencies. The decorative cloth comes in a variety of attractive muted colors, all tinged with white to make the columns unobtrusive. Initial setup effort is minimal; the tube sections simply screw together.
From this extensive treatise covering their usage, it's obvious that ASC Tube Traps are superbly powerful and flexible, a true breakthrough product for turning your listening room into a final link that is worthy of the quality of the rest of your playback system. Alas, like most good things in life, they are also expensive, especially when you deploy them all around your room as we have described.
What is the bottom line? The Tube Traps are usually sold in sets of 4, with various combinations. We'll give you the approximate price breakdown by type of tube section. The 11 inch diameter section that is 2 feet long retails for $147.25; the 3 foot long version costs $184.75. The 9 inch diameter section that is 3 feet long costs $132.25. The 9 inch half cylinder model costs half of this for a 3 foot long section. Pricing has not yet been set on a forthcoming 15 inch diameter model, which will extend coverage into the low bass. These are the retail prices through ASC dealers. The Tube Traps are also available through select Monster Cable dealers, but at an unconscionable price premium of about 31%. Prospective ASC dealers and customers can write to: Acoustic Sciences Corp, PO Box 1189, Eugene OR 97440 (phone 800-ASC-TUBE).
A few hobbyists might try to literally roll their own imitation Tube Traps. But most of you should regard this crucial acoustic treatment of your listening room as a serious investment. Think of all the money you've aleady spent on your record collection and your playback equipment. Would you play all this through chintzy dime store loudspeakers? Well, remember, the loudspeaker is not the final link in your playback system. Your listening room is. And the chintzy dime store acoustics of most listening rooms out there are literally destroying the sound you have spent so much money and effort to create. Most listening rooms butcher the sound that your system is actually putting out, in the many ways described above (the mud factor, poor bass in many ways, reverberant ringing hangover, tonal colorations, poor stereo imaging, etc.). You'll never hear what your system is actually putting out unless you fix your listening room. And now, at long last, here in the ASC Tube Trap is an effective tool to accomplish this critical mission in your pursuit of the holy grail.