W1 does achieve an excellent rating in freedom from upper bass colorations. And the W1 is positively outstanding in its flexibility, allowing it to be partnered with the Evolution M6 or M5 satellites in a variety of ways, and also making it this NHT W1 an excellent partner for most other brands of satellites, many of which have pretty sorry sounding subwoofers within their own brand.
      All four of the Evolution subwoofer models employ the exactly the same 12 inch cone driver. All four operate on the same acoustic suspension (sealed box) principle. All four utilize the same cabinet volume per driver, so all four have the same operating parameters. Yet they sound very different from one another (thereby proving that there's more to bass system design than merely following Thiele-Small parameter guidelines). The two dual-woofer models (the B6 and W1) obviously can pump out louder bass with lower distortion, per subwoofer cabinet, than the single-woofer models (the B5 and W2).
      The dual-woofer W1 has a further advantage over the single-woofer W2, besides greater piston area. In the W1, the two woofers are mounted facing in opposite directions. This means that the Newtonian reactive kick-back forces from the two woofers cancel, so the whole W1 subwoofer cabinet inherently stays put in one place, thereby providing audibly superior quality and quantity of bass. It also means that certain cabinet flexings and resonances are excited far less, thereby yielding lower cabinet coloration.
      If your budget is limited and you can't manage two W1 subwoofers, then and only then would we recommend that you buy two W2 subwoofers, as preferable to getting just one monaural W1, so you can experience the spatial richness and immersion that requires at least two widely spaced subwoofers. If you do opt for the W2, we would suggest mounting it on the floor with its single woofer pointing upward. In this way the Newtonian kick-back forces will at least be directed into your relatively non-yielding floor. This will give you better bass quality and quantity from the W2 than if the woofer were pointing sideways in any direction, since the latter tactic could allow the whole subwoofer to slide slightly backwards with each Newtonian kick-back, thereby canceling some of the bass quantity, and also thereby degrading the bass quality, because this sliding would have its own lagging time constants.
      What about the B6 and B5 subwoofers? They would seem to be the natural choice for most of you, to partner with the M6 or M5, since their greater height (than the W1 and W2 cubes) is expressly meant to be at the correct altitude for them to act as pedestals for the M6 and/or M5 satellite monitors. The B6 and B5 might seem to be the natural choice, but unfortunately their bass performance is markedly inferior, indeed so inferior to the very good W1 that we regard the B6 and B5 as being completely out of the running (our recommendation is that you don't even consider them). Their bass sound is much weaker in quantity and impact, and is much more woolly and poorly defined in quality.
      Why is the bass so inferior from these taller pedestal subwoofers? Several reasons. The woofer driver or drivers point sideways, and in the dual-woofer B6 both woofers face the same way. So the Newtonian kick-back forces from the woofers' motion makes the whole subwoofer cabinet move in the opposite direction, thereby canceling some of the bass quantity.
      To make matters worse, the enclosures are taller, so their center of gravity is higher in altitude, which means that the B6 and B5 cabinets rock sideways, back and forth, thereby also degrading the bass quality, since the cabinet's initial rocking motion and subsequent rock-back restoration impose their own lagging time constants (slowing bass attack and also causing lingering overhang). The outrigger feet for the cabinets reduce this rock and roll dance somewhat, but don't entirely cure it. To make matters even worse, the woofer drivers are mounted higher in the B6 and B5 than they are in the floor hugging cubical cabinets of the W1 and W2, so their Newtonian kick-back force acts with a greater moment arm (the same force at a greater distance from the floor pivot point produces worse effects).
      To make matters yet worse, the fact that the drivers are mounted higher in the B6 and B5 means that the interference from floor bounce is worsened, thereby degrading both the quantity and quality of bass (one might think that this factor would be irrelevant at the very long wavelengths of low bass frequencies, but in point of surprising fact we've found in our research that every inch counts, even at these low frequencies and long wavelengths, as will be discussed further below even for the floor hugging W1). And, to make matters even yet worse, the two drivers in the B6 are at different distances from the listener, thereby degrading bass quality due to phase interference effects (here too we've found in our research that subwoofer phase and timing is amazingly critical, even at these low frequencies and long wavelengths, and we'll see a concrete example when we discuss optimum W1 orientation below).
      Incidentally, the B6 and B5 have a further potential problem that has been recognized and well addressed by NHT: the backside panel of these pedestal cabinets (to the rear of the woofer or woofers) has large dimensions, much larger than the dimensions of any panels of the diminutive W1 or W2 cube cabinets, so it is prone to worse flexing, which could reduce bass quantity and quality yet further, as well as introducing cabinet flexing colorations above the bass region - but NHT has addressed this via thorough internal bracing of this large area panel.
      The W1 subwoofer has none of these technical problems. And, now that you understand the problems that degrade the bass quantity and quality of the B6 and B5, you can now see why the W1 (or W2) is a far better sounding choice as a subwoofer model, in spite of the B6 or B5 seeming to be the natural choice because of their pedestal height. The W1 produces much better bass quantity and quality than the B6 (or B5) because it doesn't do their rock and roll dance. Instead, thanks to the W1's dual opposing woofers (as well as its low center of gravity and the low mounting height of its woofers, which enhance immunity to any slight woofer driver mismatch), the W1 cabinet just sits there immobile, as though it were welded to your floor. This means that what you hear from the W1 is the full bass power, bass quality, and articulate definition of which the drivers themselves are capable. Similarly, because the W1 woofers are close to the floor (unlike the B6), the interference problems from floor bounce are minimized. And, because the W1 can easily be oriented such that both its woofers are precisely the same distance to the listener, the phase and time misalignment problems of the two staggered-distance woofers in the B6 are obviated.

Bass Equalization for Subwoofer

      As noted, the four Evolution subwoofer models all operate as sealed box, acoustic suspension bass systems, with the same cabinet volume per subwoofer driver. Now, acoustic suspension systems are renowned for operating with surprisingly small cabinet volumes, given how low their bass extension can reach. But even with the acoustic suspension principle there are limitations imposed by the laws of physics. Specifically, cabinet volume cannot be made very small if you want to achieve very deep bass extension (as of course you do for a subwoofer) and if you also want reasonable loudspeaker system efficiency, so that reasonable size power amplifiers can be used to obtain reasonably loud bass levels. But today's market, which is not only décor conscious but also must fit plural subwoofers within the modest size rooms of modern housing, demands very compact subwoofer cabinets. Compact subwoofer cabinets are also advantageous for sonic reasons, since smaller cabinet panels flex and resonate at higher frequencies, which are beyond the subwoofer pass band and thus don't get triggered by the subwoofer driver's motions - and this means better bass quantity and quality, and less cabinet coloration to muddy and artificially color the sound.
      The cabinet volume of the Evolution subwoofers is therefore, with good reason, made quite small, indeed so small that, with the chosen NHT subwoofer driver of reasonably high efficiency (i.e. reasonably low moving mass), the natural in-box system resonance of all the Evolution subwoofers is up at a relatively high 50 Hz. Below this 50 Hz point, the intrinsic bass response of the subwoofer starts its 12 dB per octave rolloff. How then do these Evolution subwoofers deliver flat bass response down to 27 Hz? The X1 electronic crossover equalizes the bass signal that it feeds to your power amplifier driving the subwoofer, progressively boosting it at the same 12 dB per octave slope, within the frequency span from 50 Hz down to 27 Hz, so that the subwoofer output with this equalized input signal winds up being truly flat down to 27 Hz.
      This means that the Evolution subwoofers must be used with the Evolution X1 electronic crossover, in order to deliver flat response below 50 Hz. So you should think of the X1 electronic crossover as being part and parcel of any of the Evolution subwoofer models you choose. Fortunately, the X1 is generally a very good crossover, with a host of other sonically beneficial (and sonically very important) features, and the X1 is very reasonably priced ($350), and each X1 is a two channel unit, so you need buy only one X1 for every two Evolution subwoofers you buy (further incentive to actually use plural subwoofers). Note by the way that this also means that the X1, though a wonderful general purpose electronic crossover on many counts, cannot be used with other subwoofers, since its bass (low pass) output is an equalized signal, equalized specifically for the Evolution subwoofer models.
      The fact that the bass signal to your chosen subwoofer power amplifier is progressively boosted below 50 Hz means that you should choose this power amplifier with some care and attention to its bass power, current, and drive capabilities (low source impedance and large safe operating area into reactive loads would be important desiderata). This progressive boost also means that the drivers in the Evolution subwoofers are worked very hard, with magnified excursions, to overcome the natural intrinsic rolloff of the subwoofer system in such a small cabinet volume. Fortunately, the drivers chosen by NHT seem very capable of handling the arduous work of these magnified excursions. Of course, these drivers can handle this arduous work even better if this work is shared amongst plural drivers, thereby giving you more bass output and impact with lower distortion - which is further reason for buying the dual driver W1 instead of the single driver W2, and for buying plural W1s instead of just one.

Evolution Subwoofer vs. Vented Enclosure Subwoofer

      What happens to the bass at and below 27 Hz? At 27 Hz the equalizing boost from the X1 stops, in order to prevent excessive excursion demands upon the subwoofer drivers from yet lower frequencies that would be electrically boosted even more. And below 27 Hz the X1 actually begins electrically rolling off the signal fed to your subwoofer power amplifier (instead of merely becoming flat below 27 Hz). The ultimate rolloff slope of the X1, far below 27 Hz, becomes 12 dB per octave. This 12 dB per octave rolloff slope adds to the intrinsic 12 dB per octave acoustic rolloff slope of the subwoofer's sealed bass system that intrinsically began below 50 Hz, thereby making the ultimate bass rolloff slope 24 dB per octave.
      This 24 dB per octave ultimate slope is significant because it means that the Evolution subwoofers are ultimately fourth order bass systems, just like the very common vented enclosure bass systems. Why is this fact so important? A pure sealed bass system (without equalization) is a second order bass system, and this inherently has superior bass transient response to fourth order bass systems (which include all vented or ported bass systems). The superior bass transient response of a pure second order sealed bass system means that there is less overhang and ringing after each musical bass transient, which yields tighter bass, better bass definition, better bass tunefulness, less of the one-note boom phenomenon, and less of that woolly, heavy fullness that characterizes most vented bass loudspeaker systems (except those with ports specifically "mistuned" for flat top response, which can achieve much better bass transient response, almost as good as a second order sealed bass system).
      But the Evolution subwoofers, even though they use sealed enclosures, are not pure second order systems, since they also employ boosting electrical equalization that then itself rolls off below 27 Hz and thereby adds two more orders (12 dB per octave extra slope) to the intrinsic acoustic rolloff of the Evolution's sealed bass, acoustic suspension enclosure. The bottom line is that the bass transient response of the Evolution subwoofers, including the W1, is not as good as a pure sealed bass second order subwoofer, and instead is much like the bass transient response of a very good vented bass subwoofer system. That's what keeps the W1's bass transient response from earning an excellent rating. The slight overhang and heaviness we hear from the W1, which demotes its bass quality rating from excellent to very good, is explained by and is directly attributable to this important fact, that its transient response actually behaves like a fourth order vented bass subwoofer system instead of a second order sealed enclosure subwoofer system. Incidentally, it's worth noting that even pure second order sealed enclosure bass systems do not have ideal bass transient response. In order to achieve ideal bass transient response, you'd need a single order bass system with a rolloff slope of just 6 dB per octave (dipole woofers can achieve this, but only for a limited range of frequencies).
      Although the W1 is a fourth order system, it does have the potential for better quality bass, at the low end of its bass passband, than common vented bass fourth order systems. One reason is that the four orders of common vented bass systems are entirely achieved by very imperfect acoustical and electromechanical means, whereas two of the W1's orders are achieved by virtually perfect electrical means, and the remaining two are achieved by well controlled acoustical and electromechanical means.
      Why does this contrast make a sonic difference? In this modern engineering age, fourth order vented bass loudspeaker systems are designed in accordance with the equations involving the major Thiele-Small parameters. The problem is that these equations make many simplifying assumptions about other minor parameters, wrongly ignoring them or assuming that they are static or well behaved, when in fact these other minor parameters do dynamically affect the sound, in particular the bass quality. To make matters worse, many of the vented bass alignment choices are very critical of even the major parameters having exact values, and these exact values, though plottable and calculable in the abstract on paper, cannot be achieved in concrete practical reality, due to tolerance variations in various parts and systems. The result is that the actual bass performance of most fourth order vented bass loudspeaker systems does not fulfill the promise that the simplified equation assumptions suggest on paper.
      In contrast, two orders of the W1's rolloff below 27 Hz are achieved by a purely electrical filter, wherein all parameters are known and no simplifying assumptions need be made, and furthermore whose behavior is virtually perfect within very tight tolerances.
      What then about the other two orders of the W1's bass rolloff, the two orders governed by acoustical and electromechanical means? They too can be far better predicted and controlled than in a common vented bass system, for several reasons. First, an acoustic suspension design such as the W1 depends on the linear springiness of a fixed and well controlled amount of air, without many complicating minor factors that must be assumed, oversimplified, or ignored, as was the case with fourth order vented systems.
      Second, the behavior of a second order system like the W1 does not depend for its success on exact alignment of many factors requiring exact alignment and exact tolerances, as do most fourth order vented bass alignments. If the parts tolerances are slightly off in an acoustic suspension system, the quality of its bass performance will be only slightly affected, instead of going to hell in a hand basket as do most vented bass systems.
      Third, the functioning of the W1's second order acoustical and electromechanical filter occurs up at 50 Hz, which is almost an octave removed from the critical 27 Hz frequency where the equalized W1 actually begins its rolloff. Thus, by the point we get down to this critical frequency of 27 Hz, the two orders of W1's acoustical and electromechanical filter have already assumed a stable 12 dB per octave downward slope, even if there were to have been some slight deviation due to parts tolerances when it turned its corner up at 50 Hz. In other words, the W1's behavior at its critical corner frequency of 27 Hz is the sum of a virtually perfect electrical filter for two orders, plus (for the other two orders) the well behaved straight line of the acoustical cum electromechanical filter that had already turned its corner an octave away. In the W1, no critical alignment factors are even present at this 27 Hz corner frequency, in sharp contrast to vented bass systems. Therefore, the quality of the W1's bass performance is not compromised by all the many factors that compromise the bass quality of most vented bass systems. The quality of the W1's bass performance can be as good in practical reality as it ideally promised to be on paper.
      Indeed, if NHT wanted to, they could easily make the quality of the W1's bass even better than it is. You see, because the W1's intrinsic second order acoustic suspension filter has already become a substantially straight line down at 27 Hz, NHT could shape the 27 Hz corner of the equalized W1's response to any shape they wanted, using purely electrical means (in the equalizing signal coming from the X1 crossover).

Shape of 27 Hz Corner

      Why does the shape of the bass rolloff corner at 27 Hz affect the sonic quality of bass? The corner shape of any filter has a strong effect on how good the transient response of that filter is. Even with fourth order vented bass systems, the design engineer has choices among various filter types, each with a different corner shape, and each with a consequently different transient response.
      Generally speaking, the sharper the corner, the worse will be the transient response, with worse overshoot and more prolonged ringing, which sonically yields poorer quality bass (lingering heaviness, one-note boom, poor definition, obscuring of subsequent music, etc.). Some engineers mistakenly choose sharper corners, because this gives them prettier looking frequency response plots

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