listener and the other driver was at the rear.
      Now, in this orientation the W1 cabinet is 13.5 inches deep, so the back bass driver is 13.5 inches farther away from you than the front driver. These two bass sources are temporally misaligned by a whopping 13.5 inches. This is 27 times worse than the 1/2 inch that was proven to cause sonic degradation of bass quality, and moreover was proven to cause degradation in exactly those same sonic aspects that we heard the W1 suffering when we tried orienting it with one woofer facing forward. Admittedly, 13.5 inches might not seem like much compared to a 10 foot wavelength. But this is obviously a red herring. What counts here is the fact, proven by our research measurements, that the bass waveform measurably and sonically changes and degrades with misalignments as small as 1/2 inch, that the ear/brain can hear these degradations, and that orienting the W1 with one driver facing forward creates a 27 times worse misalignment, thereby producing a concomitant degradation in bass quality that is very undesirable (say 27 times worse, in terms of lost bass impact and definition, and woolly lingering boom).
      Fortunately, there's a very easy fix. Simply orient the W1 so that both drivers are precisely the same distance to the central listening seat, so that both bass sources are precisely aligned with each other. And this is achieved by merely orienting the W1 so that the drivers point to the sides, toed in so that the blank end panel of the W1 points directly at the central listening seat. Since the low bass notes are radiated omnidirectionally, you'll hear just as strong bass, even though the woofers are not pointed at you.
      By the way, we speculate that there might be a further sonic advantage to this orientation. There will be some air turbulence where the expanding hemispherical wavefronts of the two woofers meet (especially since the two bass drivers are not precisely matched), and you will be listening at exactly this point. Thus, you will also experience a velocity component to the bass sound, from this air turbulence.
      In the concert hall, you hear both pressure and velocity components of the sounds produced by the live musical instruments, but microphones capture and record only the pressure component, and most loudspeakers can only play the pressure component. It has been hypothesized that one of the barriers, preventing reproduced sound from truly seeming like the real thing, has been the fact that reproduced sound lacks this velocity component that you hear from live music. Bass is particularly bad in this regard, since our bass systems only pressurize a room (single mono subwoofers are especially bad in this regard, which is why we so strongly recommend you use at least two widely spaced subwoofers).
      The W1, if oriented precisely as we recommend, can provide a perceived sense of this sorely missing velocity component for bass, thereby making bass sound more naturally real and lifelike. It's not the full fledged real thing, since the velocity component was not recorded in any one channel, and so does not exist in any one channel for a single W1 to reproduce. But it still helps the natural veracity of the experience to be hearing a velocity component in your room's air from each subwoofer, as opposed to a conventional subwoofer that just pumps pressure at you.
      And this sense of experiencing velocity is dramatically further enhanced when you employ two or more widely spaced subwoofers, reproducing stereo or surround bass from distinctly recorded channels. That's because the velocity component of bass can be recorded by two or more widely spaced microphones, which can pick up the propagation velocity of the bass wave as it wafts across a large hall, and you can reproduce this wafting velocity effect by employing two or more widely spaced subwoofers to play back the two or more recorded channels, jointly containing between them this recorded velocity propagation.
      If you opt for the less expensive, single driver W2 subwoofer, the most important consideration is to orient it with the woofer driver pointing straight upwards, in order to minimize Newtonian kick-back motion and thereby maximize bass quantity and quality, as noted above. Of course this means that you can't use the W2 as a pedestal for satellites. And it also means that the woofer will be 14 inches away from the floor, much farther away than the W1's twin side-firing woofers, so the floor bounce interference phenomenon will be worse with the W2 than with the W1, making its bass quality and quantity somewhat poorer (and of course its bass quantity capability is also halved by the fact that it has only half the driver area as the W1).
      The Evolution W1 is inherently such a good subwoofer, at such a bargain price, that NHT ought to also market it as a general purpose subwoofer, with its low price inviting people to employ it in multiples, for stunning effect. In order to enable the W1 to perform to its full intrinsic potential, we'd like to see NHT introduce a slightly modified X1 crossover. This X1A would have a simple rear panel switch allowing the user to defeat the subsonic filter below 27 Hz, thereby improving the W1's bass extension and also the quality of its bass transient response.
      It might also be helpful to allow the user to select a shallower 6 dB per octave crossover slope for the W1's upper frequency rolloff. This could take advantage of the fact that the W1 is so well behaved to such high frequencies for a subwoofer (beyond 500 Hz), to create a more transient perfect crossover transition with many satellites (including the M6), and a sonically better feed to any satellite, since a simple series capacitor (with much less sonic degradation than any electronic crossover, including the X1) could be used to set a single dominant rolloff pole for the power amplifier channels feeding the satellites, say at least an octave above the satellite's intrinsic bass resonance.

X1 Electronic Crossover

      The X1 offers an imaginatively rich array of very useful and sonically very important features, to optimize subwoofer performance for your room and taste, and also to optimize the sonic integration with the M6 or other satellites. In our evaluation, we found that these X1 control features really work, and are very effective at making the bass an integral, seamless part of your system sound, rather than a sonically separate event as it is in so many other competing systems. The X1 has evolved from earlier NHT crossovers, which boasted similar features, so NHT has had long experience developing and refining these innovative features.
      To start with, the X1 offers the obvious control features. One knob on the front panel, called master gain, controls the overall volume of the low pass output to the two subwoofer outputs, but does not control the gain of the X1's high pass output feed to your satellites, so it controls the relative loudness of your subwoofers (but not your overall system loudness). A rear panel slide switch selects between two sensitivities for the X1, spaced 10 dB apart, so that the front panel master gain control is easier to use over the majority of its range (depending on how sensitive your satellites are, and also on how your two chosen types of power amplifier, one for driving the subwoofers and the other for driving the satellites, might differ in their gain).

Choosing Crossover Frequencies

      Another front panel knob chooses continuously among frequencies for the subwoofer's upper rolloff (low pass filter), anywhere between 50 Hz and 140 Hz. A 3 position back panel switch chooses among 3 possible frequencies for the complementary high pass filter feed to your satellites, either 50, 80, or 110 Hz. You can of course bypass the X1 entirely for the signal feed to your satellites, and run your satellites either full range or via an alternative high pass filter (such as a simple capacitor in series with the input of each channel of your power amplifier feeding the satellites). But you must use the X1 to feed the low frequency signal to your power amplifier that drives the Evolution subwoofers, since all the Evolution subwoofers require the X1's flattening equalization from 50 Hz down to 27 Hz.
      With the W1 and M6, you can freely explore the X1's whole gamut of crossover frequencies, thanks to the W1 subwoofer's superiority in smoothly reaching higher in frequency than most other subwoofers, and thanks to the M6's superiority in richly reaching lower than most other satellites. Simply play with both X1 crossover frequency controls until you find the sound that best suits you, and your room, and your chosen loudspeaker locations within that room. As you rotate the front knob (low pass crossover to the W1 subwoofer), it is helpful to slide the rear switch (high pass crossover to the satellite) to the frequency choice (50, 80, or 110 Hz) that's nearest the frequency indicated on the front panel knob setting.
      Just these two X1 controls give you extraordinary flexibility in tailoring the Evolution system's warmth and upper bass sound to your liking, allowing (indeed encouraging) you to fully exploit the superb flexibility that the W1's upper frequency reach and the M6's lower frequency reach afford you.
      Note that, at every point in this experimental process where the front panel knob indicates a higher frequency than the rear slide switch choice, there will be a richer, warmer, fatter sounding overlap, in just the frequency span between these two frequencies, since both subwoofer and satellite will be contributing slightly more than normal in this span (and that richer sounding overlap is fine, if that is the sound you like). You can specifically choose just where in the spectrum this richer sounding overlap occurs, by simply choosing the two frequencies bracketing this span, via the front panel knob and rear slide switch. The front panel knob sets the upper frequency of this span, if you want enriching overlap, and the rear slide switch sets the lower frequency of this span. This tactic is also useful if you use a satellite (other than the M6) that happens to be sonically thin or lean in the upper bass or warmth region, since this enriching overlap allows the W1 to supplement the upper bass or warmth missing from that satellite.
      Conversely, if you want the sound to be leaner or thinner in some portion of the upper bass or warmth region, that's also easy to achieve, by simply reversing the roles of the front knob and rear switch. If you simply set the rear switch to a higher frequency than the front knob, then you'll get a leaner sounding underlap valley. And you can specifically choose just where in the spectrum this leaner sounding underlap occurs, by simply choosing the two frequencies bracketing this span. In this case, for a leaner underlap, it is the rear panel switch (instead of the front knob) that sets the upper frequency of this span, and the front panel knob (instead of the rear slide switch) sets the lower frequency of this leaner valley. This tactic is also useful if the M6 or some other warm sounding satellite is sounding too richly warm in some spectral area, perhaps from a half wavelength reinforcement due to its distance from a nearby wall.
      As a further factor to consider, it is wise to set the rear panel slide switch to a frequency that's different from (and above, or perhaps below) the satellite's intrinsic bass resonance or rolloff corner frequency. If the rear panel switch frequency matches the corner frequency of the satellite's intrinsic low end response, then the transient response of the system will be worsened, with a worse overhang and lingering one-note boom at this corner frequency. That overhang and boom will be audible on most music, since most music is transient in nature, and most transients (even "high frequency" violin plucks) actually contain some spectral energy down at this corner frequency, so they will all trigger the obnoxious boom, which will in turn both color all music and also muddy or obscure all music via the prolonged lingering boom.
      For example, if the satellite's corner frequency is right at 80 Hz, avoid setting the X1's rear slide switch at 80 Hz Instead, in such a case, you can (1) set the slide switch at 110 Hz and bring the W1 up to approximately 110 Hz via the front panel knob (set this knob higher than 110 Hz for enriching overlap, or below 110 Hz for leaning underlap); or (2) run this satellite full range (not through the X1 at all); or (3) set this slide switch at 50 Hz (which will act as an excursion-saving subsonic filter for the satellite), and bring the W1 up to approximately 50 Hz via the front panel knob (again, set this knob higher than 50 Hz if you desire enriching overlap).
      The corner frequency of the M6 satellite is about 65 Hz, so you would be pretty much OK using any of the three slide switch frequencies, 50, 80, or 110 Hz (the last being the best for transient response from the M6). If you opt for the smaller M5 satellite, its corner frequency is about 70 Hz, which is getting a bit close to the 80 Hz slide switch choice, and the 50 Hz choice might stress the M5's smaller woofers too much, so 110 Hz would probably be the best choice for the M5.
      The X1 is a two channel stereo unit, so each X1 can handle a stereo system or two bass channels of a surround system. A rear panel switch allows you to do a mono blend of the two input channels, so you can drive a single subwoofer (or plural subwoofers) with a mono bass signal derived from two input channels if you wish (we don't recommend this if you can avoid it). There's an extra X1 input for the LFE channel, and a front panel LFE gain control gives you good control of how loudly you blend this LFE channel information into the two X1 output channels. A rear panel LFE output jack makes it easy to daisy chain the LFE signal to multiple X1s, so you can put all your W1s to work pumping out those LFE sound effects.

Boundary EQ Control

      An unusual X1 control is a front panel knob called Boundary EQ. This continuously variable control allows you to boost or cut (or keep flat) the X1's nominal frequency response curve, at the fixed frequency of 40 Hz. You can boost the energy at 40 Hz by any amount up to 8 dB, producing a hump in response, or cut it by any amount up to 7 dB, producing a valley in response, with either the hump or valley both centered at 40 Hz. The response hump, peaking at 40 Hz, gradually slopes downward on both sides, extending its (gradually declining) effect about one octave on either side (i.e. down to 20 Hz and up to 80 Hz). Likewise, the response valley, bottoming at 40 Hz, gradually slopes upward on both sides by the same octave. By rotating this control knob to its center position, you can choose nominally flat X1 response through the 40 Hz region. Note that in this nominally flat response setting the X1 still boosts the response below 50 Hz, to equalize the Evolution subwoofers and make them flat down to 27 Hz.
      Since the frequency of this boost or cut control is fixed, we might well ask why it is fixed at 40 Hz rather than some other frequency. After all, 40 Hz is above the low bass spectral region, and at the border of upper bass, so we might wonder why such a control would be boosting a frequency this high. Why not boost low bass instead, by having this control act at say 30 Hz?
      It turns out that 40 Hz is a more practical frequency to boost or cut, for several reasons. First, changes at 40 Hz are far more audible, hence sonically rewarding, than changes at some lower frequency, because the ear's sensitivity declines at lower frequencies. Second, demands on subwoofer driver excursion increase at lower frequencies, and even more so to produce an equivalent sonic effect (given the ear's declining sensitivity at lower frequencies), so boosting a frequency lower than 40 Hz would stress the subwoofers more. Third, most music and most film sound effects contain a lot more spectral energy around 40 Hz than they do at lower frequencies, so 40 Hz is the proper frequency to boost or cut if you want to achieve maximum sonic effectiveness on real world program material. It's still very important to extend subwoofer system response far below 40 Hz if you want to achieve accurate bass quality and impact, but the 40 Hz region affords the maximum opportunity for audible manipulation.
      Where should you set this boundary EQ control knob on the X1? You should first experiment with it on a variety of music and film sound effects, to get acquainted with its particular sonic effects (as you hear them in your room). Then simply set this control where you think its effect on the mid bass sounds best to you. We recommend that you ignore the diagrams in the owner's manual, which suggest specific knob settings of this control for different subwoofer locations relative to your room boundaries. We think they are misleading. Indeed, even the control's name, Boundary EQ, is misleading.
      In our judgment, this Boundary EQ control is sonically very effective, but not for boundary EQ. Why not? To understand this, we have to realize that there are really three distinct boundary acoustic phenomena at work here.

-- Boundary Reinforcement

      First, there is the boundary reinforcement effect, here as it relates only to subwoofers. A subwoofer is ominidirectional throughout its spectral range. This means that if you hang it in true open space, it will radiate equally in all spherical directions, so you as the listener, situated at just one point in that omnidirectional spherical radiation pattern, will intercept a certain amount of bass energy. Now suppose that you put the subwoofer down on a floor, instead of hanging it in open space. The half of the subwoofer's omnidirectional spherical radiation pattern that is aimed downward will reflect off that immediately adjacent floor, and will be directed upward, to add to that hemisphere half of the woofer's omnidirectional spherical radiation pattern which was directed upward. Thus, the subwoofer's bass will be twice as loud (assuming a rigid floor) in that upward hemisphere. Since you are also sitting in that upward hemisphere, you will intercept twice as much bass energy from your listening position, so you will hear the subwoofer playing bass twice as loudly as if that subwoofer had been hung in open space. Likewise, your entire room is within that upward hemisphere (since all of your room is above the floor), so all of your room will experience bass twice as loud from the subwoofer.
      Next, suppose that you back the subwoofer up against a single wall, while still keeping it on the floor. The wall will reflect back the bass energy just as the floor did, so the bass will now become twice as strong again. The wall and floor junction acts to confine the total free radiation pattern of the subwoofer to merely one quarter of a full sphere, and, more importantly, they reflect back the bass energy that the subwoofer is still ominidirectionally radiating to the remaining three

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