and more indirect. It sounds just like live music sounds when you are sitting in the back of a concert hall and hearing virtually only indirect reverberant hall energy from the live music on stage (i.e. hearing virtually no direct sound from the live music on stage). Coming from the M6, this makes for a very relaxing listening experience.
      But you could also choose to deliberately disobey NHT's instruction, and connect all your M6s in inverted polarity, by connecting the minus polarity outputs of your power amplifier to the red input terminal of all your M6s. This would invert the whole polarity picture of the M6. The upper bass and warmth regions from the M6's woofers would now be radiating in inverted polarity, which makes only a small sonic difference (note that your connections to all subwoofers should also then be inverted, to allow the bass to be consistent and strong). But there would be a dramatic difference in the sonic quality of most of the music (all frequencies above 350 Hz). With the input connection inverted, the M6's midrange and tweeter would be radiating all midrange and treble regions in correct absolute phase polarity. When midrange and treble regions are made to be reproduced (by whatever means) in correct polarity, they sound more direct, with a more articulate and incisive attack to all transients. This sounds just like live music sounds when you sit closer to the stage at the front of a hall, and thereby hear a greater proportion of direct energy coming straight at you from the instruments on stage. Coming from the M6, this alternative sonic personality is more exciting and realistic (truer to the recording) for most of the spectrum, especially the upper frequencies. The choice is yours for the M6's sonic personality in upper frequencies: direct or indirect.
      The third factor pertains to radiation pattern, and how you orient your listening seat relative to the M6's radiation pattern. All multi-driver loudspeakers exhibit lobing in their radiation patterns. This lobing arises from the fact that, when the system is heard (or measured) from various angles, the relative distances from you the listener (or the measuring microphone) to the various drivers on the front panel changes. For example, if you sit directly in front of the midrange driver, then you are closer to it but slightly farther away from the tweeter, whereas if you sit directly in front of the tweeter, then you are closer to it but slightly farther away from the midrange driver. These differences in path lengths produce interference patterns, wherein the outputs from these two drivers will algebraically add to produce greater amplitude (a lobe) at some listening angles for some frequencies, and lesser amplitude (even a null) at some other listening angles for some other frequencies. This lobing phenomenon produces its greatest lobe vs. null effects when both drivers are equally contributing output energy, and are contributing at nearly their full output level. This occurs in the spectral vicinity of the crossover frequency, which in the M6 is placed at 2300 Hz.
      The designers of the M6 are fully aware of this lobing phenomenon, and they have deliberately engineered the M6 to take advantage of it. In the M6 design, there is a lobe of bright energy (especially near the crossover frequency of 2300 Hz), and it is deliberately designed to directionally point away from the M6 at an angle, off toward the part of the room on the tweeter side of the M6. And there is a complementary null in the lobing pattern, the null pointing off from the midrange side of the M6.
      NHT instructs you to point the main front M6s straight ahead with respect to your room, and have the tweeter on the inboard side of the front M6s as you view them. Since you the listener are sitting at the apex of the V between the front M6s, you will be sitting squarely in the bright lobe that angles off from the tweeter side. Likewise, with all the surround M6s, you would orient them so that you the listener are hearing them at an angle from the tweeter side (instead of having them point straight at you), so that, again, you are consistently sitting in the bright lobe of the radiation pattern from each M6. The stated goal of this design approach is to increase the amount of direct sound that you hear coming directly from each M6, and to simultaneously decrease the amount of sound that you hear reflected from your room's walls (since there is a complementary null pointing at your walls from the midrange side of each M6). There is a school of thought which believes that increasing direct sound and decreasing room reflections, from a playback system, improves spatial imaging, as well as improving clarity and articulation.
      But you can choose to deliberately disobey this NHT instruction. If you find the M6's sound too bright and aggressive when you listen in this bright lobe, you can dramatically change the M6's sonic personality by simply flipping every M6 upside down. You would then be listening from the midrange side of each M6, essentially where the lobing pattern subtracts instead of adds. This produces a sonic personality for the M6 that is much less bright and aggressive, and much more mellow. Again, you can easily listen to both possibilities, and the choice is yours.
      As you see, there are 3 binary choices, which yields a total of 8 possible sonic personalities for the M6 you can choose from. For example, if you like the aggressive boom and sizzle sound of typical home theater loudspeakers, you can get all the emphasis of the spectrum's bottom and top ends that your heart could desire from the M6, by making the requisite choices. Alternatively, if you want a relaxing loudspeaker sound that is easy to live with, that too is simply attainable by making the appropriate choices. The M6's sonic personality can become the loudspeaker personality you want, and you can even change it easily to suit different types of program material or different listening moods.
      We mentioned that the M6 has more than 8 different sonic personalities. How do we get more than 8? For the third factor, we started playing around with the listening angle at which you orient the M6 relative to your listening seat. In this third factor, we only considered two choices, and only with respect to horizontal listening angle. But of course there are literally an infinite number of listening angles, with respect to both horizontal and vertical axes, to which you can orient the relationship between the M6 and the listening seat. We explored some variations in our research, and we'll discuss these below.

Boundary Control Switch

      Now, which choices do we recommend for the above factors? Let's start again with the first factor, warmth control. NHT calls that little switch on the terminal block the Boundary Switch. The intent of this switch is based on solid engineering thinking. When loudspeakers are placed near room boundaries (walls and even more so corners), the amount of warmth energy they output is increased, by reinforcement from those nearby boundaries. On the other hand, when loudspeakers are placed out in the open, far away from room walls and corners, they output less warmth energy, since there is no reinforcement from nearby boundaries. Most loudspeaker designs don't even take this phenomenon into account, and they certainly don't give you any means for dealing with it. Your only tool for dealing with this phenomenon with most loudspeakers is to physically move them closer to or farther away from room boundaries, until you get the amount of warmth energy you want to hear. But the resulting location for most loudspeakers will probably be far from the optimum location for obtaining the best spatial imaging from those loudspeakers, so you'll be torn between two incompatible desiderata. Furthermore, if your room is medium to small in size you probably won't even have enough room to position these loudspeakers for optimum warmth energy, since the loudspeakers would then be too close to the listening position, resulting in an artificial in-your-face sound and poor integration among the loudspeaker system's various drivers (space would be especially tight if you are doing a surround system instead of just a stereo system).
      Enter the clever and thoughtful engineering of NHT. NHT not only recognizes and takes into account the phenomenon of warmth reinforcement by nearby boundaries, but also gives you a means for dealing with it. The basic idea of that Boundary Switch is that you can locate the all the M6s for optimum spatial imaging, and then simply flick the switch to hear more warmth energy or less warmth energy. This switch certainly does make a big difference in the audible amount of warmth energy, so it definitely works effectively to that extent. But in our evaluative experiments with this witch, we found that, while we agree with its intent and its change, we disagree with its calibration. Specifically, in our judgment this switch provides too much warmth energy in the boost position, even in open free field conditions where the M6 is getting minimal warmth reinforcement from room boundaries.
      As part of our research experiments, we deliberately tried positioning the M6s far from the room walls, so there would be no boundary reinforcement at all of the warmth region (the M6s were 10 feet from all walls, which means that the boundary reinforcement phenomenon occurred way down at 56 Hz, far below the warmth region and indeed at the very bottom end of the M6's bass response capability). In this position, with no warmth reinforcement from room walls, the M6 still put out plenty of rich warmth energy on its own from its two woofers, in the flat setting of the boundary switch. If we moved the M6 progressively closer to the room walls than this 10 feet, then of course the amount of warmth energy would only increase, beyond this already rich amount heard out in the open free field location.
      The implication of this research experiment is that, in our judgment, the very worthwhile Boundary Switch feature of the M6 should be recalibrated, so that it cuts or reduces warmth energy in one position (for being near room walls) and is flat in the other position (for being far away from room walls), rather than being flat in one position and then boosting warmth energy above flat in the other position (as its present design does). Until such time as NHT might recalibrate the warmth energy profiles of this Boundary Switch, we would suggest that you leave it in the flat "1" position.
      A brief technical footnote here. Nearby room wall boundaries also cause a cancellation dip in frequency response, one octave below the frequency at which they cause a reinforcement hump in frequency response. The hump provided by the M6's warmth boost position could conceivably be used to offset the dip caused by a nearby wall boundary. However, in practice this is not very helpful. That's because the M6's warmth hump, in the boost position of this switch, is centered at 200 Hz. And you would have to place the M6 within 1.5 feet of a wall to have the dip from boundary cancellation occur at 200 Hz. But you should never place a loudspeaker this close to any wall. So doing would cause bad colorations in the midrange from comb filtering interference patterns. And so doing would also severely degrade clarity and spatial imaging, since you would hear the reflections from such nearby walls very soon after you heard the direct sound coming from the loudspeaker (well below the Haas window), so your ear/brain would integrate the reflected sound with the direct sound, producing the perception of a blurry muddle instead of clear sound, and confused spatial imaging because there would be two conflated sources of sound and spatial cues instead of just one (the loudspeaker).
      There's another reason for not using the warmth boost position "0" of the M6's Boundary Switch. We found that the warmth boost position made the M6's overall sound get thick and opaque, obscuring the midranges that are so crucial to musical clarity. It's as if the excess of warmth energy overshadowed the musical information in the spectrally nearby midrange region, thereby partially obscuring the music's vital midrange information, and making the sound less clear. It sounded like the difference between clear broth (in the flat position) and an opaque porridge (in the warmth boost position). We like rich warmth as much as the next person, indeed even more so, so we can take a large dose of warmth per se in stride. But we don't want anything obscuring our music's midranges, not even our beloved warmth.

Choice of Polarity Connection

      Which choice do we recommend for the second factor, the choice of your polarity connection to the M6? Let's assume that you follow our recommendations for the third factor below, thereby evoking the mellow tonal balance side (as opposed to the aggressively bright tonal balance side) of the M6's sonic personality. In this case, both choices for polarity connection produce very good sound, and we would recommend that you try both and choose for yourself. I personally like both very much, and I could even see myself alternating between the two polarity connections, depending on the program material and on my listening mood.
      The nominally correct polarity connection (power amplifier plus to red input terminal) puts most of the music (above 350 Hz) in inverted absolute polarity, and this inversion combines very consistently and convincingly with the M6's mellow tonal balance, to re-create a realistic semblance of live music heard from a distant seat in the concert hall. The M6's warm, mellow tonal balance (with our recommended choice for the third factor below) accurately mimics what music really sounds like toward the rear of a large, plush concert hall (since considerable high frequency energy is absorbed by the hall and by the clothing of the audience). And the indirect, softened quality you obtain, by having most of the musical spectrum in inverted polarity, perfectly meshes with this tonal balance by accurately mimicking the softer, more indirect quality of transients that you hear from live music when you sit in the reverberant far field of a distant concert hall seat. In short, the M6's tonal balance profile (as selected under factor 3 below) works hand in glove with the indirect, softened sonic quality that polarity inversion wreaks, to give you a believably realistic portrait of live music heard from a distance, a portrait that is very relaxing to listen to.
      This polarity connection is also very effective at counteracting and offsetting the excessive hardness that is incorporated into many program sources. Music recordings that are too closely miked tend to have an excessively hard and bright quality, which might represent what the musical instruments do sound like when heard from a few inches away, but does not represent the sound that you have ever heard from these musical instruments, since you have always heard them from at least several feet away. Likewise, film soundtracks often sound too hard and bright, because the voices and sound effects especially are typically miked much too closely, and also because the solid state signal processing equipment of the movie studios typically adds artificial solid state hardness and brightness. The softening wrought by this polarity choice for the M6 counteracts the excessive hardness contained in these recordings, and the mellow tonal balance profile achieved for the M6 by our recommended choice for factor 3 below counteracts the excessive brightness contained in these recordings.
      Alternatively, you can also achieve very good sound by choosing to invert the polarity connection to all your M6s (and also to all your subwoofers). This polarity connection puts most of the music (above 350 Hz) in correct absolute polarity (note the counterintuitive flip here - inverting your connection to the M6 puts most of the music in correct absolute polarity). This polarity connection makes musical transients, especially the transient attacks of upper frequencies, more articulate and incisive. If the previous polarity choice above sounded too soft for your taste, this polarity choice will put more ting in the triangle, more bite in the brass, more zing in the cymbal, and more sass in sibilants.
      This polarity choice, like the previous polarity choice above, also gives you a realistic and convincing portrayal of a live music concert hall experience, but a somewhat different live experience. It puts you at a somewhat closer seat in the concert hall, and it also puts you in a hall that is a little less plush and a little more lively. It's a little more engaging and exciting to listen to, whereas the previous polarity choice is a little more relaxing.
      This polarity choice actually makes the M6 more accurate in reproducing the true transient attack quality encoded in recordings. It makes the M6 more hi-fi in the good sense of that term, i.e. more accurately representing the input signal, and more accurately revealing of all musical transients as contained in the recording.
      Incidentally, you'll hear an obvious difference between these two polarity choices, and that proves that absolute phase polarity is indeed a very audible phenomenon (especially at upper frequencies), and is a pretty important factor to pay attention to, throughout the recording and playback chain.
      Note that this polarity choice will put the warmth and bass regions into inverted absolute polarity, including the bass from your subwoofers (their connection needs to be inverted, in order to properly blend with the now inverted upper bass coming from the M6). But polarity inversion of the frequencies below 350 Hz makes only a small sonic difference, so it is the sound of your polarity choice for the music above 350 Hz that should be the deciding factor. Absolute polarity is much less audible for the bass and warmth regions than it is for the midrange and treble regions.
      That's especially true for the bass that you play back on your system from any program source. The absolute phase and polarity of bass is of very dubious pedigree by the time it reaches your loudspeaker, because it is mangled by the many bass cutoff filters in the recording and playback chain. Thus, by the time bass reaches your loudspeaker, its absolute phase and polarity is so mangled (and variable among various recordings) that it makes little sense to worry about adhering to correct absolute polarity for bass just in the loudspeaker link. Absolute polarity for bass matters only when bass transients are heard live. Our reference standard for bass polarity (and bass impact and transient accuracy) are the artillery explosions we occasionally hear from a nearby military firing range. Needless to say, no audio component (loudspeaker or electronics) has been successful in reproducing this reference standard of bass impact and accuracy (though the Citation 16 power amplifier came impressively close).
      By the way, please note that we are saying here that what matters little is absolute phase polarity of bass, not the relative phase and phase alignment of bass. It turns out, as we'll discuss below for the Evolution subwoofers and X1 crossover, that relative phase and phase alignment matter a lot, and indeed are amazingly critical in view of the long wavelengths involved.

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