What choice do we recommend for listening angle, and for orientation of the M6? Here the story gets more complicated, and more interesting. Let's start simply.
The NHT manual specifically instructs you to orient the M6 pointing straight ahead into the room, so you'll be listening to the M6 horizontally off axis. And the manual instructs you to flip the M6 vertically so that the tweeter is on the inboard side. Also, the intended pedestals for the M6, i.e. either the P6 pedestal or the B6 subwoofer, put the M6's midrange and tweeter (which are side by side) exactly at ear height, so you're listening right in line with the vertical axis of maximum brightness.
If you choose to follow these NHT instructions, you will be putting the prime listening seat in the maximally bright lobe of the M6's radiation pattern. As noted above, the M6 is deliberately designed so that the maximum amplitude lobe for the upper frequencies (especially the frequencies near the 2300 Hz crossover) points at an inward angle from the tweeter side of the M6 (instead of pointing straight ahead as it does in some other loudspeaker designs). Meanwhile, a minimum amplitude null points at an outward angle from the midrange side of the M6. The design intent is to project maximum direct energy at you the listener, and to project considerably less energy at the side walls of your room, thereby increasing the ratio of direct to reverberant sound you hear in your listening room. One reason for doing this is that some engineers believe that this improved a loudspeaker's spatial imaging. This also naturally provides the maximally bright tonal balance from the M6, since you will be sitting right in the maximum lobe of upper frequency output.
Conversely, you could disobey these instructions by simply flipping the M6 upside down, thereby putting the tweeter on the outboard side. This directs the maximally bright lobe of the M6's radiation pattern at the side walls, and. more importantly, places you the listener in the least bright portion of the M6's radiation pattern.
Thus, your binary choice of putting the M6 tweeter on the inboard or outboard side (by simply flipping the M6 upside down one way or the other) achieves a marked change in the tonal balance personality of the M6 as you hear it. With the tweeter inboard as instructed, the M6 sounds much brighter, but with the tweeter outboard the M6 sounds much more mellow. Remember that in both cases the M6 is pointing straight ahead into the room, so in both cases you are listening to the M6 horizontally off axis by the same amount, about 20 degrees in the manufacturer's recommended setup.
Now, up to this point in our discussion, we would be prepared to say that you can freely take your choice of which M6 orientation, and therefore which tonal balance personality, you prefer. Tonal balance is not an absolute sonic virtue or vice. With other sonic parameters like distortion, foreign material coloration, and spatial imaging, it is usually possible to say that one option is absolutely better and the other option is absolutely worse (less distortion and less foreign material coloration is always absolutely better than more). But tonal balance, barring gross anomalies, is open to subjective preference and other factors. You might personally prefer a brighter sound to a mellow sound. Or your room might be full of drapes and stuffed furniture that absorb high frequencies, thus making a brighter loudspeaker sound from the M6 more appropriate for your room. Or your playback chain might contain a mellow tube amplifier or a bright solid state amplifier that could usefully be offset by a counterbalancing tonal balance choice from the M6. It's great to have a tonal balance brightness choice available from a loudspeaker like the M6, so you can pick the brightness or mellowness that best suits your taste, your room, and your system.
If the only change wrought by flipping the M6 upside down, so the tweeter is inboard or outboard, were merely a change in tonal balance personality, we'd be happy to recommend that you simply take your pick. But there's more to the story. Much more. And here's where things get complicated. To make these complications understandable, let's put this new story in context.
The NHT Evolution system, as noted, gives you outstanding engineering sophistication, flexibility, and a very good overall performance envelope, including very good coverage of the entire spectrum and loudness capability. Its overall capability is indeed worthy of a high end loudspeaker system. So, at its bargain price, the NHT Evolution is a minor miracle, and a superb value. But there are some sonic limitations in the Evolution system, including limitations in the M6 satellite monitor. And these limitations do relate to the bargain price of the Evolution. You see, NHT has generously given to you in the Evolution a huge helping of sophisticated and clever engineering, and a generous serving of product design (a full three-way system with dual woofers for the satellite alone), which takes the overall performance of the Evolution far beyond what most other loudspeaker systems offer you for a similar price.
But NHT's munificence obviously has to stop at the cost of the parts included in the product. NHT could not stay in business if they followed the business model that says, "We lose money on every one, but we make it up in volume." Thus, the raw cost of the parts making up each M6 has to have a pretty low ceiling, given the bargain price they charge for this four-driver system with a three-way crossover.
We hear some limitations in the M6's performance, and these limitations are intimately related to the necessary ceiling on the cost of parts going into a loudspeaker selling for this bargain price. Before we discuss these performance limitations in detail, note that we judge all products on an absolute scale of performance, so that you and we can always be conducting meaningful sonic comparisons among products. In other words, price does not affect what we tell you about the absolute sonic performance.
Price does come into play when we relate the sonic performance (judged on an absolute scale) to price, in order to arrive at a judgment of value. Value of course is not the same thing as sonic performance. The Evolution system offers you outstanding value because its cost is so amazingly low, for a system that transcends its admitted sonic limitations, with an overall performance envelope that is worthy of far more expensive systems. Also, since our research discovered ways for you to work around and bypass most of these sonic limitations, you can overcome most of them if you follow our setup recommendations.
You should also know that we reached all our listening judgment conclusions before we had any clue about the price of the Evolution. Thus, we were not prejudiced toward expecting to hear the sonic limitations of a bargain system. Indeed, given the engineering sophistication, complexity, and overall performance envelope of the Evolution, we would have guessed that it cost about four times its actual price, so we were expecting not to hear these performance limitations in what looked to be such an obviously expensive system. Thus, our listening observations and judgments were reached in spite of our predisposition and expectations, not because of them. We were stunned when we later learned that the Evolution sells for about one fourth the price we would have expected. In view of its bargain price, the Evolution's sonic limitations are reasonable, and you can still get a lot of sonic performance out of this system, especially if you follow our specific setup recommendations, which work around most of the sonic limitations.
The first sonic limitation pertains to the crossover. The sound of the M6 is gently veiled throughout the spectrum, with a very slight defocus and grundge. With this veiling, the M6 is a couple of notches below the best state of the art loudspeakers in transparent revelation of musical information. It sounds as if the M6 puts a gauze veil over the pellucid, articulate acuity we can hear from the best (and far more expensive) competing loudspeakers. We deliberately drove the Evolution with our very revealing reference system, in order to be able to accurately assess the Evolution's absolute performance abilities, so we know that this veiling we heard is intrinsic to the M6. For example, all modules of the Evolution were driven via Nordost's extraordinarily transparent Valhalla cable; lesser cables might have introduced some of this veiling we heard, but not Valhalla.
Our extensive experience with loudspeaker systems and parts thereof leads us to identify this veiling as coming from inexpensive, sonically mediocre parts in the crossover (capacitors, inductors, and wiring). In other words, drivers do not introduce this particular kind of sonic veiling. If you are hobbyist, you could consider upgrading the parts in the M6 crossover to more expensive, better sounding parts. If not, then you can simply accept this slight veiling as an aspect of the M6 you'll be living with.
Fortunately, this veiling involves mostly a passive loss of information, which is more benign than active spurious additions to the music (as the old adage says, errors of omission are more benign than errors of commission). This means that, insofar as this veiling is concerned, you can fully enjoy listening to the M6 without realizing that you are missing anything, and you would only hear that this veiling is losing information if you directly compare the M6 to a more transparent (and surely more expensive) competing loudspeaker.
The second limitation related to cost relates to the cabinet. The M6 cabinet is admirably rigid overall, but the box panels have not been made as dead or inert as they could be, or as they are in more expensive competing loudspeakers (as a simple knuckle rap test in the middle of each panel will demonstrate). The consequent panel ringing could have the potential for coloring and muddying the sound of the M6, as it does in every box loudspeaker with lively sounding box panels. And this panel ringing would indeed surely impart an audible coloration to the M6 if the M6 were more pellucidly transparent. But fortunately, because of the M6's aforementioned slight veiling, the cabinet resonances that are surely there do not play a significantly audible role in coloring the music. The slight sonic murkiness from the aforementioned overall veiling seems to effectively conceal any panel colorations that are lying beneath the surface. Thus, you don't need to be concerned about this limitation in practice, even though it is surely there.
Spurious Misbehavior by Drivers
The third limitation pertains to the M6 drivers. Ideally, a driver, like every other part in an audio system, should reproduce everything that is in its input signal, and should not add anything spurious of its own to that reproduced signal. With a device like a loudspeaker driver that necessarily involves motion and vibration to accomplish its mission, it is very difficult to prevent the device from adding its own spurious vibrations, and hence spurious noises, to the music. These spurious noises are sonically very pernicious, for a number of reasons.
First, they are added to the music signal, so they audibly stick out like a sore thumb. Gentle passive omissions (like the veiling above) are more sonically benign, because they still present you with only the music itself, merely subtly failing to present you with all the music's details. But spurious added noises from a loudspeaker driver sound different from the music, and it's very easy for the human ear/brain to detect that some strange new unmusical noises have been superimposed on top of the music. Musically sensitive listeners become very irritated and quickly fatigued by the perceived obnoxiousness of even slight amounts of added spurious noises.
Second, since these spurious added noises are triggered by the music signal varying over time, the noises likewise vary over time, so they are modulated by the music signal, and hence can sound like a kind of modulation distortion. The human ear/brain is very sensitive to even small amounts of intermodulation distortion, and quickly gets very irritated and fatigued by this.
Third, these spurious added noises linger after the triggering musical transient has stopped. That's because the mechanism causing these spurious noises is the fact that portions of the driver are failing to be completely controlled by the music signal, so they are spuriously vibrating on their own. And, since they are spuriously vibrating on their own, they will naturally continue to vibrate on their own, even after the supposedly controlling musical transient has stopped and gone silent. These uncontrolled spurious vibrations will die away and become quiet only on their own time schedule (in accordance with the time constants of the misbehaving mechanical parts). This means that these ugly spurious noises will linger after each musical transient has gone silent. And this in turn means that these ugly spurious noises will stick out even worse as an audibly obvious and obnoxious sore thumb, since you can easily hear them against the background of silence after each musical transient has gone silent.
Fourth, these lingering spurious noises obscure and blur some of the music. Since they linger after a strong triggering musical transient has already gone silent, they will block and obscure the subtle musical details that immediately follow a strong transient (e.g. the subtle string resonances that immediately follow a strong guitar pluck transient).
Fifth, these lingering spurious noises will make the music sound smeared and defocused. Because these spurious noises are triggered by the amplitude of musical transients, they are to a degree modulated by the music signal, and therefore sound somewhat like a very garbagy form of the music. Thus, when these spurious noises linger after the triggering musical transient itself has gone silent, they sound somewhat as though the musical transient itself has become extended and smeared (or elongated) in time, in garbagy distorted form. Note that this extension in time of the garbagy shadow exacerbates and compounds the modulation distortion discussed as the second problem above. In effect, the lingering, of this garbagy alter ego of the music transient, makes the distortion of the music sound even worse, since this distortion has been extended in time, and also since this distortion has been increased in total energy over time, and also since this distortion has been further highlighted by the fact that its lingering tail is all the more audible and obnoxious because the original music transient has already gone silent.
Sixth, these spurious noises will often add foreign tonal colorations to the music. Many of these driver misbehaviors involve parts vibrating uncontrollably on their own, and a misbehaving driver part will often tend to vibrate in a particular intrinsic pattern, based on its material composition and its physical dimensions. That pattern of vibration may well involve a few select frequencies, in which case the spurious noises will have a perceivable pitch or pitches, corresponding to these frequencies of its spurious vibration pattern. This means that the spurious noises added by the driver to the music will be at certain pitches, which means that they will impart tonal colorations to the driver's reproduction of music. For example, the spurious noises added by misbehaving driver parts might make the driver's tonal balance artificially brighter, in effect adding bright distortion energy to the output of the driver.
By the way, even if these driver misbehaviors have a certain pitch or frequency, they will nevertheless be triggered by virtually all musical transients, regardless of the seeming frequency or pitch of the musical transient. That's because virtually every musical transient actually contains a very broad spectral spread of frequencies, and an infinitely dense spread. Thus, virtually every musical transient contains at least some energy in the vicinity of the frequency or pitch of the driver's misbehavior, and will thereby trigger that misbehavior.
Seventh, the added spurious noises are often especially audible and obnoxious because they are not just random noises. Instead, they have a special pattern which we can readily identify as belonging to a foreign material, in particular the physical material in the driver that is misbehaving and is spuriously vibrating on its own. For example, if the misbehaving, spuriously vibrating part of the driver is the rubber surround, its added spurious vibrations will be readily identifiable by us as being rubbery. So such a driver would impart a foreign rubbery coloration to all musical notes, regardless of the materials in the musical instrument making the original sound. Guitars and pianos don't have any rubber parts, but such a driver would make every guitar note and every piano note sound rubbery. Likewise, a driver with a misbehaving paper cone would add a papery sound to all musical notes, and a driver with a misbehaving plastic dust cap would add a plasticky sound to all musical notes.
Drivers sometimes use very rigid materials for diaphragms, in an attempt to obtain the benefits of accurate pistonic reproduction over a wider frequency range. But such rigid materials can often backfire, since they exhibit very severe misbehavior when they finally do break up into non-pistonic uncontrolled vibrations. Furthermore, as we have all known since Lord Rayleigh, the misbehavior of such rigid materials is especially complex. This complex misbehavior means that the spurious noises may pop up in unexpected places. For example, the hard domes of metal tweeters might have a breakup resonance that is supersonic, or at the upper end of audibility, yet they might exhibit misbehavior that is clearly audible within the spectrum, and is audible as an obnoxious foreign material (metallic) coloration. The complex misbehavior in these cases might be causing interference patterns that produce easily audible difference frequencies well within the audible spectrum, and which are readily audible as foreign material colorations. For example, many aluminum dome tweeters impart a readily audible metallic coloration to all treble musical transients, and a coloration that is even easily identifiable as being that of say aluminum. Such a tweeter makes nylon guitar strings sound as if they are made of aluminum.
Eighth, these spurious driver noises usually have a disastrous effect in degrading the loudspeaker's spatial imaging. Your ear/brain can easily localize these foreign colorations as coming directly from the loudspeaker location in your room, and as not being part of the re-created spatial image between loudspeaker locations, as was encoded in the recording. So you hear an excessive amount of energy coming just from the loudspeaker locations, and these hot spots are enough to degrade spatial imaging, the loudspeaker's ability to portray the space that was recorded and encoded into the recording. To make matters worse, your ear/brain is distracted by these spurious noises, since they sound different from the music. So, unconsciously, your ear/brain pays extra attention to these intrusive foreign noises, and less attention to the music. Thus, your ear/brain
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