is not just triggered by bass musical notes in its spectral vicinity. Instead, this artificially colored and musically obscuring boom is actually triggered by virtually every musical note, regardless of pitch or frequency. That's because virtually every musical note is a singular transient, and as such contains a broad, infinitely dense spectral energy content (extending nearly down to zero Hz or DC), so virtually every musical note will have some spectral energy in the vicinity of this vented system's corner frequency, which will trigger that one-note boom. In fact, in our research we once measured a single pluck on the highest string of one of the highest frequency musical instruments, the violin, and its spectral energy content actually extended all the way down to (but not including) zero Hz or DC!
In contrast, a sealed bass system like that employed in the NHT M6 can offer superior transient response for its lower frequency reproduction. Basically, that because it has a more gentle corner for its low frequency cutoff point, and a gentler curve in the frequency domain generally translates into sharper, more accurate tracking of the music waveform curve in the time domain. The sealed bass system of the M6 satellite monitor gives the M6 more accurately natural reproduction of the bass and warmth regions, with better definition. And it also does not produce nearly as much of that lingering, musically obscuring one-note boom as a typical vented bass system, which means that the M6 can be more revealing of musical information, not just for its more accurate bass and warmth regions, but also for all music and sounds throughout the rest of the spectrum.
The second benefit of the M6's sealed bass system is that its low frequency response below the system's bass cutoff frequency is much stronger than that of a vented system. A vented system rolls off steeply at 24 dB per octave, but a sealed system like the M6 rolls off much more gently, at just 12 dB per octave. This means that, at a frequency one octave below the nominal low frequency system cutoff, a sealed system like the M6 puts out over 10 times stronger bass than a vented system. This can mean that bass from a sealed system has much better kick and impact, since those desirable aspects of bass depend on how much energy the system puts out at very low frequencies, usually below the nominal cutoff frequency.
This stronger output below cutoff also means that a sealed system like the M6 can better sonically integrate with a subwoofer than a typical vented system can. All crossovers involve some overlap, and the loudspeaker drivers or systems on both sides of the crossover frequency should be able to handle this overlap and put out enough energy to cover this overlap, in order for the two drivers or systems to dovetail, blend, and integrate properly. The sealed system satellite like the M6 can put out over 10 times more bass energy one octave below its cutoff than a vented system can, so it can handle this overlap much more capably, which means that it can dovetail and integrate with a subwoofer more capably.
Additionally, the filter cutting off the upper frequency response of the subwoofer (the low pass filter, which is often electronic, ahead of the power amplifier driving the subwoofer) should complement the slope of the satellite's low frequency cutoff, so that the two responses dovetail and blend properly. If the satellite's low frequency cutoff slope is gentler, as is the case with a sealed system like the M6, then the subwoofer's upper frequency cutoff slope can (and should) be likewise made gentler, so that they properly dovetail. And a gentler upper frequency cutoff slope for a capable subwoofer (one that does not itself have an obnoxious spurious peak at 300 Hz that must be filtered out) is in itself intrinsically very desirable. That's because a subwoofer's frequency response essentially looks like a mountain, with one falloff slope below about 30 Hz, and another falloff slope at its upper end, which is merely a bit more than one octave away (say about 80 Hz). That's a very narrow mountain shape for any system, a narrow mountain shape determined by the nearness in frequency of the two filters delimiting the subwoofer's pass band (the spectral width of its performance range). Now, the narrower the mountain of any system's pass band, and the steeper the slopes of the mountain, the worse will be that system's transient response (sharper slopes in the frequency domain mean worse time smear and lingering overhang in the time domain).
Thus, subwoofers are very prone to poor transient response, regardless of how much money and design effort is poured into their driver and enclosure, simply because their pass band incorporates such a narrow mountain. Therefore, the slopes of this subwoofer mountain assume extra importance in deciding the quality of the subwoofer's transient response. The subwoofer itself will produce better quality bass if the slopes of its mountain can be made more gentle, in this case specifically if the upper frequency slope of its mountain can be made more gentle. Since the upper frequency slope of the subwoofer's mountain should match the lower frequency slope of the satellite, for the two systems to dovetail and integrate properly, it follows that the upper frequency slope for the subwoofer's mountain can be made more gentle only if the satellite's low frequency cutoff slope is gentle.
This means that a sealed system like the M6, with a low frequency cutoff slope that is far more gentle than that of a vented satellite system, has the benefit of allowing the subwoofer to employ a gentler upper frequency cutoff slope, thereby allowing the subwoofer itself to give you superior transient response. In short, the sealed system of the M6 also allows your mating subwoofer to produce better quality bass. And this better quality bass from the subwoofer not only has the sonic benefit of sounding intrinsically better, but it also has the sonic benefit of having less lingering overhang, so it allows the M6 as a satellite to itself be heard more clearly, since that lingering overhang from the subwoofer would obscure subsequent musical information coming from the satellite.
There's also a similar complementary benefit to the low frequency transient response of the satellite. In order to reduce excessive low frequency woofer excursion, reduce distortion, and allow the satellite to play louder, most satellites are not fed a full spectrum signal, but instead are fed a signal that has had its bass rolled off by an electrical filter, usually preceding the power amplifier that is driving the satellite. This electrical filter introduces an additional pole (bass rolloff) or poles, making the bass rolloff slope for the satellite more steep than its intrinsic bass rolloff slope. If the satellite is a sealed bass system like the M6, then its intrinsic rolloff slope is a moderate 12 dB per octave, and if the added electrical filter at a nearby frequency is also a 2 pole filter with a 12 dB per octave slope, then the total steepness of the slope for the satellite system will be about 24 dB per octave. But if the satellite is a vented bass system as is common, then its intrinsic bass rolloff slope is already 24 dB per octave, and adding the electrical filter will make the total steepness of its rolloff slope 36 dB per octave, which is very steep, and which will therefore produce worse transient response from the satellite. Again we see that a sealed satellite system, like the M6, produces superior transient response for musical notes, in a complete real world system environment, than a vented satellite system. And remember again that virtually all sound effects and musical notes (even high pitched ones) will trigger the transient behavior or misbehavior of the satellite at its low frequency end, since as transients all these sounds contain some spectral energy at the low frequency end of the satellite's operating range.
Most satellites, especially those built to a cost conscious price like the M6's $600, are merely two-way systems, with a single woofer-midrange driver and a single tweeter, since most manufacturers don't want to spend the extra money for extra drivers and crossover complexity into a system that will sell for just $600. But the M6 is designed like an expensive high end satellite, being a full three-way system, with two woofers instead of one, and a dedicated midrange driver, as well as the usual dome tweeter. This three-way design of the M6 also gives you several notable sonic benefits.
First, the lower midrange and midrange can be covered by a single driver, which can potentially make these midrange regions sound more coherent and accurate. The M6's midrange driver is a 4 inch cone unit, and it is assigned a spectral pass band from 350 Hz to 2300 Hz. The fact that this 4 inch driver is smaller than a typical woofer driver (which usually has a 6.5 to 8 inch cone) means that it can be more pistonically accurate in reproducing the signal than a woofer-midrange combination driver of a typical two-way system could, especially in the 1000-2000 Hz region, where almost all woofer-midrange combination drivers are no longer accurately pistonic and are instead radiating inaccurate breakup mode energy (the fact that such drivers are still outputting relatively flat energy in the 1000-2300 Hz region, as seen on a frequency response plot, does not mean that their output is accurately pistonic here).
Second, the smaller 4 inch midrange driver of the three-way M6 can have a wider radiation pattern into your room than the larger woofer-midrange driver of a typical two-way system would. This means that the total radiated power into your room can be flatter through the midrange from the M6 than from a competing two-way satellite system. And that in turn means that you could perceive a flatter tonal balance through these midrange regions from the M6. that's because the human perception of tonal balance is tied to total perceived energy, which consists primarily of total reverberant energy you hear reflected from your room's walls, originating from the off-axis radiation of the loudspeaker as directed toward those walls (the on-axis direct sound you hear from a loudspeaker, which corresponds to the loudspeaker's on-axis frequency response measurement graph, plays only a minor role in determining the loudspeaker's tonal balance as you perceive it in a real room).
Third, the three-way configuration of the M6 means that the M6's woofers can be electrically cut off at a relatively low frequency, and in the M6 the twin woofers are cut off at a very low frequency, 350 Hz, far below the operating reach of a typical 6.5 inch driver. This low cutoff in turn provides several sonic benefits. All driver diaphragms (cones and domes) inevitably break up at some upper frequency. But the inevitable breakup modes of the M6 woofer cones are at a frequency so far above this 350 Hz cutoff that they are not excited, since the crossover has electrically cut off virtually all signal being fed to the woofers by the point this much higher frequency of cone breakup is reached. Thus, the M6 woofers are accurately pistonic throughout their operating range, and they do not reproduce part of the spectrum via non-pistonic inaccuracies (as woofer-midrange drivers in two-way systems all do). Also, thanks to this 350 Hz cutoff the M6 woofers do not contribute artificial colorations, of a foreign material breaking up, to the sound (these artificial colorations, imposed on the music by the woofer-midrange breaking up in two-way systems, typically sound like the cone material that is spuriously vibrating out of control, i.e. they impose a rubbery, plasticky, or papery sound upon the music).
The low cutoff at 350 Hz of the M6 woofers also can make the regions above 350 Hz, the lower midrange and midrange, sound clearer and cleaner. In a two-way system, the large excursions of the combination woofer-midrange driver, in handling its lowest frequencies, cause modulation distortion of the higher (midrange) frequencies being handled by the same driver. As Paul Klipsch's research showed, human hearing is very sensitive to this modulation distortion (modulation distortion by low frequencies makes the midranges sound garbled and grundgy, and, incidentally, modulation distortion by higher frequencies makes music's upper frequencies sound fuzzy, smeared, and artificially bright). But in the M6's three-way design, the midranges are handled by a separate midrange driver, so they are not modulated or distorted by the large excursions of the separate woofer drivers.
The tweeter of the M6, a 1 inch aluminum dome driver, takes over at the common crossover frequency of 2300 Hz, and extends well through the upper treble. The crossover for this M6, with crossover points being at 350 Hz and 2300 Hz, employs second order (12 dB per octave) slopes throughout. The drivers are connected to this second order crossover network in polarities such that the woofer is in correct absolute phase polarity for its operating range, while the midrange and tweeter are both connected such that they radiate in inverted absolute phase polarity for their operating ranges (it is characteristic of classic second order crossovers that some drivers be connected to radiate in opposite polarity from other drivers, in order to secure flat measured frequency response on axis through the crossover regions).
The M6 is intended to be used in a vertical orientation for all main and surround loudspeaker positions. In this orientation, one woofer is at the top of the front panel, the other woofer is at the bottom, and the midrange and tweeter are located in the middle, situated side by side. To use the M6 as a center channel loudspeaker, you simply turn it on its side, so that the woofers are at the extreme left and right of the front panel, with the midrange and tweeter in the middle. A small but nice touch is the fact that the rear terminal block can be rotated 90 degrees, so that you can easily read the label and dress the input cable neatly, even when the M6 is turned on its side. This kind of thorough, thoughtful, user-friendly engineering is usually found only in high end products that are far more expensive than the Evolution. Incidentally, the M6 offers only a single set of input terminals, so bi-wiring or tri-wiring is not an option.
Flexible Setup Choices for M6
You might think that evaluating a product is just a passive activity, and indeed it is probably so for most reviewers. You just set up the product according to manufacturer instructions, and sit and listen and pass judgment. But, for us, product evaluation is often an dynamic, exciting, interactive process. Yes, we first follow the standard passive route of setting up the product as the manufacturer instructs, and listening carefully. But then, as we start analyzing the product's performance, we can begin to see ways in which the horizons of the product's strong points might be enhanced, and ways in which the product's weak points might be ameliorated or even overcome. Our background technical knowledge, coupled with the experimental curiosity and creative imagination that is in the lifeblood of every research scientist, allows us to constructively stray from the manufacturer's instructions, and share with you ways of improving the product's performance, so that the sonic results you can obtain are (in our professional judgment) better than even the manufacturer himself has imagined or heard.
The M6's design afforded especially challenging (and ultimately rewarding) opportunities to work around the sonic problems we heard when we first evaluated the system as set up in accordance with the manufacturer's instructions. After we heard these sonic problems, we stretched our creative imagination and background technical knowledge, trying to come up with easy ways for you to cure some of these problems, and thereby hear much better sound from the M6. In some cases we had to deliberately disobey some of the manufacturer's setup instructions, think outside the box, and research alternative possibilities as constructive solutions. After many, many research experiments, we came up with paydirt, namely some alternative setup suggestions that do indeed enhance the strong points of the M6 and virtually eliminate some of its key weaknesses. Think of the following, then, as a guide to getting the most out of your M6.
Loudspeakers are like people - every one has a different personality. Most of us can't stand the most obnoxious personalities. But, even given a group of 10 of the most honest people or loudspeakers, you'll still find that each has a different personality. Which personality suits you best is up to you, with loudspeakers as it is with people. We can fully describe the personality of each to you, and tell you the good and bad points to take notice of, but ultimately the personal auditioning and choice is up to you.
Now, most loudspeakers have just one essential sonic personality, so we can describe that one personality, and tell you whether it is tonally neutral or colored, and in what ways it is tonally colored. But the NHT M6 has at least 8 markedly different sonic personalities (hence the unusual plural in the chapter heading above), each with a different tonal balance profile. So, instead of you being forced to accept and live with a single tonal balance personality as with other loudspeakers, with the M6 you can tailor the tonal balance personality to suit your liking. We'll tell you what the M6's various different sonic personalities are, and how they occur, and then we'll make specific recommendations to simplify your final choice.
Let's start with a brief overview of the factors that produce the M6's different sonic personalities. To produce the 8+ different sonic personalities, there are 3 primary factors for you to choose from, each factor offering you essentially a binary choice.
The first factor is the amount of warmth. A tiny two position switch on the M6's input terminal block offers you the choice of two warmth settings (effected by a change in the crossover network). At the "1" setting the warmth region is essentially flat. At the "0" setting a hump is introduced in the warmth region, with its primary enrichment in the 170 Hz to 270 Hz region, although the upper bass is boosted slightly as well. After you get accustomed to the counterintuitive notion that the "zero" setting has more (and boosted) warmth than the "one" setting, you can settle in for some listening and decide for yourself which sonic personality you prefer. There is a striking sonic contrast between the two settings, so you won't have any trouble hearing the difference on a variety of program material.
The second factor is overall polarity for your cable connection to the M6. NHT recommends that you connect the red terminal to the plus polarity output of your power amplifier. If you follow this manufacturer instruction, the M6's woofers will be in correct absolute phase polarity, but the midrange and tweeter will be radiating their spectral range with inverted polarity, so most of the music (from 350 Hz upward) will be heard in inverted polarity, especially all upper frequencies. When music's upper frequencies are inverted, by whatever means, they sound softer
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