In a way, this is a speaker the world has been waiting more than twenty years for. It has taken that long for driver technology to catch up with one of the grand visions in speaker design—the symmetric-driver, phase-linear, two-way speaker.
Almost from the beginning, TAS was interested in phase linearity. By Issue 29 (the 10th Anniversary), there was a long survey by AHC on phase-linear, time-aligned speakers, where it was taken at least somewhat for granted that these issues were important. Then in the later 1980s, John Dunlavy of Duntech brought out a line of phase-linear speakers using symmetric (MTM or WMTMW) driver arrangements, which ameliorated the “lobing” of phase-linear asymmetric designs.
But phase linearity done via analog crossovers requires gentle crossover slopes (“first-order”) and thus makes big demands on the drivers. It is hard to make, say, a midrange driver that remains really clear octaves above where it crosses over to the tweeter. And phase-linear speakers of decades ago tended to lack ultimate clarity and resolution, however outstanding their basic engineering.
Times changed, and drivers got better. Clarity and resolution are, in fact, strong suits of the ESS AMT 450 ($11,800). And phase-linear it most definitely is.
Without going off into the mathematics of phase linearity and the theory of filter design, let me just describe what it is in practical terms: “Impulse in gives impulse out” covers the whole story. The ultimate “hard transient,” a burst of energy with no spread in time, gives an acoustic output that is just like that— no spread in time—or it gives that at the listening position at least.
It may be a surprise that most speakers don’t do this. You may be tempted to say, “You mean speaker X is time-smearing my audio?” Well, yes, it is, almost no matter what speaker you’re talking about. You can see the difference easily by graphing any speaker’s “impulse response.”
I am bringing this issue up before I say much of anything about the ESS sound, because it is a major point of the speaker’s brief. A measurement system from Kirchener is used in the design work, specifically to give a visual presentation of the time distortion (if any) of speakers. Time/energy problems show up as mountain ranges. Most speakers look like the Rockies. The ESS, by design, looks like the Great Plains. ESS aimed for time perfection, and it got very close to it—the AMT 450 has as nearly perfect an impulse response as I’ve ever seen.
OK, that was the goal, and the goal was achieved. But how does the thing sound? It would be bad science to attribute its sonics to its phase linearity, without making controlled experiments. On the other hand, the speaker really does sound the way intuition suggests a speaker without time smear would. It is ultra-precise, transparent, and clear. And it is not only clear in the “mono” sense that everything is articulated and pure sounding. It is also spatially clear—things are cleanly separated in space, and the space around them is delineated. (You can really hear the hall.) How clean and detailed everything sounds with these speakers. While the company’s representatives and its Web site attribute this to phase linearity, I personally think the design’s low distortion also has a lot do with it. In any case, the clean clear sound is both musically revealing and, as it happens, very relaxing and appealing to listen to.
Male audiophiles are always wondering in a guilty sort of way why women don’t like the speakers they like. But the AMT 450s are a conspicuous exception. My girlfriend Paige likes them a lot (and she thinks they look cool, too). This was one review speaker that started in our living room and stayed there.
The ESS speakers have remarkable dynamic ease. The Heil tweeter does not harden up when the going gets tough, and the SEAS magnesium mid/bass drivers similarly remain clean and unruffled at the big moments. ESS has a larger model, but I can’t imagine you’ll need it unless you have a gigantic room.
If you allow enough space around the AMT 450s to minimize wall reflections, they do a great job of extracting spatial information both in terms of delineation and extension. I played the Mahler Fifth [Waterlily] on the bigger model at CES 2005 in a very big room, and the only word for that was “wow.” This was about as close as you get to the sense of a fullsized orchestra in front of you.
In short, these speakers really deliver the high-end goods—clarity, resolution, spatial performance, dynamics, you name it. Among other things, they make it clear why top-quality drivers really matter, not to mention good design.
Of course, at this point in an REG review, a still small voice always pipes up with a caveat or two.
So what limitations and defects do the ESS speakers have? Not very many, in fact, as speakers go.
The first is practical. You have to sit some considerable distance from the speakers to get them to integrate correctly. This is not a design flaw—it is an intrinsic feature of the geometry of timealignment. Speakers with broad driver overlap and physically separated drivers cannot be integrated correctly at all distances. At suitable distances, the ESS is, indeed, very coherent and sounds remarkably like a point source. Closer, it isn’t and doesn’t. The AMT-450s do not work as “nearfield monitors.” You need to sit on the order of ten feet away from them, minimum. Moreover, because their offaxis behavior is a bit, shall we say, complex, you also need space from the sidewalls. A fairly big room is desirable if you are to fully appreciate their virtues.
The second caveat is not roomdependent. The ESS has a prominence from around 800Hz–2kHz, and a rather complex peculiar sound in this region in addition to the prominence. It appears as though the Heil driver does not roll off quite as rapidly at the bottom of its range as it ought to, and a little extra sound from it shows up around 1k or so. This gives the ESS what the British reviewers used to call a “shouty” coloration, and a tendency to project some parts of, say, a human voice over other parts. The overall midrangy bulge is also made more obvious by the absence of any excess warmth below it. In many locations, indeed, there is some shortage of warmth; careful placement is required to avoid Allison Effect holes in the 100–300Hz area. In practice, I could not manage to get the 100–300Hz region up to level and free of dips, no matter where I put the speakers. But in any case, placement will not alleviate the midrange prominence. You’ll have to listen for how troublesome you find all this. (People seem to vary a lot about how much such things disturb them— and I admit being a bit obsessive about them.) I got used to the lack of warmth fairly quickly, but the coloration remained bothersome.
The deeper bass, below the lower mid/upper bass warmth region, is actually very good. The passive radiator system works quite well, apparently— there is no hint, not any, of “one-note” sound or frequency-dependent bloating, and bass instruments sound very convincing. This was a constant source of satisfaction—to hear the bass of an orchestra, especially, so well delineated.
Readers have been raising the question of why there is so little comparative reviewing within a given price range in TAS—so many different-sounding speakers all greeted so enthusiastically and so little made of their differences. Of course, there is a fundamental difficulty in making linear comparisons of the “A is better than B is better than C” kind: speaker performance is a multidimensional thing. Even the kinds of naïve statistical analyses involved in consumerpreference testing usually use a number of separate rating categories initially, even if these are lumped together eventually. Still, inquiring readers have a point. One has, after all, to buy one or another of the possibilities, usually. (Well, I have six pairs of large speakers, but then I am in this professionally.) So let me try to put the ESS speakers in perspective in different sonic categories. In some categories it is all but incomparable, while in others it is more middle-ofthe- pack in its quite-expensive-but-notphenomenally- expensive price range.
I noted earlier the areas where the ESS is weakest. To my ears, it is insufficiently warm in the lower mids/upper bass so that, while deep bass is quite well served, the overall balance is too midrange oriented. Also, the speaker does not have the absolute tonal neutrality of models for which tonal perfection is the main goal, e.g., the BBC-tradition monitors. This is not to say that the ESS is at all bad along this line, just that it is not leading the charge here. It does, however, relate very well to the room around it, with smooth if not ideal inroom balance. Only the midrange coloration is really intractable.
Now let us talk about the strong points, of which there are many. The ESS has very low perceived distortion, being as clean and clear as the proverbial whistle. Box effects are truly minimal. This is a box speaker that does not sound like a box at all, really. At a proper distance it is very coherent. It is, in particular, far more cohesive and coherent than first-ordercrossover speakers that do not use symmetric- driver arrays (or coaxial drivers).
In many respects, it will be hard to better these strong points at any price. Audiophiles who do not always think technically often have the idea that enough money can solve any problem. But if you approach the matter logically, you will find that this cannot be so. Many of the constraints of speaker design are the same for all. To take a random example, for a given driver geometry, how far you must be from the speaker to get a given level of equality in distances from the driver is determined not by price but by Euclid. The amount of beaming that a pistonic driver of a given diameter has is a calculation of physics, not the marketplace. What money can buy is high-quality drivers and careful design work, and both are in evidence here.
What I am trying to get at is that there is no intrinsic reason why a speaker in this price range cannot reach for the best possible results in certain directions. Of course, it is a fairly small speaker and will not fill an auditorium or the like. But in other directions, it is pushing the outer limits quite nicely.
Let me also extol the Heil tweeter. Though it has been around a while, its present incarnation is extraordinary. Its off-axis behavior is a little less regular than domes can offer, but the on-axis sound, which is what counts the most in a reasonably behaved room, is superb, with real clarity, precision, and freedom from overload, even from hints of overload about to happen. The most demanding material, like cymbal crashes, which in real life are very loud for short intervals, is handled with aplomb. Again, one could spend more money on a tweeter, but I doubt that one could do much better. (I really like this tweeter!)
Finally, we come to the raison d’être, very nearly, of the speaker, its time/energy correctness, or phase linearity if you prefer. The ESS people definitely grabbed the prize they were after here. This speaker has an impulse response that makes most others look like a confused mess. You have to form your own impression of how important this is in audible terms. My own experience with DSP phase-linearity in and out is that it is audible but not quite as significant as it looks to be graphically. Still, there is no denying the clarity of the ESS—whatever the reason for it may be. In any case, ESS stands on top of the phase-linearity mountain, with few other speakers standing nearby. High-end audio has become, for better or worse, largely about resolution. Everyone will have noticed that expensive speakers are often not pursuing increasingly neutral sound so much as highly resolved sound. The big ultra-expensive monsters are, as often as not, no flatter than smaller speakers, and indeed less nearly flat than the box monitors designed for flat response. And some of the approaches to resolution seem based more on hope or obsession than knowledge. ESS has tried to do something in the resolution line that strikes me as sensible: It has tried to make a speaker that has very rapid energy decay. And in the ultimate test, listening, its approach does work. For whatever reason, this speaker seems to do a truly exceptional job of preserving the clarity of live music. In that vital matter, it will be hard to beat.
The Air Motion Tranformer Tweeter of Oskar Heil
The AMT tweeter invented by Dr. Oskar Heil in 1972 is a driver with a unique operating principle. It moves air to generate sound by a mechanism no other driver uses and is unusually effective at delivering clean and pure high-frequency sound, particularly at high levels.
To appreciate the mechanism of the AMT, think for a moment about how other drivers work. Dome tweeters and indeed almost all other drivers—woofers, midranges, full-range electrostatics, etc.—operate by moving a diaphragm or membrane back and forth. When the diaphragm moves out, the air is compressed. When it moves in, the air is rarified. This succession of compressions and rarefactions traveling towards the listen are the sound wave.
Now the diaphragm can be made to move back and forth in various ways: electrostatic force (electrostats), magnetic force on a coil attached to a diaphragm (“dynamic” drivers), magnetic force at the edge of a diaphragm (dome tweeters), or direct magnetic drive of the diaphragm as a whole (planar magnetics and ribbons). But, however various the driving forces and the geometry of the drivers, the basic motion is the same: The driver moves in and out, rarefying or compressing the air as it does its back-and-forth thing.
Now when a small diaphragm—and tweeters need to be small in general to avoid beaming—moves back and forth, the air does not just sit there getting compressed or rarefied. It tends to move out of the way to the sides, as well. As a result, the coupling of the diaphragm to the air is not very good, and it is hard to transfer a lot of acoustic energy without huge driver motion. Physics people call this situation an “impedance mismatch.” It limits the dynamic power of tweeters. (It is also why horn-loading has been used when high levels are needed.) Heil’s insight was that one could make the energy transfer much better by using a quite different physical mechanism of creating compressions and rarefactions.
The Heil driving element is in effect “planar magnetic”—conductive strips laminated on a plastic diaphragm are acted on by a magnetic field surrounding the diaphragm. But the diaphragm is accordion-folded, highly pleated as it were. The magnetic force is set up not to move the ribbon back and forth but to push the pleats together or pull them apart, depending on which way the current in the driver element flows. This pleating-and-unpleating, bellows-like motion pushes the air out and in to make the required compressions and rarefactions. But the energy transfer is much higher than a simple back and forth motion would be. Only a small amount of driving element motion is required to produce a large acoustic effect. Impedance matching is better—hence the “transformer” name (electrical transformers are impedance-matching devices).
The AMT tweeter looks like a black box, six inches tall, seven wide, and about four deep, with openings front and back to let the sound out from the drive element in the center. The drive element itself looks like a five-inch-wide ribbon. But it has the effective radiation of an eight-inch driver! It also has the non-beamy pattern of a ribbon with the air-coupling of a large electrostatic element. What you hear is pure sound that hardens not at all as the level rises. Unique and truly remarkable.
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