Rockport Technologies Altair Loudspeaker (TAS 214)

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Rockport Technologies Altair
Rockport Technologies Altair Loudspeaker (TAS 214)

High-end-audio loudspeaker designers are an elite group. From the thousands of engineers around the world, they have risen to the top through raw talent, a single-minded pursuit of perfection, and decades of dedication to the art. Within this select group is an even smaller sub-set of designers who have pushed beyond the standards of the high-end to create truly transcendental products.

Among this handful of the world’s most innovative loudspeaker designers is Rockport Technologies’ Andy Payor. He’s been at the forefront of heroic enclosure construction and radical driver development for decades. But Payor’s public profile isn’t commensurate with his products’ performance. Rather than engage in self-promotion, he seems content to let audiophiles discover his products on their own. Payor takes a very low-key approach to marketing that’s in sharp contrast with that of many of his peers, perhaps because he’s 100% engineering nerd and 0% marketer. Moreover, Rockport loudspeakers are made in very limited quantities, with each pair tested, hand-calibrated, and auditioned by Payor himself. Consequently, very few music lovers have heard one of the world’s great loudspeaker lines.

Design Overview

The $97,500 Rockport Altair is a formidable loudspeaker, weighing in at 515 pounds out of the crate. Seen from the listening position, the Altair doesn’t look all that big. But step around to the side and the speaker’s volume becomes apparent. The relatively narrow baffle provides an ideal wave-launch platform for the front-firing drivers, and the depth provides the enclosure volume as well as a baffle for the side-firing 15" woofer.

The Altair is the second model down in Rockport’s seven-product line. (The entry-level is the $6300 Mira Monitor; the top is the $225,000 Arrakis.) In addition to that 15" side-firing woofer, the driver complement includes a front-firing 9" mid/bass, 5¼" midrange, and a 1" beryllium dome tweeter. The three cone drivers, custom built for Rockport, feature diaphragms of carbon-fiber skins on either side of a Rohacell core. Payor designs and builds the diaphragms, and then sends them to Audiotechnology in Denmark for assembly with custom motors. The 15" side-firing woofer is ported in the rear through a huge flaired-duct, machined from aluminum. Input is via a single pair of binding posts. The loudspeaker rests on four machined aluminum feet with rounded bottoms that screw into threaded holes in the base.

Unlike most loudspeaker enclosures that are made from panels joined together, the Altair’s composite enclosure is laminated as a single unit (actually, in three sections—baffle, base, and main enclosure). This molded monocoque approach reportedly not only results in a stronger enclosure, but also allows compound curvatures that would be impossible with panel construction. The finish is available in any automotive color; my review samples were painted in a gorgeous Porsche Atlas Grey Pearl. The smoothness of the finish and luster of the paint were stunning.

The crossover is built from custom inductors and capacitors, with parts matched to 1% tolerance. Each network is then hand-tuned to the particular set of drive units with which it will be mated. Crossover components are connected with point-to-point wiring rather than a printed-circuit board, and then encapsulated in a potted module. All internal wiring is made by Transparent Audio. (See my accompanying interview with Andy Payor for more on the Altair’s design and construction.)

Listening

Setting up the Altair was relatively straightforward after they were out of the crates. The speaker is shipped lying on its back, and is best removed from the crate in your listening room. With the crate disassembled around the speaker, you screw the four large threaded feet into the base and then tilt the speaker up onto four furniture sliders, one beneath each foot. Once standing upright, one person can slide the speaker into position. Small changes are easily made while the speaker is on the sliders, which are removed after finding the optimal placement. The rounded feet serve as the final foundation; no spikes are necessary.

The Altair’s bass was different from other loudspeakers I’ve reviewed in two respects. First, the 15" side-firing woofer seemed to couple to the listening room’s air in a way that fostered a sense of great solidity. I’m not describing just bass heft, weight, dynamics, or extension (all of which the Altair has in spades), but a different phenomenon that gave the entire bottom end an “anchored” feeling. Bass-rich instruments—bowed or plucked doublebass, bass guitar, timpani, and the left-hand lines on some piano recordings—just seemed more “there” and tangible than I’ve heard from any other loudspeaker. The bass had a visceral grip that rendered a palpability of bass-rich instruments like I’ve never before heard in reproduced music. The woofer seemed to “lock” to the listening room’s air volume. I don’t mean that I heard the “locking” phenomenon directly, but rather that the Altair produced the impression that there was no woofer moving back and forth to create the sound. Rather, bass-rich instruments just seemed to exist, fully formed and fleshed out, right in my listening room.

The second way in which the Altair’s bass distinguished itself was the tremendous sense of bass power, particularly in the mid- and upper-bass region. The range from about 80Hz to 200Hz had tremendous timbral warmth and even more stunning dynamic impact. I hate to use the word “warmth” because of the negative connotation of a tonal imbalance; perhaps “densely textured” is a more accurate description. In fact, the Altair was anything but colored. This “warmth” wasn’t manifested as an excess of energy, but rather as a forcefulness of presentation that’s related to the “visceral grip” and sense of solidity described in the preceding paragraph. Frankly, the Altair makes many other loudspeakers sound slightly anemic in this range. The deeply tuned tom-toms that open “Sing, Sang, Sung” from Swinging for the Fences by Gordon Goodwin’s Big Phat Band produced a sense of physical impact that was startling. Or listen to the thundering left-hand lines on Nojima Plays Liszt and try not to be shaken to your core. The Altair easily filled and pressurized the considerable volume of air in my listening room. Although substantial in size, the Altair delivered a far bigger sound—in bass weight and dynamics—than you’d think.

Bass extension was also outstanding. Although physically smaller than many loudspeakers that aspire to be full-range, reference-quality transducers, the Altair gave up nothing in the bottom octave. Organ pedal points pressurized the room convincingly, with I might add, no chuffing from the port, doubling-distortion from the woofer, or other artifacts.

Despite the sheer amount of bass and midbass energy, the Altair sounded like a featherweight in its portrayal of bass detail and micro-dynamic shadings. The Altair’s bottom end had a lithe, agile quality that was at odds with its iron-fisted impact. The delicacy of timbre, resolution of pitch, clarity of instrumental lines, and stunning rendering of even the slightest dynamic nuances elevated the Altair to a league of its own. Moreover, the Altair had no hint of thickness, congestion, or confusion, even at high listening levels. These qualities, when coupled with the solidity and sheer output in the bass, produced many listening experiences that I’ll never forget. The massive timpani rolls in “Dream of a Witches’ Sabbath” from Symphonie Fantastique [Reference Recordings] exemplify everything I’ve been saying about the Altair’s bass. The timpani had the frightening, thundering power of a freight train, yet within this barrage I could hear the individual strokes on the heads, the heads’ vibration decaying, and the resonant body of the timpani. I’ve never heard this combination of bass power and delicacy; it’s one I’m going to find difficult to live without.

Interestingly, the bass performance changed with the amount of toe-in because of the side-firing woofer. Even small changes in toe-in affected how the woofers drove the room, but experimentation paid off when Andy Payor, who visited for the fine-tuning, found the spot where the bass locked in.

But spectacular bass performance was only the beginning of the Altair’s greatness. This loudspeaker is vanishingly low in tonal coloration, from top to bottom. I consistently had the impression that the Altair was a transparent window on musical timbres, imposing so little of itself on the presentation. Instrumental textures were stunningly vivid and alive, yet not in a Technicolor way that could become fatiguing. This vividness came from revealing the natural timbres of instruments and voices themselves rather than from some editorial interpretation. By vivid I mean palpable, present, and realistic, not overly forward. The realism of timbre was a result, I believe, of extremely low driver coloration, very fast transient ability, integration between the drivers, and zero contribution from the cabinet. Tone colors were rich, dense, detailed, and saturated, with extraordinary resolution of the finest inner detail of instrumental timbre. In addition, the Altair had a top-to-bottom coherence, tonally, spatially, and dynamically, that made it speak with one voice. The integration between the drivers was outstanding, with absolutely no change of tone color or density with an instrument’s register. I had a sense that an instrument’s harmonics were fully integrated with fundamentals, not a separate component riding on top. Moreover, the Altair’s character didn’t change with volume, sounding just as pure and clean during the most demanding musical peaks as in quiet passages. This all added up to an impression not of listening to a pair of loudspeakers, but of hearing spontaneous music-making.

The Altair’s overall tonal balance was extremely flat and neutral, but sources, electronics, and cables with a smooth treble balance were the best match with this loudspeaker. The Altair is highly revealing of everything upstream of it, and isn’t forgiving of treble brightness or a forward balance in amplification or sources. The BAlabo preamplifier and power amplifier, for example, were a perfect match for the Altair—so much so that this combination could be the paradigm of system synergy.

The Rockports completely disappeared as a sound source, throwing a huge, deep, and highly precise soundstage. Image focus was razor-sharp, and the sense of space between images was as good as it gets. The overall perspective was immediate, bold, and incisive, but not to the point of being pushy or forward. This was not a relaxed-sounding loudspeaker that imposes a sense of distance and depth on every recording.

What really distinguished the Altair’s spatial reproduction was the sense of blackness behind the instruments that allowed me to hear extremely fine spatial details. This extraordinary resolution of the lowest-level signal components, such as discrete reflections and reverberation decay, greatly added to the Altair’s stunning sense of realism. Instrumental decay, as well as hall ambience, seemed to hang in space longer. Moreover, the sounds of instruments decaying and reverberation tails “held together” and sounded coherent at the very lowest levels rather than degenerating into an undifferentiated noise. The Altairs opened up a transparent window on fine spatial detail, particularly with LPs and high-resolution digital sourced from my music server. This quality was particularly apparent when driving the Altairs with the Constellation Audio Altair preamp (no relation to the Rockport Altair) and Hercules power amplifiers, which in my experience have unrivaled resolution of low-level detail that emerges from a deep blackness. I think the Altair’s heroic cabinet construction is a large contributing factor to its ability to vanish as a sound source—spatially as well as timbrally.

Dynamically, the Altair achieved the best of both worlds; it had the ability to reproduce the most demanding dynamic contrasts with tremendous authority and slam, yet it had a delicacy and grace when reproducing finely filigreed micro-transient information. The way in which notes started and stopped played a large role in delivering the sense of realism and vibrancy I mentioned earlier. Leading-edge transients were extremely fast and sharply defined, yet completely devoid of etch. Listen, for example, to Ralph Towner’s superbly recorded acoustic guitar on Oregon’s Beyond Words on the Chesky label; the dynamic envelope of each note’s attack was perfectly defined. Listen also to the way in which the notes resonated and decayed in a totally believable fashion. By reproducing such dynamic starts and stops in a way that’s closer to what I hear in life, the Altair removed just one more clue that I was listening to a loudspeaker and not to live music.

Conclusion

One way to judge an audio product is how easily it makes you forget you’re listening to an electro-mechanical reproduction of music rather than to music itself. By that criterion, the Rockport Altair was transcendental. I consistently had the impression of music-making coming alive in my room, not of listening to a hi-fi system. Not every loudspeaker that satisfies a sonic checklist of audiophile priorities—tonal balance, dynamics, soundstaging, for examples—rises to the rarified air that the Altair occupies.

Highly resolving, effortlessly dynamic, utterly transparent, and full in balance, the Altair is one of the world’s great loudspeakers. I must caution you however, that the Altair is highly revealing of any shortcomings in the signal feeding it. The Altair deserves and demands to be driven by the finest sources, electronics, and cables.

In addition to this sonic performance, the Altair’s build-and-finish quality is as good as it gets. This loudspeaker is clearly the creation of a fanatical dedication to perfection in every aspect of its design and execution.
You should audition the Altair at your own risk; once you hear its magical ability to conjure up musicians right in front of you, your standards will forever be altered. I know that mine have.

SPECS & PRICING

Configuration: Four-driver dynamic loudspeaker
Driver complement: One side-firing 15" woofer, one 9" mid-bass driver, one 5-1/4" midrange cone, one 1" beryllium dome tweeter
Sensitivity: 91dB
Impedance: 4 ohms
Weight: 515 lbs. each (net), 780 lbs. each (crated)
Price: $97,500

ROCKPORT TECHNOLOGIES
586 Spruce Head Road
South Thomaston, ME 04858
(207) 596-7151

ASSOCIATED COMPONENTS
BAlabo BC-1 Mk-II preamplifier and BP-1 Mk-II amplifier, Constellation Altair preamplifier and Hercules power amplifiers, Mark Levinson Nº53 power amplifiers;
Meridian 808.3 and Meridian Sooloos system (Ethernet connected), dCS Puccini/U-Clock, and Berkeley Audio Design Alpha DAC, custom fanless and driveless PC server with Lynx AES16 card; Basis Inspiration turntable with Basis Vector 4 tonearm, Air Tight PC-1 Supreme cartridge; Aesthetix Rhea Signature phonostage; Shunyata V-Ray V2 and Audience aR6t power conditioners; Shunyata CX-series AC cords; Transparent XL Reference interconnects; Transparent XL Reference loudspeaker cables; Billy Bags equipment racks, ASC 16” Full-Round Tube Traps.

FEATURE INTERVIEW: ROCKPORT TECHNOLOGIES’ ANDY PAYOR TALKS WITH ROBERT HARLEY

Within fifteen minutes of meeting Rockport Technologies’ founder Andy Payor at his workshop/factory in Rockport, Maine, I had a strong sense of his values. Payor has spent his life tackling the engineering challenges of designing and building world-class audio products—and it shows. He talks about his work with an intensity and depth that’s extreme even for a high-end audio designer.

Payor has always been ahead of the curve. His Rockport Sirius turntable, introduced in 1991, presaged the modern movement toward mega-turntables. At its introduction, the Sirius was considered the world’s best turntable, and many regard its successors’ performance (the Sirius III) as unmatched to this day. It is an engineering marvel, with a self-leveling pneumatic support structure, vacuum hold-down, an air-bearing platter, and air-bearing tonearm—all of Payor’s design.

He is smart, totally engineering-driven, highly opinionated, has no tolerance for mumbo-jumbo, and holds deep convictions about how high-end audio products should perform. He’s a walking engineering encyclopedia, ready to deliver a detailed treatise on seemingly any aspect of audio-system design. But unlike many engineering types, Payor has a strong aesthetic and artistic sense, whether it’s music, restoring wooden boats, wine, or charcoal cooking. My visit was scheduled to last an afternoon but I ended up spending nearly two days with this fascinating, multifaceted individual whose approach embodies the ethos of high-end audio.

Robert Harley: How did you get into loudspeaker and turntable design?
Andy Payor: I was exposed to a great deal of music at a very young age, and much of it came from a huge console that played New York’s WQXR in our living room. During that period, I bought an old Garrard turntable from a friend of mine and my very first Cat Stevens album. I was enthralled with the notion that I could play whatever music I wanted whenever I wanted to. I also really enjoyed all things electronic from a young age and began to tinker with speakers in college. But one day in the early 1980s I saw an ad in an audio magazine with Matthew Polk in his white lab coat, and I said to Tracie [Payor’s wife]: “If this guy can do it, I can, too.” So, I set off with zeal to build a loudspeaker.

I was selling my first designs around the New England area when a purchasing manager at Tweeter [a retail chain] said, “It’s an excellent design, but we don’t have a place for it. But there’s someone you have to talk to, and that’s Karen Sumner of Transparent Audio Marketing.” So I took my creation to Transparent, but before we listened to my speaker she wanted me to hear what she considered to be the state of the art at the time. She had a Goldmund Studio turntable, Electrocompaniet electronics, and a big pair of Respons Grand speakers from Sweden. I was immediately struck by how different it was from anything I’d heard, and certainly anything I was building. We then listened to my speakers and she graciously critiqued them. I took back a revised version a couple of weeks later and she was really taken aback by how much progress I had made in that short amount of time.

Some months later Transparent asked me if I would design a loudspeaker that would be a smaller companion to the large Respons Grand. I co-designed that speaker with Reidar Persson, and worked on some other products for Transparent, and then they asked me if I would be interested in building turntables for them. Transparent was marketing the Well-Tempered Turntable that Bill Firebaugh had designed, and that his wife Kay was building at the time. Transparent was backordered by 300 units when I started, and I think we built 25 turntables a week for five years. That was a lot of turntables.

What inspired you to make such a leap to the Sirius, which was perhaps the most ambitious turntable created at the time?
There were things I wanted to do in a turntable design but couldn’t do them under the auspices of Well Tempered because they were not consistent with Bill’s designs. I wanted to solve the problems that I saw in LP playback and not have any constraints. There was enough evidence, experiential and otherwise, to determine that there were very distinct problems in analog playback that needed to be addressed; otherwise it simply wouldn’t sound good. While the Well Tempered had exceptional performance in some regards, there were other things that I felt could be improved upon. As in any design, you can’t elevate the performance of one set of parameters of a product without elevating the other parameters at the same time. If you do, you’ll wind up with something whose overall performance is limited by the least well-thought-out and executed component of that design. So, I decided to go on my own and design a turntable from a blank sheet of paper with no limitations on the performance.

Let’s move on to loudspeakers. What was your overall goal for the Altair?
My overall goal for the Altair was to make a full-range loudspeaker that would actually fit in someone’s home and do everything that a big, full-range loudspeaker should do but in a reasonably sized package. I do recognize that the Altair isn’t reasonable by some people’s standards. It’s a direct descendant of the Arrakis, and because it’s essentially half the drive-unit complement of the Arrakis, it shares a lot of the same technology. The idea was to deliver Arrakis-type sound for smaller rooms.

What are the advantages to a side-firing woofer in the narrow, deep enclosure?
A narrow-baffle profile affords you a less obstructed view into the soundstage, and is capable of “disappearing” better than a baffle that would be wide enough to mount a large woofer on. They’re also usually more pleasant to look at from the listening position. It is true, though, that narrow baffles don’t support the wave launch to as low a frequency as a wider baffle before its radiation pattern transitions from 2-pi to 4-pi space. However, this phenomenon occurs in all freestanding enclosures, so care must always be taken to compensate for the “diffraction loss” that arises from this transition—in the narrower enclosure it may just happen at 800Hz instead of 650Hz. In addition, the side-firing woofer does have a distinct advantage in that the woofer doesn’t excite room modes as strongly, particularly the transverse axial mode. In a conventional speaker if the woofers are positioned optimally to minimize this transverse mode, the midrange drivers and tweeters are usually not quite far enough apart for the best imaging. The side-firing woofer allows you to simultaneously optimize the woofer placement to minimize standing-wave modes as well as optimize imaging by moving the midranges and tweeters to their correct locations within the room. Again, you also have the freedom to use a larger woofer whose impedance match to the air mass load is much more efficient at low frequencies than that of a smaller woofer—i.e. there’s more “traction” in the bass.

Tell me about the enclosure construction—how the cabinets are designed and why they’re built that way.
The thing that separates the Altair from any of its competitors, and even the rest of the speakers in our own line with the exception of the Arrakis, is the molded composite enclosure. One of the beauties of a composite laminate construction is that you’re not constrained by the geometry of sheet stock; you’re able to create shapes optimized for the wave launch, including compound curvatures.

In addition to the correct shape, there are three fundamental characteristics of an ideal loudspeaker enclosure: 1) It has to be as stiff as possible; 2) It should be as well damped as possible—and I’m speaking here about the cabinet structure, not the bass alignment; and 3) It should be as heavy as realistically possible. In an ideal loudspeaker the combined acoustic output of the drive units should be an exact analog of the signal that’s going into it. Any output from the cabinet is a distortion because it’s going to be frequency-specific, delayed in time, and will have variable amplitude that’s not consistent with the musical input signal. The surface on which the drivers are mounted must be extremely rigid because you don’t want energy imparted to the enclosure from the drive units causing motion of the driver’s mounting surface. Any energy coming from the driver should come only from the cone motion, not from additional motion of their mounting surfaces with unknown relative phase relationships. Also, the stiffer a cabinet’s construction, the higher its frequency of resonance will be. Typically, there will be less excitation of a cabinet’s resonance—so it will be less noticeable—as frequency increases, due to the average spectral distribution of music, which has significantly greater energy in the lower octaves.

You also want something that’s really massive, since additional mass, which should be accompanied by greater stiffness if the design is properly executed, will require more force to excite it. That’s a simple F = MA thing—it takes a lot more force to accelerate something that has a lot more mass.

A problem arises when you try to achieve these multiple goals with a single material. Materials that are very stiff and rigid have very little internal loss mechanism, and therefore exhibit high-Q resonances—they tend to ring. Well-damped materials with high internal losses don’t ring but they don’t provide stiffness. If we can agree that an ideal loudspeaker cabinet should be infinitely stiff, as well as totally damped, then the design has to use materials with dissimilar characteristics to achieve these dual goals. The art, or dare I say it, science, lies in utilizing appropriate materials correctly to optimize both of these attributes, thereby improving the overall performance of the enclosure design.

To achieve these dual goals we make a composite structure that utilizes the very high tensile strength of the glass-fiber shell at the inner and outermost region of the enclosure, separated by a core. This core is a specially formulated, proprietary epoxy that bonds the inner and outer shell together, and also has a very, very high hysteresis loss at room temperature. The epoxy core has excellent compressive strength, exceptionally high sheer strength, and it’s very heavy—about 14 pounds a gallon. When you bond the whole thing together you are essentially building a beam section for the cabinet wall. So you have the high tensile strength inner and outer shell separated by a high-hysteresis-loss, heavy, massive core material. The stiffness of the composite goes up exponentially as you increase the distance between the skins.

The other beauty is that there’s no joinery or fasteners. Except where the baffle and base are attached, there are no fasteners or glue joints. The baffle is attached via bonding a continuous rabbet with high-performance structural epoxy, so it’s a completely monocoque construction. All the corners, radiuses, and internal features are made as a single element. 

What are the sonic benefits when you take a cabinet to that extra level of stiffness and mass?
Well, it’s immediately apparent when you listen to it that there’s a complete freedom from any kind of congestion that is caused by low-level noise that’s always present in virtually all cabinets. So even though a cabinet resonance signature may be very low, like it might be 30 or 40dB down, it’s still not low enough. Cabinet resonance releases energy over time, which really colors the sound and veils low level information. Also, again there is the problem of the drive-unit mounting surfaces moving when they should not, creating Doppler distortions in the drive units.

Another thing that’s very obvious audibly is that the Altair has an absolutely black background, and its resolution floor that is much lower than other loudspeakers. In our world, what’s going on at 50dB is as important as what’s going on at 90dB. We want to make sure that low-level signals are not masked and obscured by cabinet resonant signatures. That’s a very, very difficult thing to do with a cabinet as large as the Altair’s if it’s not really ambitiously designed and executed. In addition to this freedom from congestion, and improved low-level resolution, this kind of enclosure yields far better rendering of dynamics. The dynamic shadings and the dynamic continuum of our loudspeaker from loud-scale dynamics all the way down to very, very low-level dynamics is quite different than most loudspeakers.

You’ve been making very light, very stiff carbon-fiber-skin drivers for some time. Tell us about the drivers in the Altair.
The drivers in the Altair were initially developed for the Arrakis in 2005. They are based on the structure and motor assemblies of AudioTechnology drivers, who we’ve been working with for almost 20 years now. They understand a great deal about drivers, but as good as their motors are, I thought it would be spectacular to put some carbon-fiber-sandwich composite cones with those motors. I tooled up three different cone profiles—a 5¼-inch unit, a 9-inch unit, and a 15-inch, all for the Arrakis project. Those are the same drivers used in the Altair.

We use a pre-preg carbon-fiber skin on either side of a Rohacell core in the aluminum tooling we’ve machined, and then consolidate them at high temperature and high pressure. It gives us an extremely light, very stiff cone that also has high intrinsic damping. Again, we’re making a beam section out of the carbon-fiber skins which are very, very thin on either side of the Rohacell core.

The other thing that’s different about our cones is that the section thickness varies over its radial dimension. This helps eliminate resonant breakup modes that occur in typical cone structures. They are far stiffer for a given weight than any monolithic structure, so their primary breakup modes are higher in frequency, and better damped than all other materials such as aluminum, magnesium, titanium, or ceramics. An added benefit is that they have very nice rolloff characteristics which allows us to use simpler crossovers.

You’ve recently started using a beryllium dome tweeter.
Going from the D-30 soft dome, which is a very, very good tweeter, to the beryllium dome was really tremendous in a number of ways. The purity of the sound that comes from the beryllium dome is absolutely remarkable. It has the ability to be extremely resolved at high frequencies but is also very expressive throughout the whole range. So if you listen to a female vocal, for example, it’s so much more expressive with the beryllium dome than it was previously with the D-30.

The downside is that it’s a little tricky to use, but the upside is that this is the first beryllium dome tweeter that we’ve ever seen—actually, the first metal dome tweeter I’ve ever seen—that really is a significant improvement over the best soft dome. The guys at Scanspeak have done a tremendous job with this—it’s an unbelievable tweeter.

Tell us about the Altair’s crossover.
The topology of all our crossovers yields very steep attenuation in the stop band with gradually lessening attenuation into the passband. This crossover design gives us ideal phase response where the two sections are summed, and very rapid stop-band attenuation. You don’t want excessive driver overlap, or the drivers to be reproducing frequencies outside their intended passband. I know a great deal has been made about “elliptical” crossovers recently, and these are the types of crossovers we’ve been using for nearly 20 years.

We also have our own custom capacitors and custom-wound inductors made for us. I had the privilege of learning a lot about capacitors when I was working with Richard Marsh while he was developing the first MIT Multi-Caps.

Lastly, the crossovers are all point-to-point wired to avoid the parasitics of PC-board type construction. This also allows me to adjust every crossover individually, which would be impossible if we used a PC board.

Let’s go back to the enclosure and discuss the unusual shape.
The reason for the loudspeakers’ unusual shape is to minimize diffraction problems. What most people don’t recognize is that there are two types of diffraction that we’re concerned about; diffraction loss, and edge diffraction. Diffraction loss is the phenomenon that occurs as the wavelengths being reproduced by the loudspeaker approach the loudspeaker baffle dimensions. As frequency decreases and the wavelength gets longer, the cabinet’s radiation pattern will transition from 2-pi space, or forward radiation only, to 4-pi space, or spherical radiation with an attendant 6dB drop in sound pressure on axis. Edge diffraction is when the sound wave moves across the baffle surface and then “snaps” off a sharp discontinuity at the edge of the enclosure, thereby creating a secondary source of sound which is physically disparate from the original source, and has a whole host of nasty phase relationships which are frequency and distance dependent because of this difference in origin. It’s a misconception that if you have a relatively small radius or chamfer on the edge of a cabinet that it will somehow eliminate diffraction. It doesn’t. You need the drive units to be at dissimilar distances to the edge chamfer, and that chamfer needs to be big, broad, and preferably somewhat rounded and of a size that approaches the wavelengths in question. A half-inch or three-quarter-inch chamfer or radius around the edge of the box doesn’t do anything to reduce diffraction effects, nor does having a gentle crown on the surface of the baffle.

If you look at the profiles of our speakers, you’ll see a very large, sweeping chamfer with ever changing dimensions around the front. It creates dissimilar path lengths between the drivers and the cabinet edges, which facilitate a smooth transition from 2-pi to 4-pi space. The chamfer must be large because diffraction is not just a problem from the tweeter, but also from the midrange. It’s a mistake to think that a speaker is devoid of diffraction problems because the baffle dimensions are larger than the wavelengths that the tweeter reproduces, since the problem merely moves down in frequency into the midrange. It’s vital that the cabinet’s shape is acoustically optimized so that it gets out of the way. Even our smallest speakers have similar baffle profiles. But we’re able to take it to a greater level of execution with the Altair because of the composite construction and ability to shape the enclosure any way we want it.

Could you talk about why you choose ported over sealed enclosures?
There’s been a lot of talk lately about the supposed superiority of sealed enclosures, but what is not widely recognized is that both ported and sealed enclosures use forms of resonance to extend the bass response. While the two resonant mechanisms differ somewhat, the extension and bass response of both systems is achieved through resonance.

The reason we use ported enclosures is that for a given enclosure size and a given –3dB point a ported enclosure can actually have greater sensitivity and lower distortion. The reason the distortion is lower is that as long as you are above port resonance, the driver excursion in a ported enclosure is less than that of a driver in a sealed enclosure. It’s driver excursion that creates distortion. If your port-resonance tuning is low, you’re unlikely to have musical stimulus below the port resonance. That frequency in the Altair is 24Hz.

We also use a proprietary tuning where the bass rolloff is approximately 12dB per octave, not the 24dB per octave of a standard ported enclosure. The notion that ported enclosures are hard to position in a room because of their rapid rolloff, or that they have poor transient response, is simply not true. It is true of a poorly done ported design, but the same could be said for a poorly done sealed design.

You’ve been making advanced-technology loudspeakers for 20 years, yet Rockport has a low profile in the marketplace. It’s one of those brands that audiophiles seem to discover on their own after years of living with other makes of products. Is that because of the limited production capability?
Partly. I decided to keep the production limited because I don’t want to be building and shipping loudspeakers that I’m not directly involved with. I have personally tweaked every crossover on every single loudspeaker that leaves this building. If we’ve just done a run of ten Miras, for example, we make ten Mira crossovers. If we took one crossover and connected it in turn to each of the ten Miras and measured them, their responses would be very slightly different. Every driver’s response varies—far more than the variation in the crossovers—even when you use the best drivers. That’s why I tune every crossover specifically for the loudspeaker that the crossover is going into. I might take a couple of turns off an inductor, or add a quarter of a microfarad to a capacitor. When someone buys one of our speakers, they are getting something that is truly “dialed in.”

We also test each speaker by reversing the polarity of the tweeter and measuring the phase cancellation. This confirms how well it will sum in-phase when the tweeter is connected with the proper polarity. We also perform the same test between the midrange and the woofer. What we look for is a very well defined, deep cancellation notch at the crossover frequency. It’s not uncommon for us to measure 35dB or 40dB of acoustic cancellation at the microphone a meter away. We look at the phase summation of every single loudspeaker, as well as other things. You can’t do that if you’re selling too many loudspeakers.

More importantly, I also have other interests and don’t want to create a situation where I’m enslaved by work. I work to live, not live to work. I’m very passionate about my work and I love being able to do things at a very high quality level like this, because there’s a lot more intrinsic reward in it. But I would not want it to be the only thing I had time to do. I have a family, and I like to barbecue, and go out on the boat, sip a little wine by the lake. As you get a little past 50, you recognize clearly that time is the most valuable thing that anybody has, and so how you spend that time is ultimately of the highest importance.

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