Impact Airfoil 5.2 loudspeaker system

Brian Damkroger, June, 2002

"You're a toy junkie!" laughed Trish, my fiancée. "Your life is a search for bigger and better toys!"

I don't remember what particular treasure I'd been gushing about, but she found it pretty amusing. With two small children, Trish's view of toys consists of Barbies and GameBoys, which she enjoys comparing to my sports cars, motorcycles, and treasured Japanese-pressing jazz LPs.

I wouldn't characterize my life as "a search for bigger and better toys," but I am intrigued by interesting things—like Impact Technologies' Airfoil 5.2 loudspeaker system. When I first encountered the Airfoil system, at the 2001 WCES, it grabbed my attention for several reasons. The first was its strange appearance. It's a four-tower system, but instead of boxes or panels, the tweeter-midrange units are graceful winglike affairs, with a stack of funny curved, pliable, polymer membranes along their leading edges. And the drivers—assuming that's what those membranes were—pointed at the wall between and behind the speakers!

Talking with designer Paul Paddock revealed that those membranes were a genuinely new and patented type of driver (see Sidebar, "Paul Paddock on the Airfoil Bending Wave Driver"). Even the subwoofers—two 12" woofers jammed into an art deco cube—are used in a novel configuration (the subject of another patent application). But the most intriguing thing about the Airfoil system was its sound. Even jammed into a tiny, untreated hotel room, it sounded wonderfully smooth and coherent, and was re-creating a spacious soundstage that completely evaporated the room's walls—and put to shame most of the super systems being shown in more expansive settings.

All that New Technology
The Airfoil's gracefully winglike tweeter-midrange tower actually is constructed like an airplane wing: a curved, veneered surface covers a skeleton of ribs and spars. The towers handle 170Hz up, and each houses nine Bending Wave Drivers. Each driver itself is a thin, clear plastic sheet about 6" tall and 12" wide, wrapped loosely around the wing's leading edge. The front vertical edge of the diaphragm is terminated in a linear voice-coil, and the rear is fastened to the enclosure. The top and bottom edges are unattached. Behind the diaphragms, the cabinet's leading edge is open and filled with damping foam.

At the base of the towers are the Coupling Woofers, which cover the octave from 80Hz to 170Hz. Each channel uses two 6.5" polymer-cone drivers, made by Eminence to Impact's specifications and mounted side by side in a sealed enclosure. A first-order crossover links Bending Wave drivers and woofers.

The subwoofers' drivers are conventional 12" doped paper-cone units, but their Balanced Force Compression loading scheme is unusual. Each subwoofer uses two drivers mounted with their magnets butted up against each other: one fires up, the other down. The drivers are wired in phase, meaning that both cones move outward—away from their magnets—at the same time. But rather than firing into open air, the drivers fire into sealed chambers at the unit's top and bottom. It's the back side of the drivers that communicate with the outside world, through a pair of complex, curved, unequal-length ports that exit through the grille on the tower's front.

The electronic crossover module is pretty straightforward. It has two inputs, with balanced (XLR) and unbalanced (RCA) connections for each. Similarly, the high-pass output—which feeds the user's amplifiers—has both balanced and unbalanced connections. The subwoofer output is balanced only; Impact provides XLO interconnects.

The battery-powered crossover is supplied with a wall-wart charger unit. A front-panel switch toggles between Standby, in which mode the crossover is muted and the batteries are charging, and Operate, which disconnects the charger from the circuit. A nifty LED in the tip of the switch glows red for Standby, green for Operate. Other front-panel controls include Volume, Subwoofer Level, and toggle switches to select between the two inputs and reverse the subwoofer's absolute phase.

System and Setup
During installation, Impact president Mark Conti stressed that the tweeter-midrange units' even horizontal dispersion made their positioning less critical than other speakers', and suggested several different setups. We ended up with the Airfoils firing across my 17' by 23' listening room, the Bending Wave Drivers approximately 5' out from the front wall and about 7' 2" apart, which put them 9' 4" in from the left wall and 6' 6" in from the right. The subwoofers were located behind the towers and slightly inboard, putting the centerlines of their cylindrical enclosures about 2' 8" from the front wall and 6' apart.

My listening position ended up a little farther out into the room than usual—about 2' 6", which put the Bending Wave Drivers about 10' 4" from my ears. The subwoofers were toed in to point directly at the listening position. The towers' unusual asymmetry made it hard to tell exactly where they were pointed, but if I extended lines that bisected their winglike cross-sections, the lines intersected midway between the speakers and about a foot in from the front wall.

I experimented with room treatments as well, and settled on a pretty minimal setup—even though my room is fairly live. I used floor-to-ceiling columns of Echo Busters Bass Busters in the room's corners, Double Buster diffuser panels behind the listening position, and another column of Bass Busters, flanked with Double Busters, on the front wall between and behind the speakers.

I did most of my listening driving the Airfoils with a 70Wpc VAC Renaissance 70/70 or with Classé's 350W CAM-350 monoblocks. Although the two varied widely in power output, they were remarkably similar in their reproduction of dynamic transients, lending support to Conti's assertion that the Airfoil is a very benign load.

Getting Started
You know how you're not supposed to buy a car the first year it's produced, because all of the bugs haven't yet been worked out? Well, the Airfoil is totally new—a new product based on a new technology resulting from a new collaboration between designer and manufacturer. I guess it's really no surprise that Impact has experienced some teething pains; and, as the recipient of one of the very first systems, I was along for the ride.

The first hiccup was a series of delays that stretched over three months as Impact worked out the details of their first production run. When the system finally did arrive, I wasn't terribly surprised to see that it had evolved somewhat from the prototype system I'd heard in Las Vegas. The most noticeable change was that the subwoofers had morphed from compact art-deco cubes into tall cylinders. "The internals are the same," explained Mark Conti. "They're just packaged a bit differently. The cubes were a nightmare to build."

Then I went through three electronic crossovers, each different from the one before. Paul Paddock visited midway through the review period to install new subwoofer amplifiers, running in a new configuration that bypassed much of the original circuitry. The updates included a few other tweaks as well, including replacing the foam damping material in the Coupling Woofers with loosely packed fiberglass cubes. Finally, one of the subwoofer amplifiers expired shortly after Paddock's visit—just before John Atkinson arrived to take measurements, of course. Impact offered to send Paddock out again, but with a deadline fast approaching, we decided to press on with only one subwoofer.

Aside from these growing pains, I had no serious complaints about the Airfoil's operation. Setup required some assembly, but was straightforward and well-documented, and the system did prove to be relatively insensitive to room placement. The Airfoils' fit and finish were excellent, and its appearance—a lovely light maple veneer and black grilles—quite striking. Unfortunately, the electronic crossovers didn't match the rest of the system, being finished in chintzy silver with black oak side panels. Maybe it's just me, but I think that for $35,000, everything should match.

Listening
When I heard the Airfoil 5.2 system at the 2001 CES, I was impressed by its abilities to create a huge, seamless soundstage and to make the speakers and listening room "vanish." In my room, its performance was even more impressive. Its reproduction of a full orchestra and the surrounding hall were, by a significant margin, the most expansive I've heard.

A lot of speakers can re-create solid, tangible images slightly outside their outer edges, but paint increasingly vague portraits from there on out. For example, violin sections will often stretch to just outside the left speaker, but areas of the stage beyond that—the space between the instruments and the sidewall and reflections from the wall itself—will be portrayed more vaguely. Plus, distances are increasingly foreshortened as you move farther outside the speakers.

While the Airfoils didn't replace my listening room with a full-sized orchestra and stage, they did do a noticeably better job than other speakers I've used. Tangible, detailed, full-sized images were solidly located several feet outside the speakers' outer edges. With the Airfoils, those violin sections stretched to well outside the speakers. The stage's outer reaches were better reproduced as well, painted with sharper, more vivid lines and textures.

The same was true in the depth dimension. Instruments at the rear of the stage—like the trumpet at the opening of the Martinon/LSO reading of Shostakovich's Symphony 1 (LP, RCA LSC-2322), or a timpani buried deep in the mix—were noticeably larger with the Airfoils, and more vividly portrayed than through other speakers. With the Airfoils, that trumpet seemed to emanate from a point well beyond my listening room's front wall.

All of this expansiveness came at a price paid by smaller-scaled works. Most recordings of smaller, more intimate performances—solo artists, jazz combos, string quartets—were somewhat larger than life. A friend stopped by one night to listen to a CD she'd just bought, Joni Mitchell's Hits (Reprise 46326-2). After a few songs, she turned to me. "This is really weird. Joni Mitchell's head is 4' tall."

Admittedly, many closely miked performances produce images that are larger than life, and Hits is an extreme example. Plus, as I'd just noticed at the 2002 WCES before writing these words, a lot of speakers produce too-large images. Nonetheless, the Airfoil's images were often unnaturally large, in proportion to the distances suggested by the ambience cues.

A second great strength of the Airfoils' performance in Las Vegas that carried over to my listening room was their incredible coherence. Spatially, there was a solidity and certainty to the continuousness of a given orchestra and recording space. Plus, there was a cut-from-the-same-cloth coherence to individual instruments and voices. Good examples of this coherence were provided by two solo recordings I used in my auditions: Dick Hyman's In Recital (CD, Reference Recordings RR-84CD) and Franz Helmerson's performance of solo cello works by Bach, Hindemith, and Crumb (LP, BIS LP-65). In both cases there was a seamless continuity to the instrumental image that made it feel more natural and made the performance incredibly captivating. More than one visitor commented on the natural way the Airfoils handled these recordings.

One component of the Airfoil's coherence was its freedom from any gross colorations or frequency-response anomalies across the midrange. Orchestras were well-balanced from top to bottom, and instruments that spanned a wide range of frequencies—Hyman's piano, for example—were tonally consistent throughout the midrange.

Regardless of tweaking or associated equipment, however, I never got the Airfoils to really sing at the frequency extremes. On top, they didn't sound as extended and airy as the ribbon tweeter of Magnepan's MG3.6/R, for example, and instruments seemed to soften near the very tops of their ranges. Triangles didn't cut cleanly through the air above an orchestra, and the cymbal on "What a Dif'rence a Day Made," from Ernestine Anderson's Never Make Your Move Too Soon (CD, Concord Jazz CCD-4147), had a nice ring but a lot less shimmer than it should. Also, the subtle circular motions of brush against cymbal were audible, but only as changes in pitch and level, not as clear, distinct motions.

On the bottom end, no amount of fiddling with the electronic crossover controls or subwoofer placement produced low bass of the sort I got with the Genesis 200s or Thiel CS7.2s. The fast bass runs on Fourplay's "Bali Run," from Fourplay (CD, Warner Bros. 26656-2), are one of my standard tests. The Airfoils did a good job, with decent articulation and pitch definition, but there weren't the power and precision that I've heard from speakers like the Thiels. However, my room's inherent ~100-120Hz boost seemed to be ameliorated a bit by the Airfoils, which may have contributed to my perception that the bass lacked a bit of power.

A consistent strength of the Airfoils was that music had a very natural, organic feel and flow. The presentation was always relaxed in the way that live music is, totally free from the sort of sparkle and pizzazz that can sound great in the showroom but can quickly lead to listener fatigue. I'm sure that their continuity and freedom from tonal anomalies was a major contributor. Another, perhaps, could be that the Bending Wave Drivers might have an inherent freedom from resonances—or produce resonances of a different sort than other speakers. Whatever the cause, the Airfoils were able to produce acoustic music with a wonderful, natural ease.

As captivating as that ease was, it didn't fall completely into the plus column. The Airfoils also had a slight liquid texture and a kind of soft-focus perspective on detail and dynamic transients. I struggled a bit with this, because a lot of speakers' razor-sharp details and pinpoint imaging aren't at all natural. And too often, what initially sound like explosive transients are really just artifacts of high-frequency ringing.

However, I'm convinced that the Airfoils did err slightly on the softer, warmer side of neutral. Spatial details were less distinct than they should be, and the sort of inner detail that identifies each instrument in an orchestra weren't as obvious as with other speakers. The LA Guitar Quartet's performance of Boccherini's Introduction and Fandango (CD, Delos DE 3144) usually paints a sharp, 3D portrait of the guitars. With the Airfoils, the picture just wasn't as sharp.

Another piece of the softening was the Airfoils' handling of dynamic transients, which simply weren't as large or precise as with the best speakers I've heard. Introduction and Fandango is a scorching flamenco-style piece. With most speakers, the transients will positively explode, the notes starting and stopping in a sharp transient that impresses a kind of aftershock into the air. With the Airfoils, the explosions just weren't there.

"In the Mood," from The Carnegie Hall Jazz Band (CD, Blue Note 36728), was another good example of the Airfoils' slightly soft perspective. The trumpets just didn't have the brassy bite and edge that they should, and the central solo—which concludes near the very top of the instrument's range—didn't seem to rip the air the way it does with other speakers, or the way it would in a downtown jazz club. On the flip side of the coin, the Airfoils did an absolutely superb job of reproducing the natural space around the instruments on Carnegie Hall, and while the sax's leading edge lacked bite, its woody, meaty character was perfectly portrayed. From my notes: "There's a great sense of space around the sax, and the bass is just right. It's got the right tone, weight...and the way it seems to pressurize the air around it is almost eerie."

As for the Airfoil system's horizontal dispersion and sensitivity to setup and positioning: In a gross sense, their tonal balance and imaging remained remarkably constant as I moved from the sweet spot to positions well beyond the speakers. Nearer the sweet spot, however, the imaging and tonal balance were quite sensitive to movements of my head, particularly from front to back. As I moved—from a few inches to a few feet—toward the speakers, the focus improved, dynamics seemed a little crisper, and the tonal balance shifted upward, to the point of occasionally having a bit of upper-midrange edge. Unfortunately, moving closer to the speakers also resulted in a Cinerama-style image.

I began to experiment with the orientation of the towers, and found that rotating them outward maybe 20 degrees produced noticeable improvements in focus and detail. I was able to get a bit more impact as well, and what seemed like a bit more power in the upper midrange/lower treble. The shifts didn't completely ameliorate the shortcomings I mentioned earlier, but did improve the situation a little. The punchline here is that although the Airfoils may be relatively insensitive to setup in a gross sense—it's easy to get the first 80-90% of their performance—dialing them in precisely is critical to getting their very best. And although the Impact folks were happy with their setup, I'm not certain that I ever heard the speakers at their very best.

The Bottom Line: Big Toy, New Toy, Neat Toy, Better Toy?
The Impact Airfoil 5.2 system is a new speaker technology that is definitely intriguing in its basic approach, appearance, and performance. Although I experienced a few bugs—not surprising for an ambitious all-new effort from a small manufacturer—Impact has built a luxurious, reliable, and impressive new speaker system.

The system's performance was in some ways dramatically different and better from that of any other speaker I've heard. For one, it had a top-to-bottom coherence and balance that gave instruments a natural "rightness" that was captivating. The soundstage was huge, and the Airfoils did a much better job of reproducing its outer regions than I've heard before.

But the Airfoils weren't perfect, or even as good as the "best I've ever heard" in all aspects—although that's exactly the bar set by their $35,000 price. For example, they didn't have the precision and focus of the Thiel CS7.2s, or the weight and power of the big Thiels' bottom end. Up top, they couldn't match the extended, airy treble of Magnepan's MG3.6/R. They could produce the scale of a full orchestra as well as the Genesis or Infinity supersystems do, but lacked those speakers' ability to scale down, to accurately reproduce the size and balance of a more intimate setting.

The Airfoils also had a distinctive natural feel to their overall presentation. Part of this was solidly in the plus column: coherence and freedom from distortion. Another piece of it, however, was a shortcoming: a slight softening due to their texture, lack of focus, and softened dynamics.

Amid all the pluses and minuses, one point that shouldn't be lost is that the Airfoil 5.2 is the first realization of a new technology. My comments reflect comparisons to designs that have been refined and optimized over decades. The Airfoils most definitely do have unusual strengths, and a great deal of potential. But I do think they may be a generation—or a handful of tweaks—away from fully realizing that potential.

Even in their first incarnation, however, the Impact Technologies Airfoil 5.2 was extremely enjoyable to listen to. No one who heard them left without commenting favorably on their performance and on how much they enjoyed listening to them. I urge any serious audiophile to find a set of Airfoils and give them a listen. I know my toys, and the Airfoil 5.2 is one really cool toy.

Sidebar 1: Paul Paddock on the Airfoil Bending Wave Driver

It's rare to find a truly new speaker technology, so I couldn't help but be intrigued by the Airfoil's Bending Wave Drivers. I asked designer Paul Paddock how they work.

"They fall into the class of devices called 'Traveling Wave' drivers. Basically, you're setting up a shock wave with a linear voice-coil along one edge of the diaphragm, that ripples through the membrane."

"So you're setting up a longitudinal wave in the membrane, and as it propagates along the membrane, transverse waves are given off and create sound?"

"Exactly. Imagine a traditional driver, but here, you want it to break up immediately, but in a controlled fashion—there's no pistonic motion. We set up a wave that moves smoothly, and dissipates as it moves around."

The Airfoil towers' winglike shape was "arrived at experientially. The goal was to achieve an even response curve and a smooth polar response. Actually, the back two-thirds of the diaphragm are an energy dump. They really only serve to dissipate the waves, so that they don't reflect off where the membrane meets the enclosure. There's some midrange given off, but most all the action is in the first third."

Paddock laughed when I asked about the membrane material itself. "It's a cousin of Mylar, and used throughout the speaker industry. In the audio industry, you're pretty much limited to what you can get off the shelf. An enormous buy for us represents sample quantities for the plastics industry. But it has to balance a lot of conflicting demands. It has to perform the way you want it to, it has to look good, it can't cost a fortune, and it has to last a long time—and not deteriorate in sunlight. But I don't want to get too tied up in the particular plastic, because I'm always looking at new things that come along—and five years down the road, I may want to change plastics."

"Speaking of years, how long have you been working on the Airfoil driver?"

"On this particular concept, it's bumping up on two years now. The driver first emerged about two years ago. I was trying to do something else, actually—a bit purer approach—but it kept doing what this one does. So I took a hard look at it, and found that it worked quite well.

"There are a lot of other things that play into it as well: the stuffing behind the membrane, the suspension, the strips attached to the driver to control vibrational modes...there are all the same things that you have in traditional cone drivers, a host of competing requirements."—Brian Damkroger

Sidebar 2: Specifications

Description: Five-piece, three-way, loudspeaker system with powered subwoofers. Drive-units and configurations, per channel: Main unit: 9 Airfoil Bending Wave Drivers stacked vertically in line-source array, two 6.5" polymer-cone Coupling Woofers in sealed enclosure at base of line-source tower. Subwoofer: two 12" doped paper-cone woofers in Balanced Force Compression Design (back to back) in freestanding cabinet. Passive crossover, coupling woofers to towers: 170Hz passive first-order, two-way series network. Active crossover, subwoofers to main units: 80Hz low-pass, 24dB/octave; 80Hz high-pass, 24dB/octave. Frequency responses: Main unit, 80Hz-20kHz, ±1.5dB; subwoofer, 17Hz-80kHz, +1dB/-3dB. Impedance: 8 ohms nominal (main unit). Sensitivity: 90dB/2.83V/m (main unit). Subwoofer amplifiers (per channel): 1000W class-B with switching power supply. Electronic crossover: battery-powered, AC charger unit supplied. Inputs: 2 balanced (XLR), 2 unbalanced (RCA). Outputs: high-pass, 1 balanced (XLR), 1 unbalanced (RCA); 1 subwoofer, balanced (XLR), dedicated cables supplied.
Dimensions: Main units: 73.5" H (1880mm) by 18" (460mm) W by 23" (600mm) D. Weight: 110 lbs (50kg) each. Subwoofers: 33.5" (860mm) H by 19" (490mm) W by 18" (460mm) D. Weight: 55 lbs (25kg) each. Electronic crossover: 2.5" (640mm) H by 17" (440mm) W by 15" (380mm) D. Weight: 15 lbs (6.8kg).
Finishes: Oak, curly maple, cherry and rosewood, mahogany; black subwoofer grilles.
Serial number of system reviewed: 601-107.
Price: $35,000/system. Approximate number of dealers: 7. Warranty: 1 year, transferable (with manufacturer's consent).
Manufacturer: Impact Technologies, P.O. Box 616, Ambler, PA 19002. Tel: (215) 653-7440. Fax: (215) 653-7441. Web: www.membrane.com/impact.

Sidebar 3: Associated Equipment

Analog source: VPI TNT Mk.V-HR turntable, Grado Signature Reference cartridge.
Digital sources: Simaudio Moon Eclipse, Wadia 861 CD players.
Preamplifier: VAC CPA1 Mk.III.
Power amplifiers: VAC Renaissance 70/70, VTL Ichiban, Mark Levinson No.20.6, Classé CAM-350.
Cables: Interconnect: Nirvana S-X Ltd. Speaker: Nordost Valhalla. AC: Synergistic Research and Reference Master Couplers, AudioPrism.
Accessories: MIT Z-System, Z-Center and Nirvana AC systems, AudioPrism Noise Sniffer AC analyzer and Quietline filters; VPI 16.5 record-cleaner, Decca/Hunt record brush, Sumiko Fluxbuster, Dennesen Soundtractor, Shure stylus-pressure gauge, Immedia Needle Nektar stylus-cleaning fluid, Nordost ECO3 and Music Fidelity DiskSolution CD treatments; Sheffield/XLO Test & Burn-in CD, Stereophile Test CD, Test CD 2; Bright Star Rack isolation systems, Tiptoes; Echo Busters room-treatment products.—Brian Damkroger

Sidebar 4: Measurements

Given the Airfoil 5.2 system's bulk, it made sense to take advantage of a road trip I was making through the Southwest last fall to measure the speaker in Brian Damkroger's Albuquerque listening room. The advantage of this was that I could actually get a handle on the speaker's in-room behavior, which its unusual design made imperative. The disadvantage was that I couldn't take all the measurements that I usually do in my Brooklyn test lab. (Because of my need to keep my traveling rig as portable as possible, I used the Mitey Mike II from Joe D'Appolito rather than my usual B&K 4006. All measurement were performed using DRA Labs' MLSSA system.)

Because I had to test the speaker in-room, I could get only a rough idea of the 5.2's voltage sensitivity, which appeared to be around 80dB(B)/2.83V/m—considerably below the specified 90dB figure. However, this will be ameliorated in-room both by the Airfoil's line-source behavior and by its wide horizontal dispersion.

The main Airfoil tower's plot of impedance magnitude and phase is shown in fig.1. The speaker is a moderately demanding load, with an impedance dropping below 4 ohms for much of the treble region, and reaching a minimum value of 3 ohms just below 2kHz. Above that frequency the impedance gradually rises, presumably due to the Bending Wave driver's voice-coil inductance. While the impedance phase angle reaches an extreme value of 67.3 degrees capacitive at 66Hz, the magnitude at this frequency is a fairly high 13.9 ohms, which will mitigate the effect of the phase.

Fig.1 Impact Airfoil 5.2, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Deciding on what axis to measure the speaker's frequency response was rather arbitrary, given the nature of the drive-unit. In the end, I chose an axis perpendicular to the driver at the point where it is driven by the voice-coil. (With the speakers as set up by the manufacturer, this was the axis that actually pointed toward Brian's listening chair.) The result at a microphone distance of 50", averaged across a window 15 degrees to either side of this axis, is shown to the right of fig.2. The peaks and dips in the lower midrange are due to room effects. (I used a pink-noise signal to assess the response in the crossover region between the Bending Wave driver array and the two coupling woofers.) The suckout centered at 600Hz is not a room effect, but appears to be characteristic of the speaker. It is very position-dependent, however, and did not affect the Airfoil's measured room response at the listening position (see later).

Fig.2 Impact Airfoil 5.2, anechoic response on listening axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the nearfield woofer and subwoofer responses plotted below 400Hz and 280Hz, respectively.

Eyeballing the trace in fig.2 indicates that the Impact's response gently slopes down as the frequency increases. This will be partly due to the Proximity Effect—the speaker's radiating dimensions are not small compared with the speaker-microphone distance, which means that, even at 50", the mike is still in the nearfield. But it's possible that the top octaves are also depressed in absolute terms—BD did comment on the depressed top octaves in his auditioning notes. And note how ragged they appear; above 5kHz or so, the Bending Wave driver appears to be operating in breakup mode.

To the left of fig.2 is shown the nearfield response of the powered subwoofer module. It exhibits the classic bandpass behavior of a coupled-cavity system, except that it rolls off at 24dB/octave above and below the passband rather than the usual 12dB/octave. Because the subwoofer is powered, its level in this graph has been arbitrarily chosen to give a crossover point at the specified 80Hz.

The Airfoil's lateral dispersion over the ±15 degrees window used to derive fig.2 is shown in fig.3, with the response differences on the drive-unit side (facing away from the listener) to the graph's rear. You can see that the depth of the 600Hz suckout depends very much on the axis, and worsens toward the speaker's rear. Note also how position-dependent the pattern of peaks and dips in the top two audio octaves is—again, the bending-wave drivers are operating in breakup mode at these frequencies and the overall in-room effect may well be quite flat. In the vertical plane (not shown), the line-source behavior of the drive-unit array means that there are very few response changes as the listener moves up and down. (The curves in figs. 2 and 3 were taken at the exact midpoint of the nine-element array, 42.5" from the floor.)

Fig.3 Impact Airfoil 5.2, lateral response family at 50", from back to front: differences in response 15 degrees-5 degrees off-axis on drive-unit side, reference response on listening axis, differences in response 5 degrees-15 degrees off-axis on baffle side.

Fig.4 shows a 1/3-octave-smoothed response, spatially averaged in a grid 36" wide by 18" high centered on the position of BD's ears in his listening chair, with the speaker towers and subwoofers in the positions used in his auditioning. The first thing to note is how smooth the trace is—many of the peaks and dips visible in earlier graphs are so position-dependent that they even out with the spatial averaging. And note how extended the deep-bass response is; in-room, the Airfoil subwoofer's output is down by just a couple of dB at 20Hz. But note, too, the broad peak in the upper midrange and the generally suppressed top octaves. I know BD didn't find the Airfoil to sound colored, but this measurement reveals that it is not a neutrally balanced transducer.

Fig.4 Impact Airfoil 5.2, spatially averaged, 1/3-octave-smoothed, farfield response in BD's listening room.

I must admit that, listening to music, I didn't hear nearly as much emphasis of the upper midrange as I would expect from this graph. There was also an astonishing separation of the acoustic objects on the recordings we auditioned from the speakers' physical locations. Having an array of identical drive-units reproducing everything from the lower midrange up certainly has a lot going for it.

In the time domain, the Airfoil's impulse response (fig.5) is time-coherent and in the correct positive polarity, though the tail of the response is overlaid with the ringing associated with the upper-frequency peaks seen in the amplitude-response graphs. The step response (fig.6) is essentially perfect, with a steep rise away from the time axis followed by a good right-triangle-shaped decay.

Fig.5 Impact Airfoil 5.2, impulse response at 50" (5ms time window, 30kHz bandwidth).

Fig.6 Impact Airfoil 5.2, step response at 50" (5ms time window, 30kHz bandwidth).

The cumulative spectral-decay or waterfall plot of the Airfoil's bending-wave array (fig.7), windowed to eliminate reflections of the sound from the floor, ceiling, and walls of BD's listening room, is a mess, with multiple ridges of delayed energy visible throughout the treble. To be fair, this behavior appears to be typical of speakers using planar or other unconventional drive-units, and in the past I have wondered if what you see in graphs like this is really the effect of multiple arrivals at the microphone position. Certainly the Airfoil's treble doesn't sound grainy or lacking in clarity, which is what you'd expect from a conventional speaker offering a waterfall plot like fig.7.

Fig.7 Impact Airfoil 5.2, cumulative spectral-decay plot at 50" (0.15ms risetime).

Summing up, from the evidence of its measured performance, the Airfoil 5.2 is an enigma.—John Atkinson