"A Matter of Diffusion"

by Keith Yates

Like many Stereophile readers, I have often sped home from a concert to fire up the audio system and then, to the sore vexation of my wife and guests, spent the rest of the evening plunged in the morbid contemplation of what, exactly, was missing.

Every lover of music has his pet theories and every theory, it seems, its season. Speaker efficiency and wide frequency response were popular in the 1950s, during the heyday of those hulking Klipschorns and James B. Lansings. The Quad ESLs caught on a few years later, when midrange transparency seemed to be what separated hi-fi from the real McCoy. In the late ’60s Amar Bose of MIT concluded that the ratio of direct to reflected sound was lopsided, and redressed the ostensible imbalance with his model 901s.

Jon Dahlquist, Richard Vandersteen, and others converged on loudspeaker time and phase discontinuities in the ’70s. Up in Canada, Mike Wright reworked the textbook electrostatic speaker for maximum bass slam and dynamic scaling; he had to seal the resulting apparatus in a fragile shroud of sulphur hexafluoride gas to keep it from arcing itself to death. Near Los Alamos, physicist Alan Hill blamed excessive moving mass; he weighed in with a near-massless tweeter comprised of a helium-bathed plasma that expanded and contracted in mid-air at audio frequencies.

While Pass (Threshold), Iverson (Electro Research), Curl (Mark Levinson), Otala, and others tackled amplifier linearity problems, over in Scotland Ivor Tiefenbrun impishly insisted the front end was to blame, eventually elevating the turntable in general and the Linn Sondek in particular to new import.

Without wishing to deny the aforementioned trailblazers their due, it is my lot to report that a peculiar-looking device called a “quadratic residue diffusor” leaves little doubt that what largely separates the listening room from the concert hall is the acoustic “fingerprint” of the listening room itself.

Most anybody could have told you so (my grandma for one: “Can’t put a gallon in a quart jar, son”), but I’m here to say these diffusors represent a powerful new way to address the fact. They’re already turning up in recording studios and some concert venues, and once you’ve heard them, as you will, in the soundrooms of dealers and the living rooms of fellow audiophiles, you’re likely to accuse your own listening chambers of having heretofore obstructed your very pursuit of the high end—of having bludgeoned sonic detail, mocked correct instrumental timbre, and thwarted the replication of concert-hall soundfields.

The concept of diffusion will be new to most audiophiles. By and large, we’re accustomed to thinking that sound striking a surface can be either reflected (as when a billiard ball bounces off a side cushion) or absorbed (as when it drops into a pocket).

Think of diffusion this way: The billiard ball strikes the cushion and is instantly shattered into sawdust-sized microballs that scatter across the whole table. You need reflection and (especially) absorption to win at billiards; to get your living room to sound like a symphony orchestra playing in a good concert hall, you’ll need diffusion, too.

To grasp why, consider that, at your ear, the arrival schedule of acoustic reflections supplies the brain the information it needs to get a “fix” on the listening room: its approximate dimensions and geometry, and to some extent the treatment of wall surfaces and other large objects.

Let’s assume that, with software of appropriate quality, your audio system successfully resolves the nuances of timbre, imaging, and acoustic space that mark the recording site as Carnegie Hall. What happens in your listening room? The ear-brain is presented with two conflicting acoustical field reports: Carnegie Hall on the one hand and your living room on the other. Maybe if you trained yourself long enough, you could become deaf to the living-room layer and experience only the Carnegie Hall layer. (If so, I promise you’ll be the subject of great curiosity among psychoacoustics researchers.) At best you’ll learn to live with the dichotomy you’ve got.

Theoretically, diffusion chips away at the listening-room layer in several important ways. First, it tampers with the arrival schedule of normal room reflections in ways that make it hard for your brain to get a clear fix on the listening-room layer. In an untreated room there will be clearly defined energy spikes over time as the sound ricochets off hard surfaces and arrives at the listening position. The position of these spikes on the time track is what tips off the ear-brain as to the location of side walls, ceiling, floor, and other large planar surfaces. Increase the distance between your speakers and room boundaries and you move the spikes further down the time track, away from the direct sound. A strong energy spike arriving 15ms after the direct sound suggests a reflecting surface, say a side wall, nearby. Because sound travels about a foot every millisecond, the reflected path length is only about 15′ longer than the direct path length, putting the speaker rather near the reflecting surface. (If the time gap between the direct sound and the first reflected spike is less than 10ms, the ear-brain processes the information as a disruption, or shift, in image.)

Place a diffusor at the reflecting site and you scatter that energy across a very broad angle (functionally 180 degrees), meaning only a very tiny fraction of the reflected energy will be steered directly to the listening position. Deprived of the clear 15ms spike, the ear-brain should not be able to accurately “fix” the location of the side wall.

Secondly, the diffusor breaks up irritating slap and flutter echoes that often plague playback in domestic listening environments. These echoes—artifacts of reflective, parallel room surfaces—overlay the music that follows and obscure its inner details.

Thirdly, the diffusor tames comb-filter-type frequency colorations, previously addressed only by laboriously tweaking the physical position of the speakers relative to room boundaries. By decreasing the depth and increasing the density and irregularity of comb-filter notches, the diffusor makes optimum speaker placement decidedly less critical.

The fourth and potentially most remarkable benefit of diffusion is that the flooding of the listening room with diffuse sound fosters a sensation of complete envelopment or immersion in a musical event, as opposed to the disengaged puzzling over sonic minutiae that too often passes for the audiophile listening experience.

In sum, with adequate diffusion, you can theoretically get a small listening room to sound rather like a very good concert hall because, psychoacoustically, all the same basic ingredients are there: a large enough time gap between direct sound and first reflected spike; lack of slap and flutter echoes and comb-filter colorations characteristic of small rooms; and the envelopment that comes from a dense, smoothly decaying reverberation that permeates the whole room.

The trick to designing an efficacious diffusor is to get it to exhibit no prejudice for frequencyamplitude, or angle of incidence or reflection. This is not an easy task, for physical structures generally operate, acoustically, only on narrow slices of the frequency band and within a restricted range of incidence angles, making them effective only for a small percentage of the total sound energy impinging on them. To be sure, recording studios and the like have relied on large polycylindrical diffusors for years, but these have been cumbersome, restricted in frequency range, and impractical in domestic environments. The upshot is that diffusion—at least up until now—has been more a theoretical solution than a practical one.

What amounts to the first commercially viable broad-range diffusor is the main enterprise of RPG Diffusor Systems of Largo, Maryland. Working off the findings of Dr. Manfred Schroeder, the German mathematics professor and Bell Labs researcher who conceptualized an acoustic “reflection phase grating” (RPG) device a few years ago in the Journal of the AES, the RPG is a 2′ by 4′ by 8″-deep affair of wood and aluminum.

For all the higher math involved, it is a low-tech-looking affair comprised of thin aluminum slats defining wooden “wells” of constant width (about 31/2 inches) and varying depths. The exact depth of these wells and their sequence are what, according to the literature, imbue the RPG with its nearly ideal diffusion characteristics. The RPG diffusors I tested cost $295 each, and are available through RPG directly at 12003 Wimbleton Street, Largo, MD 20772.

The Set-Up
Coached over the phone by RPG president Dr. Peter D’Antonio, I deployed 24 of his model QRD(R)-734W diffusors in my store’s main soundroom, a purpose-built facility some 19′ wide, 23′ long, and 10′ at its highest point, the ceiling being a coffered affair. The wall directly behind the speakers (hereinafter called the front wall) is a 55-ton construction of concrete, cork, and sand; it is well over 2′ thick and nearly 20′ high (the upper floor is used for storage). Walls and ceilings are of conventional 5/8″ sheetrock nailed to 2×6 studs. The room is of the “single-speaker demonstration” variety, meaning that speakers not under direct audition are normally ensconced in a separate, acoustically isolated closet to prevent interaction with the pair being played.

Over the years the room has seen a fair amount of acoustical experimentation, including full Sonex treatment on the front wall, dozens of ASC tube-traps crammed into corners, and half-rounds scattered around side walls. For critical auditioning, 6 RPGs were elevated about 2′ off the floor and screwed into the front wall; up to 7 were set along each side wall; and up to 12 were stacked in a 16′ by 6′ monolith a few feet behind the listening positions (see diagram).

It was an imposing sight, to be sure. A wide variety of high-grade components was used during more than a full year of listening evaluations, beginning in late 1986. Front-ends included a Goldmund Studietto/T5 ‘table/arm set-up with Linn Karma or Koetsu Black Gold Line cartridges; a Linn Sondek/Ittok with Grace F9E Super, Linn Troika, or Linn Karma; several Michell Gyrodecs with Zeta, SME, or Eminent Technology tonearms fitted with Madrigal Carnegie, sundry Koetsu, or Grado Signature series cartridges; and The Source turntable from Scotland, fitted with SME V and a Carnegie One. Original master tapes and dubs, played on a Don Alley-modified Revox B77 and a stock Tandberg TD20A-SE, were also used on occasion.

On the digital side, CD players seeing the most action were the Meridian 207 Pro, Nakamichi OMS7A MkII, Tandberg 3015A, and California Audio Labs Tempest Revised; an Accuphase DC80/DP81, Kinergetics, and Stax Quattro were also used for shorter times.

Electronics included the Mark Levinson ML7A and prototype No.26 preamps; Levinson No.23 and No.20 Class A monoblock amps; the full assortment of Conrad-Johnson Premiers; a Spectral DMA-100; the Jadis JA-30 class-A monoblocks; Bryston 3B and 4B; and various loaner or trade-in electronics from Audio Research, Krell, Cello, Rowland, and Accuphase.

I used a range of speakers familiar to most Stereophilereaders: the Apogee Diva, Scintilla, and Duetta ribbons; Quad ESL-63 electrostatics; Magnepan MGIIIa planar magnetics; and KEF 107, Vandersteen 2C, Spendor SP1, and Spica TC50 electrodynamics. I also included two new electrodynamics that may be unfamiliar to most: the Waveform Loudspeakers and Booth Curvefront IIs. Although many others were tried as well, I shall restrict my comments to the aforementioned, as they cover a broad and representative range of operating principles, polar patterns, tonal personalities, and, as it happened, results.

For the formal listening sessions I settled on a Gyrodec turntable fitted with SME V tonearm and Carnegie One cartridge feeding the Levinson ML7A preamp (L3A phono modules) and Levinson No.23 power amp. The amplifier was selected after much care because it was found to be particularly well suited to the test, displaying both the iron grip to deal firmly with the unruly Scintillas, and the finesse to coax the Quads and Spicas to their delicate best. Cabling was by Monster, MIT/Brisson, and Belden.

I brought in my recording engineer, John Oster, to record the sound of the system playing both with and without the RPGs deployed. A pair of the new B&K omni mikes, placed at the listening position, fed a modified Revox B77 reel-to-reel and a Nakamichi Dragon cassette deck. Later I went back and “A/B’ed” the room with and without the diffusors by replaying the tapes over Stax Lambda Pro electrostatic headphones and comparing my original notes.

Although the more controlled part of the test involved evaluating the diffusors with speakers of various types, it is worth noting that substituting any of the other source components, electronics, or cables for those in the reference playback system had little or no effect on results found for the RPGs. In other words, there was no case in which a different combination of source components, electronics, and/or cables produced results that were at odds with those produced with the reference system and described in this overview.

However, the RPGs did alter the sonic environment in ways that often made it easier to get a handle on the sound of a particular component; they illuminated certain equipment characteristics that previously lurked only as vague impressions. I suspect this was due to the fact that the diffusors remove sonic grundge (comb filter and echo effects) that otherwise masks some of the subtleties of certain components. At any rate, the fact that the diffusors did not turn component preferences topsy-turvy is interesting.

The Particulars
I’ll begin with the clearest success story: the KEF 107s. In the untreated soundroom the much-praised 107s betrayed a blare and honk that deepened my appreciation for unalloyed silence. After finding the speakers and the supplied equalizer (the KUBE) to be in proper working order, I got a feeling worse than buyer’s remorse: I felt I’d been saddled with a patient with severe personality disorders—and one that was hard to move, took up valuable floor space, and cost me a fair chunk of change into the bargain.

When the diffusors were deployed it was as if the patient had undergone electroshock therapy. A ratty blare gave way to a sweetness and harmonic balance of which I had never thought the speaker capable. Violins and violas shed their pinched, hollow sound; and the brass—jeeminee, I about wore out the preamp’s mute switch every time they started that insufferable blatting and hooting—actually bore a commendable similitude to the real thing. The soundstage virtually uncorked itself, opening up in all dimensions, and the clumping of instruments around the speakers gave way to a handsome delineation of placement along all three axes—left/right, front/back, and up/down.

The whole room seemed acoustically and infectiously energized. Especially on orchestral, big band, and Dire Straits-type pop music, the effect was big, fleshy, and compelling. If not in quite the same league as the admittedly much more expensive WAMM, Infinity IRS, or Apogee Diva systems, the 107s began to sound like they deserved some of their glowing press and word-of-mouth accounts. During one listening episode, a customer who had heard the 107s in the room’s pre-RPG days walked in and asked, incredulously, “What did youdo to those things?”

I’m going to lump the Apogees and Magnepans together—ah, what strange bedfellows acoustics make—not because I’m ignorant of their political differences, but because the diffusors are. Being dipolar line-source designs, both are sensitive not only to rear and side wall proximity, but to the physical attributes of those walls as well. Highly absorptive surfaces like Sonex or Owens-Corning acoustical panels rob the speakers of their characteristic lively, expansive sound, while plain reflective walls can impart a “slap” that creates an interesting but sometimes unnatural spatial effect. If you want the best from them, you seem forever consigned to fussing with their placement.

With diffusors in place, the hard slap gave way to a more evenly distributed foldback of the rear wave into the room. Here I mean “evenly distributed” in both the tonal sense (viz, no erratic or spiky colorations) and the spatial one (e.g., no more listening-position hot spots).

The Divas offered a ferocious grip on the music coupled with a rich ambience—almost a “wraparound” effect—that seemed to deposit the listener nearer the stage, but in a larger and more acoustically inviting space. It seemed the overall energy level had been turned up.

The change affected the very way one experiences music. The urge to dissect the sound gave way to just basking in it. Recordings like Gary Karr’s Albinoni Adagio, the old Rachmaninov Symphonic Dances on Vox Turnabout, and Ernest Bloch’s Schelomo on Mercury Living Presence unfurled a concert-hall acoustic you wanted to reach out and touch. Once-submerged details of orchestration and phrasing were presented with a tangibility that often brought a new sense of proportion and wholeness to complex orchestral music.

On many recordings, music flowed with a rolling or “speaking” quality with the Diva/RPG combination. Certainly I have never heard Apogees anywhere to rival the hall-type glow that music regained with the Divas played in the diffusor-treated soundroom. With the Scintillas, Duettas, and MGIIIas, the diffusored systems improved markedly, yet it must be said that never were the speakers “transformed” into sounding like something else—such as Divas or Quads or Acoustats, for example. Rather, they were clearly themselves, minus some familiar warts, and stretching out in a friendlier and more exciting space.

With all three, musical images appeared to move off the surface of the speakers and into the room, where they assumed more realistic shapes and positions. It should be noted that the diffusors also ameliorated a dry middle-treble range on the MGIIIas, yielding a fuller, more harmonically natural lighting to violins, violas, clarinets, and trumpets. Owners of the various Apogees and Magnepans (and other dipolar line sources) will likely find that a cluster of RPGs on the walls behind and around the speakers will facilitate placement of these otherwise fussy monoliths.

In the untreated soundroom the Booth Curvefront IIs—$3500 hand-finished beauties resembling the Dahlquist DQ-20s or the larger Keveks and employing a choice selection of Dynaudio drivers—had suffered a lack of top-to-bottom tonal consistency that bordered on discombobulation. Powerful and “physical” nearly throughout their range, they unfortunately never allowed one to settle in and unwind with the music. They displayed an unhappy knack for taking a recording with a full soundstage, collapsing it into two small boxes, then megaphoning what was left of it into your face.

With the RPGs, the Curvefront IIs were a somewhat different animal, with a generous and sometimes downright comfortable presentation, and at least hinting at an octave-to-octave seamlessness that had previously seemed far away indeed. The megaphoning gave way almost to a dipole-like ability to illuminate all corners of the room effortlessly. A nasal, ringing coloration subsided, bringing a more natural timbre to male voices of all types, from Fischer-Dieskau and Hermann Prey to Johnny Hartmann and Mick Hucknall (Simply Red).

The change—”transformation,” rather—was strikingly effective, and again I suspect that, given normal, sheetrock sidewalls, the Booth’s significant off-axis energy output had previously exaggerated and distorted their true midrange potential.

The Quad ’63s have always seduced more than impressed. By enlarging and enlivening the room acoustic, the diffusors rendered the Quads more extroverted, more “commanding” and less reticent. Oh, they’re still seducers, these, but they’re now seducers of a saucier type. On Chicago Pro Musica’s Till Eulenspiegel(Reference Recordings), the effect was one of lusty and infectious merriment, altogether closer in spirit to this ensemble’s live performance of the piece.

The specificity of acoustic images the speakers suspended in the room (always a forte with Quads) was complemented by a wider, deeper soundstage, a decidedly less dry-sounding acoustic, and an addicting sensation of being in the middle of the action.

Listening analytically, you’d have to say the changes were almost purely spatial or related to power level (the speakers often sounding as if they’d been harnessed to a more assertive amplifier). This is probably because, even in the untreated main soundroom, the ’63s seem miraculously free of tonal aberrations, grundge, detail smudging, or other problems—problems that the RPGs had a knack for mitigating with other speakers.

Listening naturally (emotionally), though, the RPGs seemed to represent a giant step toward the real, live experience of participating in the original music-making event. This was heard on many recordings, independent of genre or technical quality. Perhaps the somewhat different nature of sonic improvement with the ESL-63s has to do with their unique, electronically steered directional characteristics: The speaker develops relatively little energy much off-axis, largely to minimize unpredictable interaction with nearby surfaces. At any rate, the spatial realism afforded by the RPGs seemed to energize the “retiring” Quad personality in an altogether useful and flattering way.

With or without the companion powered subwoofer (the 2W), the Vandersteen 2Cs have always sounded coherent and believable in the main soundroom. Pushed to around 98dB or more, though, the upper midrange/lower treble begins to “trumpet” and coarsen, an effect owing to either limitations of drivers or room acoustics, I could never clearly determine. The RPGs revealed the blame to be with the room, not the speakers. With the full diffusor complement the Vandersteens showed that they can be pushed quite hard in a large-ish room without distress. (A good test for this is the Vaughan Williams Tallis Fantasia, Nimbus CD 5019.)

The diffusors did not seem to alter the specificity of the 2Cs’ already sharply drawn images, but they wrapped these images in a larger, warmer, and more alive space. They also suppressed a slight “a” (as in “cat”) coloration in the speakers, previously evident at moderately loud levels and up.

These were unambiguous improvements: On a purely commercial level, we found sales of 2Cs to increase dramatically during the evaluation period. The diffusors had no detectable effect on the sound of the matching subwoofer, but did reveal the supplied crossover unit, with its 8-switch blocks, to be wholly unsatisfactory. (Vandersteen suggests using it only to establish the correct capacitor value, then ordering that specific value in a single-value, in-line package.)

Imaging on the Spendor SP1s has always been strong at center-fill, even without toeing-in; with the diffusors in place the space around instruments seemed to expand literally, well past the side walls. The speakers seemed to energize the room’s entire air space, not just the area between and behind the speakers themselves. As with the Divas, Vandersteens, and Quads, the diffusors often seemed to escalate the overall musical energy level in the room. The diffusors did not alter the SP1s’ characteristic upper-bass “plumminess,” which probably issues from the speaker’s thin-wall cabinet construction rather than dispersion problems or room interaction per se.

The Spica TC50s fared similarly, sounding far bigger than they look, and again seeming to drive the 4500 cubic feet of listening room with relative ease. This effect was also clearly in evidence when I substituted a modestly powered integrated amp, the British Fidelity A1. With diffusors, treble reproduction sweetened tonally and deepened spatially.

Now we come to the puzzler—the $17,000 Waveforms. Looking like nothing so much as a chopped-and-channeled pyramid in the Art Deco style, this 200-lb. bruiser is based on research done at the National Research Council’s facility in Ottawa, Canada. Furniture designer John Otvos—a charming young fellow from the old school—decided to leave his imprint on the speaker architecturally rather than sonically. For the sonics he sought Paul Barton of the NRC, requesting a design that, on the NRC’s kind of controlled, double-blind listening tests, would consistently outscore all competitors. No constraints were placed on driver technology, parts costs, or enclosure design.

Barton’s prescription is surprisingly conventional: a 15″ woofer, two 6-inchers, a soft-dome tweeter, and ribbon supertweeter. An active crossover is supplied, and the speaker must be biamped.

This was the first time Otvos had played the speakers in a US dealer’s showroom, and he worried aloud about the effects the RPGs might have on the sound. Having experienced positive results with all other speakers, and having considered the supporting technical rationale, I wondered if Otvos wasn’t voicing a preliminary objection in order to later dismiss the auditioning results if they turned out unfavorable.

We played the speakers—the Levinson No.23 on top, the stout Bryston 4B on bottom—and found that Otvos had not worried in vain.

But let me give you the good news first. Without the RPGs, the Waveforms had the upfront, grab-you-by-the-glands presence that lifted you from mid-hall to the conductor’s platform. Music sounded zesty and sure-footed, an effect like that of a crack orchestra triumphantly returning from a long road trip to play a favorite old lollipop for eager hometown supporters. Bass was generous, lively, yet relatively free of overhang. Up to what I would guess to be 100dB or so, dynamic gradations were first-rate, harking back to those mammoth designs of the JBL era. (It turns out those 15″ woofers issue from that very firm.)

The Waveforms set up a nicely delineated lateral soundstage, receiving markdowns for a foreshortened and somewhat clouded front-to-back perspective. I quibbled with some lower-treble peakiness, caused, it later turned out, by a minor miscalculation in the crossover, and probably exacerbated by those side walls. Yet on balance the musical effect was one where the whole was greater than the sum of the parts. Here then were speakers for the sybarite looking for a rollicking good time from his music, and who will trade away some finesse to get it.

With the RPGs, however, the speakers sounded tidier but strangely less involving. The sound lost much of its spirit and hence its fun. Although I was more engaged acoustically—immersed in the soundfield, really—emotionally I was oddly disengaged. This effect was commented upon by other listeners who were not predisposed by Otvos (or me or my staff) to the Waveforms’ “unsuitability” with the RPGs. The soundstage became larger and more credible, and even the lower-treble peak seemed to lose its edge, but these were “improvements” of an oddly unflattering type.

After living with the Waveforms and RPGs for some months, I called “Dr. Diffusor” D’Antonio about it and he, too, was at a loss to account for the effect. Otvos could offer nothing more than the observation that the speakers were designed to be used in a fairly reflective living-room-type environment. But there you have it: a case where the untreated room “outperforms” the same room with the full RPG complement.

The Big Picture
Without question, the diffusors did more to upgrade the overall musical experience in the main soundroom than any previous mitigation measure, whether it happened to be installing 4’x8′ Sonex sheets on the front and forward walls, standing Tube Traps in corners, or isolating all undriven speakers in the separate holding area.

The reduction in sonic grundge, the exposure of previously obscured inner detail and interleavings, the marked sense of envelopment within a musical event, and the perception that the walls had been blown down all contributed to a heightened sense of involvement in the music-making.

The results obtained in the main soundroom were duplicated in a smaller adjacent soundroom as well. This room has a low ceiling and a somewhat unpleasant sound, mitigated (partially) by the installation a few years ago of Owens-Corning floor-to-ceiling acoustical wall material on the front wall.

When the diffusors were “parked” in this room (during the times we were “A/B’ing” the main soundroom), the sound was transformed in ways consistent with my descriptions of the treated main room.

Final Thoughts and Remaining Questions
Because all of us audiophiles—consumers, retailers, manufacturers, critics—are accustomed to seeing audio “solutions” take the more familiar form of devices that can be set on a shelf, plugged in, auditioned, paid for, and carted home in the back seat of the car, there is a chance the diffusor may not get the real “hearing” it so richly deserves. During the course of my year-long evaluation, a prominent high-end firm informally introduced the diffusors to select dealers at Chicago CES. The feedback they got suggested that most audiophiles would not purchase anything as unwieldy, expensive, and aesthetically unflattering as the RPG units, despite clear-cut and often dramatic sonic improvements.

I hope this pessimistic prediction proves untrue, for it seems unlikely we’ll have much meaningful audio progress unless and until we deal with the fundamental acoustic limitations of the average listening room. I would also like to point out that in order to reduce the variables in the test, I avoided mixing in other acoustical treatments, such as Sonex or ASC Tube Traps, with the diffusors. Yet it is almost a surety that the diffusors will work best as part of an overall listening-room overhaul that would include products like the Tube Traps in corners (for dipolar speakers), and broadband absorbers against the front wall and forward portions of side walls for most forward-firing designs. The diffusors seem to have little or no effect in the sub-150Hz region where Tube Traps can be effective in “equalizing” pressure buildups in room corners. I would like to experiment with diffusion and/or absorption on the floor and ceiling (to scatter or kill those strong and largely monophonic early reflections) and Tube Traps in the corners.

In the meantime, RPG has introduced their own line of additional products, including the ABFFUSOR(R), a broadband absorber offering some diffusion, and the TRIFFUSOR(R), an equilateral triangular prism that can be rotated to expose absorptive, diffusive, and reflective sides. (The TRIFFUSOR(R) is also available as the Korner-Killer[tm], which is one diffusive and two absorptive sides, for “Tube Trap”-type effects.)

There is the issue of whether similar results could be had without resort to brute-force acoustics, say with active electronic manipulation a la surround-sound or time-delay systems. My experience to date is that existing aftermarket components do not offer the same degree of blow-down-the-walls enhancement because they leave intact the normal acoustic cues that aurally define the small room; they only superimpose another set of aural cues that suggests a bigger room. I wonder if the mental “dissonance” of receiving several sets of acoustic cues might not lead to confusion and perhaps listener fatigue for some. With existing electronic enhancers, the time interval between the arrival of direct sound and its first reflections remains that of a small room; the denser and longer reverberant tail of the sound suggests a bigger room, but for me it suggests more than it persuades.

Undoubtedly units like the Yamaha DSP-1 six-channel system will evolve, opening up at least the (admittedly remote) possibility of some day getting RPG-type improvements electronically.

There is no question of the diffusors’ offering good value: few component upgrades or acessories can approach their price/performance ratio. There remains, though, the question of whether mass production could significantly lower the cost of the RPG units to the consumer. As it is, you could “diffuse” a couple thousand dollars in RPG’s direction to get the results I’ve described in a typical 12’x18′ listening room.

I asked D’Antonio whether he’d considered moving to an injection-molding process instead of time-consuming assembly of furniture-grade wood slats and metal strips. “Yes,” he said, “but at the end of the day, you have one configuration, one color, a fire-rating problem, and an acoustically diaphragmatic entity. Consequently, we made a conscious decision to use wood, a material which has endured the test of time.”

Despite the diffusors’ high value and good finish, I suspect some Audio Cheapskate partisans will try to buy the raw materials locally and copy the RPG design as best they can. N.B. Cheapskates: D’Antonio hints that a lower-cost version may be in the offing.

Another suggestion: The manufacturer should provide an easy way to affix grille fabric to conceal the wells. The standard lacquered birch versions I received played to mixed reviews in the looks department during their year-plus of nearly continuous display. Of course, care would need to be taken in the choice of fabrics, as at high frequencies and very shallow angles of incidence some “acoustically transparent” material is actually quite reflective. Perhaps RPG could make available swatches of acoustically acceptable fabric, along with replaceable speaker-grille-type frames.

On a related note, D’Antonio indicates that custom finishes–from plexiglass for see-through applications to mirror, laminate, paint, and select hardwoods–are available at extra cost.

Finally, the RPG’s greatest contribution could be that it catalyzes widespread interest not only in acoustically rehabilitating existing listening rooms, but also in the next logical step: the construction of purpose-built music environments in the home. That is the high-end’s next frontier, and it is now at our doorstep.

Recalibrating the Listening Room
Sonex and ASC Tube Traps are perhaps the best-known acoustical treatments. But the successful recalibration of most listening rooms will require more diffusion than broad-range absorption (Sonex) or low-frequency corner attenuation (Tube Traps). Before spending dime one, it’s imperative that you understand just what you’re trying to accomplish. The goal of room makeovers is generally to remove the room’s characteristic small acoustic “fingerprint” and replace it with the space, warmth, envelopment, and liveness of a very good concert hall.

These—the removal of one “layer” and the installation of another—are two separate tasks. Broad-range absorbers like Sonex’s sculpted open-cell foam can remove the cues that cry “small” by killing the early reflections from nearby surfaces that tip off the ear-brain as to the real size of the room. The best locations of these absorbers are typically arrived at by placing a mirror on the front and side walls and moving it until, from the listening position, one or both speakers are visible. At these points, attach Sonex, Owens-Corning, Armstrong SoundSoak, or similar acoustical products.

The second task, the setting up of a soundfield that cries “big,” requires diffusion, especially in the areas flanking and directly behind the listening position. The ideal diffusor takes broad-range acoustical energy and scatters it in a non-frequency and non-temporally related way throughout the room, regardless of the angle at which the energy initially impinges on it.

By eliminating hard “specular” reflections and flooding the listening environment with ambient energy, the diffusor fosters the smooth, logarithmically decaying reverberation that marks the best concert halls. Poor concert halls and most all domestic listening rooms have erratic energy/time curves as a result of inadequate diffusion and/or unfavorable ratios of room dimensions. Ideally, reverberation will ramp down to -60dB in approximately 500ms (RT60=0.5s).

There is an audiophile wrinkle to this classical prescription. Many, perhaps most, audiophiles own or at least hanker for speakers that happen to have polar patterns that assume a reflective, not absorptive, area behind and around them. Most planars—e.g., those from Quad, Magnepan, Apogee, Martin-Logan, Eminent Technology, Acoustat, Sound Lab, Infinity IRS, and old-timers like the KLH 9s and the Dayton-Wright XG series—are configured as dipoles, meaning they direct as much energy backward as they do forward. Many others, like the Ohm Walsh units, Bose 901s, dbx Soundfield series, ESS/Heil air motion transformers, and dozens of others similarly not on the current “best-dressed” list, also rely heavily on broad polar shaping to achieve their characteristic sounds.

By installing too much absorption in the areas around planars and these latter devices, you disrupt their intended tonal and spatial properties, as the foldback of the rear and side waves figures prominently in establishing the speaker’s “musicality.” For these reasons, it is often desirable either to replace the absorbers with diffusors (which are, after all, a special kind of reflector) behind and around the speakers, or to mix diffusors with broad-range absorbers. Consider experimenting with absorption by constructing an absorbent “muffler” to go around the back side of the speaker, varying its distance (or angling it) to allow more or less rear-radiated sound into the room. (If you’re not a do-it-yourselfer, look into a set of Watkins “Echo Muffs.”)

The Numbers Game
Manfred Schroeder came up with the basis for the RPG diffusor after studying over 20 famous concert halls in Europe. He and his associates found that listeners preferred high, narrow halls to low, wide ones. He reasoned it had to do with the fact that in a high, narrow hall, the first reflections come from the side walls. These reflections are acoustically dissimilar, whereas in wide, low halls the first reflections will be from the ceiling, and will be acoustically similar—actually, nearly monophonic.

His next project was to design acoustic ceiling “gratings,” functionally analogous to optical diffraction gratings, to direct sound to the side walls. Schroeder turned to quadratic residue sequences for an answer. Such a sequence might be based, for example, on the prime number 17. The first sequence number is the “residue” (or remainder) after the first number, 1, is squared and divided by 17. The answer is 1.

Squaring all the numbers from 1 to 16, then dividing by 17 to find the residue, yields the sequence: 1, 4, 9, 16, 8, 2, 15, 13, 13, 15, 2, 8, 16, 9, 4, 1. For larger numbers, the pattern simply repeats. Finding the depth of a given grating well involves multiplying the appropriate number in the sequence by the longest wavelength for which the grating is designed to scatter sound efficiently, then dividing by a factor that depends on the well’s numerical position.

Mathematical analysis shows that for such an arrangement the spectrum of energies scattered into different directions is essentially flat. Schroeder says the reason number theory works so well is in the way waves cancel or reinforce each other, depending on whether a crest of one wave meets another wave’s trough or crest. For perfectly periodic waves, destructive interference (ie, partial or complete cancellation) occurs whenever one wave lags behind the other by half a wavelength or 1 1/2 wavelengths or 2 1/2 wavelengths and so on. In every case, the result is the same. Hence, in wave interference it’s not the path difference that determines the resulting pattern, but the residue after dividing by the wavelength. Similarly, in modular arithmetic what counts is not the numerical value itself, but the remainder after division by the modulus (17 in the example). —Keith Yates

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