AudioKinesis Flagship Bohemian 215 Satellite Speaker
All New for 2021
Best Value Reference Class System
- “Bohemian:” socially unconventional in an artistic way. AudioKinesis makes a wide array of some of the music industry’s most renown and respected speaker cabinets. “215” is music industry nomenclature for a speaker cabinet with two 15-inch mid-bass drivers.
- Horn/wave guide: Oblate Spheroid, proprietary Duke LeJeune design employing Dr. Earl Geddes' equations, 22-inch diameter including round-overs both inside and outside of the mouth, 75-degree radiation pattern. We believe this to be the most advanced large-format horn made today, providing constant-directivity coverage of the entire range above 700 Hz, with the coherence of a single highly refined horn and compression driver. Standard horn comprises painted poplar. At 2021 RMAF we display standard cost poplar horn w/Sherwin-Williams paint: Emerald Designer Edition, Rustic & Refined class, satin "Succulent" SW9650. Artisan wood horns as portrayed in renderings available for extra cost up to $6k/pr.
- Compression Driver: either choice is US made w/1.4" throat. Standard custom 16-ohm Textreme-diaphragm w/neodymium-motor. +$6k/pr for optional Radian beryllium diaphragm w/5 dB less sensitivity requiring different mid-woofers. Wide range user-adjustable "Treble Tilt" via external non-inductive bifilar power resistor, each step alters gain and treble curve to maximize usefulness and sensitivity.
- Crossover: 700 Hz, variable-slope, slow initial acoustic roll-off accelerates w/uniform asymmetry vs. the mid-woofers. Premium quality components.
- Mid-Bass + Loading: two series-wired 8-ohm B&C 15-inch = 16-ohm, dual drivers minimize boundary notch effects maximizing fullness and density lacking in single mid-bass systems. Dual discrete internal chambers each with multiple pluggable dual-flared ports optimizing performance for virtually any taste, room acoustic and supporting system (includes one air-tight threaded rubber-gasket pipe plug per port.)
- Mid-Bass Cabinet includes Space Generator: height x width 34 inches includes 1" glider footers x 22 inches, depth 18 @ base x 12 @ cabinet top. Baffle slopes back 10 degrees to eliminate xo lobbing effect. "Gold Core" MDF/birch ply laminate w/solid wood dress pieces flanking the 15s and grille-cloth. Standard price includes 15 veneer choices plus automotive paint finishes. Multi-layer construction with complementary materials, extensive bracing, and strategically sited constrained-layer panel damping. At 2021 RMAF we display satin dark chocolate walnut mid-bass bin.
- Cradle Support & Plywood Dress Cylinder: a cradle supports the wood cylinder covering the compression driver. Buyer selects wood type for both pieces, powder coat for the slotted alloy rear cylinder cover and quartz or granite 1.25" thick side-pieces for the cradle (side-pieces H x W 9" x 6", curved from lower front corner to upper rear corner.) At 2021 RMAF we display anigre wood cradle and cylinder, pewter rear cylinder cover and flecked-quartz cradle side-pieces.
- Grille: wood frame, black or brown knit cloth, 17-inches wide, flush with baffle face, silk loop pull tab at base center. Vertical corner radius on the grille frame and flanking solid wood decorative pieces maximize vertical height perception.
- Space Generator: comprises up-and-back firing controlled-pattern wide range horn w/response optimized for this application. User-adjustable volume level and top-end tilt.
- Outermost Dimensions: H x W 56 inches including glider footers x 22 inches; depth 18 @ base/14.5 @ horn. Horn center height 45 inches determined by listening tests.
- Sensitivity, Impedance, Frequency Range: buyer specifies 95 dB or 100 dB, nominal/minimum 11/8 ohm, tube friendly impedance curve, 70 Hz to 20 kHz.
- Binding Posts: all Cardas CPBP, each pair has one each copper and rhodium post to average the characteristics of each metal and maximize musical performance.
- One spike the user fastens after precision siting, located at the front center underside.
We call our loudspeaker design philosophy the “Two-Streams Paradigm,” positing that “presence” and “envelopment” are both highly desirable, that the direct sound conveys “presence,” the reverberant sound conveys “envelopment,” and that a reflection-free time gap between the direct and reverberant sound enables both presence and envelopment. Psychoacoustics and concert hall acoustics expert Dr. David Griesinger on the subject:
Envelopment is the holy grail of concert hall design. When reproducing sound in small spaces [home listening rooms], envelopment is often absent."
Envelopment is perceived when the ear and brain can detect two separate streams: A foreground stream of direct sound and a background stream of reverberation. Both streams must be present if sound is perceived as enveloping.
When presence is lacking the earliest reflections are the most responsible.
The earlier a reflection arrives the more it contributes to masking the direct sound. (emphasis added)
Earl Geddes on the same topic, focused more specifically on home audio:
The earlier and the greater in level the first room reflections are, the worse they are. This aspect of sound perception is controversial. Some believe that all reflections are good because they increase the listener's feeling of space – they increase the spaciousness of the sound. While it is certainly true that all reflections add to spaciousness, the early ones (less than 10 milliseconds behind the direct sound) do so at the sake of imaging and coloration...These reflections must be considered in the loudspeaker design and should be also be considered in the room as well.
We infer that later reflections (those arriving after 10 milliseconds) are desirable because they convey spaciousness without degrading imaging or introducing coloration, assuming they are spectrally correct.
Concepts associated with the Two-Streams Paradigm are decades old but were usually considered a room acoustics matter instead of a loudspeaker design matter. In effect Bohemian 215 incorporates performance benefits comparable to extensive and well-executed acoustic treatment. Other loudspeaker topologies may demonstrate the Two-Streams Paradigm, but our deliberate focus has resulted in a unique combination of attributes which sum to a unique solution.
Here are some of the design concepts we use in our coming flag ship Bohemian 215:
The way we see it the room + loudspeakers + amplifiers = a “system within a system.” We prefer high efficiency + high and smooth impedance curve for compatibility with specialty tube amps like OTL and SET. B-215 is compatible with solid state and push-pull tube amps but it is among a select group of speakers producing world-class sound w/Atma-Sphere’s smallest amp the S-30.
Since we manufacture successful distributed multi-sub systems whose bottom two octaves outperform that from full range speakers, Bohemian 215 is a satellite requiring only moderate low-frequency cutoff. This too improves performance – that which multiple subs can better do, let them do. This frees us to use mid-woofers with superb mid-range resolution and the mid-bass bin is smaller than that required for deep bass w/high efficiency. We sell custom subwoofer options; Bohemian 215 can be used with other manufacturer’s subwoofers.
Since most of our design efforts focus on the room/loudspeaker interface, radiation control is a primary concern. In the Bohemian 215 we use two 15” mid-woofers with light cones and powerful motors, crossed over to a large horn where their radiation patterns match. In fact, our 15” mid-woofer's motor-strength-to-moving-mass ratios surpass that of uber-high-end 4” through 7” mid-range cones. This immediately provides impact, pattern control and articulation that small mid-woofers cannot dream of. Superior articulation results partially from reduced early reflections, the most detrimental ones according to Geddes and Griesinger.
The baffle width reduces the baffle-step frequency so low that it is of little consequence. The two 15” cones in a vertical stack result in a very weak floor-bounce anomaly. The net result is an authoritative mid-bass lending realistic body to instruments like cello and double bass. The enclosure uses extensive bracing, panels comprising both constrained and unconstrained layers of complementary materials and is divided into an upper and a lower chamber of different dimensions to reduce standing wavelengths, resulting in more effective management via damping material. The only internal parallel walls are the side walls, and those are well treated.
The horn we use is unique. We designed it using Earl Geddes' equations and nobody else has anything quite like it: a large Oblate Spheroid designed for a 1.4” throat compression driver. The Oblate Spheroid profile is mathematically the most benign curvature for a given radiation pattern angle. Ours has a 75 degree constant-directivity radiation pattern which gives good coverage over a wide listening area and minimal early sidewall interaction. We paid attention to the details: the horn's entry angle matches the compression driver's exit angle to minimize internal diffraction, and the round-overs are large enough to be effective in the frequency ranges that matter most.
A tradeoff inherent with constant-directivity type horn correlates with on-axis efficiency at high frequencies. Tractrix, spherical, exponential, hyperbolic, and most other horns concentrate high frequencies on axis. Conversely, constant directivity horns spread high frequencies across their full pattern width, minimizing the high frequency on-axis SPL. Power response is similar among the horn types, but ours distributes consistent across the horn's pattern width instead of increasingly “beaming” w/rising frequency. For constant-directivity horns the trade-off is a practical upper limit on system efficiency of about 100 dB. Horn systems with considerably higher on-axis efficiencies exist but their radiation patterns are correspondingly narrower at high frequencies.
Most high-efficiency horn systems use at least two upper-frequency horns, one for the midrange and one for the tweeter. Our philosophy avoids the ensuing compromises: do we align the horn mouths, or the compression driver diaphragms? If the former, do we use DSP to align their acoustic centers?
Until very recently there were no large-format compression drivers with enough clean and powerful top octave energy for a 100 dB system efficiency using constant-directivity horns. This changed with the advent of Textreme diaphragms, which are lighter than Beryllium and have competitive spectral decay plots (in particular, ultra-fast settling times). Our Textreme diaphragm compression driver is a custom unit with ultralight voice coil, as we do not need the high power handling required for pro-sound applications. The result is that we avoid juggling the tradeoffs involved by adding a small high-frequency horn. Obviously, we think very highly of beryllium diaphragms as well because we offer it as an option, at somewhat higher cost and lower system efficiency.
A great deal of consideration went into choosing the horn size. The horn needs to go low enough and high enough, have large enough round-overs to minimize mouth reflection and diffraction, but be small enough to limit the distance to the mid-bass drivers to avoid venetian blind effect. As part of our optimization the mouth's inside and outside round-overs are unique, each chosen for its role.
There is a worthwhile advantage to using a single driver to cover the frequency range from 700 Hz on up. According to David Griesinger, it is particularly important that the phase of the overtones above 1 kHz (and ideally above 700 Hz) be preserved. The format we use enables this and goes one step further: the depth of our Oblate Spheroid horn corresponds with the phase-rotation-induced delay imposed on the mid-woofers, so the acoustic centers of compression driver and mid-woofers are effectively aligned. This avoids the time-domain smear often present in conventional speakers which have high-pass and low-pass drivers technically “in phase” in the crossover region, but with the woofer delayed by one wavelength relative to the tweeter. We avoid choosing between the elegant simplicity of woofer and tweeter on the same baffle, and the superior time-alignment of stepping the tweeter back by a sufficient distance to effectively align their acoustic centers. The depth of our horn is our tweeter set-back. So, the fundamentals and overtones arrive at the same time, or much more nearly so than is normally the case without DSP.
So why don't we rely on DSP? DSP is a different path with its own set of tradeoffs, and we think its use comes with an audible price. And we would like for our customers to have the option of enjoying an all-analog signal path if they so choose.
Good horn systems excel at conveying “presence” - the sensation of listening to actual voices and instruments. Conversely, in a good room, the best wide pattern speakers excel in conveying “envelopment” - the sensation of being immersed in the acoustic space of the recording. We have found a way to deliver the “best of both worlds” and have refined it for several years.
In the concert hall, those seats which simultaneously provide both presence and envelopment have a particular set of characteristics. Let us revisit the words of David Griesinger, specialist in the acoustics and psychoacoustics of concert halls, and then we will take his ideas into our home audio setting:
Envelopment is the holy grail of concert hall design. When reproducing sound in small spaces [home listening rooms], envelopment is often absent.
Envelopment is perceived when the ear and brain can detect TWO separate streams: A foreground stream of direct sound, and a background stream of reverberation. Both streams must be present if sound is perceived as enveloping.
When presence is lacking the earliest reflections are the most responsible.
The earlier a reflection arrives the more it contributes to masking the direct sound.
Transients are not corrupted by reflections if the room is large enough - and 10ms of reflection free time is enough.
We derive the “Two-Streams Paradigm” from Griesinger's findings Re. what is happening at a good seat in a concert hall. There are three requirements which enable the Two Streams which simultaneously provide presence and envelopment:
1. A clear stream of direct sound, followed by
2. Freedom from significant reflections for at least ten milliseconds, followed by
3. A well-energized stream of spectrally-correct reflections.
These three requirements are the secret formula to the “best of both worlds.” The Bohemian 215 starts out with these three requirements in mind, provided by our secret weapon: an upward-and-backward firing wide-range horn we call the “Space Generator.” The reason behind that name will soon become apparent.
The spatial cues for the envelopment we seek are already in the recording, and the proper role of the in-room reflections is to be “carriers” which convey these spatial cues to the listener from all around. At the same time, the in-room reflections also contain “small room signature” cues inherent to the playback room. In fact, there is a competition between these “small room signature” cues and the “venue spatial cues” on the recording, and most of the time in domestic audio the “small room signature” dominates. The Space Generator's job is to tip the balance in favor of the venue cues, given a good recording.
But first, some background:
The ear gets its room-size cues from the earliest reflections and from the decay times of the reverberant tails. In fact, the “small-room signature” cues are primarily conveyed by the early reflections, and the “venue spatial cues” are primarily conveyed by the reverberant tails on the recording.
Now suppose Bohemian 215 is in a normal untreated room, set up maybe a foot or two out from the wall, toed-in aggressively and spaced wide apart. Let us look at this through the lens of our three requirements in ascending time sequence:
Direct sound followed by
Reflection-free interval followed finally by
Spectrally-correct reverberant sound
Bohemian 215’s relatively narrow radiation pattern minimizes early sidewall reflections, particularly with our recommended strong toe-in. In fact, the first sidewall reflection of the left loudspeaker will be the long, across-the-room bounce off the right-side wall, and vice-versa. We still have the floor and ceiling bounces but our perception of such is rather benign (according to Griesinger and Geddes), and the two large mid-woofers in a vertical line mitigate the floor-bounce notch. So far, we have achieved the first two requirements: clean direct sound and a largely reflection-free time interval of at least ten milliseconds.
After a little over ten milliseconds the output of the upward-and-backward firing Space Generator arrives, as do the contralateral sidewall reflections of the toed-in front-firing speakers. The radiation pattern of the mid-woofer section widens below the crossover frequency, effectively filling in the spectral balance of the horn-only Space Generators.
We have a surge of reflections arriving after ten milliseconds, which shifts the center-of-gravity of the reflections back in time, contradicting the small-room signature cues which would normally be conveyed by an earlier-arriving surge of reflections. We have thoroughly disrupted the normal small-room signature cues. “But wait, there’s more!”
The increase in spectrally-correct, later-arriving reflections powered by the Space Generator's contribution results in a highly effective presentation of the reverberation tails on the recording, provided the room is not over-damped. And it is these (often subtle) reverberation tails which convey the impression of envelopment in the much larger acoustic space of the recording venue (whether that “space” be real, synthesized or both). It is highly counter-intuitive to think that by judiciously adding more reflections we hear less of the playback room and more of the recording venue, but that is exactly what happens. The spatial presentation changes significantly from one recording to the next indicating that it is dominated by the recording's ambience cues and is not an overlaid artifact of the in-room reflections. The presentation sounds like you are in the venue's larger space or a reasonable proximation thereof, hence the name “Space Generator.”
There is one thing we need to be aware of: too much additional reverberant energy can degrade clarity. So, we have made the relative volume level of the Space Generators user-adjustable. We believe this adjustability to be an advantage over other polydirectional loudspeakers (such as dipoles) whose rear-firing energy cannot readily be minimized-the rear-firing energy level is fixed and baked into the cake (if we use treatment, we alter the spectral balance which is not good). And based on our in-house blind listening tests, the optimum level for the rear-firing energy is generally lower than what we get from a dipole loudspeaker. We offer guidance to our customers in setting the levels of the Space Generators.
Improved spatial quality is not the only benefit which accrues from the Space Generators. The increase in spectrally-correct reverberant energy enhances timbre, much like the spectrally-correct reflections in a good recital hall or concert hall do. In fact, the development path which resulted in the present evolved version of the Space Generators started out as a pursuit of timbral enrichment.
So, we combine the lifelike presence and dynamics of good horns with the low coloration and sense of envelopment one might have with a good set of wide-pattern speakers in a good room and in most cases, we can do this without reliance on acoustic treatments.
We think the Two-Streams Paradigm offers worthwhile improvement over more conventional approaches. And we think the Bohemian 215's unique embodiment of the Two Streams Paradigm makes it competitive with (and even superior to) conventional systems many times more expensive.
Our sales pitch is not really that we make a better horn, use better parts, have a better crossover or a better enclosure. Those things are just details. Our sales pitch is that we have a better idea.
*Horn depicted in the following renderings is solid artisan wood, up to $6k/pr extra cost depending on buyer's wood choice. Standard is poplar horn w/paint finish of buyer's choice.