The AudioKinesis Bohemian 215 design notes (more teaser content)

Duke LeJeune

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The development of our new "statement" speaker system is now far enough along that we are releasing 3D renderings. For scale, those are 15" woofers and a 22" diameter horn:
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The Bohemian 215 will come in two different versions: A 100 dB/1 watt efficient, specialty-tube-amp-friendly version; and a 95 dB/2.83 volt solid-state-amp-friendly version.

Overall dimensions are 56 inches tall; 22 inches wide; and 18 inches deep at the bottom. The center of the horn will be at 45 inches. It is possible that these dimensions might change slightly in the production version.

We'd like to share some of the thoughts that went into the design of the Bohemian 215.
 

Duke LeJeune

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Loudspeaker design is often an interesting competition of ideas. Equally intelligent and competent designers may embrace very different philosophies. Even when they embrace the same philosophy, they may choose very different implementations. And even among speakers which are practically identical at first glance we may find very divergent yet creative thinking.

We call the general philosophy we embrace the “Two-Streams Paradigm”. The Two-Streams Paradigm holds that “presence” and “envelopment” are both highly desirable; that the direct sound conveys “presence” and the reverberant sound conveys “envelopment”; and that a reflection-free time gap between the direct and reverberant sounds enables both presence and envelopment. Concert hall acoustics and psychoacoustics expert 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."

Earl Geddes on the same general 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 very 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.”

One implication is 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.

The basic concepts of the Two-Streams Paradigm have been around for decades, but have usually been considered a room acoustics matter, rather than a loudspeaker design matter. In effect we are designing into the loudspeaker benefits comparable to fairly extensive and well-executed acoustic treatment. While there are other loudspeaker topologies which are effectively compatible with the Two-Streams Paradigm, 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 the Bohemian 215:

The way we see it, amplifiers + loudspeakers + room = a “system within a system”, so we want a fairly high and smooth impedance curve, along with high efficiency, for compatibility with specialty tube amps like OTL and SET amps. Not to leave the solid state guys out, we have developed a version specifically optimized to perform well with solid-state amplification.

Since we manufacture highly successful disributed multi-sub systems which do a better job in the bottom two octaves than a pair of loudspeakers can, we don't need to build a great deal of bottom-end extension into the Bohemian 215. Nor do we want to – that which multiple subs can better do, let them do. So we are freed up to use midwoofers which are better in the midrange region, and we don't need the enormous enclosures which would be required to combine high efficiency with deep bass. Custom subwoofer options are available for those interested in extension down into the single digits, and of course the Bohemian 215 can be used with other manufactures' subwoofers.

Since most of our design efforts focus on the loudspeaker/room interface, we pay a lot of attention to radiation pattern control. In the Bohemian 215 we use two 15” midwoofers with very powerful motors and very light cones, crossed over to a large horn where their radiation patterns approximately match. Our 15” midwoofers' motor-strength-to-moving-mass ratios surpass those of high-end 5” and 6” midbass drivers, and are competitive with high-end 5” midrangecones. So right off the bat we have impact that small midwoofers cannot dream of, along with excellent articulation and superior radiation pattern control. The excellent articulation is partially a result of the reduced early reflections (which are the most detrimental ones, according to Griesinger and Geddes).

Our midbass enclosure has a wide front baffle which pushes the baffle-step frequency down low enough 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 authoratative midbass which lends realistic body to instruments like cello and double bass. The enclosure uses extensive bracing and both constrained and unconstrained layers of complementary materials for the panels, and is divided into an upper and a lower chamber (of different dimensions) to reduce the length of the longest standing wavelengths the internal dimensions allow, 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 with minimal early sidewall interaction. We paid attention to the details: The horn's entry angle matches the compression driver's exit angle, and the round-overs use purpose-optimized curvatures large enough to be effective in the frequency ranges which matter most.

Now one of the tradeoffs involved with the use of a constant-directivity type horn has to do with the on-axis efficiency at high frequencies. Because the high frequencies are spread across a full pattern width (rather than being concentrated on-axis, as is the case with exponential, spherical, tractric, hyperbolic, Le Cleac'h, and most other horns), the on-axis SPL at high frequencies is not as high as with most horn types. The same amount of high frequency energy is going out into the room, but its distribution is consistent across the horn's pattern width, instead of being “beamed” into an angle which gets progressively narrower as we go up in frequency. This places a practical upper limit of about 100 dB as far as system efficiency goes, for constant-directivity horns. 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. This imposes a juggling of tradeoffs: Do we align the horn mouths, or the compression driver diaphragms? And if we align the horn mouths, do we use DSP delay to align their acoustic centers?

Recent developments in compression driver diaphragms have made it possible to cover a wider range with a single driver, and the compression drivers we use are optimized to go high enough without needing a separate tweeter.

A great deal of consideration went into choosing the horn size. The horn needs to go low enough and high enough, have large enough roundovers to minimize the mouth reflection and diffraction, but not be so large that the distance to the midbass drivers is an issue. As part of our optimization, we use a different curvature for the mouth's inside round-over than the one we use for the outside round-over, each being 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 midwoofers, so the acoustic centers of compression driver and midwoofers are effectively aligned. This avoids the time-domain smear often present in conventional speakers which have highpass and lowpass drivers technically “in phase” in the crossover region, but with the woofer actually delayed by one wavelength relative to the tweeter. We don't have to choose 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.
 
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Duke LeJeune

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So why don't we rely on DSP? Because that's a different path, with its own set of tradeoffs, and we do not think its use comes without an audible price. We'd 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. But they generally don't do as well as the best wide-pattern speakers (in a really good room) when it comes to 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 been refining it for several years now.

In the concert hall, those seats which deliver both presence and envelopment at the same time have a particular set of characteristics. Let's revisit the words of physicist David Griesinger, specialist in the acoustics and psychoacoustics of concert halls, and then we'll 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 reflections free time is enough." [This time interval gives the ear a good signal-to-noise ratio down to 700 Hz.]

The “Two-Streams Paradigm” is taken from Griesinger's findings about what's happening at a good seat in a concert hall. There are three requirements which enable the Two Streams, whereby presence and envelopment exist at the same time:

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 for the “best of both worlds”. The Bohemians start out with these three requirements in mind, and they have a secret weapon: An upwards-and-backwards firing horn which we call the “Space Generator”. The reason behind that name will soon become apparent.

Now 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 home 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, we need a bit of background:

The ear gets its room-size cues primarily from the earliest reflections, from the decay times of the reverberant tails, and from the time span in between the first-arrival sound and the temporal “center of gravity” of the reflections. 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 we have the Bohemians in a normal untreated room, set up maybe a foot or two out from the wall, toed-in aggressively and spaced wide apart. Let's look at this through the lense of our three requirements (direct sound, reflection-free interval, and spectrally-correct reverberant sound):

The relatively narrow radiation pattern of the Bohemians minimizes early sidewall reflections, particularly with the strong toe-in that we recommend. 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 they happen to be perceptually rather benign (according to Griesinger and Geddes), and the two large midwoofers 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 upwards-and-backwards firing Space Generators starts to arrive, as do the contralateral sidewall reflections of the toed-in front-firing speakers. The radiation pattern of the midwoofer section widens below the crossover frequency, while the Space Generators are supplying extra off-axis energy (relative to the front horns alone) above the crossover frequency, so the net result is beneficial from a timbral standpoint.

So we have a surge of reflections arriving after ten milliseconds, which shifts the center-of-gravity of the reflections back in time, and contradicts the small-room signature cues which would normally have been conveyed by a much earlier-arriving surge of reflections. So we have thoroughly disrupted the normal small-room signature cues. But we're not done yet!

The increase in spectrally-correct, later-arriving reflections powered by the Space Generator's contribution results in a very 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 or engineered or both). It is highly counter-intuitive to think that by judiciously adding MORE reflections we actually hear LESS of the playback room and MORE of the recording venue, but that is exacly 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. With a good recording, the presentation sounds like you are in the venue's larger space or a reasonable approximation thereof. Hence the name, “Space Generator”.
 

Duke LeJeune

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There is one more thing we need to be aware of: Too much additional reverberant energy can start to 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 volume-adjusted if it is too loud. 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 really 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 several times more expensive.

Our sales pitch isn't really that we make a better horn or use better parts or have a better crossover or a better enclosure. Those things are just details. Loudspeaker design being a competition of ideas, our sales pitch is that we start out with a very competitive idea, namely the Two-Streams Paradigm.
 
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Ron Resnick

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That looks amazing, Duke! Congratulations!
 
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the sound of Tao

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Congratulations Duke, great to be able to see your ideas take shape and come to life.
 

tima

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Fascinating. Thanks for the time and the write-up. Enjoyed the Griesinger material.

What size room(s) to you anticipate as workable for the Bohemian 215s?
 
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Duke LeJeune

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Fascinating. Thanks for the time and the write-up. Enjoyed the Griesinger material.

What size room(s) to you anticipate as workable for the Bohemian 215s?
Thank you!

In general the Bohemian 215 (plus its entourage of four small subs) will be unusually small-room compatible for a big system, but there IS one thing I don't yet know: Minimum listening distance for avoiding an audible vertical discontinuity.

Based on my experiences with other horn speakers, I estimate the minimum listening distance is no more than eight feet. And it might be as little as five feet.

The aggressive pattern control north of 700 Hz allows the Bohemian 215s to be placed up against the side walls as long as you toe them in strongly, as the first significant sidewall reflection for each speaker will be the long, across-the-room bounce off the opposite side wall. The multiple pluggable ports allows adaptation to near-the-walls placement which results in a lot of boundary reinforcement. The up-and-back firing Space Generator disrupts the normal "small room signature" cues that would reduce soundstage depth if the speakers were close to the front wall. And the spectral balances of both the main horn and the space-generator horns are independently adjustable, allowing a bit more adaptation to the room's acoustics.

Unfortunately I haven't figured out a way for loudspeaker radiation pattern shape to address the early reflections off the wall behind the listening position, which distance generally decreases as the room size decreases, so the smaller the room the more likely that you would benefit from treating the first reflection zones on the wall behind the listener.

If it's a dedicated listening room wherein you have free reign, I guesstimate that 12 feet by 14 feet (8 foot ceiling) might work with treatment of the first reflections zones behind the listener.

A bit further down the road is a 212 version, which will have less vertical center-to-center spacing, implying a bit closer minimum listening distance.
 
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kodomo

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Congrats Duke, I wish it for to be a successful product. We always need more horns :)
 

Duke LeJeune

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Any indication of pricing?

"Well, if you have to ask..." Ha! I've always wanted to say that!!

Seriously, pricing is not finalized yet. Mid-30's ballpark is the current estimate.

There will be some trickle-down to less expensive models. Probably a 212 in a similar form factor, then one or two models which have woofers & horn sharing the same enclosure, and so forth.
 
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ACHiPo

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Duke,
These look great. I would be very interested in hearing them!
Evan
 
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ddk

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Congratulations Duke! If I may, why did you call it Bohemian? The 215 part is obvious.
Now one of the tradeoffs involved with the use of a constant-directivity type horn has to do with the on-axis efficiency at high frequencies. Because the high frequencies are spread across a full pattern width (rather than being concentrated on-axis, as is the case with exponential, spherical, tractric, hyperbolic, Le Cleac'h, and most other horns), the on-axis SPL at high frequencies is not as high as with most horn types. The same amount of high frequency energy is going out into the room, but its distribution is consistent across the horn's pattern width, instead of being “beamed” into an angle which gets progressively narrower as we go up in frequency. This places a practical upper limit of about 100 dB as far as system efficiency goes, for constant-directivity horns. Horn systems with considerably higher on-axis efficiencies exist, but their radiation patterns are correspondingly narrower at high frequencies.
You can always offer and optional lens to focus the high frequencies for those who want them, JBL offered them for most of their designs and even Siemens designed a lens for their theater horns for the same reason. Personally I prefer the sound without the lens and can easily live with what is perceived as a slight rolloff by some but few of my customers like to use the lens. It all depends on the room, the electronics even the listening chair as well as the listener's age, not many youngsters here :p!

david
 

sbnx

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Can I come over and hear them?
 
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Duke LeJeune

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Duke,
These look great. I would be very interested in hearing them!
Evan

We plan to show them at the Rocky Mountain Audio Fest in Denver in early October. That being said, Jim Romeyn lived in the Bay area for many years and has friends there so the chances of him bringing a set there for people to hear at some point are probably pretty good.

Congratulations Duke! If I may, why did you call it Bohemian? The 215 part is obvious.

Thank you David!

Maybe I'm just verbally challenged, but imo it's easier to design a speaker than to come up with a good name for it.

Three reasons for the name:

1. I just like the way it sounds and feels. Bo-HEE-mee-yan.
2. I like the word's connotations: Someone living an unconventional life; and one of the most epic and unconventional rock songs ever.
3. Far as I can tell, no other high-end speaker companies are already using it.

You can always offer and optional lens to focus the high frequencies for those who want them, JBL offered them for most of their designs and even Siemens designed a lens for their theater horns for the same reason. Personally I prefer the sound without the lens and can easily live with what is perceived as a slight rolloff by some but few of my customers like to use the lens. It all depends on the room, the electronics even the listening chair as well as the listener's age, not many youngsters here :p!

I don't know how to design a suitable lens, though I could probably hire someone who does. But there will be some built-in adjustability in the horn's portion of the spectrum with what I'm doing:

The frequency response of the first-arrival sound will be somewhat adjustable via an external resistor. By changing the value of the resistor, the top-end of the front-firing horn can be "tilted" a bit, to either come closer to "flat" or be more downward-sloping.

The spectral balance of the energy contributed by the rear horn will likewise be user-adjustable, as will its loudness. Adjusting the loudness of the rear horn also changes the direct-to-reverberant sound ratio somewhat, north of 700 Hz or so.

Can I come over and hear them?

The first pair's home will be with my partner Jim Romeyn in northern Utah. My first set of Bohemians is more likely to be 212s than 215s, just because some of my room's dimensions are small. But once we get to that point, YES, or maybe I can bring them to you?
 
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cal3713

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Agreed with all the comments Duke. These look/read great. Thanks so much for the writeup and education.

One question about the design, and feel free to just claim trade secrets, as you've clearly already shared way more than others do... but if you're willing, could you say a bit about why the front baffle is sloped? To decrease floor bounce? Clearly it's not for the usual time-alignment here...

Regardless, I hope the timing works out and I can come up to hear a pair in person this fall.
 
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LL21

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Thank you!

In general the Bohemian 215 (plus its entourage of four small subs) will be unusually small-room compatible for a big system...
Fascinating, Duke...would love to know more about the 4 small subs. A few questions:

1. You have the SWARM already...will this be an updated or somehow custom-tuned version of the SWARM to match these particular speakers? Or will you go bigger (ie, 15" cones for each sub instead of the 12" I think you currently use with SWARM)?

2. Will the subs be powered or passive?

3. Since you have 4 subs (which normally would require running cables to 6 different locations (2 main speakers + 4 subs), I note that some manufacturers use wireless to send the sound signal from the main system to the sub (REL does this?)...I wonder if that would be possible here either as standard or as an option?
 
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Duke LeJeune

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One question about the design, and feel free to just claim trade secrets, as you've clearly already shared way more than others do... but if you're willing, could you say a bit about why the front baffle is sloped? To decrease floor bounce? Clearly it's not for the usual time-alignment here...
With a two-way loudspeaker and a passive crossover, the signal going to the lowpass driver (a pair of 15" woofers in this case) is effectively "delayed", relative to the highpass driver (the horn) by the inevitable phase shift imposed by the crossover. One effect of this phase shift is a downward-tilted "lobe" in the crossover region. The 10 degree slope of the front baffle helps to mitigate the downward-tilt of this lobe. So does the depth of the horn itself, and in fact the compression driver being physically behind the woofer actually makes more of a difference in this case.

The midrange quality may also be improved a bit. The 15" woofers each have their own separate airspace, with somewhat different internal dimensions because of the tilt-back. So we are spreading out their internal standing wave modes somewhat, as well as any residual panel resonances.

Having two big midwoofers in a vertical stack DOES reduce floor bounce effects, but the tilt-back itself has very little additional effect on that.

Finally, since the use of subwoofers is part of the system design, we don't need to maximize low-end extension from the 15" woofers. So the loss of internal volume due to the front baffle's tilt-back has no detrimental effect on the system performance.
 

cal3713

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Awesome, you're great Duke. As before, thank you so much for the education.
 

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