The AudioKinesis Bohemian 215 design notes (more teaser content)

Duke LeJeune

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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?

1. The regular Swarm (which uses 10" woofers) will work very well with the Bohemian 215, unless the room is very large and/or unusually high sound pressure levels are required, or extension well below 20 Hz is required. In those cases, we can offer a larger and more capable version of the Swarm. We have solutions down to about 9 Hz (in-room), on a custom basis, and can theoretically extend that with EQ.

2. We have no plans to incorporate amplification within the subwoofer enclosures, for reliability reasons. The failure-in-the-field rate for our subwoofer amplifiers is extremely low, and we think freedom from vibrations has something to do with it. Also if replacing the amplifier is ever called for, much easier to do without having it bolted into the sub, and much easier down the road if the replacement amp doesn't have to fit the cut-out.

3. We have no plans to do wireless subs, mainly for the reasons described above. However if you find a wireless subwoofer system (or any subwoofer system) from somebody else which does what you want, then get that!

The Bohemian 215 will be easier to integrate with subwoofers than most speakers for two reasons: First, multiple pluggable ports so that you can adjust its low end, where it blends with the subs. Second, those twin big 15's do not NEED a protective highpass filter unless you plan on playing them very loud (like well north of 120 dB at 1 meter). So in most applications, adding a protective highpass filter to the main speakers' signal path will be optional.
 
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sbnx

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Regarding the crossover: So reading the posts it looks like there is no crossover network on the woofers. What is on the mid/tweeter? Just a single cap?
 

Duke LeJeune

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Regarding the crossover: So reading the posts it looks like there is no crossover network on the woofers. What is on the mid/tweeter? Just a single cap?
Sorry if I gave that impression!

There will be a lowpass filter on the woofers and a highpass filter on the compression driver. In my experience it isn't feasible to combine the kind of radiation pattern control I want with a "single cap on the tweeter" crossover, at least not for this type of loudspeaker. The amplifier will still see a very benign load, but the eye would not see a correspondingly simple crossover board.

Every horn + compression driver combination whose impedance curve I've looked at has at least one significant impedance peak associated with the compression driver's resonance frequency. A single capacitor will fail to adequately suppress the driver's output in this region, resulting in a frequency response anomaly (which may or may not be objectionable) plus possibly audible distortion if the driver is pushed hard. The solutions include either a second-order highpass filter (capacitor in series with the voice coil plus an inductor in parallel with the voice coil) OR an impedance compensation circuit to smooth the impedance peak. A second-order highpass filter is MUCH more effective into a variable-impedance load than a simple capacitor is. These solutions add more components to the crossover circuit, but neither necessarily places more components in the signal path.

While there are some types of horns which can give good on-axis frequency response without any additional equalization, they are not "constant directivity" types. Rather, their radiation patterns are wide at the low end and progressively narrow as we go up in frequency. Not everyone thinks this is undesirable, but I do. So I use a different horn type which squeezes all of the compression driver's output into the SAME radiation pattern width (or at least aspires to do so), which results in the off-axis response having approximately the same frequency response as the on-axis response, BUT now BOTH are generally downward-sloping as we go up in frequency. That downward-sloping trend has to be corrected in the crossover. The good news is that once we have corrected the on-axis response, we have simultaneously corrected the off-axis response.

My "Two-Streams Paradigm" calls for the reverberant energy to have approximately the same spectral balance as the first-arrival sound, so I make my system and crossover design tradeoffs with that priority. But I have heard excellent-sounding horn speakers which use a different type of horn and in some cases just use a single capacitor.

If I do my job well, the tradeoffs arising from my prioritizing radiation pattern control will hopefully be negligible, or a least will still result in a viable alternative to approaches which can be implemented with a simpler crossover topology.
 
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LL21

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Dec 26, 2010
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1. The regular Swarm (which uses 10" woofers) will work very well with the Bohemian 215, unless the room is very large and/or unusually high sound pressure levels are required, or extension well below 20 Hz is required. In those cases, we can offer a larger and more capable version of the Swarm. We have solutions down to about 9 Hz (in-room), on a custom basis, and can theoretically extend that with EQ.

2. We have no plans to incorporate amplification within the subwoofer enclosures, for reliability reasons. The failure-in-the-field rate for our subwoofer amplifiers is extremely low, and we think freedom from vibrations has something to do with it. Also if replacing the amplifier is ever called for, much easier to do without having it bolted into the sub, and much easier down the road if the replacement amp doesn't have to fit the cut-out.

3. We have no plans to do wireless subs, mainly for the reasons described above. However if you find a wireless subwoofer system (or any subwoofer system) from somebody else which does what you want, then get that!

The Bohemian 215 will be easier to integrate with subwoofers than most speakers for two reasons: First, multiple pluggable ports so that you can adjust its low end, where it blends with the subs. Second, those twin big 15's do not NEED a protective highpass filter unless you plan on playing them very loud (like well north of 120 dB at 1 meter). So in most applications, adding a protective highpass filter to the main speakers' signal path will be optional.
Fantastic. Thank you. That is really exciting. Look forward to reading more about this speaker. In fact, it was because of this thread...when I happened to come across the Diesis Roma speaker via one of the videos on this site, I was immediately intrigued because of your own design/thread.

I do not wish to ask you as a professional to start comparing your product to the Diesis. I emphasize, that is not my intent. However, to the extent we could learn [even] more about how you came to your decision to use the current large horn design...that would be interesting. What is it about this particular design that attracted you...what key sound reproduction priorities does this design represent vs some of the [generally] other basic key designs within the horn + cone world?
 
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Duke LeJeune

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... to the extent we could learn [even] more about how you came to your decision to use the current large horn design...that would be interesting. What is it about this particular design that attracted you...what key sound reproduction priorities does this design represent vs some of the [generally] other basic key designs within the horn + cone world?

Thank you for the invitation! Wish I had time now, but unfortunately I have to leave on a short-notice trip. I'll write a decent reply soon as I can.
 

LL21

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Thank you for the invitation! Wish I had time now, but unfortunately I have to leave on a short-notice trip. I'll write a decent reply soon as I can.
Thank you and look forward to it!
 

James Romeyn

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May 27, 2017
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Re. the horn design I hope it's helpful to cut/paste these paragraphs from the B-215 page at my website:

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.
Duke designed the horn employing Dr. Earl Geddes' latest mathematical formulas. One claim to fame for this horn is extremely high tolerance matching the horn's entry angle to the compression driver's exit angle.

Re. the mid-bass bin: I presume Duke mentioned above that our friend and business associate Hans Looman of Infigo Audio gave us the posted renderings (Hans was the chief designer/engineer @ Resonessence Labs.)

We plan to include a grille about 17" wide; its cloth being flush with one piece of solid wood flanking each side of the 15s. I am currently thinking the vertical corners of the solid wood and grille frame shall have a radius: the former for cosmetic beauty for rare cases when the grille is removed, the latter to help stretch and fasten cloth on the frame. Two adjoining radii should accent the vertical and "slim" the appearance of the baffle.

The 15s cross circa 700 Hz, a 1.6 foot wavelength, hence radius or not there is zero risk of diffraction from the 3/4" grille frame and flanking solid wood. Ditto, grille cloth is invisible to a 1.6 foot wavelength. The grille frame has a silk pull tab at the base center.

Also included standard shall be an aluminum dress ring covering the mounting flange and fastening screws for each 15. Ring surface and outer machined circumference is black powder coat; the inner machined surface is natural.

Our first demo Bohemian 215 appears @ 2021 RMAF. The mid-bass bin shall be an even darker version of our so-called "chocolate walnut," with the aforementioned solid walnut flanking the 15s.

The horn front/sides/back shall be a dark green, kind of a smoky gray/blue/US Army green. The 8" cylinder around the compression driver and the 9" x 6" (H x W) sides of the cradle that support it shall be beech or anigre. Fastened atop each beech or anigre side panel is a 1" thick piece of blue/gray/white granite with random small flecks ranging from light gray to dark gray blue. Each granite piece has a radius on the forward-facing 9" dimension (from the lower front corner to the upper rear corner, mirror imaged pairs.)
 
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Robh3606

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Hello Duke

Nice speaker system! Hope you come east at some point. Love to give them a listen! Are you running both woofers up to your horn at crossover?? Quite a bit of thought went into them thanks for sharing!



Rob :)
 
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cal3713

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Re. the horn design I hope it's helpful to cut/paste these paragraphs from the B-215 page at my website:


Duke designed the horn employing Dr. Earl Geddes' latest mathematical formulas. One claim to fame for this horn is extremely high tolerance matching the horn's entry angle to the compression driver's exit angle.

Re. the mid-bass bin: I presume Duke mentioned above that our friend and business associate Hans Looman of Infigo Audio gave us the posted renderings (Hans was the chief designer/engineer @ Resonessence Labs.)

We plan to include a grille about 17" wide; its cloth being flush with one piece of solid wood flanking each side of the 15s. I am currently thinking the vertical corners of the solid wood and grille frame shall have a radius: the former for cosmetic beauty for rare cases when the grille is removed, the latter to help stretch and fasten cloth on the frame. Two adjoining radii should accent the vertical and "slim" the appearance of the baffle.

The 15s cross circa 700 Hz, a 1.6 foot wavelength, hence radius or not there is zero risk of diffraction from the 3/4" grille frame and flanking solid wood. Ditto, grille cloth is invisible to a 1.6 foot wavelength. The grille frame has a silk pull tab at the base center.

Also included standard shall be an aluminum dress ring covering the mounting flange and fastening screws for each 15. Ring surface and outer machined circumference is black powder coat; the inner machined surface is natural.

Our first demo Bohemian 215 appears @ 2021 RMAF. The mid-bass bin shall be an even darker version of our so-called "chocolate walnut," with the aforementioned solid walnut flanking the 15s.

The horn front/sides/back shall be a dark green, kind of a smoky gray/blue/US Army green. The 8" cylinder around the compression driver and the 9" x 6" (H x W) sides of the cradle that support it shall be beech or anigre. Fastened atop each beech or anigre side panel is a 1" thick piece of blue/gray/white granite with random small flecks ranging from light gray to dark gray blue. Each granite piece has a radius on the forward-facing 9" dimension (from the lower front corner to the upper rear corner, mirror imaged pairs.)
Really looking forward to seeing them in person. Thanks for posting the details James.
 

James Romeyn

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May 27, 2017
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Someone in this or Duke's first B-215 thread (early '20) asked about potential trickle down models. Duke mentioned a potential model with the B-215's 22" horn atop a 212 mid-bass bin. For a particular nearfield/SET application in the North San Francisco Bay Area, Duke defined a 96 dB 112 Tower Satellite with the next-size smaller horn (already in Duke's possession,) a baffle-mount only 12.5" (Gina's 10.5" horn +2".)

To avoid diluting this B-215 thread, upon request I'll start a new thread describing the planned 96 dB 112 Tower Satellite, with built-in 12.5" wood horn and Big Horn Space Generator, targeted for nearfield/SET.
 

James Romeyn

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May 27, 2017
10
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70
...
The Bohemian 215 will be easier to integrate with subwoofers than most speakers for two reasons: First, multiple pluggable ports so that you can adjust its low end, where it blends with the subs. ...
The last full range speaker in my sound room was Duke's Azel Tower, each speaker with 2 ports directly behind the single 12" mid-bass just below the horn and 2 more rear-firing ports just above floor.

Prior to Azel Tower I had long used Duke's satellites + his distributed sub array to smooth my room's offensive and considerable bass mode effects. I expected much less smooth bass from two full range towers. Instead I was happily surprised, with infinitely smoother and more enjoyable bass than I expected.

Azel L/R pair comprise a total of 8 rear firing ports, 4 behind the 12s and 4 just above the floor. We can think of the 8 ports as one set of low bass sources, separate from the two 12s (1 per channel.) I could only conclude the bass linearity resulted from those 8 ports covering such a wide geographic area in the room. IIRC I ended up with 1 port plugged behind a 12 and one port plugged in the opposite channel near the floor.

I have also had good results with another woofer/mid-bass bin with a ported 15 and sealed 10. Coming back to B-215's mid bass bin: I generally prefer to leave as many ports open as possible. There will almost certainly be at least two ports behind each 15 in its own sub-enclosure. One could theoretically seal one 15 and leave the other ported.

The point which I took a long time to make is that IMO, the 215 mid-bass bin, with its wide array of tuning options, results in very smooth bass, even in troubling rooms, especially considering its cutoff in the 70 Hz range.
 
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Mikem53

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Duke, Those speakers look Fab !! Great Design ! Thanks for the write up and good luck with your product !
 
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morricab

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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.



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.



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.



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?
So, you are crossing at 700hz to the horn? I am running my horn from 800hz with the larger horn and 1200jz with the smaller. Both get to 20khz...so no super tweeter.
 

morricab

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Apr 25, 2014
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Sorry if I gave that impression!

There will be a lowpass filter on the woofers and a highpass filter on the compression driver. In my experience it isn't feasible to combine the kind of radiation pattern control I want with a "single cap on the tweeter" crossover, at least not for this type of loudspeaker. The amplifier will still see a very benign load, but the eye would not see a correspondingly simple crossover board.

Every horn + compression driver combination whose impedance curve I've looked at has at least one significant impedance peak associated with the compression driver's resonance frequency. A single capacitor will fail to adequately suppress the driver's output in this region, resulting in a frequency response anomaly (which may or may not be objectionable) plus possibly audible distortion if the driver is pushed hard. The solutions include either a second-order highpass filter (capacitor in series with the voice coil plus an inductor in parallel with the voice coil) OR an impedance compensation circuit to smooth the impedance peak. A second-order highpass filter is MUCH more effective into a variable-impedance load than a simple capacitor is. These solutions add more components to the crossover circuit, but neither necessarily places more components in the signal path.

While there are some types of horns which can give good on-axis frequency response without any additional equalization, they are not "constant directivity" types. Rather, their radiation patterns are wide at the low end and progressively narrow as we go up in frequency. Not everyone thinks this is undesirable, but I do. So I use a different horn type which squeezes all of the compression driver's output into the SAME radiation pattern width (or at least aspires to do so), which results in the off-axis response having approximately the same frequency response as the on-axis response, BUT now BOTH are generally downward-sloping as we go up in frequency. That downward-sloping trend has to be corrected in the crossover. The good news is that once we have corrected the on-axis response, we have simultaneously corrected the off-axis response.

My "Two-Streams Paradigm" calls for the reverberant energy to have approximately the same spectral balance as the first-arrival sound, so I make my system and crossover design tradeoffs with that priority. But I have heard excellent-sounding horn speakers which use a different type of horn and in some cases just use a single capacitor.

If I do my job well, the tradeoffs arising from my prioritizing radiation pattern control will hopefully be negligible, or a least will still result in a viable alternative to approaches which can be implemented with a simpler crossover topology.
Are you using butterworth or L/R filters in this design? I tried both in my design and ended up preferring the L/R filter (2nd order).
 

Tim Link

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That 700Hz horn looks awesome! I've been dreaming of building something like that myself. A lot of very intriguing ideas being put together here. I use DSP but often wonder what sort of audible losses it really creates, assuming it's used correctly. Very subtle frequency response changes in a speaker can cause a noticeable difference so it's really hard to get a DSP crossover and a passive to match perfectly enough to compare them and rule out simple response and phase characteristics causing the audible difference rather than anything related to it being digital. Regardless, if it can be done just as well passively without DSP it simplifies usability considerably. And if you're using an all analog signal chain, any worries of digital issues can be put completely to rest.
 
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