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Chris A

A K-402-Based Full-Range Multiple-Entry Horn

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On mine I built these speaker pads/mounts and sandwiched the K402 horn in under a little bit of pressure.  Mine is very, very solid when you wrap your knuckles inside the horn.

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Your K-402 mounted in a cabinet and wrapped with rock wool looks good and solid. My question about rigidity came to mind while looking at a picture of a free standing treble horn image.

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7 minutes ago, Khornukopia said:

Your K-402 mounted in a cabinet and wrapped with rock wool looks good and solid. My question about rigidity came to mind while looking at a picture of a free standing treble horn image.

IMHO - it would never handle it.  The bracket for the K402 is designed to support all of the driver weight.  If you add woofers they'll need their own supports, no question.

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16 hours ago, Khornukopia said:

Somebody once mentioned a desire to build a wood version of the K-402. That would be interesting.

Maybe this summer

 

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Any progress on the horns?

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19 hours ago, ellisr63 said:

Any progress on the horns?

Soon, I hope.

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Any updates?



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Not yet...but soon.

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On 10-11-2016 at 9:28 PM, Chris A said:

I use 475 Hz within the K-402 horn (48 dB/octave). 

 

Did you ever use, or try, a higher crossover? If so, what was the highest it performed well at (or you think it would perform)?

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The placement of the off-axis ports relative to the throat (apex) compression driver determines the low-pass behavior of the off-axis ports.  I placed the off-axis ports where they would have a low-pass of 475 Hz--and hit it right on the nose with the prototype.

 

If you want to cross over at a higher frequency, those ports must be closer to the throat.  The issue with that is that you're making a trade against the off-axis coverage angles (vertical and horizontal) due to the ports, or you're giving up horn efficiency (a LOT of it) if you make the off-axis ports much smaller in area.  So there's a trade that must be made. 

 

I could've placed the off-axis ports at 1 kHz crossover frequency point in the horn (which turns out to be the 1/4 wavelength from the horn throat at the center frequency of interest), but then I'd be giving up a lot in terms of off-axis performance at and just below that frequency, and a lot of woofer-horn efficiency. 

 

The trade-off point (475 Hz) that chose was just right for my needs, and results in an extremely clean-sounding MEH that fills the room with coherent, clear, and tonally balanced acoustic energy.

 

Chris

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On 15-2-2018 at 9:00 PM, Chris A said:

The placement of the off-axis ports relative to the throat (apex) compression driver determines the low-pass behavior of the off-axis ports.  I placed the off-axis ports where they would have a low-pass of 475 Hz--and hit it right on the nose with the prototype.

 

If you want to cross over at a higher frequency, those ports must be closer to the throat.  The issue with that is that you're making a trade against the off-axis coverage angles (vertical and horizontal) due to the ports, or you're giving up horn efficiency (a LOT of it) if you make the off-axis ports much smaller in area.  So there's a trade that must be made. 

 

I could've placed the off-axis ports at 1 kHz crossover frequency point in the horn (which turns out to be the 1/4 wavelength from the horn throat at the center frequency of interest), but then I'd be giving up a lot in terms of off-axis performance at and just below that frequency, and a lot of woofer-horn efficiency. 

 

The trade-off point (475 Hz) that chose was just right for my needs, and results in an extremely clean-sounding MEH that fills the room with coherent, clear, and tonally balanced acoustic energy.

 

Chris

I was (wrongly) under the misunderstanding that the distance determined the MINIMUM crossover, not one specific frequency. Also, what kind of woofer horn loading are you looking at in your model? 

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13 hours ago, Droogne said:

I was (wrongly) under the misunderstanding that the distance determined the MINIMUM crossover, not one specific frequency.

It's the maximum frequency before the woofers encounter the apex reflection frequencies, a band of frequencies that are cancelled due to the off-axis port acoustic waves bouncing off the throat of the horn and returning through the mouth of the horn.  This is a very strong low pass phenomenon that is characteristic of multiple entry horns.

 

The throat-located compression driver can play through these frequencies, albeit with a loss of off-axis SPL due to the loss of off-axis port area reflectivities. Usually, MEHs are set up to overlap the crossover region by the higher frequencies and lower frequency drivers in order to cover the missing off-axis horn reflectivities in the off-axis port regions. 

 

13 hours ago, Droogne said:

Also, what kind of woofer horn loading are you looking at in your model? 

I used Hornresp to verify that the T/S parameters of the woofers that I chose were well suited to both the high frequency and low frequency performance of the woofer pass band, i.e., that their SPL output relative to the throat-mounted compression driver was the same output levels without pronounced dips in SPL on their high frequency end of their response, and that their lower frequency response was at a relative maximum and smooth with frequency vs. other woofer driver choices that could have been made. 

 

Hornresp is actually rather limited in terms of what it can do outside of that type of performance.  It would take a fairly detailed Boundary Element Model (BEM) to delve deeper into the acoustic performance of the MEH geometries and driver electro-mechanical parameters to understand much more.  The difficulty of using BEMs is at least 100x higher than using a lumped model like Hornresp.  I've found it to be easier to use rules of thumb and "cut-and-try" approaches rather than to presently go down the path of BEMs. That might change in the future if more MEH designs are attempted, but that's another discussion entirely.

 

The size of the off-axis ports and the relative compression ratio of the woofers (woofer diaphragm area divided by the off-axis port area) were factors that were selected based on rules of thumb and testing of the resulting off-axis port area/placement and woofer diaphragm area.  The nominal efficiency of the woofers was calculated using Olson's formula for driver/horn throat area efficiency.

 

Chris

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21 minutes ago, Chris A said:

The size of the off-axis ports and the relative compression ratio of the woofers (woofer diaphragm area divided by the off-axis port area) were factors that were selected based on rules of thumb and testing of the resulting off-axis port area/placement and woofer diaphragm area.  The nominal efficiency of the woofers was calculated using Olson's formula for driver/horn throat area efficiency.

 

Chris

Excellent summary of your methods, Chris!

 

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On 17-2-2018 at 3:33 PM, Chris A said:

It's the maximum frequency before the woofers encounter the apex reflection frequencies, a band of frequencies that are cancelled due to the off-axis port acoustic waves bouncing off the throat of the horn and returning through the mouth of the horn.  This is a very strong low pass phenomenon that is characteristic of multiple entry horns.

 

On 17-2-2018 at 3:33 PM, Chris A said:

The throat-located compression driver can play through these frequencies, albeit with a loss of off-axis SPL due to the loss of off-axis port area reflectivities. Usually, MEHs are set up to overlap the crossover region by the higher frequencies and lower frequency drivers in order to cover the missing off-axis horn reflectivities in the off-axis port regions. 

Thanks for clarifying! 

On 17-2-2018 at 3:33 PM, Chris A said:

I used Hornresp to verify that the T/S parameters of the woofers that I chose were well suited to both the high frequency and low frequency performance of the woofer pass band, i.e., that their SPL output relative to the throat-mounted compression driver was the same output levels without pronounced dips in SPL on their high frequency end of their response, and that their lower frequency response was at a relative maximum and smooth with frequency vs. other woofer driver choices that could have been made. 

Is hornresp difficult to work with? In any case, what were the most important T/S parameters that worked for you? 

On 17-2-2018 at 3:33 PM, Chris A said:

Hornresp is actually rather limited in terms of what it can do outside of that type of performance.  It would take a fairly detailed Boundary Element Model (BEM) to delve deeper into the acoustic performance of the MEH geometries and driver electro-mechanical parameters to understand much more.  The difficulty of using BEMs is at least 100x higher than using a lumped model like Hornresp.  I've found it to be easier to use rules of thumb and "cut-and-try" approaches rather than to presently go down the path of BEMs. That might change in the future if more MEH designs are attempted, but that's another discussion entirely.

I doubt I would ever be able to master something as complex as that ;) I might have a look at horn resp though. When looking at designing my own synergy/unity I followed your 10:1 ratio.

On 17-2-2018 at 3:33 PM, Chris A said:

 

The size of the off-axis ports and the relative compression ratio of the woofers (woofer diaphragm area divided by the off-axis port area) were factors that were selected based on rules of thumb and testing of the resulting off-axis port area/placement and woofer diaphragm area.  The nominal efficiency of the woofers was calculated using Olson's formula for driver/horn throat area efficiency.

 

Chris

And what was that efficiency? I'm guessing with 2 15"ers that will be pretty high. If you'd have to take a guess (or of you know it ofcourse), what would the overal effeciency of the MEH be? 

 

Maarten 

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15 minutes ago, Droogne said:

 

Thanks for clarifying! 

Is hornresp difficult to work with? In any case, what were the most important T/S parameters that worked for you? 

I doubt I would ever be able to master something as complex as that ;) I might have a look at horn resp though. When looking at designing my own synergy/unity I followed your 10:1 ratio.

And what was that efficiency? I'm guessing with 2 15"ers that will be pretty high. If you'd have to take a guess (or of you know it ofcourse), what would the overal effeciency of the MEH be? 

 

Maarten 

Hornresp takes a little practice, but it's not difficult. The Electro Mechanical to Acoustic conversion efficiency starts with the Magnet/Voice Coil (BL product) coupled into horn loading and directivity of the horn. If we are talking on-axis, there's also whether the two woofers are in series or parallel, which affects Sensitivity, not efficiency. The ports and horn do the same thing for a given hole size within the horn regardless of the drivers spects in terms of efficiency, but you have to specify a frequency or average over bandwidth. It's not that simple.

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1 hour ago, Droogne said:

Is hornresp difficult to work with? In any case, what were the most important T/S parameters that worked for you? 

The biggest issue with Hornresp is the user interface, which has been an evolutionary creation beginning a long time ago when its capabilities were far less powerful.  So you have to learn the UI with the "Multiple Entry Horn Wizard", which is easy...once you see how to do it.  There are no instructions, however.   If you go to the diyAudio forum, you will see the application's creator (David McBean) answering questions on how to use the "new" facilities, like the MEH Wizard.  Once you get in the swing of things, it's pretty easy.

 

1 hour ago, Droogne said:

I doubt I would ever be able to master something as complex as that ;) I might have a look at horn resp though. When looking at designing my own synergy/unity I followed your 10:1 ratio.

Yes, the interface is just like a finite element analysis (FEA) environment with 3-D mesh builders and time step visualization facilities (I'm talking about ABEC3 in the present case, but there are other BEM tools available).  It's a real engineering job to run these models--not really home hobby stuff unless you've got a little background in the theory.

 

1 hour ago, Droogne said:

And what was that efficiency? I'm guessing with 2 15"ers that will be pretty high. If you'd have to take a guess (or of you know it of course), what would the overall efficiency of the MEH be? 

Actually, it's lower than you might think due to the 10:1 compression ratio used.  I've not measured the sensitivity/efficiency, since it doesn't really make much difference when bi-amping.  But the resulting loading on the woofers allows extension on both the high end (up to the low pass-port frequency) and low end coupling to the room boundaries.  That's a subject that many folks really don't understand well--using eighth-space or quarter space coupling to really extend the bottom end without penalty of dramatically increased harmonic or modulation distortion, as well as group delay/phase distortion below the same point where the Khorn or Jubilee bass bins also lose their horizontal directivity (about 110 Hz).  That's "full-range MEH design 201".  ;)

 

Chris

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Since it's a little too cold to be doing horn stuff in the garage today, I thought that I'd reveal a little known reason why the basic shape of the K-402 and all straight-sided horns, such as conical horns, including those with mouth flares can be used at much lower frequencies than exponential or hyperbolic horns.  Below you will see an impedance plot of your typical exponential horn (like that of the Khorn, Jubilee, La Scala and Belle).  The normalized resistive "real" part of the complex impedance vs. frequency is the top curve:

 

5aca31da5d110_Exphornimpedancevsfreq.thumb.PNG.0465298de443cbc60ac427b3268a052f.PNG

 

The wiggly line is due to the undersized mouth and the finite length of the horn.  An infinitely long horn with a mouth circumference that is as least as long as the longest wavelength produced would not be "wavy" vs. frequency.  Note how the resistance at the horn throat goes to essentially zero just below the cutoff frequency of the horn (corresponding to ~1/2 wavelength of sound--equal to the horn's length--25 cm in this case, which is about 690 Hz). 

 

Here is the same type of plot but in this case the horn is instead a straight-sided conical horn of the same axial length

 

5aca330117890_Conicalhornimpedancevsfreq.thumb.PNG.61e756eef74a2022bcd6cb6c6c7d4b37.PNG

 

Notice how the resistive (real) part of the impedance drops off very gradually below the 4 kHz breakpoint frequency.  But also notice how the resistive part of impedance is still not below 20% (i.e., 0.2 normalized) even at 300 Hz. 

 

Since the increase in SPL on-axis of a driver using a horn is between 5-10 times its output not using a horn, you can see that the conical horn-loaded driver will still have an increased output over the driver being used as a direct radiator.  So depending on the horn gain achieved,  straight-sided horn loading will provide more output than an exponential horn below the exponential horn's cutoff frequency, and will also still provide more output than the direct radiating case--even down to 200 Hz for this length and size of horn. 

 

Other advantages of the straight-sided horn is that it doesn't have a high frequency roll-off off-axis like that of any other curved horn with positive curvature, and therefore doesn't have any "horn sound". 

 

Chris

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One other thing that I should mention: straight-sided horns have a property known as "boundary gain", like other horn profiles (exponential, hyperbolic, parabolic, etc) due to the presence of nearby walls, floor and ceiling.  The Khorn bass bin explicitly makes use of corner gain to complete the horn's last fold, and to provide boundary gain loading.  This also works the same for other horn-loaded bass bins mentioned above (as well as direct-radiating woofers in the corner of a room).  Here is an acoustic impedance plot of a "manifold exponential horn" using three sections in series, like that of a folded horn bass bin in a room corner:

 

5aca3acbc4c70_Manifoldexponentialhornimpedancevsfreq.thumb.PNG.e9a9019f9945dcf41ece7fa494b029c5.PNG

 

Even having one wall and a floor nearby to the horn's mouth will provide added boundary gain, thus further increasing the low frequency throat resistance of the horn, and improving low bass frequency output below the typical "rule of thumb" low frequency cutoff point of the horn.

 

Because the straight-sided horns of the MEH type have very large mouths compared to their conventional horn brothers, they actually load more efficiently via boundary gain than do exponential and hyperbolic horns of the folded-horn type. 

 

So the K-402-type full-range MEH loudspeaker is good to 30 Hz--20 kHz by using two woofers of 15" diameter and only a K-402-sized horn in boundary gain.  Its low frequency output is increased relative to direct radiating woofers due to horn loading, and this reduces the modulation distortion vis-à-vis direct radiating mode. 

 

Chris

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One other thing that I should mention: straight-sided horns have a property known as "boundary gain", like other horn profiles (exponential, hyperbolic, parabolic, etc) due to the presence of nearby walls, floor and ceiling.  The Khorn bass bin explicitly makes use of corner gain to complete the horn's last fold, and to provide boundary gain loading.  This also works the same for other horn-loaded bass bins mentioned above (as well as direct-radiating woofers in the corner of a room).  Here is an acoustic impedance plot of a "manifold exponential horn" using three sections in series, like that of a folded horn bass bin in a room corner:
 
5aca3acbc4c70_Manifoldexponentialhornimpedancevsfreq.thumb.PNG.e9a9019f9945dcf41ece7fa494b029c5.PNG
 
Even having one wall and a floor nearby to the horn's mouth will provide added boundary gain, thus further increasing the low frequency throat resistance of the horn, and improving low bass frequency output below the typical "rule of thumb" low frequency cutoff point of the horn.
 
Because the straight-sided horns of the MEH type have very large mouths compared to their conventional horn brothers, they actually load more efficiently via boundary gain than do exponential and hyperbolic horns of the folded-horn type. 
 
So the K-402-type full-range MEH loudspeaker is good to 30 Hz--20 kHz by using two woofers of 15" diameter and only a K-402-sized horn in boundary gain.  Its low frequency output is increased relative to direct radiating woofers due to horn loading, and this reduces the modulation distortion vis-à-vis direct radiating mode. 
 
Chris
Great info Chris, thanks for posting.

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