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Bacek

Superelliptical tractrix horn for tweeter

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Superelliptical tractrix horn for tweeter - a.k.a small rain from a big cloud.

 

Why a.k.a. It took me several years to actually implement this idea and originally I was planing it for 400Hz horn.
I started to think about it in Forte days, I was planing to build some external mid/high horn for fun.
Then as replacement horn for La Scala. Original material suppose to be something else. Don't know English name
but I was planing it as cast made of polymer resin with mineral filler. Friend of mine was doing round horns with it
and I really liked the product.

 

So as I said got idea maybe 5 or more years ago, I did math, CAD project and laid it into a drawer for a long time.
Doing large horns is really challenging. Inspired by Dave A work on tweeters I thought why not try something much smaller fist.
So I ended with small solid beech wood tweeter horn instead of huge cast resin one.

 

OK so why Superellipse? It's fun curve. Special cases of this curve are circle, ellipsis and in the end rectangle.
By manipulating basic curve parameters we can do smooth transition from circle to rectangle
or in this specific case rectangle with rounded corners and all the way be compliant with Tractrix horn flare.

 

Why to bother when we have elliptical horns? Because using same front area we can make horn with lower cut of frequency without having
any sharp edges on wave path. So basically join elliptical and rectangle horn strong points.
Of course it's not always desirable to have lowest cut of freq but in this case math looked good.

 

I have created parameterized xls sheet witch which I can control how transition from circular throat changes to semi rectangular mouth.

 

Horn is calculated for 1544Hz and has good loading from 4147Hz (strange tails but I was shooting for particular mouth size)
For this particular horn for half of length it goes smoothly from circle to ellipsis and then shape starts to be more rectangular.
But this is fluent and I could make it almost rect from the beginning.

 

From math I did FreeCAD project (or actually my xls almost generates it).

SET_T_Cad.jpg.fae9a2995e75764b1405edec9f3190b7.jpg

 

Then I had to find someone who will make wooden prototype. It's not so easy to find someone who wants to play with such "large", 4 items, orders, but finally I did.

 

As always with prototype not everything is 100% correct so cut off is probably bit higher  but as proof of concept it went well. I stained it little bit and painted with semigloss lacquer
I decided to use B&C DE120 for it. Guess I was not reading too carefully because to use it I needed to scrap some wood from top hat to fit drivers magnet.

SET_T_1.jpg.6f992add121c8aee14aa9aa33af1cf8e.jpg

 

SET_T_2.jpg.4374e16b54beae50182fc028006035d2.jpg

 

I did some measurements to compare K77 square, K77 round in original horns and DE120 to mine. Looks fine.  Green is round, red is square, blue is DE120. Don't pay attention for reference level, my mike has +3 or more dBa from 16kHz but as reference to other K77 measurements it's fine.

 

SET_T_Measure.jpg.b851e9f141f29bec00a0bf1777761349.jpg


Currently my system plays better then ever. Unfortunately I cannot say it's all because of horn/tweeter change.
At the same same I went from passive to fully active system. So currently I'm using active digital crossover (Najda DSP) with 6 channel Rotel amp.
Additionally to normal crossover points I did full time delay alignment for all drivers.
With everything changed I really love to listen live recordings. For good realizations it sometimes gives me an illusion of being actually there at time of a concert.

 

Now it's time for bass extension with La Scala bass reflex mod

 

 

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55 minutes ago, Westcoastdrums said:

Indeed 

Definitely I copied Dave's mounting scheme, older one, monolithic that ALKENG was selling too. It's good idea for DIY/wooden project. I'm not aware of what exact flares Dave's horns are. Except of obvious, different mouth shape, mine are shallower (28mm deepth) as they don't have any mounting spacer, just pure tractrix flare (or sadly almost, thanks to CNC production error).

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15 hours ago, Bacek said:

Definitely I copied Dave's mounting scheme, older one, monolithic that ALKENG was selling too. It's good idea for DIY/wooden project. I'm not aware of what exact flares Dave's horns are. Except of obvious, different mouth shape, mine are shallower (28mm deepth) as they don't have any mounting spacer, just pure tractrix flare (or sadly almost, thanks to CNC production error).

There is only one reasonable mounting scheme that will work on older speakers that are rear mounted so not hard to extrapolate where you came from.  

I had always wondered how the narrowing of the ellipse to the sides on the other horns affected the off axis sound.  I notice yours are not full ellipses and it would be interesting to see if there is a perceived benefit on dispersion and sound.

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Unfortunately I'm in microphone modification phase (decomposed internal preamp). But off axis measurements are planned.

I did not notice that sweet spot is especially wider or narrower with those than k77. But I don't believe it could be. When you are moving left or right you are changing distance from one or other speaker so time delay aspects come in play.

 

Any way I'm thinking about another version of those, which will be almost full rectangle and without any "dead space" on front. But I guess this time I'll try with 3D printing. Its more widely available in my neighborhood.

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On 10/28/2019 at 1:56 PM, Bacek said:

Additionally to normal crossover points I did full time delay alignment for all drivers.

On 10/28/2019 at 1:56 PM, Bacek said:

 

 

I often wonder how audible the time alignment is, the weak spot of the LaScalas on paper.

I should probably test the resistance on my k-77 tweeters to see if they are in spec.

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2 hours ago, Bubo said:

I often wonder how audible the time alignment is, the weak spot of the LaScalas on paper.

The La Scala is much like the Belle (i.e., there is a slightly higher bass bin--midrange crossover frequency in the Belle, and slightly different midbass "hump" in response, but otherwise, it's basically the same design in a slightly different form factor).  The difference in sound tri-amping and time-aligning the Belle was the difference in my retaining the Belles vs. selling them.  Having Jubilees on either side of one of the Belles as a center to judge the sound was a motivating factor, but the sound quality was transformed by time alignment.  The following thread de of that experience--before switching to a JuBelle, etc.:

 

 

Chris

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It's LaScala with K55M and B&C DE120 in horn from topic. NAJDA DSP XO. Green is  time aligned, blue not alligned. I tought XO point was at 5200 but I probably made mistake checking files. It looks like 4500Hz -48dBa reject high/reject low FIR slope filter.

 

time_align.png

 

There was a longer discussion in 

 

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That MDF horn ALK sells was my inspiration.

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