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I seem to remember that in 1976 there was AGAIN some experimentation with compression slots for the K-horns.  With PWK checking the builds at the plant to ensure the specific changes met his requests for testing.

 

Your La Scala has the oversized compression slot on its motorboard, so the splitter is wider than in the 1977 dimensions which I posted on the forum over 15 years ago, and which you are referring to.

 

It appears that your compression slot on your La Scala motorboard was altered (widened) AFTER production, since it is easy to see that the width is a much newer cut than the remnants of the original compression slot.  The splitter was likely changed out at the same time to a wider one due to the increased width of the compression slot.

 

As for the difference in the two K-horns, the splitter just starts the ball rolling to have the compressed sound waves begin their bifurcated path to first flares of the bifurcated bass horn, instead of some of it being reflected back to the woofer or diffused in all directions.  It is the horn lens itself which determines the expansion of air volume.

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On 5/2/2017 at 10:13 AM, HDBRbuilder said:

It appears that your compression slot on your La Scala motorboard was altered (widened) AFTER production, since it is easy to see that the width is a much newer cut than the remnants of the original compression slot.  The splitter was likely changed out at the same time to a wider one due to the increased width of the compression slot.

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On 5/2/2017 at 10:13 AM, HDBRbuilder said:

As for the difference in the two K-horns, the splitter just starts the ball rolling to have the compressed sound waves begin their bifurcated path to first flares of the bifurcated bass horn, instead of some of it being reflected back to the woofer or diffused in all directions.  It is the horn lens itself which determines the expansion of air volume.

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So you decided to alter the frequency response curves by making these changes to the bass bins and replacing the drivers, but are overly worried about splitter sizes?  Doesn't make much sense to me...just saying.  The changes you made do more to alter the response curve than ensuring that the splitters are perfectly dimensionally uniform.  On the LaScala, the best splitter "improvement" would be to install longer ones and notch the ends for a much closer fit to the wing assemblies, which would reduce air turbulence within the soundpathway at those points.  The same thing goes for the K-horn splitters.

 

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What lead you to increase the slot to what looks like about 5" x 12+"?  If my memory is correct, Mr. Paul first used 6" x 13" slots and later switched to 3" x 13" slots because it gave the bass horn and K-33 more output in the 300 to 400 Hz region, leading to a smoother overall frequency response. 

 

The bass horn doubles the impedance of the woofer.  Did you adjust your crossovers to account for the ~16 ohm bass section you have now? 

 

What does the frequency response of you horns look like with the new woofers? 

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5 hours ago, mach-1 said:

My K-33P are the older square magnet offerings.  Not much of a horn driver to begin with,  perhaps a band-aid fix?

 

I replaced the woofer inductors with lower DCR  3.0mH  units from Bob Crites.

 

My older speakers were given a +5 to -5 dB  bass to treble frequency response (-10 at lower bass),  just by ear they sound better.  One could go deaf running response curves on horn loudspeakers!

 

When I looked at the T-S parameters I  found for the EVM-15L, I thought the same thing.  Mr. Paul used EV 15WKs in early bass horns, with the 6 x 13 "slot".  The K-33-P and -E were purpose built for the K-horn.  If you had said it was not much of a woofer for a ported speaker like a Cornwall, I would have agreed.   You can consider the -E an upgrade, especially for the models with a 3" wide slot. 

 

If you'd run the response curves, you'd know whether that 3 mH inductor was big enough.  The calculated value is still larger. 

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I suspect that the ramp or prism is not very sensitive to size.  Granted it may have some effect in directing the high pressure coming out of the slot to the two paths. 

 

The illustration of the slot is more interesting.  It is reported as a low pass filter.  

 

I've written on this before in connection with the mystery of the slot.

It appears to me that we have a low pass filter formed by mass of the diaphragm.  Keele shows how high freqs roll off because of the mass of the diaphragm.  In a way, that can be called a low pass filter.  Essentially, the horn loading is taxing the force available from the motor. and moving the mass of the diaphragm bogs it down. further.  It is a first order filler.

 

What to do?

 

It would be nice if we can set up a resonance between the mass of the diaphragm and a chamber in front of it.  However if the horn throat is about half the size of the diaphragm we don't have much of a chamber.  The solution is to close off the path to the throat with the slot..

 

Now we have a mass in the diaphragm and a spring in the semi closed chamber.  We have to make sure that the acoustic impedance of the mass and the acoustic impedance of he chamber are equal at about the freq where the freq response is rolling off.  Now we have a second order filter which can have a peak just where we need it.  It becomes a second order filter with a peak where we need it.  Baranek shows something like this.  

 

I will admit that there might be something to the Klipsch design.  Bruce Edgar did an analysis which IMHO missed this point.  He apparently made some straight horns and could not make the slot work. If I understand correctly he thought the improvement was limited to the Klipsch structure. Maybe that is correct in that the prism and the right angle turns act to seal off the slot a bit.  We have a more effective sealed off chamber.

 

This may come as a bit of a surprise.  With a first order filter there can't be peaks.  Usually we see second order filters designed so that there is just a sharper roll off.  However, they can be designed to make a peak before rolling off.  

 

WMcD

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peaking of high freq.jpg

 

This is for a high freq horn of course.  There is an electrical circuit model where CM1 is the chamber in front of the diaphragm.  If there is none Cm1=0

PWK would have seen this and I believe this made him experiment with a front chamber for the bass horn.  Again, I believe that he knew it would be necessary to form a semi-closed chamber by the smaller slot.  All conjecture on my part but it makes sense.

 

At one time I did some calculations and found that the reactance of the weight of a K-33 diaphragm at about 350 Hz to be X.  I guestimated the volume of air in the chamber in front of the diaphragm to be something like a cylinder 12 inches in diameter and 1 inch tall.  The reactance was about X.  When reactances are equal we get a resonance.  Therefore I concluded this was a correct analysis.

 

Then I used Don Keele's electrical model on Spice plugged in the values.  It showed what is shown above, basically.  I can't find all that right now.  However since it was close and not off by an order of magnitude I was happy.

 

Thinking further: there is the issue that resonances are damped by the loading caused by the throat.  It could be that the slot reduces the loading.  Some diagrams out there show the design has somewhat lower overall output.  I thought the slot was lossy.  But maybe reduces loading and does not damp out the resonance as much as a full sized slot would.

 

This has some similarity to using a ported box in the back of a woofer.  Response in a closed box is rolling off.  However the port and box causes a resonance just as it rolls off to goose up the overall response (through the port) just where it is rolling off. Roughly, we get the lower half octave out of the port.  The penalty is that below box-port resonance the response drops off very quickly.

 

My best thoughts.  Sorry if I talk too much.

 

WMcD 

    

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