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Old 2nd May 2009, 10:57 AM   #1
domtw is offline domtw  France
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Default Short Tractrix tweeter wave guide

Hello,
I am planning to concentrate on the drivers time alignment for my next 2 way system.
I will measure the delta time between input and output signal through both drivers, using a fixed distance between the driver and the microphone.
From this measurement I will calculate how much I should set the tweeter back from the woofer.
Because I want to use a single panel, I will have to dig a short wave guide for the tweeter.
I am not sure which curve I should use. For the moment I am thinking of using a tractrix one.
The tweeter (Vifa XT25SC90-04)
(Vifa XT25SC90-04) has a dome diameter (outer ring of the dual ring) of 41mm.
Let's pretend that I will have to place the tweeter 25mm behind the woofer. My "short horn" will then be 25mm deep.
If we look at this curve:
Click the image to open in full size.

Should I use the curve from x=0 to x=25?
Or from x=120 (where the diameter is around 41mm, r=20.5) to x=145 (120+25) ?

I would be glad to have your opinion about this.
Thank you
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Old 2nd May 2009, 09:31 PM   #2
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I've always had tweeter in seperate enclosure then had wife slide it back and forth till it sounded best (using 24db LR active crossover). And passive parts also shift driver positions. I think 1 cap moves it back, but I'm unsure.

I can't speak for a tractrix, but this is a very good horn. But it is a round, spherical mouth, and I have no idea of the contour.
http://www.mcmelectronics.com/product/54-580

here is an article on horn loading a dome.
http://www.zaphaudio.com/hornconversion.html

The only flaw I find is that the drooping response is due to the weaker strength magnet not able to load the horn like a compression driver. Mass corner = fs/qts * 2. Compression drivers roll off due to the sheer inductance, but some horns such as a 90 x 50 add high freq lift, unlike a constant directivity or round horn.

here is john K's project
http://www.zaphaudio.com/Waveguidetmm.html


And here is another who built it
http://www.htguide.com/forum/showthread.php4?t=30373

Norman
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Old 3rd May 2009, 01:34 AM   #3
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Include the xover's contribution to your calculations/tests.

Using a horn to give you a setback may or may not give you the desired results... You can't merely pick an arbitrary distance and build an arbitrary curve to meet that criterion.

There are issues using standard drivers to load a horn. Bruce Edgar found this out many years ago. Also the standard tweeter is designed to have a flat freq response on axis with no horn, add the horn and you will no longer have a flat freq response.

There may be means to get fixed setback without the effects of the horn?

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Old 3rd May 2009, 05:05 AM   #4
Mach5 is offline Mach5  United States
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Default Re: Short Tractrix tweeter wave guide

Quote:
Originally posted by domtw
Hello,
I am planning to concentrate on the drivers time alignment for my next 2 way system.
I will measure the delta time between input and output signal through both drivers, using a fixed distance between the driver and the microphone.
From this measurement I will calculate how much I should set the tweeter back from the woofer.
Because I want to use a single panel, I will have to dig a short wave guide for the tweeter.
I am not sure which curve I should use. For the moment I am thinking of using a tractrix one.


Should I use the curve from x=0 to x=25?
Or from x=120 (where the diameter is around 41mm, r=20.5) to x=145 (120+25) ?

I would be glad to have your opinion about this.
Thank you
about the curve. Tractix flares are different for the cutoff freqeancy. This curve is almost certainly to low for the tweeter. just change the parameters in the spreadsheet you got that off of and it will give you a better one
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