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RE: More horn ruminations: Tractrix? Or La Cleac'h?

Posted by John Sheerin (A) on February 11, 2015 at 18:36:58

In Reply to: RE: More horn ruminations: Tractrix? Or La Cleac'h? posted by Presto on February 11, 2015 at 08:47:15:

On my horn (1.5" throat JBL 2435 - beryllium diaphragm), I find the polar pattern goes screwy above 9k (c/d) and the horn / driver starts to store energy, so on your 1.9" throat, I'd try to stay below 7k as a start. So your idea sounds good to me.

I use a JBL 2402 tweeter on a custom Le Cleac'h horn / phase plug I designed. It gets me the DI I want to match up to my midrange horn plus low diffraction. Response is crap (some very sharp notches), but I don't find that to be a problem once the average response is EQ'd to my target. That's just in the design of the driver as far as I can tell - haven't investigated it too much more. I know one notch is a circumferential mode in the plug, but there are others that don't go away when I fix that. I just know my FEA model of the original driver had the same response issues, but the DI was way worse. I'm not sure it matters at those frequencies though - the notches are very narrow. I currently have it crossed at 2nd LR at 9k acoustic, but some modeling is showing that it might be better crossing it 1st order acoustic at 9k with another 2nd order highpass at 2.5k. I measured it as being good for distortion at the levels needed to do 115dB/1m in the passband with this acoustic roll off, so it should be okay with this for my purposes. For this I would roll my mid off at 6k 2nd order butterworth acoustic, then another 1st order butterworth acoustic at 15k. Then playing with the delay gets it working right - it needs to be within about a 7mm window to not completely screw up the response at crossover. The actual acoustic crossover is about 7k, but it's at around -5dB - kind of an oddball. This gets me on axis, listening window, and power response without a dip at crossover. The downside is that there is kind of a funky interference pattern vertically, but I think it's dense enough that it's not perceptually audible. I found you can do a crossover without this interference, but it has other downsides that I thought were bigger risks. Still have to try listening to this though.

I would avoid the slot horn - never played with it, but the polar is going to be very wide in one direction which won't match to anything you're likely to cross it to. This will color the in room response.

Every time I've looked at a coax-mounted tweeter (not in the driver itself, but just hung in the horn mouth), it hasn't looked good.

On the rollback, a long time ago I did some simulations and found you don't really need the full rollback to get most of the benefits. You do want it to go beyond 90 degrees to on axis though. You could just add a large radius to the end of a tractrix that looks about like a Le Cleac'h and it would probably be fine, but I've always liked the sound and simulated polar responses of the Le Cleac'h horns better than tractrix.

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