Horns for tweeters

I'm thinking of replacing the metal horn flares of my Ionovac tweeters with curcular machined aluminium ones. The stock crossover for the Ionovacs is 3.5k hz, but whether the stock horn does any loading or not, I don't really know.

My questions are...

1) Does the type of flare (Tractrix, exponential etc.) matter? Or would a straight cone suffice? Witness those Fostex types.

2) If it is a straight cone, how long & how wide at the mouth should the the horn profile be?

The quartz cell aperature (throat) of an Ionovac is about 5mm sq.

Thanks

Ken
 
Circular is good

There are trade offs to the shape of a horn flare. Circular is good if you want a uniform wave front with minimal distortion. But there are a good many designs that use a partial conical expansion that gives good directivity. The directivity will provide a more controlled pressure response. That basically means that the sound where you sit will be more uniform in it's real loudness at your ears.

In terms of imaging a round horn is the best. A tractrix profile is also one of the best available. There are more such as a Jean Michel Le Cléac'h profile. But most are variants that again have pros and cons.

However the machining is not going to be easy! If you are a machinist you will know what I mean. The profile would have to be roughed in carefully. It would probably be easiest to grind a custom profile cutter and have it well supported on the tool post to finish it off. Nasty work. For size ideas you want a low end at least an octave and a half below the cross over point. A minimum of 1000hz should be the low end flare length. The mouth size is what will help out the tweeter the most to actually project it's sound best. A wall horn size is best. Not a pi/4 but a pi/2 size. ( pi is the 3.14......... )

Mark