AJ wrote:

Hello,This is my first post to this site, which I have been absorbing for over a year.A horn project that I have been considering would include either a 1 inch or 1.5 inch throat compression driver into a Tractrix profile horn of which I have not yet finalized the frequency range that would be covered. The drivers I have been researching come in either a radial or annular phase plug version. I would like to know some of the audible differences of radial vs annular and the musical end result.

your question has no answer. Unfortunately.

There is a number of different phase plugs but they all pretty much devised into 3 groups:

1)      Radial slots 
2)      Concentric annular slots
3)      Combination slots – a compromise between advantages of radial and circumferential topologies 

I clearly know and understand the cons and pros of each topology but unfortunately strategic reasoning not always works in sound and the specific implementation of the given driver in most of instances is much more important than any super theory behind the phase plugs topology.

Do not forget that we have no option to have an identical driver, to use for instance radial and annular slots and compare results. Sure we can but who said that annular phase plug to demonstrate own advantage would need the SAME driver. Would it be possible that annular plug might need to move diaphragm a few millimeters closer, or to change damping juts a bit or to rise crossover point for 100 Hz or zillion other factors…  Who would consider all of it? In the best scenario people would do analyses of high frequency cancellations caused by standing waves into phase-plug slots and it theoretically would be enough. The really it much more complex and not predictable, not to mention that the best sounding drivers do have almost induced standing waves in the plugs….

So, unless you are a machinist and make your own drivers (if you are then let to be friends) then you shall not wary about radial vs. annular phase plug as if you buy drivers you do not buy a plug topology but you buys a complete driver implementation.  There are latterly hundreds reason why a driver  you bought might or might not sound as you expect – how would you define what in that Sound came from the phase plug?

Do not forget also that is real word 99% of drivers out there are used with misalign diaphragms. So, if one of the driver with the specific phase plug topology sound better then it is because of the plug topology or because the given misalignment of the diaphragms complicit with the given phase plug slot patter and sizes…