Compression Driver with diaphragm at back of magnet vs at front

[ewsclass66]

Hi all

Just wondering if anyone would care to explain the pros and cons of having the CD diaphragm at the back vs at the front of the pole piece. I have seen both configurations on a range of CDs and wondered what the reasons are.

Thanks

 

[chris661]

Compression drivers need phase plugs to “assemble” the wavefront at the exit/horn throat. Since that needs a bit of distance to work, it makes sense to put the donut-shaped magnet around the perimeter of the phase plug. Keeps things neat.

Chris

 

[JMFahey]

Geometry.

Since diaphragm dome is a section of a sphere, and sound from all of its surface must converge at the same time at the horn throat; diaphragm-at-front is clumsy imperfect while diaphragm at back is natural, so better and simpler.

All radial paths are same length if diaphragm on back, while on front only a clumsy approximation is possible, so first is used in PRO drivers, latter only in cheap cheesy "PA horns" or cheap horn tweeters.

Good:

Wonder why Altec call this "Tangerine" 😉

Cheesy-cheap street PA horn driver, notice bulge of driver and (poor) phase plug at front:

Cheap horn tweeter (Fostex) cast zinc or plastic front phase plug.
Those holes are only an approximation of what´s actually needed.

 

[ewsclass66]

That's great, thanks guys for the info. I have come across good compression drivers with the 'bullet' style phase plugs at the front of the diaphragm which have had a nice sound.