Just a question, which compression drivers would you consider sota as of now? I assume your horn speaker project uses one.
I have tested and used the JBL 2440 (375) and the JBL 2441. These are both 4" aluminum dome drivers with 2" exits. I have also tested the TAD TD-4001 which is a rework of the JBL 2440 (375) with a slightly larger magnet and a beryllium diaphragm instead of the AL of the JBL original. I have also tested the TAD TD-4003 which is a more modern design with an even more powerful neodymium magnet, reworked phase plug, a new design for the 4" Be diaphragm with only 1.3 grams of moving mass, and a 1.5" exit. I also measured and compared the JBL 476Be with the TAD TD-4003. Lastly I measured a few different JBL drivers with the Materion made beryllium replacement diaphragms. (Sold under the TruExtent name.)
Based on all of this research carried out over several years, the TAD TD-4003 is definitely the best compression driver that I have encountered. I have heard the extremely costly ALE drivers in the Magico Ultimate speakers... they sounded great, but I can't comment on the drivers as I have no data.
I saw a compression driver with aluminum diaphragm. Is that better than titanium?
Compression drivers have been made with phenolic resin impregnated cloth, mylar, aluminum, titainium, magnesium, and beryllium. One of the principal differences is the mass break point of the different materials. This is the point where the diaphragm produces and resonant peak and exhibits relatively high distortion. The non metallic diaphragms produce less audible distortion products, but they also do not typically operate as high in frequency. In general you want the mass break point to be as high in frequency as possible and ideally out of the passband. The better the material the higher in frequency the mass break point is.
Because of its extremely low mass, beryllium (number 4 on the atomic chart!) and well damped characteristics, it is absolutely the best material to use for transducers. Magnesium is the next best material, followed by aluminum and finally titanium. As has been mentioned. Ti has been used due to its low failure rate.
On the subject of Be. Virtually all of the manufacturers who use real Be in their drivers, be it the dome of a Focal or Revel speaker, a JBL, or a Radian... they have their domes (compression driver or standard domes) produced by Materion out of beryllium foil that is drawn into shape. Materion is one of the world's leaders in beryllium production and fabrication. TAD and Yamaha back in the day are exceptions. They both use vapor deposition to produce their domes, and in the case of the TAD they also use this process for their mid drivers cones.
The vapor deposition process produces a much more rigid Be diaphragm. This gives it better acoustic properties, but it does produce a diaphragm that is more fragile. When a TAD driver is driven excessively hard the diaphragm can shatter. In a worst case scenario this can produce toxic Be dust. The Materion Be foil will not shatter, but it can tear.
Sorry... probably more info than was needed.
Radian 951BePB. TAD TD-4003 is also a great driver but not manufactured anymore and price tag at the end was way too high IMO.
I have talked with the new US importer of TAD Professional. They have plans to bring certain TAD drivers back to the market including the costly but excellent TD-4003.