Here's some background on JBL compression driver and diaphragm evolution. There is more detail here:
http://www.audioheritage.org/html/pr...logy/435be.htm
From the very first JBL compression driver of 1948 (the D175), until 1980, there was no change in the basic engineering of JBL's small format compression drivers. They all used an Alnico motor with an aluminum diaphragm and tangential surround. 1980 saw the first major change with the development of the diamond surround. While still constructing the diaphragm out of aluminum, the surround was pressed into a series of diamond shaped polygons. The reason for this was to increase the HF extension by taking advantage of a phenomenon known as parasitic resonance. The smaller diamond polygons increase the second resonance to the point that it reinforces HF response in the highest audible octave. This was primarily intended for the large format 375/2440, which otherwise was restricted in response to 10khz. Using parasitic resonance, extension could be pushed to 18khz and eliminate the need for three-way systems in most pro applications. While the small format compression drivers were in less need of this HF boost, they were found to benefit as well and they were converted to diamond surrounds at the same time.
However, there was a penalty to be paid for this change that did not become fully apparent until a high profile incident in 1982. The problem was that the diamond peaks concentrated stresses to a much greater degree than the previous tangential geometry. The stress concentration led to premature failures of the diaphragms. The incident in question was the premier of the brand new JBL installation at the Academy of Motion Picture Arts and Sciences theatre. It was considered quite a coup for JBL to replace the old Altec sound system with their then state-of-the-art cinema system (for which JBL would later win an Academy Technical Award). However, during the premiere, the diaphragms on nearly all of the 2441’s failed. As a result, an edict was issued from on high at JBL to immediately fix this issue. Six months later, the result was the titanium diaphragm.
Titanium is an order of magnitude more resistant to fatigue failure. The diamond surrounds, with their increased HF response, could still be employed, but power handling was increased by 30% with greatly increased longevity. Nonetheless, there were tradeoffs involved. As Giskard stated, titanium has poorer internal damping than aluminum. Further, titanium has reduced stiffness meaning that it is driven to breakup at a lower frequency than aluminum. The net result is increased distortion. This increase is relatively miniscule, but many listeners find it audible in critical applications like monitoring or home hi-fi.
For this reason, JBL has reverted to aluminum and introduced beryllium diaphragms on their latest compression drivers. The use of a composite mylar surround has solved the fatigue problem. The use of beryllium has solved the diaphragm breakup issue so that these drivers have the lowest levels of distortion of any compression driver yet built by JBL. The smaller diaphrams (3" vs 4") of these newer drivers furhter pushes the frequency of breakup modes and increases overall HF response. Finally, applying EQ allowed these drivers to have the same UHF extension without the need for parasitic resonance and its attendent distortion.
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