I'd guess a little more than what is required to "keep up" with the dual 12" at 650Hz ;^).
The difference between displacement between the DCX464 and the DCX354 is about the same as between a 4" and 3" diaphragm, Peter Morris has stated previously in this thread that a 4" diaphragm driver is required to "keep up" with the 12" at 650Hz.
Compression drivers don't have "a whole lot of displacement", the lower annular ring probably has around 0.8mm.
Four times the displacement (excursion) is required for each halving of frequency to maintain the same level.
The DCX464 and DCX354 charts are for the "Driver mounted on 320 Hz exponential horn".
A 320Hz horn loads the diaphragm to a much lower frequency than the smaller horn used in the PM60.
The Eighteen Sound XT1464 horn "Usable Frequency Range" is specified as "above 500 Hz", with a recommended crossover frequency of 800Hz or above, but the response of the 12" requires a crossover in the 650Hz range.
Anyway, if you think the smaller diaphragms in the DCX354 will keep up to the 2x12" in the 650Hz range on the XT1464 horn, go ahead, and report back how they compare when the 12" are reaching 135+dB.
Art
