Alternate drivers for the PHOENIX dipole woofer - 2
As I indicated in FAQ4 I have been trying to find a large excursion 12" driver that would give me twice the useable volume displacement as the 1252DVC or X6100 for 6 dB more acoustic output at about the same level of non-linear distortion. The cabinet size should increase only marginally and the cost of the driver should not more than double.
I have come to the conclusion that it is not possible to obtain this. What is possible, though, are lower distortion at the same displacement, and about 3 dB more useable output at twice the cost and in a 2.5" wider cabinet than for the 1252DVC or X6100.
The major problem with increased excursion is the disproportionate increase in aero-dynamic noise. It is generated by the turbulent flow of air in and out of the magnet vent and by the air trapped behind the spider if it is not completely vented. The typical, low cost stamped basket design is not suited for venting the spider. The extraneous noise becomes especially noticeable in an open baffle speaker, because it contains many high frequency spectral components and is intermodulated with the desired signal.
One possible lower distortion alternative driver is the Adire Audio "Shiva". As most drivers designed for box applications this driver (SHIVA 121947 8ohm, #758) has a low Qts=0.49. In combination with a higher Fs=26 Hz it causes the response to be down 5 dB at 20 Hz compared to the 1252DVC. The early roll-off could be equalized with a shelving lowpass filter if deemed necessary.
All measurements on this page are taken at about 1"
in front of the dust cap of the driver without baffle.
The Shiva is clearly a lower distortion driver for the same terminal voltage. This corresponds to the same output level around 60 Hz, but since the Shiva has less output below this frequency than the 1252DVC, it also has less total cone excursion. The frequency response would have to be equalized to match the 1252DVC to make this a complete comparison.
A more conventional harmonic distortion test with a single 30 Hz tone and 0.5" peak-to-peak cone excursion confirms the Shiva's better performance.
At 0.5"pp the Shiva's distortion looks quite respectable, especially when the driver arrangement in the PHOENIX woofer gives an additional reduction of even order distortion products. This would seem to apply even at 0.72"pp excursion, but now the aero-dynamic distortion takes over. It does not show up in the measurement in front of the dust cap, but on the magnet side.
In effect, the turbulence noise limits the useable excursion. The Shiva can effectively only give about 3 dB more maximum useable output. If more is needed, then the solution is to go to 4 drivers and a larger cabinet. Within those limits the Shiva will always give a lower distortion output than the 1252DVC or X6100 and does represent a worthwhile alternative, if the slightly larger cabinet and increased cost are not an issue.
The comparison has focused on the Shiva driver, but there are other 12" drivers of similar construction available which claim 1"pp excursion capability. Unless they have solved the aero-dynamic noise problem, they will also be limited to less than 1"pp maximum useable excursion in open baffle applications. It also must not be assumed that their non-linear distortion behavior at lower excursions is similar to the Shiva's. Only measurements can tell. To be useable at 1"pp excursion a driver must have, amongst other things, no dust cap and a fully vented spider, so that no air is trapped and forced to flow at high velocity through narrow openings with sharp edges.
The NHT1259 driver has a response that is almost identical to the 1252DVC except 2 dB lower in sensitivity. Though it claims Xmax = 13 mm, only 6 mm are useable in open baffle application due to noise and rapidly increasing distortion. The same applies also to the Sonicraft SC-1250. Its sensitivity is 3 dB lower than the 1252DVC. Either driver could be substituted for the 1252DVC/X6100 in a slightly wider cabinet. It would have the advantage of lower distortion at the same excursion level as the 1252DVC, but only marginally higher useable output. If significantly higher SPL levels are required, then it will take four of either the Shiva, NHT1259, SC-1250 or the 1252DVC/X6100 in a double height PHOENIX style cabinet.
For a higher output alternative see Woofer3.
Dipole woofer cabinet for Shiva size driver