A Tale of Three Twelves


Here is an interesting graph of some data I took recently. This graph shows the sensitivity of three 12" drivers in very different enclosures. Altough the data was taken indoors, a calibration run was done to remove the effect of the room and give effective 1/2 space efficiency figures. The two Peerless drivers were fed with 2.8V RMS (1W into 8R) and the Thunder 12.3 was fed with 2V RMS (1W into 4R). What is really interesting when looking at this graph is just how effective the tapped horn is. At double the internal volume you would expect it to have around 3dB more acoustic output for a given input power than the sealed box, which has a similar cutoff. When driven at a sensible maximum input power the Thunder 12.3 manages to edge out the sealed 830847 by 3dB at 32Hz, but the Tapped Horn is still a clear winner with another 7dB of output at the same frequency. (Power level chosen for comparison: 700W (+28dB) for the Thunder 12.3, 200W (+23dB) for the sealed 830847 and 400W (+26dB) for the Tapped Horn.)

The Thunder 12.3 is an 85 liter enclosure with an AESpeakers AV12 woofer and two 1400gm 15" PR's with a box resonant frequency of around 20Hz. This sub is pretty flat to 18Hz in a small box so it's efficiency suffers compared to the other two, at around 86 or 87dB/W/M. The driver in my Thunder 12.3 is an AESpeakers C12.1, which is similar to the old AV12, but with a copper sleeve over the pole piece to reduce distortion and increase thermal power handling. This driver is very well made, has low distortion and good excursion. Thermal power handling is rated as 700W RMS, even though it has the excursion to handle quite a bit more power in the pass band.
Here are the C12.1 spec's with the voice coils in series:
Fs: 27Hz
Qts: 0.5
Qms: 9
Qes: 0.48
Re: 3ohm
Bl: 16Tm
Z: 4ohm
Le: 2.3mH
Vas: 37L
Mms: 265g
Sd: 446sqcm
Xmax: 23mm

The 830847 is a dual voice coil driver from Peerless' XXLS series. Here it is installed in an 85 liter heavily stuffed sealed enclosure and in this test only one voicecoil is driven. This increases the Q of the system resulting in a flatter response down low, but also robbing the system of 3dB of efficiency, even so it comes out at about 89dB/W/M. In a sealed box it doesn't go as low as the 12.3 in a PR assisted box, but the sound quality is excellent and the efficiency is good too. I could have used this driver in a flat to 20Hz vented alignment, but it would have been much larger in internal volume compared to the Thunder 12.3. 160 liters tuned to 20Hz seems to work very well, and gives a useful efficency advantage over the sealed box I tested with only one coil driven. Maximum system output would be about the same as the 12.3, but with half the input power.
Here are the 830847 spec's with the voice coils in parallel (One Voice coil driven for the measurements):
Fs: 21.8Hz
Qts: 0.36
Qms: 10.24
Qes: 0.37
Re: 2.5ohm
Bl: 10.2Tm
Z: 4ohm
Le: 1.8mH
Vas: 144.2L
Mms: 110.9g
Sd: 466sqcm
Xmax: 12.5mm
With one coil only the Qe increases to 0.74

The 830564 is a dual voice coil driver from Peerless' XLS series. It is specifically designed to be used in small sealed enclosures in cars, but I found it works very well in a 30Hz Tapped Horn. Total enclosure volume is around double either of the other two boxes in this test.
Here are the 830564 spec's with the voice coils in parallel (Voice coils in series for the measurements):
Fs: 48.8Hz
Qts: 0.61
Qms: 8.06
Qes: 0.66
Re: 1.2ohm
Bl: 9.3Tm
Z: 2ohm
Le: 0.9mH
Vas: 20.4L
Mms: 156.8g
Sd: 466sqcm
Xmax: 12.5mm
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