Drivers ListRE Audio DriversMX-18 D2

ManufactureRE Audio
Motor TypeUnderhung
Driver Size18 "
Driver Weight38 lbs
Coil Diameter4 "
Coil MaterialAL
Coil Length2.28 "
Manufacture's Xmax22 mm
MSRP$749 USD
Year2011

TS Parameters (Measured by Data-Bass.com)

Qts0.487
Qes0.57
Qms3.34
Fs18.3 Hz
Res2.8 Ω
Le 1khz2.64 mH
Sd0.121 m2
Vas306.4 liters
Mms504.2 grams
Cms150 μm/N
BL16.88 Tm
BL2/Res101.8 N2/W
L/R Time Constant0.94 milliseconds
SplSens87.02 dB

Notes

This driver is now discontinued due to the dramatic increase in neodymium magnets over the last couple of years. This driver shares many characteristics with the XXX model including the cone, one piece concave dust cap cone reinforcement, 10 piece bolt together octopus frame with rubber front gasket, 9" spiders with sewn in voice coil leads, allen head screw down wire terminals and massive half roll rubber surround. 

The voice coil is a standard outside wind and not a split coil. It is 4" diameter 4 layer aluminum wire wound to a roughly 58mm length. The former appears to be stainless steel but it is not mentioned in any literature. The motor is rather unusual to say the least. It is based on neodymium magnets as previously mentioned in order to keep weight down and features a greatly extended steel top plate that comes up about 2.5" from the base of the motor where the frame bolts on. It also appears that the flat steel part of the top plate with the mounting screw holes is not very thick as judging from the hollow ring it produces when tapping it with a wrench. Also the there is a gap of about 1/8" around the entire perimeter of the extended top plate from the flat part of steel with the mounting screw that appears to vent air up from out of the motor. this appears to be the only venting. The motor uses the same cover plate as on the XXX but the means necessary to facilitate it's removal was not obvious as it was with the XXX. It may have been a bolt in the top of the pole piece but removal of that bolt is sketchy as it may have a role in holding all of the components of the motor in alignment. This is unfortunate as what is under the back cover plate would have been interesting. Speaking of the pole piece it is greatly extended and is machined with cooling chamfers into it. The top portion is a large chunk of machined aluminum also containing the cooling chamfers. Overall this is an odd motor design and it is unclear whether it is obviously an overhung design and underhung design or some in between variation. 

The specifications for this driver are no longer on the RE Audio site but their accuracy was questionable to begin with. some of their specs did not seem to calculate out correctly and it appeared that the 18" model had lower Q values than the 15" and 12" which would be very odd for drivers all using the same basic motor. 

Looking at the free air impedance trace shows that the dc resistance for the driver is about 2.8 ohms or 1.4 ohm per coil. The motor force is mediocre and curiously low for a driver employing a neodymium magnet system which usually results in high motor force. The FS is very low and the moving mass is heavy at over 500g which results in a driver with low top end sensitivity. There is no mention of demodulation circuits in the motor but examining it showed a very large block of aluminum and what looked to be some more further down into the gap so apparently some measures have been taken to control inductance. The relatively shallow increase in impedance above 100Hz appears to bear this out. 

Running the driver to large excursion levels in free air produces a lot of air noise from the motor. The motor has little venting other than a ring around the top plate and up from the gap itself. At about 1" peak to peak it starts to become pronounced. It increases with excursion and at about 2" peak to peak the air noise is substantial with a lot of hissing coming from the motor as air is jetted through these narrow spaces. On other drivers mechanical noise dominates but on this one the mechanical sounds aren't much up until very long excursions but the air noise is substantial. It is unclear how audible this air hissing noise will be once the driver is put inside of an enclosure with some damping material. The manufacturers xmax is labeled as 22mm one way but the physical xmech is at 48mm inward where the triple joint of the bottom spider will contact the extended top plate. The driver was pushed a little past 3" peak to peak where it was producing a lot of mechanical clatter on top of the motor air noise. Some of this may be distortion due to loss of most of the motor force that far from coil center position. Still this driver offers as much or notably more excursion ability than some other drivers claiming a bigger xmax figure. The free air excursion video in the forum gives example of the noise with excursion level.

This driver is odd. The weight is only 38lbs due to the neo motor, but the motor force is mediocre, it is very mechanically and thermally rugged, but has a very low resonance and sensitivity. The xmax is rated at only 22mm but there is plenty of excursion support available from the suspension and clearance in the parts. it doesn't have the titanic stroke that the XXX does. The cosmetics look good but the motor is somewhat of an oddball. This driver is a little bit of an enigma as to what they were going for from an engineering standpoint.

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RE Audio 18" MX-18 D2 Systems